Seasonality of foliar respiration in two dominant plant species from the Arctic tundra: response to long-term warming and short-term temperature variability Mary A. Heskel A,B,E , Danielle Bitterman A , Owen K. Atkin B , Matthew H. Turnbull C and Kevin L. Griffin A,D A Department of Ecology, Evolution, and Environmental Biology, Columbia University, New York, NY 10027, USA. B Research School of Biology, The Australian National University, Canberra, ACT 0200, Australia. C School of Biological Sciences, University of Canterbury, Private Bag 4800, Christchurch 8041, New Zealand. D Department of Earth and Environmental Sciences, Lamont-Doherty Earth Observatory, Columbia University, Palisades, NY 10964-8000, USA. E Corresponding author. Email: [email protected]Abstract. Direct measurements of foliar carbon exchange through the growing season in Arctic species are limited, despite the need for accurate estimates of photosynthesis and respiration to characterise carbon cycling in the tundra. We examined seasonal variation in foliar photosynthesis and respiration (measured at 20 C) in two field-grown tundra species, Betula nana L. and Eriophorum vaginatum L., under ambient and long-term warming (LTW) conditions (+5 C), and the relationship of these fluxes to intraseasonal temperature variability. Species and seasonal timing drove most of the variation in photosynthetic parameters (e.g. gross photosynthesis (A gross )), respiration in the dark (R dark ) and light (R light ), and foliar nitrogen concentration. LTW did not consistently influence fluxes through the season but reduced respiration in both species. Alongside the flatter respiratory response to measurement temperature in LTW leaves, this provided evidence of thermal acclimation. The inhibition of respiration by light increased by ~40%, with R light : R dark values of ~0.8 at leaf out decreasing to ~0.4 after 8 weeks. Though LTW had no effect on inhibition, the cross-taxa seasonal decline in R light : R dark greatly reduced respiratory carbon loss. Values of R light : A gross decreased from ~0.3 in both species to ~0.15 (B. nana) and ~0.05 (E. vaginatum), driven by decreases in respiratory rates, as photosynthetic rates remained stable. The influence of short-term temperature variability did not exhibit predictive trends for leaf gas exchange at a common temperature. These results underscore the influence of temperature on foliar carbon cycling, and the importance of respiration in controlling seasonal carbon exchange. Additional keywords: Betula nana, Eriophorum vaginatum, Kok effect, photosynthesis, seasonality. Received 9 May 2013, accepted 22 September 2013, published online 31 October 2013 Introduction Recent climate change in the Arctic tundra has resulted in a cascade of warming-mediated ecological changes (Post et al. 2009; Wookey et al. 2009), many of which directly and indirectly impact terrestrial carbon cycling in this carbon-rich region (Shaver et al. 1992; Chapin et al. 1995). Among these changes, warming is associated with the lengthening of the Arctic tundra growing season through the promotion of earlier spring snowmelt (Stone et al. 2002), which is often linked to increased woody shrub cover (Chapin et al. 2005; Pomeroy et al. 2006), and delayed snow cover in the autumn. The potentially two-tailed extension of the snow-free season and warmer ambient growth temperatures may alter the carbon balance of this system by increasing the duration of carbon assimilation via photosynthesis and carbon release through plant and soil respiration. However, the seasonal dynamics of many aspects of the terrestrial carbon cycle, especially at the leaf level, remain unclear. This is especially true when considering the light inhibition of respiration, which may significantly control the amount of C respired by vegetation in Arctic species (Heskel et al. 2012, 2013). As the tundra becomes increasingly modified via warming, it is important to understand the relationship of foliar gas exchange, photosynthesis and respiration to seasonal timing and temperature variability in order to strengthen predictive ability for understanding C dynamics in this landscape. Previous studies have examined growing season C exchange in Arctic tundra, primarily at the ecosystem scale using data from eddy covariance (Vourlitis and Oechel 1999; Vourlitis et al. 2000; Loranty et al. 2011; Rocha and Shaver 2011) and large chamber methods (Oberbauer et al. 1998; Oechel et al. 2000; Welker et al. 2004; Sullivan et al. 2008; Natali et al. 2011). Experimental warming can enhance both ecosystem respiration and gross CSIRO PUBLISHING Functional Plant Biology http://dx.doi.org/10.1071/FP13137 Journal compilation Ó CSIRO 2013 www.publish.csiro.au/journals/fpb
Transcript
Seasonality of foliar respiration in two dominant plantspecies from the Arctic tundra response to long-termwarming and short-term temperature variability
Mary A HeskelABE Danielle BittermanA Owen K AtkinB Matthew H TurnbullC
and Kevin L GriffinAD
ADepartment of Ecology Evolution and Environmental Biology ColumbiaUniversity NewYork NY 10027USABResearch School of Biology The Australian National University Canberra ACT 0200 AustraliaCSchool of Biological Sciences University of Canterbury Private Bag 4800 Christchurch 8041 New ZealandDDepartment of Earth and Environmental Sciences Lamont-Doherty Earth Observatory Columbia UniversityPalisades NY 10964-8000 USA
ECorresponding author Email maryheskelanueduau
Abstract Directmeasurements of foliar carbonexchange through thegrowing season inArctic species are limited despitethe need for accurate estimates of photosynthesis and respiration to characterise carbon cycling in the tundra We examinedseasonal variation in foliar photosynthesis and respiration (measured at 20C) in twofield-grown tundra speciesBetula nanaL and Eriophorum vaginatum L under ambient and long-term warming (LTW) conditions (+5C) and the relationship ofthese fluxes to intraseasonal temperature variability Species and seasonal timing drove most of the variation inphotosynthetic parameters (eg gross photosynthesis (Agross)) respiration in the dark (Rdark) and light (Rlight) and foliarnitrogen concentration LTWdidnot consistently influencefluxes through the season but reduced respiration in both speciesAlongside the flatter respiratory response to measurement temperature in LTW leaves this provided evidence of thermalacclimation The inhibition of respiration by light increased by ~40withRlight Rdark values of ~08 at leaf out decreasingto ~04 after 8 weeks Though LTW had no effect on inhibition the cross-taxa seasonal decline in Rlight Rdark greatlyreduced respiratory carbon loss Values of Rlight Agross decreased from ~03 in both species to ~015 (B nana) and ~005(E vaginatum) driven by decreases in respiratory rates as photosynthetic rates remained stable The influence of short-termtemperature variability did not exhibit predictive trends for leaf gas exchange at a common temperature These resultsunderscore the influence of temperature on foliar carbon cycling and the importance of respiration in controlling seasonalcarbon exchange
Received 9 May 2013 accepted 22 September 2013 published online 31 October 2013
Introduction
Recent climate change in the Arctic tundra has resulted in acascade of warming-mediated ecological changes (Post et al2009 Wookey et al 2009) many of which directly andindirectly impact terrestrial carbon cycling in this carbon-richregion (Shaver et al 1992 Chapin et al 1995) Among thesechanges warming is associatedwith the lengthening of theArctictundra growing season through the promotion of earlier springsnowmelt (Stone et al 2002) which is often linked to increasedwoody shrub cover (Chapin et al 2005 Pomeroy et al 2006) anddelayed snow cover in the autumn The potentially two-tailedextension of the snow-free season and warmer ambient growthtemperatures may alter the carbon balance of this system byincreasing the duration of carbon assimilation via photosynthesisand carbon release through plant and soil respiration Howeverthe seasonal dynamics of many aspects of the terrestrial carbon
cycle especially at the leaf level remain unclear This isespecially true when considering the light inhibition ofrespiration which may significantly control the amount of Crespired by vegetation in Arctic species (Heskel et al2012 2013) As the tundra becomes increasingly modified viawarming it is important to understand the relationship offoliar gas exchange photosynthesis and respiration to seasonaltiming and temperature variability in order to strengthenpredictive ability for understandingCdynamics in this landscape
Previous studies have examined growing season C exchangein Arctic tundra primarily at the ecosystem scale using data fromeddycovariance (Vourlitis andOechel 1999Vourlitis et al 2000Loranty et al 2011 Rocha and Shaver 2011) and large chambermethods (Oberbauer et al 1998 Oechel et al 2000Welker et al2004 Sullivan et al 2008 Natali et al 2011) Experimentalwarming can enhance both ecosystem respiration and gross
ecosystem productivity creating little difference in seasonal netecosystem exchange and maintaining a role as a C sink thoughtundra type and variations in interannual environmentalconditions such as water availability and ambient light caninfluence this effect (Welker et al 2000 2004 Oberbaueret al 2007 Sullivan et al 2008) Alternatively other casesexhibit changes of net ecosystem exchange through anincrease in ecosystem respiration leading to a C source inboth experimentally warmed sites (Natali et al 2011) andnon-manipulated sites with generally warmer soil and canopytemperatures (Cahoon et al 2012) Given the difficulty ofpartitioning soil and autotrophic CO2 fluxes in tundra fieldstudies few studies examine the effect of experimentalwarming on C exchange in vegetation alone (Chapin andShaver 1996 Shaver et al 1998 Starr et al 2008)
Short-term intraseason temperature responses ofphotosynthesis and respiration must be considered in concertwith long-termwarming responses tomechanistically understandand robustly predict rates of foliar and ecosystem carbon cyclingin the Arctic tundra given the highly fluctuating temperatureenvironment during the growing season Short-term ambienttemperature conditions may drive significant change in bothecosystem and leaf carbon balance in addition to the currentand dramatic warming trend in this region (Serreze et al 2000)For example foliar respiration can be sensitive to small rapidshifts in temperature and can thermally acclimate over time(Atkin and Tjoelker 2003 Atkin et al 2005) In both warm-grown and warm-treated leaves this flexibility can lead todecreased respiration compared with cold-grown and cold-treated leaves when measured at a constant temperature(Atkin and Tjoelker 2003) This may be attributed toreorganisation of the mitochondrial alternative and cytochromepathways (Armstrong et al 2006b 2008 Searle et al 2011)Many factors are at play in warming Arctic ecosystems ndash growthforms are likely to respond to an altered temperature regime inmorphologically and physiologically different ways (Gough andHobbie 2003 Heskel et al 2013) and the duration of warmingmay strongly influence these changes (Elmendorf et al 2012)Kornfeld et al (2013) recently reported a significant decrease inrespiration in the two focal species of our current study Betulanana L and Eriophorum vaginatum L grown under the samelong-term warming treatment conditions suggesting potentialthermal acclimation at a longer timescale Although the Kornfeldet al (2013) study demonstrated the potential role of underlyingmitochondrial mechanisms in response to long-term warmingtreatment it is still unclear how this long-term warming willimpact temperature sensitivity at shorter time scales through thegrowing season
Many aspects of the seasonality of foliar C cycling and theinfluence of long-term warming and shorter-term intraseasonaltemperature variation on these processes remain unknown in theC-rich tundra landscape In our current study we examine foliarphotosynthesis respiration and the associated leaf traits in twoabundant species B nana a woody shrub and E vaginatum atussock-forming graminoid to acquire accurate estimates of thespeciesrsquo contributions to the ecosystemrsquos carbon balance duringthe growing season This study also investigates the relationshipof these mechanistic responses to seasonal timing long-termwarming treatment and short-term ambient temperature
variability Additionally a related objective of this study wasto quantify the light inhibition of respiration through the growingseason As Arctic tundra plants experience near-complete orcomplete 24-h photoperiods during the growing seasonestimates of respiration in the light (Rlight) which account forthe known inhibition of respiration in light (the lsquoKok effectrsquo)were quantified in addition to dark respiration (Rdark) In thisstudy we focus on the less frequently measured but ecologicallyrelevant flux of Rlight and its relationship to the more commonlymeasured flux of Rdark to tease apart controls of light inhibitionand emphasise the importance of light inhibition in the carboncycling of Arctic plants Respiration in the dark is an inaccurateestimate of actual plant carbon release in the nightless growingseason but it provides a point of comparison across Arctic andnon-Arctic sites and serves as an assumed uninhibited rate Wehypothesise that the seasonal dynamics of gas exchange willcorrespond to the energy demand of the growing vegetation (1)rates of photosynthesis should increase as leaves develop thoughthis trendmay be stronger in the deciduous shrubB nana than theevergreen graminoid E vaginatum (2) respiration rates will begreatest in the early growing season when energy demandfor growth is highest and (3) the elevated energy demand forgrowth will be associated with relaxed levels of light inhibitionof respiration as shown in elevated CO2 studies (Shapiro et al2004 Crous et al 2012) Further we hypothesise that growthunder long-term warming will suppress the respiratory responseto temperature due to thermal acclimation (Atkin and Tjoelker2003) To test these hypotheses in tundra vegetation we utilisedglobal change experimental plots that have been continuouslymaintained for nearly 30 years by the Arctic Long-termEcological Research (ARC LTER) site at Toolik Lake Alaskaand measured foliar gas exchange and related leaf traits atcondensed intervals over 8 weeks during the 2010 growingseason Our study represents a high temporal resolutionexamination of the mechanisms controlling plant carbonbalance through the tundra growing season and is the first toquantify the seasonal response of the light inhibition of foliarrespirationmaximum rates of ribulose-15-bisphosphate (RuBP)carboxylation and RuBP regeneration in these common tundraspecies
Materials and methodsField site and species
The study took place over 8 weeks during the 2010 growingseason (6 Junendash28 July) at the ARC LTER field site near ToolikLake (68380N 149360W) on Alaskarsquos North Slope located254 km north of the Arctic Circle Air temperature relativehumidity light and precipitation were all measured andrecorded at this site by the Toolik Field Station EnvironmentalData Center (Environmental Data Center Team 2011) All leaveswere sampled from experimental plots in moist acidic tundra(MAT) established andmaintained by theARCLTER since 1989(similar to anolder experiment described byChapin et al (2005))The MAT site consists of four randomised blocks of 5 20mtreatment plots separated by a 1-m buffer arrayed two-by-two ona slightly sloped and poorly drained hillside Leaves weresampled from control plots and plots that have been warmedvia greenhouses since 1989 These wood-framed greenhouses
B Functional Plant Biology M A Heskel et al
covered with transparent 015-mm plastic sheeting passivelyincrease the air temperature by ~5C during the growing seasonData on thawdepth and soilmoisture though likely influenced bythe passive warming treatment as well as to influence foliar gasexchange rates were not available for this study The focalspecies for our study are common and abundant at the MATsite Eriophorum vaginatum L (cottongrass) the tussock-forming sedge that is widely distributed over much ofAlaskarsquos North Slope northern Canada and northern EurasiaandBetula nana nanaL (dwarf birch) a deciduous woody shrubthat is also abundant and often dominant over much of the entireArctic region
Foliar carbon exchange
CO2 fluxes of photosynthesis and respiration were measuredusing an infrared gas analyser (IRGA LI-6400XT PortablePhotosynthesis System LI-COR Lincoln NE USA) Allmeasurements were made on clipped leaves of E vaginatumand small branches forBnana collected from the long-termMATgreenhouse and control plots re-cut under water in the field andtransported in water to the laboratory where they acclimated tomeasurement temperatures as inHeskel et al (2012) Preliminarytests on these species showed no difference in the rates of gasexchange and stomatal conductance between field-measured andlaboratory-measured leaves (Griffin unpublished) as has beenshown previously in other species (Mitchell et al 1999 Turnbullet al 2003) This sampling technique was required to provide agreater degree of temperature control maximise the numberof replicates and minimise the time between replicates over theshort growing season and during potentially rapidly changingenvironmental conditions in the field For each E vaginatumsample we collected approximately 10 leaves from a singletussock in order to provide sufficient leaf area in the leafchamber (~6 cm2) For B nana the one or two measuredleaves from the sampled branch that were smaller than the6 cm2 cuvette area were measured for leaf area after IRGAmeasurements and gas exchange values were corrected to thatarea for analysis To obtain high-resolution seasonal monitoringofgas exchange leavesofboth specieswere sampled fromeachofthe four control andgreenhouseplots at theMATsite (n= 4) every~4ndash5 days comprising a measurement round For 12 days inmid-July there was a gap in measurement as the equipment wasbeing used for another research project
Prior to measurement in the laboratory leaves were enclosedin the cuvette at high light conditions to activate photosynthesisthough this exposure was kept to a minimum time to reducethe potential for photoinhibition CO2 assimilation wasmeasuredunder ambient (400 parts per million (ppm)) CO2 concentrationsunder 26 levels of PAR 1500mmolmndash2 sndash1 1200mmolmndash2 sndash1800mmolmndash2 sndash1 400mmolmndash2 sndash1 200mmolmndash2 sndash1100mmolmndash2 sndash1 and every 5 PAR between 100 and 0mmolmndash2 sndash1 This range of light fully encompasses the lightenvironment experienced by the species in their growthenvironment Following the light response curve leaf sampleswere treated to 900mmolmndash2 sndash1 PAR and 400 ppm CO2 for5ndash10minbeforeCO2assimilationwasmeasured in response to11levels of increasing [CO2] ~0 ppm 50 ppm 100 ppm 150 ppm200 ppm 300 ppm 400 ppm 600 ppm 800 ppm 1200 ppm and
1500 ppm Following the CO2 response curve measurement andafter 10min in darkness at 400 ppm CO2 it was assumed that nophotosynthesis or photorespiration was taking place and all CO2
flux could be attributed to mitochondrial respiration in the dark(Rdark)
All measurements were taken at a relative humidity of~40ndash60 and potential diffusion in and out of the cuvettewas accounted for as was diffusion through the gasketaccording to corrections presented in the Li-Cor 6400Instructional Manual The average leaf vapour pressure deficitwas 0942 032 kPa (se) for the light response curves and1142 029 for CO2 response curves across both species andtreatments The cuvette block temperature was set to 20C forall measurements to control for leaf temperature and isrepresentative of temperatures experienced during the growingseason (Fig 1) leaf temperatures in situ ranged across 19ndash23Cin both species Light-saturated net photosynthetic rate (Asat) was
0
140 150 160 170 180 190 200 210 220 230
5
10
15
20
Pre
cipi
tatio
n (m
m)
Julian day
20
40
60
80
100
Rel
ativ
e hu
mid
ity (
) 0
10
20
30
40
50
0
10
20
30
40
Max
Min
Tem
pera
ture
(degC
)P
AR
(m
ol m
ndash2 d
ayndash1
)
(a)
(b)
(c)
(d)
Fig 1 Environmental data from the moist acidic tundra experimental plotsat the Arctic Long-term Ecological Research field site at Toolik Lake Alaskaincluding (a) growing season maximum and minimum air temperature(b) daily total PAR (c) relative humidity and (d) precipitation for ambient(control solid lines) and passively warmed (dotted lines) tundra The vapourpressure deficit calculated from seasonal means of relative humidity andtemperature was 0187 kPa in control plots and 0472 kPa in warmedplots Photosynthesis and respiration measurements were made on leavessampled between the dashed lines (Julian days 162ndash209 or 11 Junendash28 July2010)
Seasonal carbon exchange under long-term warming Functional Plant Biology C
estimated by fitting data from the light response curve to arectangular hyperbolic function (Excel Solver MicrosoftRedmond WA USA) CO2 response curves were analysed forthe maximum carboxylation velocity of Rubisco (Vcmax) and themodelled maximum rate of electron transfer (referred to here as Jdue to data inconsistency at high CO2 levels) using the netphotosynthetic ratendash internal CO2 (ci)curve fitting utility (ver20071) provided and detailed in Sharkey et al (2007) Examplesof light andCO2 response curves can be seen in Fig S1 (availableas Supplementary Material to this paper) A portion of thereplicate measurements gave inaccurate readings of CO2
saturated photosynthesis at the highest levels of CO2 limitingstatistical comparison of maximal photosynthesis Values ofphotosynthesis and respiration are expressed on an area massand nitrogen basis
Quantifying respiration in the light using the Kok methodTo estimate Rlight we used the Kok method which is convenientfor field measurements and can be used under ambientatmospheric conditions This method is based on theobservation that the quantum yield of photosynthesis usuallydecreases abruptly above a certain level of light intensity ndash oftennear the light compensation point where carbon flux is zero (Kok1948) This leads to a noticeable nonlinearity or lsquobendrsquo in theotherwise linear lower range of the light response curve which isinterpreted as the saturation point of light inhibition of respiration(explained in detail in Shapiro et al 2004) Since respiration isassumed to be constant above this point an extrapolation to0mmolmndash2 sndash1 PAR of the linear portion of the curve above thispoint is assumed to give Rlight Here the irradiance range we usedto calculate Rlight spanned 25ndash90mmolmndash2 sndash1 (Fig S1b)
The Kok method assumes that the CO2 assimilation rateresponds only to light thus corrections must be made toaccount for changes in ci As rates of photosynthesis slowunder decreasing light intensity CO2 tends to accumulatewithin the leaf increasing ci which in turn affect the shapeof the light curve by decreasing the rate of photorespiration atlower light levels We corrected all measurements to a constantci to account for these effects according to Kirschbaum andFarquhar (1987) as described in Ayub et al (2011) toachieve a more accurate extrapolation of Rlight
Short-term in situ temperature response of respiration
To estimate respiratory fluxes under field conditions that varydaily we need descriptive high-resolution thermal responsecurves to extend our interpretation of measurements at20C Following methods similar to those described by Huumlveet al (2012) and OrsquoSullivan et al (2013) we sampled leavesof both species (similar to the sampling method describedabove) to quantify the short-term respiratory response ofleaves to measurement temperature Data presented in thisstudy represent replicates measured in late June on fullyexpanded leaves Leaves were kept in darkness for ~30minbefore measurement and then placed in a 155 110 65 cmwater-jacketed glass-topped aluminium chamber that wasthen covered by a dark fabric Two fans mixed air within thechamber and chamber temperature was controlled via aconnected programmable circulating waterbath (MRC300
Melcor Trenton NJ USA) that warmed from 5C to 35Cover ~45min Leaf temperature in the chamber was measuredwith a small gaugewire copper-constantan thermocouple pressedagainst the lower surface of the leaf and recorded via a data-logger(CR1000 Campbell Scientific Logan UT USA) Dry air waspassed through the cuvette at a rate of 045 L hndash1 and exited thecuvette via tubing connected to an IRGA (LI-840A LI-COR)which measured the flux of CO2 from the leaf Values ofrespiratory flux were corrected for the leaf area and mass ofthe measured replicate The standard equations used to calculatethe temperature sensitivity of respiration (Q10) the basalrespiration rate at 10C (R10) and a variable related to theenergy of activation calculated from values of respiration at20C and R10 (Eo) are presented in the Supplementary Material(Eqns S1ndash2)
Physical leaf traits and foliar nutrients
All leaf samples used for gas exchange measurements weremeasured for leaf area using a rotating-belt leaf area meter (LI-3100CAreaMeter LI-COR) Sampleswere then dried in an ovenat 60C for a minimum of 2 days before mass was determinedAfter transport to Columbia University in NewYork all sampleswere ground weighed and packaged for elemental analysis of[CHN] (2400 Series II Perkin-Elmer Boston MA USA)
Statistical analyses
The effect of seasonal timing on foliar gas exchange and relatedleaf characteristics was analysed using a linear mixed-effectsmodel framework to perform a three-way repeated-measuresANOVA in R (ver 270 The R Foundation for StatisticalComputing Vienna Austria) assigning measurement round(the 4- to 5-day period when replicates from each speciestreatment combination were measured) species (B nana andE vaginatum) and warming (control or greenhouse) asexplanatory variables treatment block was treated as a randomeffect Prior to analysis data were tested for normality(ShapirondashWilk) and heteroskedasticity (BreuschndashPagan) andlog10-transformed when necessary Post hoc multiplecomparisons were made using Tukeyrsquos test To address therelationships between foliar physiological rates and ambienttemperature or leaf traits correlation analyses were employedIn addition stepwise linear regression models that integrated theeffects of species measurement round warming treatment andambient temperature to assess select gas exchange variableswere evaluated using corrected Akaike Information Criteria(AICc see the Supplementary Material) For all analysesP-values of lt005 were considered significant
Results
Seasonal environmental variability
Full snowmelt at the long-termexperimentalwarmingandcontrolplots occurred by ~June 9 2010 (Toolik EDC) During themeasurement period of Julian Day 162ndash209 (11Junendash28 July)themaximumandminimum temperature relativehumidity lightandprecipitationwere highlyvariable (Fig 1)During this periodthe highest recorded temperature (239C) occurred on 9 Julyand the lowest minimum temperature (04C) after snowmeltoccurred on 6 July merely 3 days prior The average temperature
D Functional Plant Biology M A Heskel et al
during the measurement period was 107C with an averageminimum temperature of 48C and an average maximumtemperature of 143C In passively warmed plots thistranslates to an average maximum temperature of 193Cnearly equal to the leaf sample measurement temperatureAcross the growing season mean vapour pressure deficitcalculated from mean values of relative humidity andtemperatures was 0187 kPa in the controls and 0471 kPa inthe passively warmed plots Three large precipitation eventsoccurred during the measurement period corresponding withlower ambient air temperatures high relative humidity and lowdaily PAR (Fig 1)
Specific leaf traits and chemistry
Due to the contrasting leaf forms of B nana which producessmall flat circle-shaped leaves each season and E vaginatumwhich grows elongated three-sided tillers that are retainedthrough the snow-covered winter it is not surprising thecorresponding specific leaf area (SLA) measurements differgreatly between the species taken as a whole (Table 1Fig S2) and within both control and warming treatments(P lt 00001) Though individual measurement rounds showsome variation in SLA across the growing season elevatedgrowth temperature did not affect either species comparedwith leaves in the control (Fig S2) Leaf C concentration wasgreater and much more variable in B nana than in E vaginatum(P lt 00001) and warming treatment and seasonal timing(Round) did not affect this in either species (Fig S3adTable 1) E vaginatum decreased in proportional N content asthe season progressedB nana also decreased through the seasonbut it was less pronounced (Fig S3) Warming did not elevate Ncontent in either species The species-specific trends in N causedthe related increases in C N over the seasonal course (Fig S3)
Intraseasonal dynamics of photosynthesis and respiration
The impacts of species warming treatment and measurementround through the season on area- mass- and N-based gasexchange rates (measured at 20C) as expressed by the resultsof a three-way ANOVA are shown in Table 2 Variation inarea-based light-saturated photosynthetic rates (Fig 2) wereprimarily explained through species differences with B nanaconsistently exhibiting greater rates of carbon assimilation acrossall measurements as well as when considering measurementsin the warmed plots (P lt 001) Area-based Asat showed nosignificant response to the measurement round (Fig 2) though
mass-based ratesyieldedan interactive effect of species and round(Fig S3andashd Table 2) suggesting the effect of leaf structuraldifferences between the species when assessing photosyntheticrates Only when expressed on a per-N rate did Asat vary withmeasurement round driven by the increases in per-Nphotosynthesis later in the season related to lower foliar Nconcentration (Fig S3endashh) Similarly there were no observedchanges in rates under long-term warming treatment in eitherspecies and B nana consistently exhibited higher rates inboth warmed and control plots compared with E vaginatum Ifoneconsiders theprocessesunderlyingphotosynthetic efficiencyVcmax was significantly affected by the interaction of species andmeasurement round Mean rates of Vcmax in B nana particularlyunder warming increased slightly through the season (Fig 3)Rates of Vcmax and J were correlated (r2 = 0856) The ratio ofthese variables J Vcmaxwas generally 1ndash2but declined throughthe growing season in both species (Fig 4endashf Table 2)
Across allmeasures ofmitochondrial dark respirationB nanaexhibited higher rates than E vaginatum (Table 2) A significantinteraction term between species and measurement roundhighlighted differences in mitochondrial dark respiration (area-andN-based) between E vaginatum andB nana over the courseof the growing seasonE vaginatumdisplayed a gradual decreasein rates around Day 180 (29 June) that was not observed inB nana (Fig 2cndashd) Further considering allmeasurements long-term growth warming was associated with lower rates of area-based Rdark in both species though this effect is only observed inE vaginatum considering N-based Rdark On a mass basis Rdark
decreased through the growing season in both species (Fig 2cndashdTable 2) with the highestmean rates for both species occurring inthe first 2 weeks of the growing season
Short-term high-resolution temperature response curves ofRdark measured midseason showed a higher degree of thermalacclimation to long-term elevated growth temperature inE vaginatum than in B nana (Fig 3) As in the measurementsmade at a single temperature of 20C (Fig 2) and calculated at10C for R10 (Table 3) rates of respiration were generallygreater in B nana than in E vaginatum across themeasurement temperature range from 5C to 20C Howeverover 20ndash35C E vaginatum grown in control plots respired atincreasingly greater rates than did B nana which correspondedto its significantly higher Q10 and Eo values (Fig 4 Table 3)
One of the main objectives of this study was to obtain a betterunderstandingof the light inhibitionof respiration in these speciesand how this phenomenon may relate to other leaf traits Acrossspecies and growth conditions light significantly suppressed
Table 1 Results from three-way repeated-measures ANOVA analysing the carbon ( C) and nitrogen (N) concentrations and their ratio (C N)of dry leaf material and specific leaf area (SLA) in Betula nana and Eriophorum vaginatum
Plt 005 Plt 001 Plt 0001 S species R round W warming round df degrees of freedom
Sdf = 1 158
Rdf = 9 150
Wdf = 1 158
SRdf = 9 140
SWdf= 1 156
RWdf= 9 140
SRWdf = 9 120
F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P
Seasonal carbon exchange under long-term warming Functional Plant Biology E
respiration rates (P lt 00001) with individual leaf values rangingwidely Rlight Rdark was less in E vaginatum thanB nana underboth growth conditions (P lt 0001) and decreased further underwarming inE vaginatum (P lt 001) For both species and growthconditions Rlight Rdark decreased over the growing season(Fig 5) Warming decreased rates in only E vaginatum forarea- and N-based rates (Plt 005 for both) but not mass-basedrates which found warming to significantly lower rates whenconsidering all measurements (P lt 001) Furthermore the effecta speciesmeasurement round interaction (Plt 005) wasevident in the different trends of respiratory release over thecourse of the season (Fig 2endashf Fig S4) The ratio of carbon lossvia respiration in the light to gross photosynthesis (Rlight Agross)which describes the relative carbon loss from respirationcompared with the total carbon cycled in a leaf (as opposed tothe difference between the fluxes) decreased through the seasonin both species (Fig 6) though at different rates Theseproportional respiratory losses were significantly greater inB nana than in E vaginatum and warming significantlydecreased these values considering all measurements acrossthe season (Table 2) No clear relationships were foundbetween the degree of inhibition and foliar fluxes or traits inthese species regression analysis between Rlight Rdark andNAsat and Vcmax produced r2lt 010 (data not shown)
Intraseasonal ambient temperature influenceson gas exchange
Toevaluate the influenceof ambient temperature ongas exchangerates we compared multivariate linear models that incorporatedtemperature values from the day of andmean temperature values
from the week before gas exchange measurement Table S1shows the results of models incorporating the effect of specieswarming treatment and day-of and week-before minimummaximum and average temperatures on area-based Asat RdarkRlight and Rlight Rdark evaluating by comparing relative AICcweights The best model for photosynthesis (measured at 20C)incorporates only species with the next best only incorporatingspecies andwarming treatment suggestingno significant effect ofshort-term ambient air temperature on rates of Asat In contrastboth Rdark and Rlight (also measured at 20C) were more sensitiveto short-term ambient air temperatures with the best modelsincorporating the effect of species growth conditions andeither the prior weekrsquos average temperature (Rdark) or themeasurement dayrsquos minimum temperature (Rlight Fig S5) Thetop three models for Rlight Rdark incorporated minimumtemperature values When Rlight is considered alone a clearpredictive response is not obvious (Fig S5) though significantrelationships were observed between Rlight and day-of minimumtemperature (P lt 001) day-of maximum temperature(P lt 001) day-of average temperature (P lt 001) and week-prior minimum temperature (Plt 001) though not with theprior weekrsquos average or maximum temperature suggestinggreater sensitivity to short-term conditions colder conditionsor both
Discussion
The primary objective of this study was to evaluate themechanistic physiological responses of foliar photosynthesisand respiration to long-term warming conditions and naturalvariations in ambient temperature through the growing season
Table 2 Results from three-way repeated-measuresANOVAanalysing foliar gas exchange variables including light-saturating photosynthesis (Asat)dark respiration (Rdark) and respiration in the light (Rlight) expressed on an area mass and nitrogen basis the ratio of Rlight to Rdark (Rlight RDark)the ratio of photosynthetic carbon assimilation to respiratory carbon release (Rlight Agross) and the maximum carboxylation rate of Rubisco (Vcmax)and the electron transport rate (J) both measured at ambient leaf temperature and corrected for a constant temperature of 258C in Betula nana and
Eriophorum vaginatum Plt 005 Plt 001 Plt 0001 S species R round W warming df degrees of freedom
Sdf = 1 158
Rdf = 9 150
Wdf = 1 158
SRdf = 9 140
SWdf = 1 156
RWdf = 9 140
SRWdf= 9 120
F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P
in two dominant Arctic plants We hypothesised that the within-growing season rates of photosynthesis respiration and theinhibition of respiration will relate to growth-induced energydemand These rates will acclimate to long-term warming and be
responsive to short-term temperature fluctuations Our resultsshow stronger seasonal and thermal responses in respiration thanin photosynthesis (when both are measured at 20C) which wasfurther influenced by species differences and growth under long-
15
Betula Eriophorum
10
5
0
15
10
5
0
6
4
2
0
6
4
2
0
64
3
2
1
0
4
2
0
Asa
t (microm
ol C
O2
mndash2
sndash1
)R
dark
(microm
ol C
O2
mndash2
sndash1
)R
light
(microm
ol C
O2
mndash2
sndash1
)
160 170 180 190 200 210160 170 180 190 200
Julian day
(a)
(c)
(e)
(b)
(d)
(f )
Fig 2 (a b)Saturating photosynthesis (Asat measured at PAR=1500mmolmndash2 sndash1 and 400 parts per million (ppm)CO2) (c d) foliar dark respiration (Rdark measured at PAR=0mmolmndash2 sndash1 and 400 ppmCO2) and (e f) respiration inthe light (Rlight estimated via the Kok method at low PAR levels) through the growing season for leaves of (a c e)Betula nana (circles) and (bd f)Eriophorumvaginatum (triangles) grown in ambient (unfilled symbols) andpassivelywarmed (filled symbols) conditions All variables were measured at 20 C in laboratory conditions (n= 4)
Seasonal carbon exchange under long-term warming Functional Plant Biology G
termwarmingThis studyalsoprovided theopportunity toexpanduponexisting knowledge about biotic and environmental controlson foliar Rlight and how seasonal timing and temperaturevariability may mediate Rlight Rdark Together these findingsprovide a more detailed mechanistic understanding of thecontrol of carbon exchange in tundra ecosystems
Species-mediated influences of seasonal and environmentaleffects on carbon exchange
In our study the rates of photosynthesis and respiration ofB nana when measured at a common temperature tended tobe higher than those of E vaginatum during the Arctic tundragrowing season (Fig 2 Table 2) grown under control conditionsand long-term passive warming The higher rates of carbonexchange in B nana both when measured at 20C (Fig 2)and over the 5ndash35C temperature range (Fig 3) wereconsistent with previous studies that sampled plants grownunder warming at midseason (Chapin and Shaver 1996) andwith the addition of soil nitrogen and phosphorus (Heskel et al2012) The difference in Asat between B nana and E vaginatumwas greatest directly after leaf out (Fig 2) which emphasises theimpact of growth formon foliar physiological processesB nanaa deciduouswoody shrub is likely to have a relatively high energydemand immediately after snowmelt to accommodate efficientaboveground and belowground growth during the short growingseason and thismay be further enhanced under warming (Chapinet al 1995 Sullivan et al 2007) By contrast E vaginatum atiller-producing graminoid can retain leaves for multiple years(Fetcher and Shaver 1983) and for this reason may act lessopportunistically upon snowmelt in the spring in terms ofnutrient acquisition via fine root growth (Sullivan et al 2007)andquick leaf production These trends are supported byprevious
work on deciduous and evergreen species where seasonalphotosynthesis and respiration were consistently significantlylower in the evergreen (Ow et al 2010) The species contrastis underscored by the respective leaf architectures andcomposition interspecies comparisons show a less dense(greater SLA) leaf with higher N content in B nana than inE vaginatum (Table 1 Figs S2 S3)
We found a general though not significant decline in Asat inleaves sampled from long-term warming plots suggesting apotential duration-dependent warming effect at the leaf levelthat may contribute to a similar lack of photosyntheticstimulation reported at the ecosystem level (Shaver et al2000 Elmendorf et al 2012) Also these tundra species maybe pushed beyond their photosynthetic thermal optimum underelevated temperatures (Sage and Kubien 2007) which could bea likely scenario in Arctic populations that may be locallyadapted for colder growth temperatures We observed noapparent seasonal arch in the Asat Vcmax and J rates which cangenerally characterise growing season carbon assimilation ofleaves measured at ambient leaf temperatures as previouslyreported (for Asat) in these species (Starr et al 2008) At theleaf level the highest rates of Asat Vcmax and J can occur in theearly growing season (Dungan et al 2003 Xu and Baldocchi2003) This discrepancy may be explained by the highly variableambient temperature and precipitation conditions (Fig 1) thoughmodel evaluation found no strong relationship betweenphotosynthetic variables (measured at 20C) and short-termtemperature values (Table S1) Also as we measured allleaves at a common temperature (20 C) we controlled for theinfluence of ambient temperature on leaf temperature which canaffect photosynthetic rates and allowed for comparison acrosschanges in the ambient temperature to assess thermal acclimation
Respiration rates of both species were generally highest in thefirst few weeks after snowmelt (measured at 20C Fig 2endashf)which is more apparent in Rlight than in Rdark and are similar topreviously reported values (Heskel et al 2012 Heskel et al2013) In both B nana and E vaginatum higher energy demandand potentially respiratory rates in the early season relative to themid- and late season may be attributable to new leaf growth anddevelopment (Vose and Ryan 2002 Xu et al 2007 Ow et al2010) and possibly a higher density of mitochondria in youngerleaves (Armstrong et al 2006) This respiratory release can befurther enhanced by the colder ambient temperatures experiencedby plants in the early growing season due to a short-term coldacclimation (Atkin and Tjoelker 2003 Armstrong et al 2006)The thermal response of respiration is nonlinear and variesbetween species in response curve shape and slope (Fig 3Table 3) higher respiration in B nana across the range oftemperatures experienced during the growing season (Fig 1)and the lack of thermal acclimation may favour its expansion inthe tundra under current conditions (Heskel et al 2013) Furtherthe observed warm acclimation in leaves ofE vaginatum (Fig 3)suggests lowered metabolic rates that may limit growth despitereducing respiratory C loss under a future climate
It should be noted that similar rates of respiration rates acrossspecies may not necessary equate to similar energy efficiencyB nana is reported to exhibit greater respiratory efficiency thanE vaginatum through the differential use of the alternativeand cytochrome pathways potentially lending a competitive
05 10 15 20 25 30 35
10
20
30
40
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Betula CT
Betula WG
Eriophorum CT
Eriophorum WG
Temperature (degC)
Rda
rk (
nmol
CO
2 gndash1
sndash1
)
Fig 3 The mean short-term thermal responses of foliar dark respirationfrom 5C to 35C modelled from a polynomial fit from raw data in Betulanana (circles) and Eriophorum vaginatum (triangles) sampled from control(CT open) andwarmed (WG closed) growth environments Temperature (T)response curves of respiration (R) were fit to second-order polynomialequations as follows B nana control r= 0027T2 + 0082T+ 660 B nanawarmed r= 0026T2 + 0103T+ 677 E vaginatum control r = 0045T2 ndash
0106T+ 310 and E vaginatum warmed r = 0018T2 + 0176T+ 2633 Forall thermal response curve fits R2 gt 0990
H Functional Plant Biology M A Heskel et al
advantage in growth and development (Kornfeld et al 2013)In addition Rlight Agross decreased over the growing season(Fig 6) driven by lower values of Rlight as the season progressedrather than any significant change in photosynthesis which is
discordant with the idea that the processes would increase in acoupled manner based on carbohydrate substrate supply anddemand Kornfeld et al (2013) found no difference in starchand sugar content between control and warmed leaves within
0
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(a)
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Vcm
ax (micro
mol
mndash2
sndash1
)J
Vcm
ax
J (micro
mol
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)
Fig 4 (a b) Maximal carboxylation rate (Vcmax) and (c d) electron transport rate (J) both measured in laboratoryconditions and corrected to a constant 25C and (e f) the ratio of these variables (J Vcmax) over the growing season inleaves of (a c e) Betula nana and (b d f) Eriophorum vaginatum grown under ambient (open) and warmed (filled)conditions Values shown are means with se
Seasonal carbon exchange under long-term warming Functional Plant Biology I
species when measured at a single sampling point during thegrowing season though leaves of B nana contained a greaterconcentration of both sugar and starch than E vaginatum Wewere unable to quantify foliar carbohydrate values in thesespecies through the season though future studies couldinclude this informative measurement to further relate themechanistic links between these processes
Both species exhibited the lowest values of Rlight Rdark
towards the end of the season in late July (Fig 5) suggestinga potential developmental or energy demand-related control onthe inhibition of respiration by light that allows for greater
respiratory energy production in the early season during leafexpansionTheonlyother knownstudy that explored the seasonalresponse of Rlight Rdark observed the lowest level of inhibitionearlier in the growing season before the warmest month (Crouset al 2012) This response similar to our study despite a starklydifferent ecological system supports the idea of shared seasonaltiming-mediated biochemical controls on the light inhibition ofrespiration across species The degree of inhibition is known toshow a relaxed response under environmental conditions thatcan stimulate growth due to increased demand for energy and Cskeletons as exhibited under elevated CO2 (Wang et al 2001Shapiro et al 2004) and increased soil nutrient availability(Heskel et al 2012) though this is not always the case (Tissueet al 2002 Ayub et al 2011) Long-termwarming alsomediatedRlight Rdark with lower values observed in warm-grownleaves though measurement temperature can also impact theserelationships (Atkin et al 2006)
Mechanistic explanations for the relationships amonggrowth demand cellular energy status and the degree of lightinhibition of respiration have been previously reported Theenvironmental challenges of a short growing season and ~24-hdiel light exposure in the Arctic tundra control growth ratesthat may then influence values of Rlight Rdark (Bret-Harteet al 2001 Bret-Harte et al 2002) In the light adenylatesupply from photosynthesis may decrease the energy demandfrom mitochondrial respiration Additionally photorespirationis associated with the inactivation of pyruvate dehydrogenasea precursor to the tri-carboxylic acid (TCA) cycle (Budde andRandall 1990) though previous estimations of photorespirationin these species didnotfinda strong correlationwith the inhibitionof Rlight (Heskel et al 2013) The TCA cycle can also besignificantly altered in the light to support N assimilationwhich effectively reduces CO2 release (Tcherkez et al 2005Tcherkez et al 2008 Tcherkez et al 2009) This effect may beenhanced in tundra plantswhen soil N is less limiting under theseconditions growth rates increase (Bret-Harte et al 2002) Theinhibition ofRlight is observed to be slightly increased in fertilisedsoils (Heskel et al 2013) Additionally the oxidative pentosephosphate pathway (OPPP) which contributes the necessaryreductant for the synthesis of multiple metabolites in darknessmay be relaxed in the light when reductants are provided byphotosynthesis diminishing CO2 release by the OPPP Thoughthe OPPP comprises a proportionally smaller CO2 flux comparedwith that from the TCA cycle in the light this decrease maycontribute to lower overall CO2 release causing Rlight ltRdark
(Buckley and Adams 2011) In our current study the apparentrelaxation of the light inhibition of respiration in leaves of bothstudy species (Fig 5) resulting in the greatest proportional CO2
losses (Fig 6) suggesting that high growth demand for ATP andcarbon skeletons outweigh potential carbon losses in the earlyseason
Influences of variable ambient temperature on gas exchange
In addition to quantifying the effects of long-termwarming on thefoliar gas exchange during the growing season this studyevaluated the influence of intraseasonal short-term temperaturevariability on these physiological processes Arctic tundragrowing seasons can exhibit dramatic shifts in temperature and
Table 3 Parameters describing the respiratory response tomeasurement temperature in leaves of Betula nana and Eriophorumvaginatum grown under control (CT) and long-term warming (WG)conditions including Q10 (the temperature sensitivity of respiration(estimated over 5ndash358C)) R10 (the basal respiration rate at 108C(mmol CO2 gndash1 sndash1)) and Eo (kJmolndash1) a variable related to the energyof activation calculated from values of respiration at 208C and R10
Values presented are means and se Statistical differences between speciesand treatments are denoted alphabetically
Fig 5 Means and se of the ratio of respiration in the light (Rlight) estimatedvia the Kok method to dark respiration (Rdark) both variables measured at20C and 400 parts per million CO2 through the growing season in Betulanana (circles) and Eriophorum vaginatum (triangles) under ambient controlfield conditions (CT open symbols) and under a long-term passive warmingtreatment (WGfilled symbols n= 4)A linear regression (y= ndash0009x+ 219R2 = 047) depicts the declining Rlight Rdark values with the progressinggrowing season
J Functional Plant Biology M A Heskel et al
precipitation within days as was observed in this study duringsummer 2010 in Toolik Lake Alaska (Fig 1) and this variabilityis likely to influence the temperature-sensitive processescontrolling leaf carbon cycling Respiration in both speciescan be highly temperature-sensitive especially at warmertemperatures which may become more common in a futureArctic (Fig 3 Table 3) When modelled against thetemperature of the measurement day the average temperatureof the prior week accounted for species and treatment effectsand some patterns emerged that help to characterise the nature ofthe temperature sensitivity of foliar gas exchange (Table S1)However photosynthesis which is known to respond to short-term changes in temperature in field-grown plants (Berry andBjoumlrkman 1980 Poyatos et al 2012) did not show strongrelationships to daily or weekly variation in temperatureinstead species effects explained most variation (Table S1)The underlying photosynthetic machinery maintains consistentrates of carbon assimilation (measured at a constant temperature)throughout the growing season though daily fluctuations intemperature and precipitation experiences in the Arctic mayinfluence in vivo rates Plant species from alpine and Arcticecosystems can exhibit local adaptation to the extremevariability experienced in those locations (Korner and Larcher1988) which may translate to a limited acclimation (Atkin et al2006) Further because the ambient temperature conditionsfrom this region fluctuate so unpredictably in short periodsthrough the season any analysis using temperature averages ofprevious time windows might not be as useful for estimatingthermal acclimation as they are in regions with more predictableseasonal temperature patterns (Ow et al 2010 Searle et al 2011)
In contrast respiratory rates in the light and dark whenmeasured at a constant temperature appeared to be morecorrelated with short-term ambient temperature variation TheAICc analysis shows that the strongest model for Rdark includedspecies and warming effects and the prior weekrsquos average
temperature in contrast the strongest model for Rlight includedthe former parameters and the minimum temperature from themeasurement day (Table S1) The model relationships maysuggest a short-term thermal sensitivity of respiration that maybe more responsive to temperature minimums though this isnot clearly reflected in regressions of Rlight with the ambienttemperature values (Fig S5) This sensitivity of respiration totemperature minimums has been observed previously inroots as well and suggests that thermal acclimation in coldgrowth temperatures may regulate metabolic activity to meetthe demands of growth and maintenance at the expense ofgreater carbon loss (Covey-Crump et al 2002) Low ambienttemperatures did not clearly relate to the highest rates ofrespiration when measured at a common temperature aswould be expected under respiratory cold acclimation (Atkinand Tjoelker 2003) However it is possible that developmentaltiming may also play into the leavesrsquo respiratory rates and abilityto acclimate especially in B nana in the early season when itsleaves are still expanding (Armstrong et al 2006) Through arelatively simple modelling exercise we found some evidenceof differential temperature controls on photosynthesis andrespiration in B nana and E vaginatum we suggest thatfuture work should include more detailed environmental andbiotic factors including soil temperature daily photosyntheticradiation and leaf carbohydrate data to more accurately informthe seasonal temperature sensitivity of foliar carbon exchange
Conclusions and implications
The physiological measurements collected in this study allowedfor the quantification of mechanistic responses in the nightlessand highly variable growing season of the arctic tundra andprovided new insights into foliar carbon regulation We presentnew information on the seasonal trends of foliar carbon cyclingin two dominant tundra species and relate these fluxes to short-
0
015
045
030
0
015
045
030
Julian day
Betula Eriophorum
160 170 180 190 200 210 160 170 180 190 200 210
Rlig
ht
Agr
oss
Fig 6 The proportional carbon loss through respiration in the light to gross photosynthesis in (a) Betula nana and(b) Eriophorum vaginatum over the growing season (Rlight Agross both variables measured at a common temperatureof 20C and 400 parts per million CO2) Linear regressions across growth conditions depict the relationship ofRlight Agross and measurement date during the growing season (B nana y= ndash0003 + 088x R2 = 030 E vaginatumy= ndash0005 + 117x R2 = 065) Values represent variable means and se error bars are not visible for many values dueto their small value
Seasonal carbon exchange under long-term warming Functional Plant Biology K
term temperature variability and long-term warming Our studypresents the first published values of the photosyntheticparameters Vcmax and J at multiple points during the season inthese species which can inform and refine parameterisation ofvegetationmodels atmany scalesAlsowe present thefirst reportof the seasonal flexibility of the degree of the inhibition of Rlight
under long-term warmed growth conditions in these or anyspecies which will enable more accurate calculations ofecosystem respiration Based on these measurements aseasonal estimate (not accounting for potential temperaturesensitivity as discussed above see Eqn S3) shows thatneglecting the inhibition of foliar Rlight in Arctic tundravegetation could lead to overestimations of foliar carbon lossof~4molmndash2 leaf (~176 g)over thegrowingseason or evenmore(~42molmndash2) when considering plants grown under warmedconditionsAlthough the temperature effect on respiratory carbonloss in the light may beminimal the overall discrepancy in fluxesbetween Rdark and Rlight must be considered for realisticdescriptions of Arctic carbon cycling Our study contributesimportant data that allow for an increased understanding ofdominant tundra speciesrsquo responses to environmental changeboth long- and short-term and will allow for more predictivepower in models estimating ecosystem carbon storage
Acknowledgements
We are grateful to the researchers and support staff at the Arctic LTER andToolik Field Station for their help in this study as well as Greg PendergastThis study was made possible through funding from the NSF 0732664OKA acknowledges grant support from the Australian Research Council(DP0986823)
References
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Armstrong AF Logan DC Tobin AK OrsquoToole P Atkin OK (2006b)Heterogeneity of plant mitochondrial responses underpinningrespiratory acclimation to the cold in Arabidopsis thaliana leavesPlant Cell amp Environment 29 940ndash949 doi101111j1365-3040200501475x
Armstrong AF Badger MR Day DA Barthet MM Smith P Millar AHWhelan J Atkin OK (2008) Dynamic changes in the mitochondrialelectron transport chain underpinning cold acclimation of leafrespiration Plant Cell amp Environment 31 1156ndash1169 doi101111j1365-3040200801830x
Atkin OK Tjoelker MG (2003) Thermal acclimation and the dynamicresponse of plant respiration to temperature Trends in Plant Science 8343ndash351 doi101016S1360-1385(03)00136-5
Atkin OK Bruhn D Tjoelker MG (2005) Response of plant respiration tochanges in temperature mechanisms and consequences of variations inQ10 values and acclimation In lsquoPlant respirationrsquo (Eds H Lambers ampM Ribas-Carbo) pp 95ndash135 (Springer Dordrecht The Netherlands)
Atkin OK Scheurwater I Pons TL (2006)High thermal acclimation potentialof both photosynthesis and respiration in two lowlandPlantago species incontrast to an alpine congeneric Global Change Biology 12 500ndash515doi101111j1365-2486200601114x
Ayub G Smith RA Tissue DT Atkin OK (2011) Impacts of drought onleaf respiration in darkness and light in Eucalyptus saligna exposed toindustrial-age atmosphericCO2 and growth temperatureNewPhytologist190 1003ndash1018 doi101111j1469-8137201103673x
Berry J Bjoumlrkman O (1980) Photosynthetic response and adaptation totemperature in higher plants Annual Review of Plant Physiology andPlant Molecular Biology 31 491ndash543 doi101146annurevpp31060180002423
Bret-HarteMSShaverGRZoerner JP JohnstoneJFWagner JLChavezASGunkelman RF IV Lippert SC Laundre JA (2001) Developmentalplasticity allows Betula nana to dominate tundra subjected to analtered environment Ecology 82 18ndash32
Bret-Harte MS Shaver GR Chapin FS III (2002) Primary and secondarystem growth in Arctic shrubs implications for community response toenvironmental change Journal of Ecology 90 251ndash267 doi101046j1365-2745200100657x
BuckleyTNAdamsMA(2011)An analyticalmodel of non-photorespiratoryCO2 release in the light and dark in leaves of C3 species based onstoichiometric flux balance Plant Cell amp Environment 34 89ndash112doi101111j1365-3040201002228x
Budde RJ Randall DD (1990) Pea leaf mitochondrial pyruvatedehydrogenase complex is inactivated in vivo in a light-dependentmanner Proceedings of the National Academy of Sciences of theUnited States of America 87 673ndash676 doi101073pnas872673
CahoonSMPSullivanPF ShaverGRWelker JM PostE (2012) Interactionsamong shrub cover and the soil microclimatemay determine futureArcticcarbon budgets Ecology Letters 15 1415ndash1422 doi101111j1461-0248201201865x
Chapin FS Shaver GR (1996) Physiological and growth responses of Arcticplants to a field experiment simulating climatic change Ecology 77822ndash840 doi1023072265504
Chapin FS Shaver GR Giblin AE Nadelhoffer KJ Laundre JA (1995)Responses of Arctic tundra to experimental and observed changes inclimate Ecology 76 694ndash711 doi1023071939337
Chapin FS Sturm M Serreze MC McFadden JP Key JR Lloyd AHMcGuire AD Rupp TS Lynch AH Schimel JP Beringer J ChapmanWLEpsteinHEEuskirchenESHinzmanLD JiaGPingC-LTapeKDThompson CDC Walker DA Welker JM (2005) Role of land-surfacechanges in Arctic summer warming Science 310 657ndash660 doi101126science1117368
Covey-Crump EM Attwood RG Atkin OK (2002) Regulation of rootrespiration in two species of Plantago that differ in relative growthrate the effect of short and long term changes in temperature PlantCell amp Environment 25 1501ndash1513 doi101046j1365-3040200200932x
Crous KY Zaragoza-Castells J Ellsworth DS Duursma RA LowM TissueDT Atkin OK (2012) Light inhibition of leaf respiration in field-grownEucalyptus saligna in whole-tree chambers under elevated atmosphericCO2 and summer drought Plant Cell amp Environment 35 966ndash981doi101111j1365-3040201102465x
Dungan RJ Whitehead D Duncan RP (2003) Seasonal and temperaturedependence of photosynthesis and respiration for two co-occurringbroad-leaved tree species with contrasting leaf phenology TreePhysiology 23 561ndash568 doi101093treephys238561
Elmendorf SC Henry GHR Hollister RD Bjoumlrk RG Bjorkman ADCallaghan TV Collier LS Cooper EJ Cornelissen JHC Day TAFosaa AM Gould WA Greacutetarsdoacutettir J Harte J Hermanutz L Hik DSHofgaard A Jarrad F Joacutensdoacutettir IS Keuper F Klanderud K Klein JAKoh S Kudo G Lang SI Loewen V May JL Mercado J Michelsen AMolau U Myers-Smith IH Oberbauer SF Pieper S Post E Rixen CRobinson CH Schmidt NM Shaver GR Stenstroumlm A Tolvanen ATotland Oslash Troxler T Wahren C-H Webber PJ Welker JM WookeyPA (2012) Global assessment of experimental climate warming ontundra vegetation heterogeneity over space and time Ecology Letters15 164ndash175 doi101111j1461-0248201101716x
EnvironmentalDataCenterTeam (2011)Meteorologicalmonitoring programat Toolik Alaska Toolik Field Station Institute of Arctic BiologyUniversity of Alaska Fairbanks Fairbanks AK 99775 Available athttptoolikalaskaeduedcabiotic_monitoringdata_queryphp
L Functional Plant Biology M A Heskel et al
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
ecosystem productivity creating little difference in seasonal netecosystem exchange and maintaining a role as a C sink thoughtundra type and variations in interannual environmentalconditions such as water availability and ambient light caninfluence this effect (Welker et al 2000 2004 Oberbaueret al 2007 Sullivan et al 2008) Alternatively other casesexhibit changes of net ecosystem exchange through anincrease in ecosystem respiration leading to a C source inboth experimentally warmed sites (Natali et al 2011) andnon-manipulated sites with generally warmer soil and canopytemperatures (Cahoon et al 2012) Given the difficulty ofpartitioning soil and autotrophic CO2 fluxes in tundra fieldstudies few studies examine the effect of experimentalwarming on C exchange in vegetation alone (Chapin andShaver 1996 Shaver et al 1998 Starr et al 2008)
Short-term intraseason temperature responses ofphotosynthesis and respiration must be considered in concertwith long-termwarming responses tomechanistically understandand robustly predict rates of foliar and ecosystem carbon cyclingin the Arctic tundra given the highly fluctuating temperatureenvironment during the growing season Short-term ambienttemperature conditions may drive significant change in bothecosystem and leaf carbon balance in addition to the currentand dramatic warming trend in this region (Serreze et al 2000)For example foliar respiration can be sensitive to small rapidshifts in temperature and can thermally acclimate over time(Atkin and Tjoelker 2003 Atkin et al 2005) In both warm-grown and warm-treated leaves this flexibility can lead todecreased respiration compared with cold-grown and cold-treated leaves when measured at a constant temperature(Atkin and Tjoelker 2003) This may be attributed toreorganisation of the mitochondrial alternative and cytochromepathways (Armstrong et al 2006b 2008 Searle et al 2011)Many factors are at play in warming Arctic ecosystems ndash growthforms are likely to respond to an altered temperature regime inmorphologically and physiologically different ways (Gough andHobbie 2003 Heskel et al 2013) and the duration of warmingmay strongly influence these changes (Elmendorf et al 2012)Kornfeld et al (2013) recently reported a significant decrease inrespiration in the two focal species of our current study Betulanana L and Eriophorum vaginatum L grown under the samelong-term warming treatment conditions suggesting potentialthermal acclimation at a longer timescale Although the Kornfeldet al (2013) study demonstrated the potential role of underlyingmitochondrial mechanisms in response to long-term warmingtreatment it is still unclear how this long-term warming willimpact temperature sensitivity at shorter time scales through thegrowing season
Many aspects of the seasonality of foliar C cycling and theinfluence of long-term warming and shorter-term intraseasonaltemperature variation on these processes remain unknown in theC-rich tundra landscape In our current study we examine foliarphotosynthesis respiration and the associated leaf traits in twoabundant species B nana a woody shrub and E vaginatum atussock-forming graminoid to acquire accurate estimates of thespeciesrsquo contributions to the ecosystemrsquos carbon balance duringthe growing season This study also investigates the relationshipof these mechanistic responses to seasonal timing long-termwarming treatment and short-term ambient temperature
variability Additionally a related objective of this study wasto quantify the light inhibition of respiration through the growingseason As Arctic tundra plants experience near-complete orcomplete 24-h photoperiods during the growing seasonestimates of respiration in the light (Rlight) which account forthe known inhibition of respiration in light (the lsquoKok effectrsquo)were quantified in addition to dark respiration (Rdark) In thisstudy we focus on the less frequently measured but ecologicallyrelevant flux of Rlight and its relationship to the more commonlymeasured flux of Rdark to tease apart controls of light inhibitionand emphasise the importance of light inhibition in the carboncycling of Arctic plants Respiration in the dark is an inaccurateestimate of actual plant carbon release in the nightless growingseason but it provides a point of comparison across Arctic andnon-Arctic sites and serves as an assumed uninhibited rate Wehypothesise that the seasonal dynamics of gas exchange willcorrespond to the energy demand of the growing vegetation (1)rates of photosynthesis should increase as leaves develop thoughthis trendmay be stronger in the deciduous shrubB nana than theevergreen graminoid E vaginatum (2) respiration rates will begreatest in the early growing season when energy demandfor growth is highest and (3) the elevated energy demand forgrowth will be associated with relaxed levels of light inhibitionof respiration as shown in elevated CO2 studies (Shapiro et al2004 Crous et al 2012) Further we hypothesise that growthunder long-term warming will suppress the respiratory responseto temperature due to thermal acclimation (Atkin and Tjoelker2003) To test these hypotheses in tundra vegetation we utilisedglobal change experimental plots that have been continuouslymaintained for nearly 30 years by the Arctic Long-termEcological Research (ARC LTER) site at Toolik Lake Alaskaand measured foliar gas exchange and related leaf traits atcondensed intervals over 8 weeks during the 2010 growingseason Our study represents a high temporal resolutionexamination of the mechanisms controlling plant carbonbalance through the tundra growing season and is the first toquantify the seasonal response of the light inhibition of foliarrespirationmaximum rates of ribulose-15-bisphosphate (RuBP)carboxylation and RuBP regeneration in these common tundraspecies
Materials and methodsField site and species
The study took place over 8 weeks during the 2010 growingseason (6 Junendash28 July) at the ARC LTER field site near ToolikLake (68380N 149360W) on Alaskarsquos North Slope located254 km north of the Arctic Circle Air temperature relativehumidity light and precipitation were all measured andrecorded at this site by the Toolik Field Station EnvironmentalData Center (Environmental Data Center Team 2011) All leaveswere sampled from experimental plots in moist acidic tundra(MAT) established andmaintained by theARCLTER since 1989(similar to anolder experiment described byChapin et al (2005))The MAT site consists of four randomised blocks of 5 20mtreatment plots separated by a 1-m buffer arrayed two-by-two ona slightly sloped and poorly drained hillside Leaves weresampled from control plots and plots that have been warmedvia greenhouses since 1989 These wood-framed greenhouses
B Functional Plant Biology M A Heskel et al
covered with transparent 015-mm plastic sheeting passivelyincrease the air temperature by ~5C during the growing seasonData on thawdepth and soilmoisture though likely influenced bythe passive warming treatment as well as to influence foliar gasexchange rates were not available for this study The focalspecies for our study are common and abundant at the MATsite Eriophorum vaginatum L (cottongrass) the tussock-forming sedge that is widely distributed over much ofAlaskarsquos North Slope northern Canada and northern EurasiaandBetula nana nanaL (dwarf birch) a deciduous woody shrubthat is also abundant and often dominant over much of the entireArctic region
Foliar carbon exchange
CO2 fluxes of photosynthesis and respiration were measuredusing an infrared gas analyser (IRGA LI-6400XT PortablePhotosynthesis System LI-COR Lincoln NE USA) Allmeasurements were made on clipped leaves of E vaginatumand small branches forBnana collected from the long-termMATgreenhouse and control plots re-cut under water in the field andtransported in water to the laboratory where they acclimated tomeasurement temperatures as inHeskel et al (2012) Preliminarytests on these species showed no difference in the rates of gasexchange and stomatal conductance between field-measured andlaboratory-measured leaves (Griffin unpublished) as has beenshown previously in other species (Mitchell et al 1999 Turnbullet al 2003) This sampling technique was required to provide agreater degree of temperature control maximise the numberof replicates and minimise the time between replicates over theshort growing season and during potentially rapidly changingenvironmental conditions in the field For each E vaginatumsample we collected approximately 10 leaves from a singletussock in order to provide sufficient leaf area in the leafchamber (~6 cm2) For B nana the one or two measuredleaves from the sampled branch that were smaller than the6 cm2 cuvette area were measured for leaf area after IRGAmeasurements and gas exchange values were corrected to thatarea for analysis To obtain high-resolution seasonal monitoringofgas exchange leavesofboth specieswere sampled fromeachofthe four control andgreenhouseplots at theMATsite (n= 4) every~4ndash5 days comprising a measurement round For 12 days inmid-July there was a gap in measurement as the equipment wasbeing used for another research project
Prior to measurement in the laboratory leaves were enclosedin the cuvette at high light conditions to activate photosynthesisthough this exposure was kept to a minimum time to reducethe potential for photoinhibition CO2 assimilation wasmeasuredunder ambient (400 parts per million (ppm)) CO2 concentrationsunder 26 levels of PAR 1500mmolmndash2 sndash1 1200mmolmndash2 sndash1800mmolmndash2 sndash1 400mmolmndash2 sndash1 200mmolmndash2 sndash1100mmolmndash2 sndash1 and every 5 PAR between 100 and 0mmolmndash2 sndash1 This range of light fully encompasses the lightenvironment experienced by the species in their growthenvironment Following the light response curve leaf sampleswere treated to 900mmolmndash2 sndash1 PAR and 400 ppm CO2 for5ndash10minbeforeCO2assimilationwasmeasured in response to11levels of increasing [CO2] ~0 ppm 50 ppm 100 ppm 150 ppm200 ppm 300 ppm 400 ppm 600 ppm 800 ppm 1200 ppm and
1500 ppm Following the CO2 response curve measurement andafter 10min in darkness at 400 ppm CO2 it was assumed that nophotosynthesis or photorespiration was taking place and all CO2
flux could be attributed to mitochondrial respiration in the dark(Rdark)
All measurements were taken at a relative humidity of~40ndash60 and potential diffusion in and out of the cuvettewas accounted for as was diffusion through the gasketaccording to corrections presented in the Li-Cor 6400Instructional Manual The average leaf vapour pressure deficitwas 0942 032 kPa (se) for the light response curves and1142 029 for CO2 response curves across both species andtreatments The cuvette block temperature was set to 20C forall measurements to control for leaf temperature and isrepresentative of temperatures experienced during the growingseason (Fig 1) leaf temperatures in situ ranged across 19ndash23Cin both species Light-saturated net photosynthetic rate (Asat) was
0
140 150 160 170 180 190 200 210 220 230
5
10
15
20
Pre
cipi
tatio
n (m
m)
Julian day
20
40
60
80
100
Rel
ativ
e hu
mid
ity (
) 0
10
20
30
40
50
0
10
20
30
40
Max
Min
Tem
pera
ture
(degC
)P
AR
(m
ol m
ndash2 d
ayndash1
)
(a)
(b)
(c)
(d)
Fig 1 Environmental data from the moist acidic tundra experimental plotsat the Arctic Long-term Ecological Research field site at Toolik Lake Alaskaincluding (a) growing season maximum and minimum air temperature(b) daily total PAR (c) relative humidity and (d) precipitation for ambient(control solid lines) and passively warmed (dotted lines) tundra The vapourpressure deficit calculated from seasonal means of relative humidity andtemperature was 0187 kPa in control plots and 0472 kPa in warmedplots Photosynthesis and respiration measurements were made on leavessampled between the dashed lines (Julian days 162ndash209 or 11 Junendash28 July2010)
Seasonal carbon exchange under long-term warming Functional Plant Biology C
estimated by fitting data from the light response curve to arectangular hyperbolic function (Excel Solver MicrosoftRedmond WA USA) CO2 response curves were analysed forthe maximum carboxylation velocity of Rubisco (Vcmax) and themodelled maximum rate of electron transfer (referred to here as Jdue to data inconsistency at high CO2 levels) using the netphotosynthetic ratendash internal CO2 (ci)curve fitting utility (ver20071) provided and detailed in Sharkey et al (2007) Examplesof light andCO2 response curves can be seen in Fig S1 (availableas Supplementary Material to this paper) A portion of thereplicate measurements gave inaccurate readings of CO2
saturated photosynthesis at the highest levels of CO2 limitingstatistical comparison of maximal photosynthesis Values ofphotosynthesis and respiration are expressed on an area massand nitrogen basis
Quantifying respiration in the light using the Kok methodTo estimate Rlight we used the Kok method which is convenientfor field measurements and can be used under ambientatmospheric conditions This method is based on theobservation that the quantum yield of photosynthesis usuallydecreases abruptly above a certain level of light intensity ndash oftennear the light compensation point where carbon flux is zero (Kok1948) This leads to a noticeable nonlinearity or lsquobendrsquo in theotherwise linear lower range of the light response curve which isinterpreted as the saturation point of light inhibition of respiration(explained in detail in Shapiro et al 2004) Since respiration isassumed to be constant above this point an extrapolation to0mmolmndash2 sndash1 PAR of the linear portion of the curve above thispoint is assumed to give Rlight Here the irradiance range we usedto calculate Rlight spanned 25ndash90mmolmndash2 sndash1 (Fig S1b)
The Kok method assumes that the CO2 assimilation rateresponds only to light thus corrections must be made toaccount for changes in ci As rates of photosynthesis slowunder decreasing light intensity CO2 tends to accumulatewithin the leaf increasing ci which in turn affect the shapeof the light curve by decreasing the rate of photorespiration atlower light levels We corrected all measurements to a constantci to account for these effects according to Kirschbaum andFarquhar (1987) as described in Ayub et al (2011) toachieve a more accurate extrapolation of Rlight
Short-term in situ temperature response of respiration
To estimate respiratory fluxes under field conditions that varydaily we need descriptive high-resolution thermal responsecurves to extend our interpretation of measurements at20C Following methods similar to those described by Huumlveet al (2012) and OrsquoSullivan et al (2013) we sampled leavesof both species (similar to the sampling method describedabove) to quantify the short-term respiratory response ofleaves to measurement temperature Data presented in thisstudy represent replicates measured in late June on fullyexpanded leaves Leaves were kept in darkness for ~30minbefore measurement and then placed in a 155 110 65 cmwater-jacketed glass-topped aluminium chamber that wasthen covered by a dark fabric Two fans mixed air within thechamber and chamber temperature was controlled via aconnected programmable circulating waterbath (MRC300
Melcor Trenton NJ USA) that warmed from 5C to 35Cover ~45min Leaf temperature in the chamber was measuredwith a small gaugewire copper-constantan thermocouple pressedagainst the lower surface of the leaf and recorded via a data-logger(CR1000 Campbell Scientific Logan UT USA) Dry air waspassed through the cuvette at a rate of 045 L hndash1 and exited thecuvette via tubing connected to an IRGA (LI-840A LI-COR)which measured the flux of CO2 from the leaf Values ofrespiratory flux were corrected for the leaf area and mass ofthe measured replicate The standard equations used to calculatethe temperature sensitivity of respiration (Q10) the basalrespiration rate at 10C (R10) and a variable related to theenergy of activation calculated from values of respiration at20C and R10 (Eo) are presented in the Supplementary Material(Eqns S1ndash2)
Physical leaf traits and foliar nutrients
All leaf samples used for gas exchange measurements weremeasured for leaf area using a rotating-belt leaf area meter (LI-3100CAreaMeter LI-COR) Sampleswere then dried in an ovenat 60C for a minimum of 2 days before mass was determinedAfter transport to Columbia University in NewYork all sampleswere ground weighed and packaged for elemental analysis of[CHN] (2400 Series II Perkin-Elmer Boston MA USA)
Statistical analyses
The effect of seasonal timing on foliar gas exchange and relatedleaf characteristics was analysed using a linear mixed-effectsmodel framework to perform a three-way repeated-measuresANOVA in R (ver 270 The R Foundation for StatisticalComputing Vienna Austria) assigning measurement round(the 4- to 5-day period when replicates from each speciestreatment combination were measured) species (B nana andE vaginatum) and warming (control or greenhouse) asexplanatory variables treatment block was treated as a randomeffect Prior to analysis data were tested for normality(ShapirondashWilk) and heteroskedasticity (BreuschndashPagan) andlog10-transformed when necessary Post hoc multiplecomparisons were made using Tukeyrsquos test To address therelationships between foliar physiological rates and ambienttemperature or leaf traits correlation analyses were employedIn addition stepwise linear regression models that integrated theeffects of species measurement round warming treatment andambient temperature to assess select gas exchange variableswere evaluated using corrected Akaike Information Criteria(AICc see the Supplementary Material) For all analysesP-values of lt005 were considered significant
Results
Seasonal environmental variability
Full snowmelt at the long-termexperimentalwarmingandcontrolplots occurred by ~June 9 2010 (Toolik EDC) During themeasurement period of Julian Day 162ndash209 (11Junendash28 July)themaximumandminimum temperature relativehumidity lightandprecipitationwere highlyvariable (Fig 1)During this periodthe highest recorded temperature (239C) occurred on 9 Julyand the lowest minimum temperature (04C) after snowmeltoccurred on 6 July merely 3 days prior The average temperature
D Functional Plant Biology M A Heskel et al
during the measurement period was 107C with an averageminimum temperature of 48C and an average maximumtemperature of 143C In passively warmed plots thistranslates to an average maximum temperature of 193Cnearly equal to the leaf sample measurement temperatureAcross the growing season mean vapour pressure deficitcalculated from mean values of relative humidity andtemperatures was 0187 kPa in the controls and 0471 kPa inthe passively warmed plots Three large precipitation eventsoccurred during the measurement period corresponding withlower ambient air temperatures high relative humidity and lowdaily PAR (Fig 1)
Specific leaf traits and chemistry
Due to the contrasting leaf forms of B nana which producessmall flat circle-shaped leaves each season and E vaginatumwhich grows elongated three-sided tillers that are retainedthrough the snow-covered winter it is not surprising thecorresponding specific leaf area (SLA) measurements differgreatly between the species taken as a whole (Table 1Fig S2) and within both control and warming treatments(P lt 00001) Though individual measurement rounds showsome variation in SLA across the growing season elevatedgrowth temperature did not affect either species comparedwith leaves in the control (Fig S2) Leaf C concentration wasgreater and much more variable in B nana than in E vaginatum(P lt 00001) and warming treatment and seasonal timing(Round) did not affect this in either species (Fig S3adTable 1) E vaginatum decreased in proportional N content asthe season progressedB nana also decreased through the seasonbut it was less pronounced (Fig S3) Warming did not elevate Ncontent in either species The species-specific trends in N causedthe related increases in C N over the seasonal course (Fig S3)
Intraseasonal dynamics of photosynthesis and respiration
The impacts of species warming treatment and measurementround through the season on area- mass- and N-based gasexchange rates (measured at 20C) as expressed by the resultsof a three-way ANOVA are shown in Table 2 Variation inarea-based light-saturated photosynthetic rates (Fig 2) wereprimarily explained through species differences with B nanaconsistently exhibiting greater rates of carbon assimilation acrossall measurements as well as when considering measurementsin the warmed plots (P lt 001) Area-based Asat showed nosignificant response to the measurement round (Fig 2) though
mass-based ratesyieldedan interactive effect of species and round(Fig S3andashd Table 2) suggesting the effect of leaf structuraldifferences between the species when assessing photosyntheticrates Only when expressed on a per-N rate did Asat vary withmeasurement round driven by the increases in per-Nphotosynthesis later in the season related to lower foliar Nconcentration (Fig S3endashh) Similarly there were no observedchanges in rates under long-term warming treatment in eitherspecies and B nana consistently exhibited higher rates inboth warmed and control plots compared with E vaginatum Ifoneconsiders theprocessesunderlyingphotosynthetic efficiencyVcmax was significantly affected by the interaction of species andmeasurement round Mean rates of Vcmax in B nana particularlyunder warming increased slightly through the season (Fig 3)Rates of Vcmax and J were correlated (r2 = 0856) The ratio ofthese variables J Vcmaxwas generally 1ndash2but declined throughthe growing season in both species (Fig 4endashf Table 2)
Across allmeasures ofmitochondrial dark respirationB nanaexhibited higher rates than E vaginatum (Table 2) A significantinteraction term between species and measurement roundhighlighted differences in mitochondrial dark respiration (area-andN-based) between E vaginatum andB nana over the courseof the growing seasonE vaginatumdisplayed a gradual decreasein rates around Day 180 (29 June) that was not observed inB nana (Fig 2cndashd) Further considering allmeasurements long-term growth warming was associated with lower rates of area-based Rdark in both species though this effect is only observed inE vaginatum considering N-based Rdark On a mass basis Rdark
decreased through the growing season in both species (Fig 2cndashdTable 2) with the highestmean rates for both species occurring inthe first 2 weeks of the growing season
Short-term high-resolution temperature response curves ofRdark measured midseason showed a higher degree of thermalacclimation to long-term elevated growth temperature inE vaginatum than in B nana (Fig 3) As in the measurementsmade at a single temperature of 20C (Fig 2) and calculated at10C for R10 (Table 3) rates of respiration were generallygreater in B nana than in E vaginatum across themeasurement temperature range from 5C to 20C Howeverover 20ndash35C E vaginatum grown in control plots respired atincreasingly greater rates than did B nana which correspondedto its significantly higher Q10 and Eo values (Fig 4 Table 3)
One of the main objectives of this study was to obtain a betterunderstandingof the light inhibitionof respiration in these speciesand how this phenomenon may relate to other leaf traits Acrossspecies and growth conditions light significantly suppressed
Table 1 Results from three-way repeated-measures ANOVA analysing the carbon ( C) and nitrogen (N) concentrations and their ratio (C N)of dry leaf material and specific leaf area (SLA) in Betula nana and Eriophorum vaginatum
Plt 005 Plt 001 Plt 0001 S species R round W warming round df degrees of freedom
Sdf = 1 158
Rdf = 9 150
Wdf = 1 158
SRdf = 9 140
SWdf= 1 156
RWdf= 9 140
SRWdf = 9 120
F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P
Seasonal carbon exchange under long-term warming Functional Plant Biology E
respiration rates (P lt 00001) with individual leaf values rangingwidely Rlight Rdark was less in E vaginatum thanB nana underboth growth conditions (P lt 0001) and decreased further underwarming inE vaginatum (P lt 001) For both species and growthconditions Rlight Rdark decreased over the growing season(Fig 5) Warming decreased rates in only E vaginatum forarea- and N-based rates (Plt 005 for both) but not mass-basedrates which found warming to significantly lower rates whenconsidering all measurements (P lt 001) Furthermore the effecta speciesmeasurement round interaction (Plt 005) wasevident in the different trends of respiratory release over thecourse of the season (Fig 2endashf Fig S4) The ratio of carbon lossvia respiration in the light to gross photosynthesis (Rlight Agross)which describes the relative carbon loss from respirationcompared with the total carbon cycled in a leaf (as opposed tothe difference between the fluxes) decreased through the seasonin both species (Fig 6) though at different rates Theseproportional respiratory losses were significantly greater inB nana than in E vaginatum and warming significantlydecreased these values considering all measurements acrossthe season (Table 2) No clear relationships were foundbetween the degree of inhibition and foliar fluxes or traits inthese species regression analysis between Rlight Rdark andNAsat and Vcmax produced r2lt 010 (data not shown)
Intraseasonal ambient temperature influenceson gas exchange
Toevaluate the influenceof ambient temperature ongas exchangerates we compared multivariate linear models that incorporatedtemperature values from the day of andmean temperature values
from the week before gas exchange measurement Table S1shows the results of models incorporating the effect of specieswarming treatment and day-of and week-before minimummaximum and average temperatures on area-based Asat RdarkRlight and Rlight Rdark evaluating by comparing relative AICcweights The best model for photosynthesis (measured at 20C)incorporates only species with the next best only incorporatingspecies andwarming treatment suggestingno significant effect ofshort-term ambient air temperature on rates of Asat In contrastboth Rdark and Rlight (also measured at 20C) were more sensitiveto short-term ambient air temperatures with the best modelsincorporating the effect of species growth conditions andeither the prior weekrsquos average temperature (Rdark) or themeasurement dayrsquos minimum temperature (Rlight Fig S5) Thetop three models for Rlight Rdark incorporated minimumtemperature values When Rlight is considered alone a clearpredictive response is not obvious (Fig S5) though significantrelationships were observed between Rlight and day-of minimumtemperature (P lt 001) day-of maximum temperature(P lt 001) day-of average temperature (P lt 001) and week-prior minimum temperature (Plt 001) though not with theprior weekrsquos average or maximum temperature suggestinggreater sensitivity to short-term conditions colder conditionsor both
Discussion
The primary objective of this study was to evaluate themechanistic physiological responses of foliar photosynthesisand respiration to long-term warming conditions and naturalvariations in ambient temperature through the growing season
Table 2 Results from three-way repeated-measuresANOVAanalysing foliar gas exchange variables including light-saturating photosynthesis (Asat)dark respiration (Rdark) and respiration in the light (Rlight) expressed on an area mass and nitrogen basis the ratio of Rlight to Rdark (Rlight RDark)the ratio of photosynthetic carbon assimilation to respiratory carbon release (Rlight Agross) and the maximum carboxylation rate of Rubisco (Vcmax)and the electron transport rate (J) both measured at ambient leaf temperature and corrected for a constant temperature of 258C in Betula nana and
Eriophorum vaginatum Plt 005 Plt 001 Plt 0001 S species R round W warming df degrees of freedom
Sdf = 1 158
Rdf = 9 150
Wdf = 1 158
SRdf = 9 140
SWdf = 1 156
RWdf = 9 140
SRWdf= 9 120
F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P
in two dominant Arctic plants We hypothesised that the within-growing season rates of photosynthesis respiration and theinhibition of respiration will relate to growth-induced energydemand These rates will acclimate to long-term warming and be
responsive to short-term temperature fluctuations Our resultsshow stronger seasonal and thermal responses in respiration thanin photosynthesis (when both are measured at 20C) which wasfurther influenced by species differences and growth under long-
15
Betula Eriophorum
10
5
0
15
10
5
0
6
4
2
0
6
4
2
0
64
3
2
1
0
4
2
0
Asa
t (microm
ol C
O2
mndash2
sndash1
)R
dark
(microm
ol C
O2
mndash2
sndash1
)R
light
(microm
ol C
O2
mndash2
sndash1
)
160 170 180 190 200 210160 170 180 190 200
Julian day
(a)
(c)
(e)
(b)
(d)
(f )
Fig 2 (a b)Saturating photosynthesis (Asat measured at PAR=1500mmolmndash2 sndash1 and 400 parts per million (ppm)CO2) (c d) foliar dark respiration (Rdark measured at PAR=0mmolmndash2 sndash1 and 400 ppmCO2) and (e f) respiration inthe light (Rlight estimated via the Kok method at low PAR levels) through the growing season for leaves of (a c e)Betula nana (circles) and (bd f)Eriophorumvaginatum (triangles) grown in ambient (unfilled symbols) andpassivelywarmed (filled symbols) conditions All variables were measured at 20 C in laboratory conditions (n= 4)
Seasonal carbon exchange under long-term warming Functional Plant Biology G
termwarmingThis studyalsoprovided theopportunity toexpanduponexisting knowledge about biotic and environmental controlson foliar Rlight and how seasonal timing and temperaturevariability may mediate Rlight Rdark Together these findingsprovide a more detailed mechanistic understanding of thecontrol of carbon exchange in tundra ecosystems
Species-mediated influences of seasonal and environmentaleffects on carbon exchange
In our study the rates of photosynthesis and respiration ofB nana when measured at a common temperature tended tobe higher than those of E vaginatum during the Arctic tundragrowing season (Fig 2 Table 2) grown under control conditionsand long-term passive warming The higher rates of carbonexchange in B nana both when measured at 20C (Fig 2)and over the 5ndash35C temperature range (Fig 3) wereconsistent with previous studies that sampled plants grownunder warming at midseason (Chapin and Shaver 1996) andwith the addition of soil nitrogen and phosphorus (Heskel et al2012) The difference in Asat between B nana and E vaginatumwas greatest directly after leaf out (Fig 2) which emphasises theimpact of growth formon foliar physiological processesB nanaa deciduouswoody shrub is likely to have a relatively high energydemand immediately after snowmelt to accommodate efficientaboveground and belowground growth during the short growingseason and thismay be further enhanced under warming (Chapinet al 1995 Sullivan et al 2007) By contrast E vaginatum atiller-producing graminoid can retain leaves for multiple years(Fetcher and Shaver 1983) and for this reason may act lessopportunistically upon snowmelt in the spring in terms ofnutrient acquisition via fine root growth (Sullivan et al 2007)andquick leaf production These trends are supported byprevious
work on deciduous and evergreen species where seasonalphotosynthesis and respiration were consistently significantlylower in the evergreen (Ow et al 2010) The species contrastis underscored by the respective leaf architectures andcomposition interspecies comparisons show a less dense(greater SLA) leaf with higher N content in B nana than inE vaginatum (Table 1 Figs S2 S3)
We found a general though not significant decline in Asat inleaves sampled from long-term warming plots suggesting apotential duration-dependent warming effect at the leaf levelthat may contribute to a similar lack of photosyntheticstimulation reported at the ecosystem level (Shaver et al2000 Elmendorf et al 2012) Also these tundra species maybe pushed beyond their photosynthetic thermal optimum underelevated temperatures (Sage and Kubien 2007) which could bea likely scenario in Arctic populations that may be locallyadapted for colder growth temperatures We observed noapparent seasonal arch in the Asat Vcmax and J rates which cangenerally characterise growing season carbon assimilation ofleaves measured at ambient leaf temperatures as previouslyreported (for Asat) in these species (Starr et al 2008) At theleaf level the highest rates of Asat Vcmax and J can occur in theearly growing season (Dungan et al 2003 Xu and Baldocchi2003) This discrepancy may be explained by the highly variableambient temperature and precipitation conditions (Fig 1) thoughmodel evaluation found no strong relationship betweenphotosynthetic variables (measured at 20C) and short-termtemperature values (Table S1) Also as we measured allleaves at a common temperature (20 C) we controlled for theinfluence of ambient temperature on leaf temperature which canaffect photosynthetic rates and allowed for comparison acrosschanges in the ambient temperature to assess thermal acclimation
Respiration rates of both species were generally highest in thefirst few weeks after snowmelt (measured at 20C Fig 2endashf)which is more apparent in Rlight than in Rdark and are similar topreviously reported values (Heskel et al 2012 Heskel et al2013) In both B nana and E vaginatum higher energy demandand potentially respiratory rates in the early season relative to themid- and late season may be attributable to new leaf growth anddevelopment (Vose and Ryan 2002 Xu et al 2007 Ow et al2010) and possibly a higher density of mitochondria in youngerleaves (Armstrong et al 2006) This respiratory release can befurther enhanced by the colder ambient temperatures experiencedby plants in the early growing season due to a short-term coldacclimation (Atkin and Tjoelker 2003 Armstrong et al 2006)The thermal response of respiration is nonlinear and variesbetween species in response curve shape and slope (Fig 3Table 3) higher respiration in B nana across the range oftemperatures experienced during the growing season (Fig 1)and the lack of thermal acclimation may favour its expansion inthe tundra under current conditions (Heskel et al 2013) Furtherthe observed warm acclimation in leaves ofE vaginatum (Fig 3)suggests lowered metabolic rates that may limit growth despitereducing respiratory C loss under a future climate
It should be noted that similar rates of respiration rates acrossspecies may not necessary equate to similar energy efficiencyB nana is reported to exhibit greater respiratory efficiency thanE vaginatum through the differential use of the alternativeand cytochrome pathways potentially lending a competitive
05 10 15 20 25 30 35
10
20
30
40
50
60
70
Betula CT
Betula WG
Eriophorum CT
Eriophorum WG
Temperature (degC)
Rda
rk (
nmol
CO
2 gndash1
sndash1
)
Fig 3 The mean short-term thermal responses of foliar dark respirationfrom 5C to 35C modelled from a polynomial fit from raw data in Betulanana (circles) and Eriophorum vaginatum (triangles) sampled from control(CT open) andwarmed (WG closed) growth environments Temperature (T)response curves of respiration (R) were fit to second-order polynomialequations as follows B nana control r= 0027T2 + 0082T+ 660 B nanawarmed r= 0026T2 + 0103T+ 677 E vaginatum control r = 0045T2 ndash
0106T+ 310 and E vaginatum warmed r = 0018T2 + 0176T+ 2633 Forall thermal response curve fits R2 gt 0990
H Functional Plant Biology M A Heskel et al
advantage in growth and development (Kornfeld et al 2013)In addition Rlight Agross decreased over the growing season(Fig 6) driven by lower values of Rlight as the season progressedrather than any significant change in photosynthesis which is
discordant with the idea that the processes would increase in acoupled manner based on carbohydrate substrate supply anddemand Kornfeld et al (2013) found no difference in starchand sugar content between control and warmed leaves within
0
20
40
60
80Betula
0
20
40
60Eriophorum
0
20
40
60
80
0
40
80
120
0
1
2
3
0
1
2
3
4
Julian day
160 170 180 190 200 210160 170 180 190 200
(a)
(c)
(e)
(b)
(d)
(f )
Vcm
ax (micro
mol
mndash2
sndash1
)J
Vcm
ax
J (micro
mol
mndash2
sndash1
)
Fig 4 (a b) Maximal carboxylation rate (Vcmax) and (c d) electron transport rate (J) both measured in laboratoryconditions and corrected to a constant 25C and (e f) the ratio of these variables (J Vcmax) over the growing season inleaves of (a c e) Betula nana and (b d f) Eriophorum vaginatum grown under ambient (open) and warmed (filled)conditions Values shown are means with se
Seasonal carbon exchange under long-term warming Functional Plant Biology I
species when measured at a single sampling point during thegrowing season though leaves of B nana contained a greaterconcentration of both sugar and starch than E vaginatum Wewere unable to quantify foliar carbohydrate values in thesespecies through the season though future studies couldinclude this informative measurement to further relate themechanistic links between these processes
Both species exhibited the lowest values of Rlight Rdark
towards the end of the season in late July (Fig 5) suggestinga potential developmental or energy demand-related control onthe inhibition of respiration by light that allows for greater
respiratory energy production in the early season during leafexpansionTheonlyother knownstudy that explored the seasonalresponse of Rlight Rdark observed the lowest level of inhibitionearlier in the growing season before the warmest month (Crouset al 2012) This response similar to our study despite a starklydifferent ecological system supports the idea of shared seasonaltiming-mediated biochemical controls on the light inhibition ofrespiration across species The degree of inhibition is known toshow a relaxed response under environmental conditions thatcan stimulate growth due to increased demand for energy and Cskeletons as exhibited under elevated CO2 (Wang et al 2001Shapiro et al 2004) and increased soil nutrient availability(Heskel et al 2012) though this is not always the case (Tissueet al 2002 Ayub et al 2011) Long-termwarming alsomediatedRlight Rdark with lower values observed in warm-grownleaves though measurement temperature can also impact theserelationships (Atkin et al 2006)
Mechanistic explanations for the relationships amonggrowth demand cellular energy status and the degree of lightinhibition of respiration have been previously reported Theenvironmental challenges of a short growing season and ~24-hdiel light exposure in the Arctic tundra control growth ratesthat may then influence values of Rlight Rdark (Bret-Harteet al 2001 Bret-Harte et al 2002) In the light adenylatesupply from photosynthesis may decrease the energy demandfrom mitochondrial respiration Additionally photorespirationis associated with the inactivation of pyruvate dehydrogenasea precursor to the tri-carboxylic acid (TCA) cycle (Budde andRandall 1990) though previous estimations of photorespirationin these species didnotfinda strong correlationwith the inhibitionof Rlight (Heskel et al 2013) The TCA cycle can also besignificantly altered in the light to support N assimilationwhich effectively reduces CO2 release (Tcherkez et al 2005Tcherkez et al 2008 Tcherkez et al 2009) This effect may beenhanced in tundra plantswhen soil N is less limiting under theseconditions growth rates increase (Bret-Harte et al 2002) Theinhibition ofRlight is observed to be slightly increased in fertilisedsoils (Heskel et al 2013) Additionally the oxidative pentosephosphate pathway (OPPP) which contributes the necessaryreductant for the synthesis of multiple metabolites in darknessmay be relaxed in the light when reductants are provided byphotosynthesis diminishing CO2 release by the OPPP Thoughthe OPPP comprises a proportionally smaller CO2 flux comparedwith that from the TCA cycle in the light this decrease maycontribute to lower overall CO2 release causing Rlight ltRdark
(Buckley and Adams 2011) In our current study the apparentrelaxation of the light inhibition of respiration in leaves of bothstudy species (Fig 5) resulting in the greatest proportional CO2
losses (Fig 6) suggesting that high growth demand for ATP andcarbon skeletons outweigh potential carbon losses in the earlyseason
Influences of variable ambient temperature on gas exchange
In addition to quantifying the effects of long-termwarming on thefoliar gas exchange during the growing season this studyevaluated the influence of intraseasonal short-term temperaturevariability on these physiological processes Arctic tundragrowing seasons can exhibit dramatic shifts in temperature and
Table 3 Parameters describing the respiratory response tomeasurement temperature in leaves of Betula nana and Eriophorumvaginatum grown under control (CT) and long-term warming (WG)conditions including Q10 (the temperature sensitivity of respiration(estimated over 5ndash358C)) R10 (the basal respiration rate at 108C(mmol CO2 gndash1 sndash1)) and Eo (kJmolndash1) a variable related to the energyof activation calculated from values of respiration at 208C and R10
Values presented are means and se Statistical differences between speciesand treatments are denoted alphabetically
Fig 5 Means and se of the ratio of respiration in the light (Rlight) estimatedvia the Kok method to dark respiration (Rdark) both variables measured at20C and 400 parts per million CO2 through the growing season in Betulanana (circles) and Eriophorum vaginatum (triangles) under ambient controlfield conditions (CT open symbols) and under a long-term passive warmingtreatment (WGfilled symbols n= 4)A linear regression (y= ndash0009x+ 219R2 = 047) depicts the declining Rlight Rdark values with the progressinggrowing season
J Functional Plant Biology M A Heskel et al
precipitation within days as was observed in this study duringsummer 2010 in Toolik Lake Alaska (Fig 1) and this variabilityis likely to influence the temperature-sensitive processescontrolling leaf carbon cycling Respiration in both speciescan be highly temperature-sensitive especially at warmertemperatures which may become more common in a futureArctic (Fig 3 Table 3) When modelled against thetemperature of the measurement day the average temperatureof the prior week accounted for species and treatment effectsand some patterns emerged that help to characterise the nature ofthe temperature sensitivity of foliar gas exchange (Table S1)However photosynthesis which is known to respond to short-term changes in temperature in field-grown plants (Berry andBjoumlrkman 1980 Poyatos et al 2012) did not show strongrelationships to daily or weekly variation in temperatureinstead species effects explained most variation (Table S1)The underlying photosynthetic machinery maintains consistentrates of carbon assimilation (measured at a constant temperature)throughout the growing season though daily fluctuations intemperature and precipitation experiences in the Arctic mayinfluence in vivo rates Plant species from alpine and Arcticecosystems can exhibit local adaptation to the extremevariability experienced in those locations (Korner and Larcher1988) which may translate to a limited acclimation (Atkin et al2006) Further because the ambient temperature conditionsfrom this region fluctuate so unpredictably in short periodsthrough the season any analysis using temperature averages ofprevious time windows might not be as useful for estimatingthermal acclimation as they are in regions with more predictableseasonal temperature patterns (Ow et al 2010 Searle et al 2011)
In contrast respiratory rates in the light and dark whenmeasured at a constant temperature appeared to be morecorrelated with short-term ambient temperature variation TheAICc analysis shows that the strongest model for Rdark includedspecies and warming effects and the prior weekrsquos average
temperature in contrast the strongest model for Rlight includedthe former parameters and the minimum temperature from themeasurement day (Table S1) The model relationships maysuggest a short-term thermal sensitivity of respiration that maybe more responsive to temperature minimums though this isnot clearly reflected in regressions of Rlight with the ambienttemperature values (Fig S5) This sensitivity of respiration totemperature minimums has been observed previously inroots as well and suggests that thermal acclimation in coldgrowth temperatures may regulate metabolic activity to meetthe demands of growth and maintenance at the expense ofgreater carbon loss (Covey-Crump et al 2002) Low ambienttemperatures did not clearly relate to the highest rates ofrespiration when measured at a common temperature aswould be expected under respiratory cold acclimation (Atkinand Tjoelker 2003) However it is possible that developmentaltiming may also play into the leavesrsquo respiratory rates and abilityto acclimate especially in B nana in the early season when itsleaves are still expanding (Armstrong et al 2006) Through arelatively simple modelling exercise we found some evidenceof differential temperature controls on photosynthesis andrespiration in B nana and E vaginatum we suggest thatfuture work should include more detailed environmental andbiotic factors including soil temperature daily photosyntheticradiation and leaf carbohydrate data to more accurately informthe seasonal temperature sensitivity of foliar carbon exchange
Conclusions and implications
The physiological measurements collected in this study allowedfor the quantification of mechanistic responses in the nightlessand highly variable growing season of the arctic tundra andprovided new insights into foliar carbon regulation We presentnew information on the seasonal trends of foliar carbon cyclingin two dominant tundra species and relate these fluxes to short-
0
015
045
030
0
015
045
030
Julian day
Betula Eriophorum
160 170 180 190 200 210 160 170 180 190 200 210
Rlig
ht
Agr
oss
Fig 6 The proportional carbon loss through respiration in the light to gross photosynthesis in (a) Betula nana and(b) Eriophorum vaginatum over the growing season (Rlight Agross both variables measured at a common temperatureof 20C and 400 parts per million CO2) Linear regressions across growth conditions depict the relationship ofRlight Agross and measurement date during the growing season (B nana y= ndash0003 + 088x R2 = 030 E vaginatumy= ndash0005 + 117x R2 = 065) Values represent variable means and se error bars are not visible for many values dueto their small value
Seasonal carbon exchange under long-term warming Functional Plant Biology K
term temperature variability and long-term warming Our studypresents the first published values of the photosyntheticparameters Vcmax and J at multiple points during the season inthese species which can inform and refine parameterisation ofvegetationmodels atmany scalesAlsowe present thefirst reportof the seasonal flexibility of the degree of the inhibition of Rlight
under long-term warmed growth conditions in these or anyspecies which will enable more accurate calculations ofecosystem respiration Based on these measurements aseasonal estimate (not accounting for potential temperaturesensitivity as discussed above see Eqn S3) shows thatneglecting the inhibition of foliar Rlight in Arctic tundravegetation could lead to overestimations of foliar carbon lossof~4molmndash2 leaf (~176 g)over thegrowingseason or evenmore(~42molmndash2) when considering plants grown under warmedconditionsAlthough the temperature effect on respiratory carbonloss in the light may beminimal the overall discrepancy in fluxesbetween Rdark and Rlight must be considered for realisticdescriptions of Arctic carbon cycling Our study contributesimportant data that allow for an increased understanding ofdominant tundra speciesrsquo responses to environmental changeboth long- and short-term and will allow for more predictivepower in models estimating ecosystem carbon storage
Acknowledgements
We are grateful to the researchers and support staff at the Arctic LTER andToolik Field Station for their help in this study as well as Greg PendergastThis study was made possible through funding from the NSF 0732664OKA acknowledges grant support from the Australian Research Council(DP0986823)
References
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Armstrong AF Logan DC Tobin AK OrsquoToole P Atkin OK (2006b)Heterogeneity of plant mitochondrial responses underpinningrespiratory acclimation to the cold in Arabidopsis thaliana leavesPlant Cell amp Environment 29 940ndash949 doi101111j1365-3040200501475x
Armstrong AF Badger MR Day DA Barthet MM Smith P Millar AHWhelan J Atkin OK (2008) Dynamic changes in the mitochondrialelectron transport chain underpinning cold acclimation of leafrespiration Plant Cell amp Environment 31 1156ndash1169 doi101111j1365-3040200801830x
Atkin OK Tjoelker MG (2003) Thermal acclimation and the dynamicresponse of plant respiration to temperature Trends in Plant Science 8343ndash351 doi101016S1360-1385(03)00136-5
Atkin OK Bruhn D Tjoelker MG (2005) Response of plant respiration tochanges in temperature mechanisms and consequences of variations inQ10 values and acclimation In lsquoPlant respirationrsquo (Eds H Lambers ampM Ribas-Carbo) pp 95ndash135 (Springer Dordrecht The Netherlands)
Atkin OK Scheurwater I Pons TL (2006)High thermal acclimation potentialof both photosynthesis and respiration in two lowlandPlantago species incontrast to an alpine congeneric Global Change Biology 12 500ndash515doi101111j1365-2486200601114x
Ayub G Smith RA Tissue DT Atkin OK (2011) Impacts of drought onleaf respiration in darkness and light in Eucalyptus saligna exposed toindustrial-age atmosphericCO2 and growth temperatureNewPhytologist190 1003ndash1018 doi101111j1469-8137201103673x
Berry J Bjoumlrkman O (1980) Photosynthetic response and adaptation totemperature in higher plants Annual Review of Plant Physiology andPlant Molecular Biology 31 491ndash543 doi101146annurevpp31060180002423
Bret-HarteMSShaverGRZoerner JP JohnstoneJFWagner JLChavezASGunkelman RF IV Lippert SC Laundre JA (2001) Developmentalplasticity allows Betula nana to dominate tundra subjected to analtered environment Ecology 82 18ndash32
Bret-Harte MS Shaver GR Chapin FS III (2002) Primary and secondarystem growth in Arctic shrubs implications for community response toenvironmental change Journal of Ecology 90 251ndash267 doi101046j1365-2745200100657x
BuckleyTNAdamsMA(2011)An analyticalmodel of non-photorespiratoryCO2 release in the light and dark in leaves of C3 species based onstoichiometric flux balance Plant Cell amp Environment 34 89ndash112doi101111j1365-3040201002228x
Budde RJ Randall DD (1990) Pea leaf mitochondrial pyruvatedehydrogenase complex is inactivated in vivo in a light-dependentmanner Proceedings of the National Academy of Sciences of theUnited States of America 87 673ndash676 doi101073pnas872673
CahoonSMPSullivanPF ShaverGRWelker JM PostE (2012) Interactionsamong shrub cover and the soil microclimatemay determine futureArcticcarbon budgets Ecology Letters 15 1415ndash1422 doi101111j1461-0248201201865x
Chapin FS Shaver GR (1996) Physiological and growth responses of Arcticplants to a field experiment simulating climatic change Ecology 77822ndash840 doi1023072265504
Chapin FS Shaver GR Giblin AE Nadelhoffer KJ Laundre JA (1995)Responses of Arctic tundra to experimental and observed changes inclimate Ecology 76 694ndash711 doi1023071939337
Chapin FS Sturm M Serreze MC McFadden JP Key JR Lloyd AHMcGuire AD Rupp TS Lynch AH Schimel JP Beringer J ChapmanWLEpsteinHEEuskirchenESHinzmanLD JiaGPingC-LTapeKDThompson CDC Walker DA Welker JM (2005) Role of land-surfacechanges in Arctic summer warming Science 310 657ndash660 doi101126science1117368
Covey-Crump EM Attwood RG Atkin OK (2002) Regulation of rootrespiration in two species of Plantago that differ in relative growthrate the effect of short and long term changes in temperature PlantCell amp Environment 25 1501ndash1513 doi101046j1365-3040200200932x
Crous KY Zaragoza-Castells J Ellsworth DS Duursma RA LowM TissueDT Atkin OK (2012) Light inhibition of leaf respiration in field-grownEucalyptus saligna in whole-tree chambers under elevated atmosphericCO2 and summer drought Plant Cell amp Environment 35 966ndash981doi101111j1365-3040201102465x
Dungan RJ Whitehead D Duncan RP (2003) Seasonal and temperaturedependence of photosynthesis and respiration for two co-occurringbroad-leaved tree species with contrasting leaf phenology TreePhysiology 23 561ndash568 doi101093treephys238561
Elmendorf SC Henry GHR Hollister RD Bjoumlrk RG Bjorkman ADCallaghan TV Collier LS Cooper EJ Cornelissen JHC Day TAFosaa AM Gould WA Greacutetarsdoacutettir J Harte J Hermanutz L Hik DSHofgaard A Jarrad F Joacutensdoacutettir IS Keuper F Klanderud K Klein JAKoh S Kudo G Lang SI Loewen V May JL Mercado J Michelsen AMolau U Myers-Smith IH Oberbauer SF Pieper S Post E Rixen CRobinson CH Schmidt NM Shaver GR Stenstroumlm A Tolvanen ATotland Oslash Troxler T Wahren C-H Webber PJ Welker JM WookeyPA (2012) Global assessment of experimental climate warming ontundra vegetation heterogeneity over space and time Ecology Letters15 164ndash175 doi101111j1461-0248201101716x
EnvironmentalDataCenterTeam (2011)Meteorologicalmonitoring programat Toolik Alaska Toolik Field Station Institute of Arctic BiologyUniversity of Alaska Fairbanks Fairbanks AK 99775 Available athttptoolikalaskaeduedcabiotic_monitoringdata_queryphp
L Functional Plant Biology M A Heskel et al
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
covered with transparent 015-mm plastic sheeting passivelyincrease the air temperature by ~5C during the growing seasonData on thawdepth and soilmoisture though likely influenced bythe passive warming treatment as well as to influence foliar gasexchange rates were not available for this study The focalspecies for our study are common and abundant at the MATsite Eriophorum vaginatum L (cottongrass) the tussock-forming sedge that is widely distributed over much ofAlaskarsquos North Slope northern Canada and northern EurasiaandBetula nana nanaL (dwarf birch) a deciduous woody shrubthat is also abundant and often dominant over much of the entireArctic region
Foliar carbon exchange
CO2 fluxes of photosynthesis and respiration were measuredusing an infrared gas analyser (IRGA LI-6400XT PortablePhotosynthesis System LI-COR Lincoln NE USA) Allmeasurements were made on clipped leaves of E vaginatumand small branches forBnana collected from the long-termMATgreenhouse and control plots re-cut under water in the field andtransported in water to the laboratory where they acclimated tomeasurement temperatures as inHeskel et al (2012) Preliminarytests on these species showed no difference in the rates of gasexchange and stomatal conductance between field-measured andlaboratory-measured leaves (Griffin unpublished) as has beenshown previously in other species (Mitchell et al 1999 Turnbullet al 2003) This sampling technique was required to provide agreater degree of temperature control maximise the numberof replicates and minimise the time between replicates over theshort growing season and during potentially rapidly changingenvironmental conditions in the field For each E vaginatumsample we collected approximately 10 leaves from a singletussock in order to provide sufficient leaf area in the leafchamber (~6 cm2) For B nana the one or two measuredleaves from the sampled branch that were smaller than the6 cm2 cuvette area were measured for leaf area after IRGAmeasurements and gas exchange values were corrected to thatarea for analysis To obtain high-resolution seasonal monitoringofgas exchange leavesofboth specieswere sampled fromeachofthe four control andgreenhouseplots at theMATsite (n= 4) every~4ndash5 days comprising a measurement round For 12 days inmid-July there was a gap in measurement as the equipment wasbeing used for another research project
Prior to measurement in the laboratory leaves were enclosedin the cuvette at high light conditions to activate photosynthesisthough this exposure was kept to a minimum time to reducethe potential for photoinhibition CO2 assimilation wasmeasuredunder ambient (400 parts per million (ppm)) CO2 concentrationsunder 26 levels of PAR 1500mmolmndash2 sndash1 1200mmolmndash2 sndash1800mmolmndash2 sndash1 400mmolmndash2 sndash1 200mmolmndash2 sndash1100mmolmndash2 sndash1 and every 5 PAR between 100 and 0mmolmndash2 sndash1 This range of light fully encompasses the lightenvironment experienced by the species in their growthenvironment Following the light response curve leaf sampleswere treated to 900mmolmndash2 sndash1 PAR and 400 ppm CO2 for5ndash10minbeforeCO2assimilationwasmeasured in response to11levels of increasing [CO2] ~0 ppm 50 ppm 100 ppm 150 ppm200 ppm 300 ppm 400 ppm 600 ppm 800 ppm 1200 ppm and
1500 ppm Following the CO2 response curve measurement andafter 10min in darkness at 400 ppm CO2 it was assumed that nophotosynthesis or photorespiration was taking place and all CO2
flux could be attributed to mitochondrial respiration in the dark(Rdark)
All measurements were taken at a relative humidity of~40ndash60 and potential diffusion in and out of the cuvettewas accounted for as was diffusion through the gasketaccording to corrections presented in the Li-Cor 6400Instructional Manual The average leaf vapour pressure deficitwas 0942 032 kPa (se) for the light response curves and1142 029 for CO2 response curves across both species andtreatments The cuvette block temperature was set to 20C forall measurements to control for leaf temperature and isrepresentative of temperatures experienced during the growingseason (Fig 1) leaf temperatures in situ ranged across 19ndash23Cin both species Light-saturated net photosynthetic rate (Asat) was
0
140 150 160 170 180 190 200 210 220 230
5
10
15
20
Pre
cipi
tatio
n (m
m)
Julian day
20
40
60
80
100
Rel
ativ
e hu
mid
ity (
) 0
10
20
30
40
50
0
10
20
30
40
Max
Min
Tem
pera
ture
(degC
)P
AR
(m
ol m
ndash2 d
ayndash1
)
(a)
(b)
(c)
(d)
Fig 1 Environmental data from the moist acidic tundra experimental plotsat the Arctic Long-term Ecological Research field site at Toolik Lake Alaskaincluding (a) growing season maximum and minimum air temperature(b) daily total PAR (c) relative humidity and (d) precipitation for ambient(control solid lines) and passively warmed (dotted lines) tundra The vapourpressure deficit calculated from seasonal means of relative humidity andtemperature was 0187 kPa in control plots and 0472 kPa in warmedplots Photosynthesis and respiration measurements were made on leavessampled between the dashed lines (Julian days 162ndash209 or 11 Junendash28 July2010)
Seasonal carbon exchange under long-term warming Functional Plant Biology C
estimated by fitting data from the light response curve to arectangular hyperbolic function (Excel Solver MicrosoftRedmond WA USA) CO2 response curves were analysed forthe maximum carboxylation velocity of Rubisco (Vcmax) and themodelled maximum rate of electron transfer (referred to here as Jdue to data inconsistency at high CO2 levels) using the netphotosynthetic ratendash internal CO2 (ci)curve fitting utility (ver20071) provided and detailed in Sharkey et al (2007) Examplesof light andCO2 response curves can be seen in Fig S1 (availableas Supplementary Material to this paper) A portion of thereplicate measurements gave inaccurate readings of CO2
saturated photosynthesis at the highest levels of CO2 limitingstatistical comparison of maximal photosynthesis Values ofphotosynthesis and respiration are expressed on an area massand nitrogen basis
Quantifying respiration in the light using the Kok methodTo estimate Rlight we used the Kok method which is convenientfor field measurements and can be used under ambientatmospheric conditions This method is based on theobservation that the quantum yield of photosynthesis usuallydecreases abruptly above a certain level of light intensity ndash oftennear the light compensation point where carbon flux is zero (Kok1948) This leads to a noticeable nonlinearity or lsquobendrsquo in theotherwise linear lower range of the light response curve which isinterpreted as the saturation point of light inhibition of respiration(explained in detail in Shapiro et al 2004) Since respiration isassumed to be constant above this point an extrapolation to0mmolmndash2 sndash1 PAR of the linear portion of the curve above thispoint is assumed to give Rlight Here the irradiance range we usedto calculate Rlight spanned 25ndash90mmolmndash2 sndash1 (Fig S1b)
The Kok method assumes that the CO2 assimilation rateresponds only to light thus corrections must be made toaccount for changes in ci As rates of photosynthesis slowunder decreasing light intensity CO2 tends to accumulatewithin the leaf increasing ci which in turn affect the shapeof the light curve by decreasing the rate of photorespiration atlower light levels We corrected all measurements to a constantci to account for these effects according to Kirschbaum andFarquhar (1987) as described in Ayub et al (2011) toachieve a more accurate extrapolation of Rlight
Short-term in situ temperature response of respiration
To estimate respiratory fluxes under field conditions that varydaily we need descriptive high-resolution thermal responsecurves to extend our interpretation of measurements at20C Following methods similar to those described by Huumlveet al (2012) and OrsquoSullivan et al (2013) we sampled leavesof both species (similar to the sampling method describedabove) to quantify the short-term respiratory response ofleaves to measurement temperature Data presented in thisstudy represent replicates measured in late June on fullyexpanded leaves Leaves were kept in darkness for ~30minbefore measurement and then placed in a 155 110 65 cmwater-jacketed glass-topped aluminium chamber that wasthen covered by a dark fabric Two fans mixed air within thechamber and chamber temperature was controlled via aconnected programmable circulating waterbath (MRC300
Melcor Trenton NJ USA) that warmed from 5C to 35Cover ~45min Leaf temperature in the chamber was measuredwith a small gaugewire copper-constantan thermocouple pressedagainst the lower surface of the leaf and recorded via a data-logger(CR1000 Campbell Scientific Logan UT USA) Dry air waspassed through the cuvette at a rate of 045 L hndash1 and exited thecuvette via tubing connected to an IRGA (LI-840A LI-COR)which measured the flux of CO2 from the leaf Values ofrespiratory flux were corrected for the leaf area and mass ofthe measured replicate The standard equations used to calculatethe temperature sensitivity of respiration (Q10) the basalrespiration rate at 10C (R10) and a variable related to theenergy of activation calculated from values of respiration at20C and R10 (Eo) are presented in the Supplementary Material(Eqns S1ndash2)
Physical leaf traits and foliar nutrients
All leaf samples used for gas exchange measurements weremeasured for leaf area using a rotating-belt leaf area meter (LI-3100CAreaMeter LI-COR) Sampleswere then dried in an ovenat 60C for a minimum of 2 days before mass was determinedAfter transport to Columbia University in NewYork all sampleswere ground weighed and packaged for elemental analysis of[CHN] (2400 Series II Perkin-Elmer Boston MA USA)
Statistical analyses
The effect of seasonal timing on foliar gas exchange and relatedleaf characteristics was analysed using a linear mixed-effectsmodel framework to perform a three-way repeated-measuresANOVA in R (ver 270 The R Foundation for StatisticalComputing Vienna Austria) assigning measurement round(the 4- to 5-day period when replicates from each speciestreatment combination were measured) species (B nana andE vaginatum) and warming (control or greenhouse) asexplanatory variables treatment block was treated as a randomeffect Prior to analysis data were tested for normality(ShapirondashWilk) and heteroskedasticity (BreuschndashPagan) andlog10-transformed when necessary Post hoc multiplecomparisons were made using Tukeyrsquos test To address therelationships between foliar physiological rates and ambienttemperature or leaf traits correlation analyses were employedIn addition stepwise linear regression models that integrated theeffects of species measurement round warming treatment andambient temperature to assess select gas exchange variableswere evaluated using corrected Akaike Information Criteria(AICc see the Supplementary Material) For all analysesP-values of lt005 were considered significant
Results
Seasonal environmental variability
Full snowmelt at the long-termexperimentalwarmingandcontrolplots occurred by ~June 9 2010 (Toolik EDC) During themeasurement period of Julian Day 162ndash209 (11Junendash28 July)themaximumandminimum temperature relativehumidity lightandprecipitationwere highlyvariable (Fig 1)During this periodthe highest recorded temperature (239C) occurred on 9 Julyand the lowest minimum temperature (04C) after snowmeltoccurred on 6 July merely 3 days prior The average temperature
D Functional Plant Biology M A Heskel et al
during the measurement period was 107C with an averageminimum temperature of 48C and an average maximumtemperature of 143C In passively warmed plots thistranslates to an average maximum temperature of 193Cnearly equal to the leaf sample measurement temperatureAcross the growing season mean vapour pressure deficitcalculated from mean values of relative humidity andtemperatures was 0187 kPa in the controls and 0471 kPa inthe passively warmed plots Three large precipitation eventsoccurred during the measurement period corresponding withlower ambient air temperatures high relative humidity and lowdaily PAR (Fig 1)
Specific leaf traits and chemistry
Due to the contrasting leaf forms of B nana which producessmall flat circle-shaped leaves each season and E vaginatumwhich grows elongated three-sided tillers that are retainedthrough the snow-covered winter it is not surprising thecorresponding specific leaf area (SLA) measurements differgreatly between the species taken as a whole (Table 1Fig S2) and within both control and warming treatments(P lt 00001) Though individual measurement rounds showsome variation in SLA across the growing season elevatedgrowth temperature did not affect either species comparedwith leaves in the control (Fig S2) Leaf C concentration wasgreater and much more variable in B nana than in E vaginatum(P lt 00001) and warming treatment and seasonal timing(Round) did not affect this in either species (Fig S3adTable 1) E vaginatum decreased in proportional N content asthe season progressedB nana also decreased through the seasonbut it was less pronounced (Fig S3) Warming did not elevate Ncontent in either species The species-specific trends in N causedthe related increases in C N over the seasonal course (Fig S3)
Intraseasonal dynamics of photosynthesis and respiration
The impacts of species warming treatment and measurementround through the season on area- mass- and N-based gasexchange rates (measured at 20C) as expressed by the resultsof a three-way ANOVA are shown in Table 2 Variation inarea-based light-saturated photosynthetic rates (Fig 2) wereprimarily explained through species differences with B nanaconsistently exhibiting greater rates of carbon assimilation acrossall measurements as well as when considering measurementsin the warmed plots (P lt 001) Area-based Asat showed nosignificant response to the measurement round (Fig 2) though
mass-based ratesyieldedan interactive effect of species and round(Fig S3andashd Table 2) suggesting the effect of leaf structuraldifferences between the species when assessing photosyntheticrates Only when expressed on a per-N rate did Asat vary withmeasurement round driven by the increases in per-Nphotosynthesis later in the season related to lower foliar Nconcentration (Fig S3endashh) Similarly there were no observedchanges in rates under long-term warming treatment in eitherspecies and B nana consistently exhibited higher rates inboth warmed and control plots compared with E vaginatum Ifoneconsiders theprocessesunderlyingphotosynthetic efficiencyVcmax was significantly affected by the interaction of species andmeasurement round Mean rates of Vcmax in B nana particularlyunder warming increased slightly through the season (Fig 3)Rates of Vcmax and J were correlated (r2 = 0856) The ratio ofthese variables J Vcmaxwas generally 1ndash2but declined throughthe growing season in both species (Fig 4endashf Table 2)
Across allmeasures ofmitochondrial dark respirationB nanaexhibited higher rates than E vaginatum (Table 2) A significantinteraction term between species and measurement roundhighlighted differences in mitochondrial dark respiration (area-andN-based) between E vaginatum andB nana over the courseof the growing seasonE vaginatumdisplayed a gradual decreasein rates around Day 180 (29 June) that was not observed inB nana (Fig 2cndashd) Further considering allmeasurements long-term growth warming was associated with lower rates of area-based Rdark in both species though this effect is only observed inE vaginatum considering N-based Rdark On a mass basis Rdark
decreased through the growing season in both species (Fig 2cndashdTable 2) with the highestmean rates for both species occurring inthe first 2 weeks of the growing season
Short-term high-resolution temperature response curves ofRdark measured midseason showed a higher degree of thermalacclimation to long-term elevated growth temperature inE vaginatum than in B nana (Fig 3) As in the measurementsmade at a single temperature of 20C (Fig 2) and calculated at10C for R10 (Table 3) rates of respiration were generallygreater in B nana than in E vaginatum across themeasurement temperature range from 5C to 20C Howeverover 20ndash35C E vaginatum grown in control plots respired atincreasingly greater rates than did B nana which correspondedto its significantly higher Q10 and Eo values (Fig 4 Table 3)
One of the main objectives of this study was to obtain a betterunderstandingof the light inhibitionof respiration in these speciesand how this phenomenon may relate to other leaf traits Acrossspecies and growth conditions light significantly suppressed
Table 1 Results from three-way repeated-measures ANOVA analysing the carbon ( C) and nitrogen (N) concentrations and their ratio (C N)of dry leaf material and specific leaf area (SLA) in Betula nana and Eriophorum vaginatum
Plt 005 Plt 001 Plt 0001 S species R round W warming round df degrees of freedom
Sdf = 1 158
Rdf = 9 150
Wdf = 1 158
SRdf = 9 140
SWdf= 1 156
RWdf= 9 140
SRWdf = 9 120
F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P
Seasonal carbon exchange under long-term warming Functional Plant Biology E
respiration rates (P lt 00001) with individual leaf values rangingwidely Rlight Rdark was less in E vaginatum thanB nana underboth growth conditions (P lt 0001) and decreased further underwarming inE vaginatum (P lt 001) For both species and growthconditions Rlight Rdark decreased over the growing season(Fig 5) Warming decreased rates in only E vaginatum forarea- and N-based rates (Plt 005 for both) but not mass-basedrates which found warming to significantly lower rates whenconsidering all measurements (P lt 001) Furthermore the effecta speciesmeasurement round interaction (Plt 005) wasevident in the different trends of respiratory release over thecourse of the season (Fig 2endashf Fig S4) The ratio of carbon lossvia respiration in the light to gross photosynthesis (Rlight Agross)which describes the relative carbon loss from respirationcompared with the total carbon cycled in a leaf (as opposed tothe difference between the fluxes) decreased through the seasonin both species (Fig 6) though at different rates Theseproportional respiratory losses were significantly greater inB nana than in E vaginatum and warming significantlydecreased these values considering all measurements acrossthe season (Table 2) No clear relationships were foundbetween the degree of inhibition and foliar fluxes or traits inthese species regression analysis between Rlight Rdark andNAsat and Vcmax produced r2lt 010 (data not shown)
Intraseasonal ambient temperature influenceson gas exchange
Toevaluate the influenceof ambient temperature ongas exchangerates we compared multivariate linear models that incorporatedtemperature values from the day of andmean temperature values
from the week before gas exchange measurement Table S1shows the results of models incorporating the effect of specieswarming treatment and day-of and week-before minimummaximum and average temperatures on area-based Asat RdarkRlight and Rlight Rdark evaluating by comparing relative AICcweights The best model for photosynthesis (measured at 20C)incorporates only species with the next best only incorporatingspecies andwarming treatment suggestingno significant effect ofshort-term ambient air temperature on rates of Asat In contrastboth Rdark and Rlight (also measured at 20C) were more sensitiveto short-term ambient air temperatures with the best modelsincorporating the effect of species growth conditions andeither the prior weekrsquos average temperature (Rdark) or themeasurement dayrsquos minimum temperature (Rlight Fig S5) Thetop three models for Rlight Rdark incorporated minimumtemperature values When Rlight is considered alone a clearpredictive response is not obvious (Fig S5) though significantrelationships were observed between Rlight and day-of minimumtemperature (P lt 001) day-of maximum temperature(P lt 001) day-of average temperature (P lt 001) and week-prior minimum temperature (Plt 001) though not with theprior weekrsquos average or maximum temperature suggestinggreater sensitivity to short-term conditions colder conditionsor both
Discussion
The primary objective of this study was to evaluate themechanistic physiological responses of foliar photosynthesisand respiration to long-term warming conditions and naturalvariations in ambient temperature through the growing season
Table 2 Results from three-way repeated-measuresANOVAanalysing foliar gas exchange variables including light-saturating photosynthesis (Asat)dark respiration (Rdark) and respiration in the light (Rlight) expressed on an area mass and nitrogen basis the ratio of Rlight to Rdark (Rlight RDark)the ratio of photosynthetic carbon assimilation to respiratory carbon release (Rlight Agross) and the maximum carboxylation rate of Rubisco (Vcmax)and the electron transport rate (J) both measured at ambient leaf temperature and corrected for a constant temperature of 258C in Betula nana and
Eriophorum vaginatum Plt 005 Plt 001 Plt 0001 S species R round W warming df degrees of freedom
Sdf = 1 158
Rdf = 9 150
Wdf = 1 158
SRdf = 9 140
SWdf = 1 156
RWdf = 9 140
SRWdf= 9 120
F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P
in two dominant Arctic plants We hypothesised that the within-growing season rates of photosynthesis respiration and theinhibition of respiration will relate to growth-induced energydemand These rates will acclimate to long-term warming and be
responsive to short-term temperature fluctuations Our resultsshow stronger seasonal and thermal responses in respiration thanin photosynthesis (when both are measured at 20C) which wasfurther influenced by species differences and growth under long-
15
Betula Eriophorum
10
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64
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t (microm
ol C
O2
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)R
dark
(microm
ol C
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mndash2
sndash1
)R
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(microm
ol C
O2
mndash2
sndash1
)
160 170 180 190 200 210160 170 180 190 200
Julian day
(a)
(c)
(e)
(b)
(d)
(f )
Fig 2 (a b)Saturating photosynthesis (Asat measured at PAR=1500mmolmndash2 sndash1 and 400 parts per million (ppm)CO2) (c d) foliar dark respiration (Rdark measured at PAR=0mmolmndash2 sndash1 and 400 ppmCO2) and (e f) respiration inthe light (Rlight estimated via the Kok method at low PAR levels) through the growing season for leaves of (a c e)Betula nana (circles) and (bd f)Eriophorumvaginatum (triangles) grown in ambient (unfilled symbols) andpassivelywarmed (filled symbols) conditions All variables were measured at 20 C in laboratory conditions (n= 4)
Seasonal carbon exchange under long-term warming Functional Plant Biology G
termwarmingThis studyalsoprovided theopportunity toexpanduponexisting knowledge about biotic and environmental controlson foliar Rlight and how seasonal timing and temperaturevariability may mediate Rlight Rdark Together these findingsprovide a more detailed mechanistic understanding of thecontrol of carbon exchange in tundra ecosystems
Species-mediated influences of seasonal and environmentaleffects on carbon exchange
In our study the rates of photosynthesis and respiration ofB nana when measured at a common temperature tended tobe higher than those of E vaginatum during the Arctic tundragrowing season (Fig 2 Table 2) grown under control conditionsand long-term passive warming The higher rates of carbonexchange in B nana both when measured at 20C (Fig 2)and over the 5ndash35C temperature range (Fig 3) wereconsistent with previous studies that sampled plants grownunder warming at midseason (Chapin and Shaver 1996) andwith the addition of soil nitrogen and phosphorus (Heskel et al2012) The difference in Asat between B nana and E vaginatumwas greatest directly after leaf out (Fig 2) which emphasises theimpact of growth formon foliar physiological processesB nanaa deciduouswoody shrub is likely to have a relatively high energydemand immediately after snowmelt to accommodate efficientaboveground and belowground growth during the short growingseason and thismay be further enhanced under warming (Chapinet al 1995 Sullivan et al 2007) By contrast E vaginatum atiller-producing graminoid can retain leaves for multiple years(Fetcher and Shaver 1983) and for this reason may act lessopportunistically upon snowmelt in the spring in terms ofnutrient acquisition via fine root growth (Sullivan et al 2007)andquick leaf production These trends are supported byprevious
work on deciduous and evergreen species where seasonalphotosynthesis and respiration were consistently significantlylower in the evergreen (Ow et al 2010) The species contrastis underscored by the respective leaf architectures andcomposition interspecies comparisons show a less dense(greater SLA) leaf with higher N content in B nana than inE vaginatum (Table 1 Figs S2 S3)
We found a general though not significant decline in Asat inleaves sampled from long-term warming plots suggesting apotential duration-dependent warming effect at the leaf levelthat may contribute to a similar lack of photosyntheticstimulation reported at the ecosystem level (Shaver et al2000 Elmendorf et al 2012) Also these tundra species maybe pushed beyond their photosynthetic thermal optimum underelevated temperatures (Sage and Kubien 2007) which could bea likely scenario in Arctic populations that may be locallyadapted for colder growth temperatures We observed noapparent seasonal arch in the Asat Vcmax and J rates which cangenerally characterise growing season carbon assimilation ofleaves measured at ambient leaf temperatures as previouslyreported (for Asat) in these species (Starr et al 2008) At theleaf level the highest rates of Asat Vcmax and J can occur in theearly growing season (Dungan et al 2003 Xu and Baldocchi2003) This discrepancy may be explained by the highly variableambient temperature and precipitation conditions (Fig 1) thoughmodel evaluation found no strong relationship betweenphotosynthetic variables (measured at 20C) and short-termtemperature values (Table S1) Also as we measured allleaves at a common temperature (20 C) we controlled for theinfluence of ambient temperature on leaf temperature which canaffect photosynthetic rates and allowed for comparison acrosschanges in the ambient temperature to assess thermal acclimation
Respiration rates of both species were generally highest in thefirst few weeks after snowmelt (measured at 20C Fig 2endashf)which is more apparent in Rlight than in Rdark and are similar topreviously reported values (Heskel et al 2012 Heskel et al2013) In both B nana and E vaginatum higher energy demandand potentially respiratory rates in the early season relative to themid- and late season may be attributable to new leaf growth anddevelopment (Vose and Ryan 2002 Xu et al 2007 Ow et al2010) and possibly a higher density of mitochondria in youngerleaves (Armstrong et al 2006) This respiratory release can befurther enhanced by the colder ambient temperatures experiencedby plants in the early growing season due to a short-term coldacclimation (Atkin and Tjoelker 2003 Armstrong et al 2006)The thermal response of respiration is nonlinear and variesbetween species in response curve shape and slope (Fig 3Table 3) higher respiration in B nana across the range oftemperatures experienced during the growing season (Fig 1)and the lack of thermal acclimation may favour its expansion inthe tundra under current conditions (Heskel et al 2013) Furtherthe observed warm acclimation in leaves ofE vaginatum (Fig 3)suggests lowered metabolic rates that may limit growth despitereducing respiratory C loss under a future climate
It should be noted that similar rates of respiration rates acrossspecies may not necessary equate to similar energy efficiencyB nana is reported to exhibit greater respiratory efficiency thanE vaginatum through the differential use of the alternativeand cytochrome pathways potentially lending a competitive
05 10 15 20 25 30 35
10
20
30
40
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60
70
Betula CT
Betula WG
Eriophorum CT
Eriophorum WG
Temperature (degC)
Rda
rk (
nmol
CO
2 gndash1
sndash1
)
Fig 3 The mean short-term thermal responses of foliar dark respirationfrom 5C to 35C modelled from a polynomial fit from raw data in Betulanana (circles) and Eriophorum vaginatum (triangles) sampled from control(CT open) andwarmed (WG closed) growth environments Temperature (T)response curves of respiration (R) were fit to second-order polynomialequations as follows B nana control r= 0027T2 + 0082T+ 660 B nanawarmed r= 0026T2 + 0103T+ 677 E vaginatum control r = 0045T2 ndash
0106T+ 310 and E vaginatum warmed r = 0018T2 + 0176T+ 2633 Forall thermal response curve fits R2 gt 0990
H Functional Plant Biology M A Heskel et al
advantage in growth and development (Kornfeld et al 2013)In addition Rlight Agross decreased over the growing season(Fig 6) driven by lower values of Rlight as the season progressedrather than any significant change in photosynthesis which is
discordant with the idea that the processes would increase in acoupled manner based on carbohydrate substrate supply anddemand Kornfeld et al (2013) found no difference in starchand sugar content between control and warmed leaves within
0
20
40
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80Betula
0
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(f )
Vcm
ax (micro
mol
mndash2
sndash1
)J
Vcm
ax
J (micro
mol
mndash2
sndash1
)
Fig 4 (a b) Maximal carboxylation rate (Vcmax) and (c d) electron transport rate (J) both measured in laboratoryconditions and corrected to a constant 25C and (e f) the ratio of these variables (J Vcmax) over the growing season inleaves of (a c e) Betula nana and (b d f) Eriophorum vaginatum grown under ambient (open) and warmed (filled)conditions Values shown are means with se
Seasonal carbon exchange under long-term warming Functional Plant Biology I
species when measured at a single sampling point during thegrowing season though leaves of B nana contained a greaterconcentration of both sugar and starch than E vaginatum Wewere unable to quantify foliar carbohydrate values in thesespecies through the season though future studies couldinclude this informative measurement to further relate themechanistic links between these processes
Both species exhibited the lowest values of Rlight Rdark
towards the end of the season in late July (Fig 5) suggestinga potential developmental or energy demand-related control onthe inhibition of respiration by light that allows for greater
respiratory energy production in the early season during leafexpansionTheonlyother knownstudy that explored the seasonalresponse of Rlight Rdark observed the lowest level of inhibitionearlier in the growing season before the warmest month (Crouset al 2012) This response similar to our study despite a starklydifferent ecological system supports the idea of shared seasonaltiming-mediated biochemical controls on the light inhibition ofrespiration across species The degree of inhibition is known toshow a relaxed response under environmental conditions thatcan stimulate growth due to increased demand for energy and Cskeletons as exhibited under elevated CO2 (Wang et al 2001Shapiro et al 2004) and increased soil nutrient availability(Heskel et al 2012) though this is not always the case (Tissueet al 2002 Ayub et al 2011) Long-termwarming alsomediatedRlight Rdark with lower values observed in warm-grownleaves though measurement temperature can also impact theserelationships (Atkin et al 2006)
Mechanistic explanations for the relationships amonggrowth demand cellular energy status and the degree of lightinhibition of respiration have been previously reported Theenvironmental challenges of a short growing season and ~24-hdiel light exposure in the Arctic tundra control growth ratesthat may then influence values of Rlight Rdark (Bret-Harteet al 2001 Bret-Harte et al 2002) In the light adenylatesupply from photosynthesis may decrease the energy demandfrom mitochondrial respiration Additionally photorespirationis associated with the inactivation of pyruvate dehydrogenasea precursor to the tri-carboxylic acid (TCA) cycle (Budde andRandall 1990) though previous estimations of photorespirationin these species didnotfinda strong correlationwith the inhibitionof Rlight (Heskel et al 2013) The TCA cycle can also besignificantly altered in the light to support N assimilationwhich effectively reduces CO2 release (Tcherkez et al 2005Tcherkez et al 2008 Tcherkez et al 2009) This effect may beenhanced in tundra plantswhen soil N is less limiting under theseconditions growth rates increase (Bret-Harte et al 2002) Theinhibition ofRlight is observed to be slightly increased in fertilisedsoils (Heskel et al 2013) Additionally the oxidative pentosephosphate pathway (OPPP) which contributes the necessaryreductant for the synthesis of multiple metabolites in darknessmay be relaxed in the light when reductants are provided byphotosynthesis diminishing CO2 release by the OPPP Thoughthe OPPP comprises a proportionally smaller CO2 flux comparedwith that from the TCA cycle in the light this decrease maycontribute to lower overall CO2 release causing Rlight ltRdark
(Buckley and Adams 2011) In our current study the apparentrelaxation of the light inhibition of respiration in leaves of bothstudy species (Fig 5) resulting in the greatest proportional CO2
losses (Fig 6) suggesting that high growth demand for ATP andcarbon skeletons outweigh potential carbon losses in the earlyseason
Influences of variable ambient temperature on gas exchange
In addition to quantifying the effects of long-termwarming on thefoliar gas exchange during the growing season this studyevaluated the influence of intraseasonal short-term temperaturevariability on these physiological processes Arctic tundragrowing seasons can exhibit dramatic shifts in temperature and
Table 3 Parameters describing the respiratory response tomeasurement temperature in leaves of Betula nana and Eriophorumvaginatum grown under control (CT) and long-term warming (WG)conditions including Q10 (the temperature sensitivity of respiration(estimated over 5ndash358C)) R10 (the basal respiration rate at 108C(mmol CO2 gndash1 sndash1)) and Eo (kJmolndash1) a variable related to the energyof activation calculated from values of respiration at 208C and R10
Values presented are means and se Statistical differences between speciesand treatments are denoted alphabetically
Fig 5 Means and se of the ratio of respiration in the light (Rlight) estimatedvia the Kok method to dark respiration (Rdark) both variables measured at20C and 400 parts per million CO2 through the growing season in Betulanana (circles) and Eriophorum vaginatum (triangles) under ambient controlfield conditions (CT open symbols) and under a long-term passive warmingtreatment (WGfilled symbols n= 4)A linear regression (y= ndash0009x+ 219R2 = 047) depicts the declining Rlight Rdark values with the progressinggrowing season
J Functional Plant Biology M A Heskel et al
precipitation within days as was observed in this study duringsummer 2010 in Toolik Lake Alaska (Fig 1) and this variabilityis likely to influence the temperature-sensitive processescontrolling leaf carbon cycling Respiration in both speciescan be highly temperature-sensitive especially at warmertemperatures which may become more common in a futureArctic (Fig 3 Table 3) When modelled against thetemperature of the measurement day the average temperatureof the prior week accounted for species and treatment effectsand some patterns emerged that help to characterise the nature ofthe temperature sensitivity of foliar gas exchange (Table S1)However photosynthesis which is known to respond to short-term changes in temperature in field-grown plants (Berry andBjoumlrkman 1980 Poyatos et al 2012) did not show strongrelationships to daily or weekly variation in temperatureinstead species effects explained most variation (Table S1)The underlying photosynthetic machinery maintains consistentrates of carbon assimilation (measured at a constant temperature)throughout the growing season though daily fluctuations intemperature and precipitation experiences in the Arctic mayinfluence in vivo rates Plant species from alpine and Arcticecosystems can exhibit local adaptation to the extremevariability experienced in those locations (Korner and Larcher1988) which may translate to a limited acclimation (Atkin et al2006) Further because the ambient temperature conditionsfrom this region fluctuate so unpredictably in short periodsthrough the season any analysis using temperature averages ofprevious time windows might not be as useful for estimatingthermal acclimation as they are in regions with more predictableseasonal temperature patterns (Ow et al 2010 Searle et al 2011)
In contrast respiratory rates in the light and dark whenmeasured at a constant temperature appeared to be morecorrelated with short-term ambient temperature variation TheAICc analysis shows that the strongest model for Rdark includedspecies and warming effects and the prior weekrsquos average
temperature in contrast the strongest model for Rlight includedthe former parameters and the minimum temperature from themeasurement day (Table S1) The model relationships maysuggest a short-term thermal sensitivity of respiration that maybe more responsive to temperature minimums though this isnot clearly reflected in regressions of Rlight with the ambienttemperature values (Fig S5) This sensitivity of respiration totemperature minimums has been observed previously inroots as well and suggests that thermal acclimation in coldgrowth temperatures may regulate metabolic activity to meetthe demands of growth and maintenance at the expense ofgreater carbon loss (Covey-Crump et al 2002) Low ambienttemperatures did not clearly relate to the highest rates ofrespiration when measured at a common temperature aswould be expected under respiratory cold acclimation (Atkinand Tjoelker 2003) However it is possible that developmentaltiming may also play into the leavesrsquo respiratory rates and abilityto acclimate especially in B nana in the early season when itsleaves are still expanding (Armstrong et al 2006) Through arelatively simple modelling exercise we found some evidenceof differential temperature controls on photosynthesis andrespiration in B nana and E vaginatum we suggest thatfuture work should include more detailed environmental andbiotic factors including soil temperature daily photosyntheticradiation and leaf carbohydrate data to more accurately informthe seasonal temperature sensitivity of foliar carbon exchange
Conclusions and implications
The physiological measurements collected in this study allowedfor the quantification of mechanistic responses in the nightlessand highly variable growing season of the arctic tundra andprovided new insights into foliar carbon regulation We presentnew information on the seasonal trends of foliar carbon cyclingin two dominant tundra species and relate these fluxes to short-
0
015
045
030
0
015
045
030
Julian day
Betula Eriophorum
160 170 180 190 200 210 160 170 180 190 200 210
Rlig
ht
Agr
oss
Fig 6 The proportional carbon loss through respiration in the light to gross photosynthesis in (a) Betula nana and(b) Eriophorum vaginatum over the growing season (Rlight Agross both variables measured at a common temperatureof 20C and 400 parts per million CO2) Linear regressions across growth conditions depict the relationship ofRlight Agross and measurement date during the growing season (B nana y= ndash0003 + 088x R2 = 030 E vaginatumy= ndash0005 + 117x R2 = 065) Values represent variable means and se error bars are not visible for many values dueto their small value
Seasonal carbon exchange under long-term warming Functional Plant Biology K
term temperature variability and long-term warming Our studypresents the first published values of the photosyntheticparameters Vcmax and J at multiple points during the season inthese species which can inform and refine parameterisation ofvegetationmodels atmany scalesAlsowe present thefirst reportof the seasonal flexibility of the degree of the inhibition of Rlight
under long-term warmed growth conditions in these or anyspecies which will enable more accurate calculations ofecosystem respiration Based on these measurements aseasonal estimate (not accounting for potential temperaturesensitivity as discussed above see Eqn S3) shows thatneglecting the inhibition of foliar Rlight in Arctic tundravegetation could lead to overestimations of foliar carbon lossof~4molmndash2 leaf (~176 g)over thegrowingseason or evenmore(~42molmndash2) when considering plants grown under warmedconditionsAlthough the temperature effect on respiratory carbonloss in the light may beminimal the overall discrepancy in fluxesbetween Rdark and Rlight must be considered for realisticdescriptions of Arctic carbon cycling Our study contributesimportant data that allow for an increased understanding ofdominant tundra speciesrsquo responses to environmental changeboth long- and short-term and will allow for more predictivepower in models estimating ecosystem carbon storage
Acknowledgements
We are grateful to the researchers and support staff at the Arctic LTER andToolik Field Station for their help in this study as well as Greg PendergastThis study was made possible through funding from the NSF 0732664OKA acknowledges grant support from the Australian Research Council(DP0986823)
References
Armstrong AF Logan DC Atkin OK (2006a) On the developmentaldependence of leaf respiration responses to short and long-termchanges in growth temperature American Journal of Botany 931633ndash1639 doi103732ajb93111633
Armstrong AF Logan DC Tobin AK OrsquoToole P Atkin OK (2006b)Heterogeneity of plant mitochondrial responses underpinningrespiratory acclimation to the cold in Arabidopsis thaliana leavesPlant Cell amp Environment 29 940ndash949 doi101111j1365-3040200501475x
Armstrong AF Badger MR Day DA Barthet MM Smith P Millar AHWhelan J Atkin OK (2008) Dynamic changes in the mitochondrialelectron transport chain underpinning cold acclimation of leafrespiration Plant Cell amp Environment 31 1156ndash1169 doi101111j1365-3040200801830x
Atkin OK Tjoelker MG (2003) Thermal acclimation and the dynamicresponse of plant respiration to temperature Trends in Plant Science 8343ndash351 doi101016S1360-1385(03)00136-5
Atkin OK Bruhn D Tjoelker MG (2005) Response of plant respiration tochanges in temperature mechanisms and consequences of variations inQ10 values and acclimation In lsquoPlant respirationrsquo (Eds H Lambers ampM Ribas-Carbo) pp 95ndash135 (Springer Dordrecht The Netherlands)
Atkin OK Scheurwater I Pons TL (2006)High thermal acclimation potentialof both photosynthesis and respiration in two lowlandPlantago species incontrast to an alpine congeneric Global Change Biology 12 500ndash515doi101111j1365-2486200601114x
Ayub G Smith RA Tissue DT Atkin OK (2011) Impacts of drought onleaf respiration in darkness and light in Eucalyptus saligna exposed toindustrial-age atmosphericCO2 and growth temperatureNewPhytologist190 1003ndash1018 doi101111j1469-8137201103673x
Berry J Bjoumlrkman O (1980) Photosynthetic response and adaptation totemperature in higher plants Annual Review of Plant Physiology andPlant Molecular Biology 31 491ndash543 doi101146annurevpp31060180002423
Bret-HarteMSShaverGRZoerner JP JohnstoneJFWagner JLChavezASGunkelman RF IV Lippert SC Laundre JA (2001) Developmentalplasticity allows Betula nana to dominate tundra subjected to analtered environment Ecology 82 18ndash32
Bret-Harte MS Shaver GR Chapin FS III (2002) Primary and secondarystem growth in Arctic shrubs implications for community response toenvironmental change Journal of Ecology 90 251ndash267 doi101046j1365-2745200100657x
BuckleyTNAdamsMA(2011)An analyticalmodel of non-photorespiratoryCO2 release in the light and dark in leaves of C3 species based onstoichiometric flux balance Plant Cell amp Environment 34 89ndash112doi101111j1365-3040201002228x
Budde RJ Randall DD (1990) Pea leaf mitochondrial pyruvatedehydrogenase complex is inactivated in vivo in a light-dependentmanner Proceedings of the National Academy of Sciences of theUnited States of America 87 673ndash676 doi101073pnas872673
CahoonSMPSullivanPF ShaverGRWelker JM PostE (2012) Interactionsamong shrub cover and the soil microclimatemay determine futureArcticcarbon budgets Ecology Letters 15 1415ndash1422 doi101111j1461-0248201201865x
Chapin FS Shaver GR (1996) Physiological and growth responses of Arcticplants to a field experiment simulating climatic change Ecology 77822ndash840 doi1023072265504
Chapin FS Shaver GR Giblin AE Nadelhoffer KJ Laundre JA (1995)Responses of Arctic tundra to experimental and observed changes inclimate Ecology 76 694ndash711 doi1023071939337
Chapin FS Sturm M Serreze MC McFadden JP Key JR Lloyd AHMcGuire AD Rupp TS Lynch AH Schimel JP Beringer J ChapmanWLEpsteinHEEuskirchenESHinzmanLD JiaGPingC-LTapeKDThompson CDC Walker DA Welker JM (2005) Role of land-surfacechanges in Arctic summer warming Science 310 657ndash660 doi101126science1117368
Covey-Crump EM Attwood RG Atkin OK (2002) Regulation of rootrespiration in two species of Plantago that differ in relative growthrate the effect of short and long term changes in temperature PlantCell amp Environment 25 1501ndash1513 doi101046j1365-3040200200932x
Crous KY Zaragoza-Castells J Ellsworth DS Duursma RA LowM TissueDT Atkin OK (2012) Light inhibition of leaf respiration in field-grownEucalyptus saligna in whole-tree chambers under elevated atmosphericCO2 and summer drought Plant Cell amp Environment 35 966ndash981doi101111j1365-3040201102465x
Dungan RJ Whitehead D Duncan RP (2003) Seasonal and temperaturedependence of photosynthesis and respiration for two co-occurringbroad-leaved tree species with contrasting leaf phenology TreePhysiology 23 561ndash568 doi101093treephys238561
Elmendorf SC Henry GHR Hollister RD Bjoumlrk RG Bjorkman ADCallaghan TV Collier LS Cooper EJ Cornelissen JHC Day TAFosaa AM Gould WA Greacutetarsdoacutettir J Harte J Hermanutz L Hik DSHofgaard A Jarrad F Joacutensdoacutettir IS Keuper F Klanderud K Klein JAKoh S Kudo G Lang SI Loewen V May JL Mercado J Michelsen AMolau U Myers-Smith IH Oberbauer SF Pieper S Post E Rixen CRobinson CH Schmidt NM Shaver GR Stenstroumlm A Tolvanen ATotland Oslash Troxler T Wahren C-H Webber PJ Welker JM WookeyPA (2012) Global assessment of experimental climate warming ontundra vegetation heterogeneity over space and time Ecology Letters15 164ndash175 doi101111j1461-0248201101716x
EnvironmentalDataCenterTeam (2011)Meteorologicalmonitoring programat Toolik Alaska Toolik Field Station Institute of Arctic BiologyUniversity of Alaska Fairbanks Fairbanks AK 99775 Available athttptoolikalaskaeduedcabiotic_monitoringdata_queryphp
L Functional Plant Biology M A Heskel et al
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
estimated by fitting data from the light response curve to arectangular hyperbolic function (Excel Solver MicrosoftRedmond WA USA) CO2 response curves were analysed forthe maximum carboxylation velocity of Rubisco (Vcmax) and themodelled maximum rate of electron transfer (referred to here as Jdue to data inconsistency at high CO2 levels) using the netphotosynthetic ratendash internal CO2 (ci)curve fitting utility (ver20071) provided and detailed in Sharkey et al (2007) Examplesof light andCO2 response curves can be seen in Fig S1 (availableas Supplementary Material to this paper) A portion of thereplicate measurements gave inaccurate readings of CO2
saturated photosynthesis at the highest levels of CO2 limitingstatistical comparison of maximal photosynthesis Values ofphotosynthesis and respiration are expressed on an area massand nitrogen basis
Quantifying respiration in the light using the Kok methodTo estimate Rlight we used the Kok method which is convenientfor field measurements and can be used under ambientatmospheric conditions This method is based on theobservation that the quantum yield of photosynthesis usuallydecreases abruptly above a certain level of light intensity ndash oftennear the light compensation point where carbon flux is zero (Kok1948) This leads to a noticeable nonlinearity or lsquobendrsquo in theotherwise linear lower range of the light response curve which isinterpreted as the saturation point of light inhibition of respiration(explained in detail in Shapiro et al 2004) Since respiration isassumed to be constant above this point an extrapolation to0mmolmndash2 sndash1 PAR of the linear portion of the curve above thispoint is assumed to give Rlight Here the irradiance range we usedto calculate Rlight spanned 25ndash90mmolmndash2 sndash1 (Fig S1b)
The Kok method assumes that the CO2 assimilation rateresponds only to light thus corrections must be made toaccount for changes in ci As rates of photosynthesis slowunder decreasing light intensity CO2 tends to accumulatewithin the leaf increasing ci which in turn affect the shapeof the light curve by decreasing the rate of photorespiration atlower light levels We corrected all measurements to a constantci to account for these effects according to Kirschbaum andFarquhar (1987) as described in Ayub et al (2011) toachieve a more accurate extrapolation of Rlight
Short-term in situ temperature response of respiration
To estimate respiratory fluxes under field conditions that varydaily we need descriptive high-resolution thermal responsecurves to extend our interpretation of measurements at20C Following methods similar to those described by Huumlveet al (2012) and OrsquoSullivan et al (2013) we sampled leavesof both species (similar to the sampling method describedabove) to quantify the short-term respiratory response ofleaves to measurement temperature Data presented in thisstudy represent replicates measured in late June on fullyexpanded leaves Leaves were kept in darkness for ~30minbefore measurement and then placed in a 155 110 65 cmwater-jacketed glass-topped aluminium chamber that wasthen covered by a dark fabric Two fans mixed air within thechamber and chamber temperature was controlled via aconnected programmable circulating waterbath (MRC300
Melcor Trenton NJ USA) that warmed from 5C to 35Cover ~45min Leaf temperature in the chamber was measuredwith a small gaugewire copper-constantan thermocouple pressedagainst the lower surface of the leaf and recorded via a data-logger(CR1000 Campbell Scientific Logan UT USA) Dry air waspassed through the cuvette at a rate of 045 L hndash1 and exited thecuvette via tubing connected to an IRGA (LI-840A LI-COR)which measured the flux of CO2 from the leaf Values ofrespiratory flux were corrected for the leaf area and mass ofthe measured replicate The standard equations used to calculatethe temperature sensitivity of respiration (Q10) the basalrespiration rate at 10C (R10) and a variable related to theenergy of activation calculated from values of respiration at20C and R10 (Eo) are presented in the Supplementary Material(Eqns S1ndash2)
Physical leaf traits and foliar nutrients
All leaf samples used for gas exchange measurements weremeasured for leaf area using a rotating-belt leaf area meter (LI-3100CAreaMeter LI-COR) Sampleswere then dried in an ovenat 60C for a minimum of 2 days before mass was determinedAfter transport to Columbia University in NewYork all sampleswere ground weighed and packaged for elemental analysis of[CHN] (2400 Series II Perkin-Elmer Boston MA USA)
Statistical analyses
The effect of seasonal timing on foliar gas exchange and relatedleaf characteristics was analysed using a linear mixed-effectsmodel framework to perform a three-way repeated-measuresANOVA in R (ver 270 The R Foundation for StatisticalComputing Vienna Austria) assigning measurement round(the 4- to 5-day period when replicates from each speciestreatment combination were measured) species (B nana andE vaginatum) and warming (control or greenhouse) asexplanatory variables treatment block was treated as a randomeffect Prior to analysis data were tested for normality(ShapirondashWilk) and heteroskedasticity (BreuschndashPagan) andlog10-transformed when necessary Post hoc multiplecomparisons were made using Tukeyrsquos test To address therelationships between foliar physiological rates and ambienttemperature or leaf traits correlation analyses were employedIn addition stepwise linear regression models that integrated theeffects of species measurement round warming treatment andambient temperature to assess select gas exchange variableswere evaluated using corrected Akaike Information Criteria(AICc see the Supplementary Material) For all analysesP-values of lt005 were considered significant
Results
Seasonal environmental variability
Full snowmelt at the long-termexperimentalwarmingandcontrolplots occurred by ~June 9 2010 (Toolik EDC) During themeasurement period of Julian Day 162ndash209 (11Junendash28 July)themaximumandminimum temperature relativehumidity lightandprecipitationwere highlyvariable (Fig 1)During this periodthe highest recorded temperature (239C) occurred on 9 Julyand the lowest minimum temperature (04C) after snowmeltoccurred on 6 July merely 3 days prior The average temperature
D Functional Plant Biology M A Heskel et al
during the measurement period was 107C with an averageminimum temperature of 48C and an average maximumtemperature of 143C In passively warmed plots thistranslates to an average maximum temperature of 193Cnearly equal to the leaf sample measurement temperatureAcross the growing season mean vapour pressure deficitcalculated from mean values of relative humidity andtemperatures was 0187 kPa in the controls and 0471 kPa inthe passively warmed plots Three large precipitation eventsoccurred during the measurement period corresponding withlower ambient air temperatures high relative humidity and lowdaily PAR (Fig 1)
Specific leaf traits and chemistry
Due to the contrasting leaf forms of B nana which producessmall flat circle-shaped leaves each season and E vaginatumwhich grows elongated three-sided tillers that are retainedthrough the snow-covered winter it is not surprising thecorresponding specific leaf area (SLA) measurements differgreatly between the species taken as a whole (Table 1Fig S2) and within both control and warming treatments(P lt 00001) Though individual measurement rounds showsome variation in SLA across the growing season elevatedgrowth temperature did not affect either species comparedwith leaves in the control (Fig S2) Leaf C concentration wasgreater and much more variable in B nana than in E vaginatum(P lt 00001) and warming treatment and seasonal timing(Round) did not affect this in either species (Fig S3adTable 1) E vaginatum decreased in proportional N content asthe season progressedB nana also decreased through the seasonbut it was less pronounced (Fig S3) Warming did not elevate Ncontent in either species The species-specific trends in N causedthe related increases in C N over the seasonal course (Fig S3)
Intraseasonal dynamics of photosynthesis and respiration
The impacts of species warming treatment and measurementround through the season on area- mass- and N-based gasexchange rates (measured at 20C) as expressed by the resultsof a three-way ANOVA are shown in Table 2 Variation inarea-based light-saturated photosynthetic rates (Fig 2) wereprimarily explained through species differences with B nanaconsistently exhibiting greater rates of carbon assimilation acrossall measurements as well as when considering measurementsin the warmed plots (P lt 001) Area-based Asat showed nosignificant response to the measurement round (Fig 2) though
mass-based ratesyieldedan interactive effect of species and round(Fig S3andashd Table 2) suggesting the effect of leaf structuraldifferences between the species when assessing photosyntheticrates Only when expressed on a per-N rate did Asat vary withmeasurement round driven by the increases in per-Nphotosynthesis later in the season related to lower foliar Nconcentration (Fig S3endashh) Similarly there were no observedchanges in rates under long-term warming treatment in eitherspecies and B nana consistently exhibited higher rates inboth warmed and control plots compared with E vaginatum Ifoneconsiders theprocessesunderlyingphotosynthetic efficiencyVcmax was significantly affected by the interaction of species andmeasurement round Mean rates of Vcmax in B nana particularlyunder warming increased slightly through the season (Fig 3)Rates of Vcmax and J were correlated (r2 = 0856) The ratio ofthese variables J Vcmaxwas generally 1ndash2but declined throughthe growing season in both species (Fig 4endashf Table 2)
Across allmeasures ofmitochondrial dark respirationB nanaexhibited higher rates than E vaginatum (Table 2) A significantinteraction term between species and measurement roundhighlighted differences in mitochondrial dark respiration (area-andN-based) between E vaginatum andB nana over the courseof the growing seasonE vaginatumdisplayed a gradual decreasein rates around Day 180 (29 June) that was not observed inB nana (Fig 2cndashd) Further considering allmeasurements long-term growth warming was associated with lower rates of area-based Rdark in both species though this effect is only observed inE vaginatum considering N-based Rdark On a mass basis Rdark
decreased through the growing season in both species (Fig 2cndashdTable 2) with the highestmean rates for both species occurring inthe first 2 weeks of the growing season
Short-term high-resolution temperature response curves ofRdark measured midseason showed a higher degree of thermalacclimation to long-term elevated growth temperature inE vaginatum than in B nana (Fig 3) As in the measurementsmade at a single temperature of 20C (Fig 2) and calculated at10C for R10 (Table 3) rates of respiration were generallygreater in B nana than in E vaginatum across themeasurement temperature range from 5C to 20C Howeverover 20ndash35C E vaginatum grown in control plots respired atincreasingly greater rates than did B nana which correspondedto its significantly higher Q10 and Eo values (Fig 4 Table 3)
One of the main objectives of this study was to obtain a betterunderstandingof the light inhibitionof respiration in these speciesand how this phenomenon may relate to other leaf traits Acrossspecies and growth conditions light significantly suppressed
Table 1 Results from three-way repeated-measures ANOVA analysing the carbon ( C) and nitrogen (N) concentrations and their ratio (C N)of dry leaf material and specific leaf area (SLA) in Betula nana and Eriophorum vaginatum
Plt 005 Plt 001 Plt 0001 S species R round W warming round df degrees of freedom
Sdf = 1 158
Rdf = 9 150
Wdf = 1 158
SRdf = 9 140
SWdf= 1 156
RWdf= 9 140
SRWdf = 9 120
F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P
Seasonal carbon exchange under long-term warming Functional Plant Biology E
respiration rates (P lt 00001) with individual leaf values rangingwidely Rlight Rdark was less in E vaginatum thanB nana underboth growth conditions (P lt 0001) and decreased further underwarming inE vaginatum (P lt 001) For both species and growthconditions Rlight Rdark decreased over the growing season(Fig 5) Warming decreased rates in only E vaginatum forarea- and N-based rates (Plt 005 for both) but not mass-basedrates which found warming to significantly lower rates whenconsidering all measurements (P lt 001) Furthermore the effecta speciesmeasurement round interaction (Plt 005) wasevident in the different trends of respiratory release over thecourse of the season (Fig 2endashf Fig S4) The ratio of carbon lossvia respiration in the light to gross photosynthesis (Rlight Agross)which describes the relative carbon loss from respirationcompared with the total carbon cycled in a leaf (as opposed tothe difference between the fluxes) decreased through the seasonin both species (Fig 6) though at different rates Theseproportional respiratory losses were significantly greater inB nana than in E vaginatum and warming significantlydecreased these values considering all measurements acrossthe season (Table 2) No clear relationships were foundbetween the degree of inhibition and foliar fluxes or traits inthese species regression analysis between Rlight Rdark andNAsat and Vcmax produced r2lt 010 (data not shown)
Intraseasonal ambient temperature influenceson gas exchange
Toevaluate the influenceof ambient temperature ongas exchangerates we compared multivariate linear models that incorporatedtemperature values from the day of andmean temperature values
from the week before gas exchange measurement Table S1shows the results of models incorporating the effect of specieswarming treatment and day-of and week-before minimummaximum and average temperatures on area-based Asat RdarkRlight and Rlight Rdark evaluating by comparing relative AICcweights The best model for photosynthesis (measured at 20C)incorporates only species with the next best only incorporatingspecies andwarming treatment suggestingno significant effect ofshort-term ambient air temperature on rates of Asat In contrastboth Rdark and Rlight (also measured at 20C) were more sensitiveto short-term ambient air temperatures with the best modelsincorporating the effect of species growth conditions andeither the prior weekrsquos average temperature (Rdark) or themeasurement dayrsquos minimum temperature (Rlight Fig S5) Thetop three models for Rlight Rdark incorporated minimumtemperature values When Rlight is considered alone a clearpredictive response is not obvious (Fig S5) though significantrelationships were observed between Rlight and day-of minimumtemperature (P lt 001) day-of maximum temperature(P lt 001) day-of average temperature (P lt 001) and week-prior minimum temperature (Plt 001) though not with theprior weekrsquos average or maximum temperature suggestinggreater sensitivity to short-term conditions colder conditionsor both
Discussion
The primary objective of this study was to evaluate themechanistic physiological responses of foliar photosynthesisand respiration to long-term warming conditions and naturalvariations in ambient temperature through the growing season
Table 2 Results from three-way repeated-measuresANOVAanalysing foliar gas exchange variables including light-saturating photosynthesis (Asat)dark respiration (Rdark) and respiration in the light (Rlight) expressed on an area mass and nitrogen basis the ratio of Rlight to Rdark (Rlight RDark)the ratio of photosynthetic carbon assimilation to respiratory carbon release (Rlight Agross) and the maximum carboxylation rate of Rubisco (Vcmax)and the electron transport rate (J) both measured at ambient leaf temperature and corrected for a constant temperature of 258C in Betula nana and
Eriophorum vaginatum Plt 005 Plt 001 Plt 0001 S species R round W warming df degrees of freedom
Sdf = 1 158
Rdf = 9 150
Wdf = 1 158
SRdf = 9 140
SWdf = 1 156
RWdf = 9 140
SRWdf= 9 120
F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P
in two dominant Arctic plants We hypothesised that the within-growing season rates of photosynthesis respiration and theinhibition of respiration will relate to growth-induced energydemand These rates will acclimate to long-term warming and be
responsive to short-term temperature fluctuations Our resultsshow stronger seasonal and thermal responses in respiration thanin photosynthesis (when both are measured at 20C) which wasfurther influenced by species differences and growth under long-
15
Betula Eriophorum
10
5
0
15
10
5
0
6
4
2
0
6
4
2
0
64
3
2
1
0
4
2
0
Asa
t (microm
ol C
O2
mndash2
sndash1
)R
dark
(microm
ol C
O2
mndash2
sndash1
)R
light
(microm
ol C
O2
mndash2
sndash1
)
160 170 180 190 200 210160 170 180 190 200
Julian day
(a)
(c)
(e)
(b)
(d)
(f )
Fig 2 (a b)Saturating photosynthesis (Asat measured at PAR=1500mmolmndash2 sndash1 and 400 parts per million (ppm)CO2) (c d) foliar dark respiration (Rdark measured at PAR=0mmolmndash2 sndash1 and 400 ppmCO2) and (e f) respiration inthe light (Rlight estimated via the Kok method at low PAR levels) through the growing season for leaves of (a c e)Betula nana (circles) and (bd f)Eriophorumvaginatum (triangles) grown in ambient (unfilled symbols) andpassivelywarmed (filled symbols) conditions All variables were measured at 20 C in laboratory conditions (n= 4)
Seasonal carbon exchange under long-term warming Functional Plant Biology G
termwarmingThis studyalsoprovided theopportunity toexpanduponexisting knowledge about biotic and environmental controlson foliar Rlight and how seasonal timing and temperaturevariability may mediate Rlight Rdark Together these findingsprovide a more detailed mechanistic understanding of thecontrol of carbon exchange in tundra ecosystems
Species-mediated influences of seasonal and environmentaleffects on carbon exchange
In our study the rates of photosynthesis and respiration ofB nana when measured at a common temperature tended tobe higher than those of E vaginatum during the Arctic tundragrowing season (Fig 2 Table 2) grown under control conditionsand long-term passive warming The higher rates of carbonexchange in B nana both when measured at 20C (Fig 2)and over the 5ndash35C temperature range (Fig 3) wereconsistent with previous studies that sampled plants grownunder warming at midseason (Chapin and Shaver 1996) andwith the addition of soil nitrogen and phosphorus (Heskel et al2012) The difference in Asat between B nana and E vaginatumwas greatest directly after leaf out (Fig 2) which emphasises theimpact of growth formon foliar physiological processesB nanaa deciduouswoody shrub is likely to have a relatively high energydemand immediately after snowmelt to accommodate efficientaboveground and belowground growth during the short growingseason and thismay be further enhanced under warming (Chapinet al 1995 Sullivan et al 2007) By contrast E vaginatum atiller-producing graminoid can retain leaves for multiple years(Fetcher and Shaver 1983) and for this reason may act lessopportunistically upon snowmelt in the spring in terms ofnutrient acquisition via fine root growth (Sullivan et al 2007)andquick leaf production These trends are supported byprevious
work on deciduous and evergreen species where seasonalphotosynthesis and respiration were consistently significantlylower in the evergreen (Ow et al 2010) The species contrastis underscored by the respective leaf architectures andcomposition interspecies comparisons show a less dense(greater SLA) leaf with higher N content in B nana than inE vaginatum (Table 1 Figs S2 S3)
We found a general though not significant decline in Asat inleaves sampled from long-term warming plots suggesting apotential duration-dependent warming effect at the leaf levelthat may contribute to a similar lack of photosyntheticstimulation reported at the ecosystem level (Shaver et al2000 Elmendorf et al 2012) Also these tundra species maybe pushed beyond their photosynthetic thermal optimum underelevated temperatures (Sage and Kubien 2007) which could bea likely scenario in Arctic populations that may be locallyadapted for colder growth temperatures We observed noapparent seasonal arch in the Asat Vcmax and J rates which cangenerally characterise growing season carbon assimilation ofleaves measured at ambient leaf temperatures as previouslyreported (for Asat) in these species (Starr et al 2008) At theleaf level the highest rates of Asat Vcmax and J can occur in theearly growing season (Dungan et al 2003 Xu and Baldocchi2003) This discrepancy may be explained by the highly variableambient temperature and precipitation conditions (Fig 1) thoughmodel evaluation found no strong relationship betweenphotosynthetic variables (measured at 20C) and short-termtemperature values (Table S1) Also as we measured allleaves at a common temperature (20 C) we controlled for theinfluence of ambient temperature on leaf temperature which canaffect photosynthetic rates and allowed for comparison acrosschanges in the ambient temperature to assess thermal acclimation
Respiration rates of both species were generally highest in thefirst few weeks after snowmelt (measured at 20C Fig 2endashf)which is more apparent in Rlight than in Rdark and are similar topreviously reported values (Heskel et al 2012 Heskel et al2013) In both B nana and E vaginatum higher energy demandand potentially respiratory rates in the early season relative to themid- and late season may be attributable to new leaf growth anddevelopment (Vose and Ryan 2002 Xu et al 2007 Ow et al2010) and possibly a higher density of mitochondria in youngerleaves (Armstrong et al 2006) This respiratory release can befurther enhanced by the colder ambient temperatures experiencedby plants in the early growing season due to a short-term coldacclimation (Atkin and Tjoelker 2003 Armstrong et al 2006)The thermal response of respiration is nonlinear and variesbetween species in response curve shape and slope (Fig 3Table 3) higher respiration in B nana across the range oftemperatures experienced during the growing season (Fig 1)and the lack of thermal acclimation may favour its expansion inthe tundra under current conditions (Heskel et al 2013) Furtherthe observed warm acclimation in leaves ofE vaginatum (Fig 3)suggests lowered metabolic rates that may limit growth despitereducing respiratory C loss under a future climate
It should be noted that similar rates of respiration rates acrossspecies may not necessary equate to similar energy efficiencyB nana is reported to exhibit greater respiratory efficiency thanE vaginatum through the differential use of the alternativeand cytochrome pathways potentially lending a competitive
05 10 15 20 25 30 35
10
20
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40
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60
70
Betula CT
Betula WG
Eriophorum CT
Eriophorum WG
Temperature (degC)
Rda
rk (
nmol
CO
2 gndash1
sndash1
)
Fig 3 The mean short-term thermal responses of foliar dark respirationfrom 5C to 35C modelled from a polynomial fit from raw data in Betulanana (circles) and Eriophorum vaginatum (triangles) sampled from control(CT open) andwarmed (WG closed) growth environments Temperature (T)response curves of respiration (R) were fit to second-order polynomialequations as follows B nana control r= 0027T2 + 0082T+ 660 B nanawarmed r= 0026T2 + 0103T+ 677 E vaginatum control r = 0045T2 ndash
0106T+ 310 and E vaginatum warmed r = 0018T2 + 0176T+ 2633 Forall thermal response curve fits R2 gt 0990
H Functional Plant Biology M A Heskel et al
advantage in growth and development (Kornfeld et al 2013)In addition Rlight Agross decreased over the growing season(Fig 6) driven by lower values of Rlight as the season progressedrather than any significant change in photosynthesis which is
discordant with the idea that the processes would increase in acoupled manner based on carbohydrate substrate supply anddemand Kornfeld et al (2013) found no difference in starchand sugar content between control and warmed leaves within
0
20
40
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80Betula
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40
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160 170 180 190 200 210160 170 180 190 200
(a)
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Vcm
ax (micro
mol
mndash2
sndash1
)J
Vcm
ax
J (micro
mol
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)
Fig 4 (a b) Maximal carboxylation rate (Vcmax) and (c d) electron transport rate (J) both measured in laboratoryconditions and corrected to a constant 25C and (e f) the ratio of these variables (J Vcmax) over the growing season inleaves of (a c e) Betula nana and (b d f) Eriophorum vaginatum grown under ambient (open) and warmed (filled)conditions Values shown are means with se
Seasonal carbon exchange under long-term warming Functional Plant Biology I
species when measured at a single sampling point during thegrowing season though leaves of B nana contained a greaterconcentration of both sugar and starch than E vaginatum Wewere unable to quantify foliar carbohydrate values in thesespecies through the season though future studies couldinclude this informative measurement to further relate themechanistic links between these processes
Both species exhibited the lowest values of Rlight Rdark
towards the end of the season in late July (Fig 5) suggestinga potential developmental or energy demand-related control onthe inhibition of respiration by light that allows for greater
respiratory energy production in the early season during leafexpansionTheonlyother knownstudy that explored the seasonalresponse of Rlight Rdark observed the lowest level of inhibitionearlier in the growing season before the warmest month (Crouset al 2012) This response similar to our study despite a starklydifferent ecological system supports the idea of shared seasonaltiming-mediated biochemical controls on the light inhibition ofrespiration across species The degree of inhibition is known toshow a relaxed response under environmental conditions thatcan stimulate growth due to increased demand for energy and Cskeletons as exhibited under elevated CO2 (Wang et al 2001Shapiro et al 2004) and increased soil nutrient availability(Heskel et al 2012) though this is not always the case (Tissueet al 2002 Ayub et al 2011) Long-termwarming alsomediatedRlight Rdark with lower values observed in warm-grownleaves though measurement temperature can also impact theserelationships (Atkin et al 2006)
Mechanistic explanations for the relationships amonggrowth demand cellular energy status and the degree of lightinhibition of respiration have been previously reported Theenvironmental challenges of a short growing season and ~24-hdiel light exposure in the Arctic tundra control growth ratesthat may then influence values of Rlight Rdark (Bret-Harteet al 2001 Bret-Harte et al 2002) In the light adenylatesupply from photosynthesis may decrease the energy demandfrom mitochondrial respiration Additionally photorespirationis associated with the inactivation of pyruvate dehydrogenasea precursor to the tri-carboxylic acid (TCA) cycle (Budde andRandall 1990) though previous estimations of photorespirationin these species didnotfinda strong correlationwith the inhibitionof Rlight (Heskel et al 2013) The TCA cycle can also besignificantly altered in the light to support N assimilationwhich effectively reduces CO2 release (Tcherkez et al 2005Tcherkez et al 2008 Tcherkez et al 2009) This effect may beenhanced in tundra plantswhen soil N is less limiting under theseconditions growth rates increase (Bret-Harte et al 2002) Theinhibition ofRlight is observed to be slightly increased in fertilisedsoils (Heskel et al 2013) Additionally the oxidative pentosephosphate pathway (OPPP) which contributes the necessaryreductant for the synthesis of multiple metabolites in darknessmay be relaxed in the light when reductants are provided byphotosynthesis diminishing CO2 release by the OPPP Thoughthe OPPP comprises a proportionally smaller CO2 flux comparedwith that from the TCA cycle in the light this decrease maycontribute to lower overall CO2 release causing Rlight ltRdark
(Buckley and Adams 2011) In our current study the apparentrelaxation of the light inhibition of respiration in leaves of bothstudy species (Fig 5) resulting in the greatest proportional CO2
losses (Fig 6) suggesting that high growth demand for ATP andcarbon skeletons outweigh potential carbon losses in the earlyseason
Influences of variable ambient temperature on gas exchange
In addition to quantifying the effects of long-termwarming on thefoliar gas exchange during the growing season this studyevaluated the influence of intraseasonal short-term temperaturevariability on these physiological processes Arctic tundragrowing seasons can exhibit dramatic shifts in temperature and
Table 3 Parameters describing the respiratory response tomeasurement temperature in leaves of Betula nana and Eriophorumvaginatum grown under control (CT) and long-term warming (WG)conditions including Q10 (the temperature sensitivity of respiration(estimated over 5ndash358C)) R10 (the basal respiration rate at 108C(mmol CO2 gndash1 sndash1)) and Eo (kJmolndash1) a variable related to the energyof activation calculated from values of respiration at 208C and R10
Values presented are means and se Statistical differences between speciesand treatments are denoted alphabetically
Fig 5 Means and se of the ratio of respiration in the light (Rlight) estimatedvia the Kok method to dark respiration (Rdark) both variables measured at20C and 400 parts per million CO2 through the growing season in Betulanana (circles) and Eriophorum vaginatum (triangles) under ambient controlfield conditions (CT open symbols) and under a long-term passive warmingtreatment (WGfilled symbols n= 4)A linear regression (y= ndash0009x+ 219R2 = 047) depicts the declining Rlight Rdark values with the progressinggrowing season
J Functional Plant Biology M A Heskel et al
precipitation within days as was observed in this study duringsummer 2010 in Toolik Lake Alaska (Fig 1) and this variabilityis likely to influence the temperature-sensitive processescontrolling leaf carbon cycling Respiration in both speciescan be highly temperature-sensitive especially at warmertemperatures which may become more common in a futureArctic (Fig 3 Table 3) When modelled against thetemperature of the measurement day the average temperatureof the prior week accounted for species and treatment effectsand some patterns emerged that help to characterise the nature ofthe temperature sensitivity of foliar gas exchange (Table S1)However photosynthesis which is known to respond to short-term changes in temperature in field-grown plants (Berry andBjoumlrkman 1980 Poyatos et al 2012) did not show strongrelationships to daily or weekly variation in temperatureinstead species effects explained most variation (Table S1)The underlying photosynthetic machinery maintains consistentrates of carbon assimilation (measured at a constant temperature)throughout the growing season though daily fluctuations intemperature and precipitation experiences in the Arctic mayinfluence in vivo rates Plant species from alpine and Arcticecosystems can exhibit local adaptation to the extremevariability experienced in those locations (Korner and Larcher1988) which may translate to a limited acclimation (Atkin et al2006) Further because the ambient temperature conditionsfrom this region fluctuate so unpredictably in short periodsthrough the season any analysis using temperature averages ofprevious time windows might not be as useful for estimatingthermal acclimation as they are in regions with more predictableseasonal temperature patterns (Ow et al 2010 Searle et al 2011)
In contrast respiratory rates in the light and dark whenmeasured at a constant temperature appeared to be morecorrelated with short-term ambient temperature variation TheAICc analysis shows that the strongest model for Rdark includedspecies and warming effects and the prior weekrsquos average
temperature in contrast the strongest model for Rlight includedthe former parameters and the minimum temperature from themeasurement day (Table S1) The model relationships maysuggest a short-term thermal sensitivity of respiration that maybe more responsive to temperature minimums though this isnot clearly reflected in regressions of Rlight with the ambienttemperature values (Fig S5) This sensitivity of respiration totemperature minimums has been observed previously inroots as well and suggests that thermal acclimation in coldgrowth temperatures may regulate metabolic activity to meetthe demands of growth and maintenance at the expense ofgreater carbon loss (Covey-Crump et al 2002) Low ambienttemperatures did not clearly relate to the highest rates ofrespiration when measured at a common temperature aswould be expected under respiratory cold acclimation (Atkinand Tjoelker 2003) However it is possible that developmentaltiming may also play into the leavesrsquo respiratory rates and abilityto acclimate especially in B nana in the early season when itsleaves are still expanding (Armstrong et al 2006) Through arelatively simple modelling exercise we found some evidenceof differential temperature controls on photosynthesis andrespiration in B nana and E vaginatum we suggest thatfuture work should include more detailed environmental andbiotic factors including soil temperature daily photosyntheticradiation and leaf carbohydrate data to more accurately informthe seasonal temperature sensitivity of foliar carbon exchange
Conclusions and implications
The physiological measurements collected in this study allowedfor the quantification of mechanistic responses in the nightlessand highly variable growing season of the arctic tundra andprovided new insights into foliar carbon regulation We presentnew information on the seasonal trends of foliar carbon cyclingin two dominant tundra species and relate these fluxes to short-
0
015
045
030
0
015
045
030
Julian day
Betula Eriophorum
160 170 180 190 200 210 160 170 180 190 200 210
Rlig
ht
Agr
oss
Fig 6 The proportional carbon loss through respiration in the light to gross photosynthesis in (a) Betula nana and(b) Eriophorum vaginatum over the growing season (Rlight Agross both variables measured at a common temperatureof 20C and 400 parts per million CO2) Linear regressions across growth conditions depict the relationship ofRlight Agross and measurement date during the growing season (B nana y= ndash0003 + 088x R2 = 030 E vaginatumy= ndash0005 + 117x R2 = 065) Values represent variable means and se error bars are not visible for many values dueto their small value
Seasonal carbon exchange under long-term warming Functional Plant Biology K
term temperature variability and long-term warming Our studypresents the first published values of the photosyntheticparameters Vcmax and J at multiple points during the season inthese species which can inform and refine parameterisation ofvegetationmodels atmany scalesAlsowe present thefirst reportof the seasonal flexibility of the degree of the inhibition of Rlight
under long-term warmed growth conditions in these or anyspecies which will enable more accurate calculations ofecosystem respiration Based on these measurements aseasonal estimate (not accounting for potential temperaturesensitivity as discussed above see Eqn S3) shows thatneglecting the inhibition of foliar Rlight in Arctic tundravegetation could lead to overestimations of foliar carbon lossof~4molmndash2 leaf (~176 g)over thegrowingseason or evenmore(~42molmndash2) when considering plants grown under warmedconditionsAlthough the temperature effect on respiratory carbonloss in the light may beminimal the overall discrepancy in fluxesbetween Rdark and Rlight must be considered for realisticdescriptions of Arctic carbon cycling Our study contributesimportant data that allow for an increased understanding ofdominant tundra speciesrsquo responses to environmental changeboth long- and short-term and will allow for more predictivepower in models estimating ecosystem carbon storage
Acknowledgements
We are grateful to the researchers and support staff at the Arctic LTER andToolik Field Station for their help in this study as well as Greg PendergastThis study was made possible through funding from the NSF 0732664OKA acknowledges grant support from the Australian Research Council(DP0986823)
References
Armstrong AF Logan DC Atkin OK (2006a) On the developmentaldependence of leaf respiration responses to short and long-termchanges in growth temperature American Journal of Botany 931633ndash1639 doi103732ajb93111633
Armstrong AF Logan DC Tobin AK OrsquoToole P Atkin OK (2006b)Heterogeneity of plant mitochondrial responses underpinningrespiratory acclimation to the cold in Arabidopsis thaliana leavesPlant Cell amp Environment 29 940ndash949 doi101111j1365-3040200501475x
Armstrong AF Badger MR Day DA Barthet MM Smith P Millar AHWhelan J Atkin OK (2008) Dynamic changes in the mitochondrialelectron transport chain underpinning cold acclimation of leafrespiration Plant Cell amp Environment 31 1156ndash1169 doi101111j1365-3040200801830x
Atkin OK Tjoelker MG (2003) Thermal acclimation and the dynamicresponse of plant respiration to temperature Trends in Plant Science 8343ndash351 doi101016S1360-1385(03)00136-5
Atkin OK Bruhn D Tjoelker MG (2005) Response of plant respiration tochanges in temperature mechanisms and consequences of variations inQ10 values and acclimation In lsquoPlant respirationrsquo (Eds H Lambers ampM Ribas-Carbo) pp 95ndash135 (Springer Dordrecht The Netherlands)
Atkin OK Scheurwater I Pons TL (2006)High thermal acclimation potentialof both photosynthesis and respiration in two lowlandPlantago species incontrast to an alpine congeneric Global Change Biology 12 500ndash515doi101111j1365-2486200601114x
Ayub G Smith RA Tissue DT Atkin OK (2011) Impacts of drought onleaf respiration in darkness and light in Eucalyptus saligna exposed toindustrial-age atmosphericCO2 and growth temperatureNewPhytologist190 1003ndash1018 doi101111j1469-8137201103673x
Berry J Bjoumlrkman O (1980) Photosynthetic response and adaptation totemperature in higher plants Annual Review of Plant Physiology andPlant Molecular Biology 31 491ndash543 doi101146annurevpp31060180002423
Bret-HarteMSShaverGRZoerner JP JohnstoneJFWagner JLChavezASGunkelman RF IV Lippert SC Laundre JA (2001) Developmentalplasticity allows Betula nana to dominate tundra subjected to analtered environment Ecology 82 18ndash32
Bret-Harte MS Shaver GR Chapin FS III (2002) Primary and secondarystem growth in Arctic shrubs implications for community response toenvironmental change Journal of Ecology 90 251ndash267 doi101046j1365-2745200100657x
BuckleyTNAdamsMA(2011)An analyticalmodel of non-photorespiratoryCO2 release in the light and dark in leaves of C3 species based onstoichiometric flux balance Plant Cell amp Environment 34 89ndash112doi101111j1365-3040201002228x
Budde RJ Randall DD (1990) Pea leaf mitochondrial pyruvatedehydrogenase complex is inactivated in vivo in a light-dependentmanner Proceedings of the National Academy of Sciences of theUnited States of America 87 673ndash676 doi101073pnas872673
CahoonSMPSullivanPF ShaverGRWelker JM PostE (2012) Interactionsamong shrub cover and the soil microclimatemay determine futureArcticcarbon budgets Ecology Letters 15 1415ndash1422 doi101111j1461-0248201201865x
Chapin FS Shaver GR (1996) Physiological and growth responses of Arcticplants to a field experiment simulating climatic change Ecology 77822ndash840 doi1023072265504
Chapin FS Shaver GR Giblin AE Nadelhoffer KJ Laundre JA (1995)Responses of Arctic tundra to experimental and observed changes inclimate Ecology 76 694ndash711 doi1023071939337
Chapin FS Sturm M Serreze MC McFadden JP Key JR Lloyd AHMcGuire AD Rupp TS Lynch AH Schimel JP Beringer J ChapmanWLEpsteinHEEuskirchenESHinzmanLD JiaGPingC-LTapeKDThompson CDC Walker DA Welker JM (2005) Role of land-surfacechanges in Arctic summer warming Science 310 657ndash660 doi101126science1117368
Covey-Crump EM Attwood RG Atkin OK (2002) Regulation of rootrespiration in two species of Plantago that differ in relative growthrate the effect of short and long term changes in temperature PlantCell amp Environment 25 1501ndash1513 doi101046j1365-3040200200932x
Crous KY Zaragoza-Castells J Ellsworth DS Duursma RA LowM TissueDT Atkin OK (2012) Light inhibition of leaf respiration in field-grownEucalyptus saligna in whole-tree chambers under elevated atmosphericCO2 and summer drought Plant Cell amp Environment 35 966ndash981doi101111j1365-3040201102465x
Dungan RJ Whitehead D Duncan RP (2003) Seasonal and temperaturedependence of photosynthesis and respiration for two co-occurringbroad-leaved tree species with contrasting leaf phenology TreePhysiology 23 561ndash568 doi101093treephys238561
Elmendorf SC Henry GHR Hollister RD Bjoumlrk RG Bjorkman ADCallaghan TV Collier LS Cooper EJ Cornelissen JHC Day TAFosaa AM Gould WA Greacutetarsdoacutettir J Harte J Hermanutz L Hik DSHofgaard A Jarrad F Joacutensdoacutettir IS Keuper F Klanderud K Klein JAKoh S Kudo G Lang SI Loewen V May JL Mercado J Michelsen AMolau U Myers-Smith IH Oberbauer SF Pieper S Post E Rixen CRobinson CH Schmidt NM Shaver GR Stenstroumlm A Tolvanen ATotland Oslash Troxler T Wahren C-H Webber PJ Welker JM WookeyPA (2012) Global assessment of experimental climate warming ontundra vegetation heterogeneity over space and time Ecology Letters15 164ndash175 doi101111j1461-0248201101716x
EnvironmentalDataCenterTeam (2011)Meteorologicalmonitoring programat Toolik Alaska Toolik Field Station Institute of Arctic BiologyUniversity of Alaska Fairbanks Fairbanks AK 99775 Available athttptoolikalaskaeduedcabiotic_monitoringdata_queryphp
L Functional Plant Biology M A Heskel et al
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
during the measurement period was 107C with an averageminimum temperature of 48C and an average maximumtemperature of 143C In passively warmed plots thistranslates to an average maximum temperature of 193Cnearly equal to the leaf sample measurement temperatureAcross the growing season mean vapour pressure deficitcalculated from mean values of relative humidity andtemperatures was 0187 kPa in the controls and 0471 kPa inthe passively warmed plots Three large precipitation eventsoccurred during the measurement period corresponding withlower ambient air temperatures high relative humidity and lowdaily PAR (Fig 1)
Specific leaf traits and chemistry
Due to the contrasting leaf forms of B nana which producessmall flat circle-shaped leaves each season and E vaginatumwhich grows elongated three-sided tillers that are retainedthrough the snow-covered winter it is not surprising thecorresponding specific leaf area (SLA) measurements differgreatly between the species taken as a whole (Table 1Fig S2) and within both control and warming treatments(P lt 00001) Though individual measurement rounds showsome variation in SLA across the growing season elevatedgrowth temperature did not affect either species comparedwith leaves in the control (Fig S2) Leaf C concentration wasgreater and much more variable in B nana than in E vaginatum(P lt 00001) and warming treatment and seasonal timing(Round) did not affect this in either species (Fig S3adTable 1) E vaginatum decreased in proportional N content asthe season progressedB nana also decreased through the seasonbut it was less pronounced (Fig S3) Warming did not elevate Ncontent in either species The species-specific trends in N causedthe related increases in C N over the seasonal course (Fig S3)
Intraseasonal dynamics of photosynthesis and respiration
The impacts of species warming treatment and measurementround through the season on area- mass- and N-based gasexchange rates (measured at 20C) as expressed by the resultsof a three-way ANOVA are shown in Table 2 Variation inarea-based light-saturated photosynthetic rates (Fig 2) wereprimarily explained through species differences with B nanaconsistently exhibiting greater rates of carbon assimilation acrossall measurements as well as when considering measurementsin the warmed plots (P lt 001) Area-based Asat showed nosignificant response to the measurement round (Fig 2) though
mass-based ratesyieldedan interactive effect of species and round(Fig S3andashd Table 2) suggesting the effect of leaf structuraldifferences between the species when assessing photosyntheticrates Only when expressed on a per-N rate did Asat vary withmeasurement round driven by the increases in per-Nphotosynthesis later in the season related to lower foliar Nconcentration (Fig S3endashh) Similarly there were no observedchanges in rates under long-term warming treatment in eitherspecies and B nana consistently exhibited higher rates inboth warmed and control plots compared with E vaginatum Ifoneconsiders theprocessesunderlyingphotosynthetic efficiencyVcmax was significantly affected by the interaction of species andmeasurement round Mean rates of Vcmax in B nana particularlyunder warming increased slightly through the season (Fig 3)Rates of Vcmax and J were correlated (r2 = 0856) The ratio ofthese variables J Vcmaxwas generally 1ndash2but declined throughthe growing season in both species (Fig 4endashf Table 2)
Across allmeasures ofmitochondrial dark respirationB nanaexhibited higher rates than E vaginatum (Table 2) A significantinteraction term between species and measurement roundhighlighted differences in mitochondrial dark respiration (area-andN-based) between E vaginatum andB nana over the courseof the growing seasonE vaginatumdisplayed a gradual decreasein rates around Day 180 (29 June) that was not observed inB nana (Fig 2cndashd) Further considering allmeasurements long-term growth warming was associated with lower rates of area-based Rdark in both species though this effect is only observed inE vaginatum considering N-based Rdark On a mass basis Rdark
decreased through the growing season in both species (Fig 2cndashdTable 2) with the highestmean rates for both species occurring inthe first 2 weeks of the growing season
Short-term high-resolution temperature response curves ofRdark measured midseason showed a higher degree of thermalacclimation to long-term elevated growth temperature inE vaginatum than in B nana (Fig 3) As in the measurementsmade at a single temperature of 20C (Fig 2) and calculated at10C for R10 (Table 3) rates of respiration were generallygreater in B nana than in E vaginatum across themeasurement temperature range from 5C to 20C Howeverover 20ndash35C E vaginatum grown in control plots respired atincreasingly greater rates than did B nana which correspondedto its significantly higher Q10 and Eo values (Fig 4 Table 3)
One of the main objectives of this study was to obtain a betterunderstandingof the light inhibitionof respiration in these speciesand how this phenomenon may relate to other leaf traits Acrossspecies and growth conditions light significantly suppressed
Table 1 Results from three-way repeated-measures ANOVA analysing the carbon ( C) and nitrogen (N) concentrations and their ratio (C N)of dry leaf material and specific leaf area (SLA) in Betula nana and Eriophorum vaginatum
Plt 005 Plt 001 Plt 0001 S species R round W warming round df degrees of freedom
Sdf = 1 158
Rdf = 9 150
Wdf = 1 158
SRdf = 9 140
SWdf= 1 156
RWdf= 9 140
SRWdf = 9 120
F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P
Seasonal carbon exchange under long-term warming Functional Plant Biology E
respiration rates (P lt 00001) with individual leaf values rangingwidely Rlight Rdark was less in E vaginatum thanB nana underboth growth conditions (P lt 0001) and decreased further underwarming inE vaginatum (P lt 001) For both species and growthconditions Rlight Rdark decreased over the growing season(Fig 5) Warming decreased rates in only E vaginatum forarea- and N-based rates (Plt 005 for both) but not mass-basedrates which found warming to significantly lower rates whenconsidering all measurements (P lt 001) Furthermore the effecta speciesmeasurement round interaction (Plt 005) wasevident in the different trends of respiratory release over thecourse of the season (Fig 2endashf Fig S4) The ratio of carbon lossvia respiration in the light to gross photosynthesis (Rlight Agross)which describes the relative carbon loss from respirationcompared with the total carbon cycled in a leaf (as opposed tothe difference between the fluxes) decreased through the seasonin both species (Fig 6) though at different rates Theseproportional respiratory losses were significantly greater inB nana than in E vaginatum and warming significantlydecreased these values considering all measurements acrossthe season (Table 2) No clear relationships were foundbetween the degree of inhibition and foliar fluxes or traits inthese species regression analysis between Rlight Rdark andNAsat and Vcmax produced r2lt 010 (data not shown)
Intraseasonal ambient temperature influenceson gas exchange
Toevaluate the influenceof ambient temperature ongas exchangerates we compared multivariate linear models that incorporatedtemperature values from the day of andmean temperature values
from the week before gas exchange measurement Table S1shows the results of models incorporating the effect of specieswarming treatment and day-of and week-before minimummaximum and average temperatures on area-based Asat RdarkRlight and Rlight Rdark evaluating by comparing relative AICcweights The best model for photosynthesis (measured at 20C)incorporates only species with the next best only incorporatingspecies andwarming treatment suggestingno significant effect ofshort-term ambient air temperature on rates of Asat In contrastboth Rdark and Rlight (also measured at 20C) were more sensitiveto short-term ambient air temperatures with the best modelsincorporating the effect of species growth conditions andeither the prior weekrsquos average temperature (Rdark) or themeasurement dayrsquos minimum temperature (Rlight Fig S5) Thetop three models for Rlight Rdark incorporated minimumtemperature values When Rlight is considered alone a clearpredictive response is not obvious (Fig S5) though significantrelationships were observed between Rlight and day-of minimumtemperature (P lt 001) day-of maximum temperature(P lt 001) day-of average temperature (P lt 001) and week-prior minimum temperature (Plt 001) though not with theprior weekrsquos average or maximum temperature suggestinggreater sensitivity to short-term conditions colder conditionsor both
Discussion
The primary objective of this study was to evaluate themechanistic physiological responses of foliar photosynthesisand respiration to long-term warming conditions and naturalvariations in ambient temperature through the growing season
Table 2 Results from three-way repeated-measuresANOVAanalysing foliar gas exchange variables including light-saturating photosynthesis (Asat)dark respiration (Rdark) and respiration in the light (Rlight) expressed on an area mass and nitrogen basis the ratio of Rlight to Rdark (Rlight RDark)the ratio of photosynthetic carbon assimilation to respiratory carbon release (Rlight Agross) and the maximum carboxylation rate of Rubisco (Vcmax)and the electron transport rate (J) both measured at ambient leaf temperature and corrected for a constant temperature of 258C in Betula nana and
Eriophorum vaginatum Plt 005 Plt 001 Plt 0001 S species R round W warming df degrees of freedom
Sdf = 1 158
Rdf = 9 150
Wdf = 1 158
SRdf = 9 140
SWdf = 1 156
RWdf = 9 140
SRWdf= 9 120
F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P
in two dominant Arctic plants We hypothesised that the within-growing season rates of photosynthesis respiration and theinhibition of respiration will relate to growth-induced energydemand These rates will acclimate to long-term warming and be
responsive to short-term temperature fluctuations Our resultsshow stronger seasonal and thermal responses in respiration thanin photosynthesis (when both are measured at 20C) which wasfurther influenced by species differences and growth under long-
15
Betula Eriophorum
10
5
0
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10
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0
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64
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Asa
t (microm
ol C
O2
mndash2
sndash1
)R
dark
(microm
ol C
O2
mndash2
sndash1
)R
light
(microm
ol C
O2
mndash2
sndash1
)
160 170 180 190 200 210160 170 180 190 200
Julian day
(a)
(c)
(e)
(b)
(d)
(f )
Fig 2 (a b)Saturating photosynthesis (Asat measured at PAR=1500mmolmndash2 sndash1 and 400 parts per million (ppm)CO2) (c d) foliar dark respiration (Rdark measured at PAR=0mmolmndash2 sndash1 and 400 ppmCO2) and (e f) respiration inthe light (Rlight estimated via the Kok method at low PAR levels) through the growing season for leaves of (a c e)Betula nana (circles) and (bd f)Eriophorumvaginatum (triangles) grown in ambient (unfilled symbols) andpassivelywarmed (filled symbols) conditions All variables were measured at 20 C in laboratory conditions (n= 4)
Seasonal carbon exchange under long-term warming Functional Plant Biology G
termwarmingThis studyalsoprovided theopportunity toexpanduponexisting knowledge about biotic and environmental controlson foliar Rlight and how seasonal timing and temperaturevariability may mediate Rlight Rdark Together these findingsprovide a more detailed mechanistic understanding of thecontrol of carbon exchange in tundra ecosystems
Species-mediated influences of seasonal and environmentaleffects on carbon exchange
In our study the rates of photosynthesis and respiration ofB nana when measured at a common temperature tended tobe higher than those of E vaginatum during the Arctic tundragrowing season (Fig 2 Table 2) grown under control conditionsand long-term passive warming The higher rates of carbonexchange in B nana both when measured at 20C (Fig 2)and over the 5ndash35C temperature range (Fig 3) wereconsistent with previous studies that sampled plants grownunder warming at midseason (Chapin and Shaver 1996) andwith the addition of soil nitrogen and phosphorus (Heskel et al2012) The difference in Asat between B nana and E vaginatumwas greatest directly after leaf out (Fig 2) which emphasises theimpact of growth formon foliar physiological processesB nanaa deciduouswoody shrub is likely to have a relatively high energydemand immediately after snowmelt to accommodate efficientaboveground and belowground growth during the short growingseason and thismay be further enhanced under warming (Chapinet al 1995 Sullivan et al 2007) By contrast E vaginatum atiller-producing graminoid can retain leaves for multiple years(Fetcher and Shaver 1983) and for this reason may act lessopportunistically upon snowmelt in the spring in terms ofnutrient acquisition via fine root growth (Sullivan et al 2007)andquick leaf production These trends are supported byprevious
work on deciduous and evergreen species where seasonalphotosynthesis and respiration were consistently significantlylower in the evergreen (Ow et al 2010) The species contrastis underscored by the respective leaf architectures andcomposition interspecies comparisons show a less dense(greater SLA) leaf with higher N content in B nana than inE vaginatum (Table 1 Figs S2 S3)
We found a general though not significant decline in Asat inleaves sampled from long-term warming plots suggesting apotential duration-dependent warming effect at the leaf levelthat may contribute to a similar lack of photosyntheticstimulation reported at the ecosystem level (Shaver et al2000 Elmendorf et al 2012) Also these tundra species maybe pushed beyond their photosynthetic thermal optimum underelevated temperatures (Sage and Kubien 2007) which could bea likely scenario in Arctic populations that may be locallyadapted for colder growth temperatures We observed noapparent seasonal arch in the Asat Vcmax and J rates which cangenerally characterise growing season carbon assimilation ofleaves measured at ambient leaf temperatures as previouslyreported (for Asat) in these species (Starr et al 2008) At theleaf level the highest rates of Asat Vcmax and J can occur in theearly growing season (Dungan et al 2003 Xu and Baldocchi2003) This discrepancy may be explained by the highly variableambient temperature and precipitation conditions (Fig 1) thoughmodel evaluation found no strong relationship betweenphotosynthetic variables (measured at 20C) and short-termtemperature values (Table S1) Also as we measured allleaves at a common temperature (20 C) we controlled for theinfluence of ambient temperature on leaf temperature which canaffect photosynthetic rates and allowed for comparison acrosschanges in the ambient temperature to assess thermal acclimation
Respiration rates of both species were generally highest in thefirst few weeks after snowmelt (measured at 20C Fig 2endashf)which is more apparent in Rlight than in Rdark and are similar topreviously reported values (Heskel et al 2012 Heskel et al2013) In both B nana and E vaginatum higher energy demandand potentially respiratory rates in the early season relative to themid- and late season may be attributable to new leaf growth anddevelopment (Vose and Ryan 2002 Xu et al 2007 Ow et al2010) and possibly a higher density of mitochondria in youngerleaves (Armstrong et al 2006) This respiratory release can befurther enhanced by the colder ambient temperatures experiencedby plants in the early growing season due to a short-term coldacclimation (Atkin and Tjoelker 2003 Armstrong et al 2006)The thermal response of respiration is nonlinear and variesbetween species in response curve shape and slope (Fig 3Table 3) higher respiration in B nana across the range oftemperatures experienced during the growing season (Fig 1)and the lack of thermal acclimation may favour its expansion inthe tundra under current conditions (Heskel et al 2013) Furtherthe observed warm acclimation in leaves ofE vaginatum (Fig 3)suggests lowered metabolic rates that may limit growth despitereducing respiratory C loss under a future climate
It should be noted that similar rates of respiration rates acrossspecies may not necessary equate to similar energy efficiencyB nana is reported to exhibit greater respiratory efficiency thanE vaginatum through the differential use of the alternativeand cytochrome pathways potentially lending a competitive
05 10 15 20 25 30 35
10
20
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40
50
60
70
Betula CT
Betula WG
Eriophorum CT
Eriophorum WG
Temperature (degC)
Rda
rk (
nmol
CO
2 gndash1
sndash1
)
Fig 3 The mean short-term thermal responses of foliar dark respirationfrom 5C to 35C modelled from a polynomial fit from raw data in Betulanana (circles) and Eriophorum vaginatum (triangles) sampled from control(CT open) andwarmed (WG closed) growth environments Temperature (T)response curves of respiration (R) were fit to second-order polynomialequations as follows B nana control r= 0027T2 + 0082T+ 660 B nanawarmed r= 0026T2 + 0103T+ 677 E vaginatum control r = 0045T2 ndash
0106T+ 310 and E vaginatum warmed r = 0018T2 + 0176T+ 2633 Forall thermal response curve fits R2 gt 0990
H Functional Plant Biology M A Heskel et al
advantage in growth and development (Kornfeld et al 2013)In addition Rlight Agross decreased over the growing season(Fig 6) driven by lower values of Rlight as the season progressedrather than any significant change in photosynthesis which is
discordant with the idea that the processes would increase in acoupled manner based on carbohydrate substrate supply anddemand Kornfeld et al (2013) found no difference in starchand sugar content between control and warmed leaves within
0
20
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80Betula
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Julian day
160 170 180 190 200 210160 170 180 190 200
(a)
(c)
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(f )
Vcm
ax (micro
mol
mndash2
sndash1
)J
Vcm
ax
J (micro
mol
mndash2
sndash1
)
Fig 4 (a b) Maximal carboxylation rate (Vcmax) and (c d) electron transport rate (J) both measured in laboratoryconditions and corrected to a constant 25C and (e f) the ratio of these variables (J Vcmax) over the growing season inleaves of (a c e) Betula nana and (b d f) Eriophorum vaginatum grown under ambient (open) and warmed (filled)conditions Values shown are means with se
Seasonal carbon exchange under long-term warming Functional Plant Biology I
species when measured at a single sampling point during thegrowing season though leaves of B nana contained a greaterconcentration of both sugar and starch than E vaginatum Wewere unable to quantify foliar carbohydrate values in thesespecies through the season though future studies couldinclude this informative measurement to further relate themechanistic links between these processes
Both species exhibited the lowest values of Rlight Rdark
towards the end of the season in late July (Fig 5) suggestinga potential developmental or energy demand-related control onthe inhibition of respiration by light that allows for greater
respiratory energy production in the early season during leafexpansionTheonlyother knownstudy that explored the seasonalresponse of Rlight Rdark observed the lowest level of inhibitionearlier in the growing season before the warmest month (Crouset al 2012) This response similar to our study despite a starklydifferent ecological system supports the idea of shared seasonaltiming-mediated biochemical controls on the light inhibition ofrespiration across species The degree of inhibition is known toshow a relaxed response under environmental conditions thatcan stimulate growth due to increased demand for energy and Cskeletons as exhibited under elevated CO2 (Wang et al 2001Shapiro et al 2004) and increased soil nutrient availability(Heskel et al 2012) though this is not always the case (Tissueet al 2002 Ayub et al 2011) Long-termwarming alsomediatedRlight Rdark with lower values observed in warm-grownleaves though measurement temperature can also impact theserelationships (Atkin et al 2006)
Mechanistic explanations for the relationships amonggrowth demand cellular energy status and the degree of lightinhibition of respiration have been previously reported Theenvironmental challenges of a short growing season and ~24-hdiel light exposure in the Arctic tundra control growth ratesthat may then influence values of Rlight Rdark (Bret-Harteet al 2001 Bret-Harte et al 2002) In the light adenylatesupply from photosynthesis may decrease the energy demandfrom mitochondrial respiration Additionally photorespirationis associated with the inactivation of pyruvate dehydrogenasea precursor to the tri-carboxylic acid (TCA) cycle (Budde andRandall 1990) though previous estimations of photorespirationin these species didnotfinda strong correlationwith the inhibitionof Rlight (Heskel et al 2013) The TCA cycle can also besignificantly altered in the light to support N assimilationwhich effectively reduces CO2 release (Tcherkez et al 2005Tcherkez et al 2008 Tcherkez et al 2009) This effect may beenhanced in tundra plantswhen soil N is less limiting under theseconditions growth rates increase (Bret-Harte et al 2002) Theinhibition ofRlight is observed to be slightly increased in fertilisedsoils (Heskel et al 2013) Additionally the oxidative pentosephosphate pathway (OPPP) which contributes the necessaryreductant for the synthesis of multiple metabolites in darknessmay be relaxed in the light when reductants are provided byphotosynthesis diminishing CO2 release by the OPPP Thoughthe OPPP comprises a proportionally smaller CO2 flux comparedwith that from the TCA cycle in the light this decrease maycontribute to lower overall CO2 release causing Rlight ltRdark
(Buckley and Adams 2011) In our current study the apparentrelaxation of the light inhibition of respiration in leaves of bothstudy species (Fig 5) resulting in the greatest proportional CO2
losses (Fig 6) suggesting that high growth demand for ATP andcarbon skeletons outweigh potential carbon losses in the earlyseason
Influences of variable ambient temperature on gas exchange
In addition to quantifying the effects of long-termwarming on thefoliar gas exchange during the growing season this studyevaluated the influence of intraseasonal short-term temperaturevariability on these physiological processes Arctic tundragrowing seasons can exhibit dramatic shifts in temperature and
Table 3 Parameters describing the respiratory response tomeasurement temperature in leaves of Betula nana and Eriophorumvaginatum grown under control (CT) and long-term warming (WG)conditions including Q10 (the temperature sensitivity of respiration(estimated over 5ndash358C)) R10 (the basal respiration rate at 108C(mmol CO2 gndash1 sndash1)) and Eo (kJmolndash1) a variable related to the energyof activation calculated from values of respiration at 208C and R10
Values presented are means and se Statistical differences between speciesand treatments are denoted alphabetically
Fig 5 Means and se of the ratio of respiration in the light (Rlight) estimatedvia the Kok method to dark respiration (Rdark) both variables measured at20C and 400 parts per million CO2 through the growing season in Betulanana (circles) and Eriophorum vaginatum (triangles) under ambient controlfield conditions (CT open symbols) and under a long-term passive warmingtreatment (WGfilled symbols n= 4)A linear regression (y= ndash0009x+ 219R2 = 047) depicts the declining Rlight Rdark values with the progressinggrowing season
J Functional Plant Biology M A Heskel et al
precipitation within days as was observed in this study duringsummer 2010 in Toolik Lake Alaska (Fig 1) and this variabilityis likely to influence the temperature-sensitive processescontrolling leaf carbon cycling Respiration in both speciescan be highly temperature-sensitive especially at warmertemperatures which may become more common in a futureArctic (Fig 3 Table 3) When modelled against thetemperature of the measurement day the average temperatureof the prior week accounted for species and treatment effectsand some patterns emerged that help to characterise the nature ofthe temperature sensitivity of foliar gas exchange (Table S1)However photosynthesis which is known to respond to short-term changes in temperature in field-grown plants (Berry andBjoumlrkman 1980 Poyatos et al 2012) did not show strongrelationships to daily or weekly variation in temperatureinstead species effects explained most variation (Table S1)The underlying photosynthetic machinery maintains consistentrates of carbon assimilation (measured at a constant temperature)throughout the growing season though daily fluctuations intemperature and precipitation experiences in the Arctic mayinfluence in vivo rates Plant species from alpine and Arcticecosystems can exhibit local adaptation to the extremevariability experienced in those locations (Korner and Larcher1988) which may translate to a limited acclimation (Atkin et al2006) Further because the ambient temperature conditionsfrom this region fluctuate so unpredictably in short periodsthrough the season any analysis using temperature averages ofprevious time windows might not be as useful for estimatingthermal acclimation as they are in regions with more predictableseasonal temperature patterns (Ow et al 2010 Searle et al 2011)
In contrast respiratory rates in the light and dark whenmeasured at a constant temperature appeared to be morecorrelated with short-term ambient temperature variation TheAICc analysis shows that the strongest model for Rdark includedspecies and warming effects and the prior weekrsquos average
temperature in contrast the strongest model for Rlight includedthe former parameters and the minimum temperature from themeasurement day (Table S1) The model relationships maysuggest a short-term thermal sensitivity of respiration that maybe more responsive to temperature minimums though this isnot clearly reflected in regressions of Rlight with the ambienttemperature values (Fig S5) This sensitivity of respiration totemperature minimums has been observed previously inroots as well and suggests that thermal acclimation in coldgrowth temperatures may regulate metabolic activity to meetthe demands of growth and maintenance at the expense ofgreater carbon loss (Covey-Crump et al 2002) Low ambienttemperatures did not clearly relate to the highest rates ofrespiration when measured at a common temperature aswould be expected under respiratory cold acclimation (Atkinand Tjoelker 2003) However it is possible that developmentaltiming may also play into the leavesrsquo respiratory rates and abilityto acclimate especially in B nana in the early season when itsleaves are still expanding (Armstrong et al 2006) Through arelatively simple modelling exercise we found some evidenceof differential temperature controls on photosynthesis andrespiration in B nana and E vaginatum we suggest thatfuture work should include more detailed environmental andbiotic factors including soil temperature daily photosyntheticradiation and leaf carbohydrate data to more accurately informthe seasonal temperature sensitivity of foliar carbon exchange
Conclusions and implications
The physiological measurements collected in this study allowedfor the quantification of mechanistic responses in the nightlessand highly variable growing season of the arctic tundra andprovided new insights into foliar carbon regulation We presentnew information on the seasonal trends of foliar carbon cyclingin two dominant tundra species and relate these fluxes to short-
0
015
045
030
0
015
045
030
Julian day
Betula Eriophorum
160 170 180 190 200 210 160 170 180 190 200 210
Rlig
ht
Agr
oss
Fig 6 The proportional carbon loss through respiration in the light to gross photosynthesis in (a) Betula nana and(b) Eriophorum vaginatum over the growing season (Rlight Agross both variables measured at a common temperatureof 20C and 400 parts per million CO2) Linear regressions across growth conditions depict the relationship ofRlight Agross and measurement date during the growing season (B nana y= ndash0003 + 088x R2 = 030 E vaginatumy= ndash0005 + 117x R2 = 065) Values represent variable means and se error bars are not visible for many values dueto their small value
Seasonal carbon exchange under long-term warming Functional Plant Biology K
term temperature variability and long-term warming Our studypresents the first published values of the photosyntheticparameters Vcmax and J at multiple points during the season inthese species which can inform and refine parameterisation ofvegetationmodels atmany scalesAlsowe present thefirst reportof the seasonal flexibility of the degree of the inhibition of Rlight
under long-term warmed growth conditions in these or anyspecies which will enable more accurate calculations ofecosystem respiration Based on these measurements aseasonal estimate (not accounting for potential temperaturesensitivity as discussed above see Eqn S3) shows thatneglecting the inhibition of foliar Rlight in Arctic tundravegetation could lead to overestimations of foliar carbon lossof~4molmndash2 leaf (~176 g)over thegrowingseason or evenmore(~42molmndash2) when considering plants grown under warmedconditionsAlthough the temperature effect on respiratory carbonloss in the light may beminimal the overall discrepancy in fluxesbetween Rdark and Rlight must be considered for realisticdescriptions of Arctic carbon cycling Our study contributesimportant data that allow for an increased understanding ofdominant tundra speciesrsquo responses to environmental changeboth long- and short-term and will allow for more predictivepower in models estimating ecosystem carbon storage
Acknowledgements
We are grateful to the researchers and support staff at the Arctic LTER andToolik Field Station for their help in this study as well as Greg PendergastThis study was made possible through funding from the NSF 0732664OKA acknowledges grant support from the Australian Research Council(DP0986823)
References
Armstrong AF Logan DC Atkin OK (2006a) On the developmentaldependence of leaf respiration responses to short and long-termchanges in growth temperature American Journal of Botany 931633ndash1639 doi103732ajb93111633
Armstrong AF Logan DC Tobin AK OrsquoToole P Atkin OK (2006b)Heterogeneity of plant mitochondrial responses underpinningrespiratory acclimation to the cold in Arabidopsis thaliana leavesPlant Cell amp Environment 29 940ndash949 doi101111j1365-3040200501475x
Armstrong AF Badger MR Day DA Barthet MM Smith P Millar AHWhelan J Atkin OK (2008) Dynamic changes in the mitochondrialelectron transport chain underpinning cold acclimation of leafrespiration Plant Cell amp Environment 31 1156ndash1169 doi101111j1365-3040200801830x
Atkin OK Tjoelker MG (2003) Thermal acclimation and the dynamicresponse of plant respiration to temperature Trends in Plant Science 8343ndash351 doi101016S1360-1385(03)00136-5
Atkin OK Bruhn D Tjoelker MG (2005) Response of plant respiration tochanges in temperature mechanisms and consequences of variations inQ10 values and acclimation In lsquoPlant respirationrsquo (Eds H Lambers ampM Ribas-Carbo) pp 95ndash135 (Springer Dordrecht The Netherlands)
Atkin OK Scheurwater I Pons TL (2006)High thermal acclimation potentialof both photosynthesis and respiration in two lowlandPlantago species incontrast to an alpine congeneric Global Change Biology 12 500ndash515doi101111j1365-2486200601114x
Ayub G Smith RA Tissue DT Atkin OK (2011) Impacts of drought onleaf respiration in darkness and light in Eucalyptus saligna exposed toindustrial-age atmosphericCO2 and growth temperatureNewPhytologist190 1003ndash1018 doi101111j1469-8137201103673x
Berry J Bjoumlrkman O (1980) Photosynthetic response and adaptation totemperature in higher plants Annual Review of Plant Physiology andPlant Molecular Biology 31 491ndash543 doi101146annurevpp31060180002423
Bret-HarteMSShaverGRZoerner JP JohnstoneJFWagner JLChavezASGunkelman RF IV Lippert SC Laundre JA (2001) Developmentalplasticity allows Betula nana to dominate tundra subjected to analtered environment Ecology 82 18ndash32
Bret-Harte MS Shaver GR Chapin FS III (2002) Primary and secondarystem growth in Arctic shrubs implications for community response toenvironmental change Journal of Ecology 90 251ndash267 doi101046j1365-2745200100657x
BuckleyTNAdamsMA(2011)An analyticalmodel of non-photorespiratoryCO2 release in the light and dark in leaves of C3 species based onstoichiometric flux balance Plant Cell amp Environment 34 89ndash112doi101111j1365-3040201002228x
Budde RJ Randall DD (1990) Pea leaf mitochondrial pyruvatedehydrogenase complex is inactivated in vivo in a light-dependentmanner Proceedings of the National Academy of Sciences of theUnited States of America 87 673ndash676 doi101073pnas872673
CahoonSMPSullivanPF ShaverGRWelker JM PostE (2012) Interactionsamong shrub cover and the soil microclimatemay determine futureArcticcarbon budgets Ecology Letters 15 1415ndash1422 doi101111j1461-0248201201865x
Chapin FS Shaver GR (1996) Physiological and growth responses of Arcticplants to a field experiment simulating climatic change Ecology 77822ndash840 doi1023072265504
Chapin FS Shaver GR Giblin AE Nadelhoffer KJ Laundre JA (1995)Responses of Arctic tundra to experimental and observed changes inclimate Ecology 76 694ndash711 doi1023071939337
Chapin FS Sturm M Serreze MC McFadden JP Key JR Lloyd AHMcGuire AD Rupp TS Lynch AH Schimel JP Beringer J ChapmanWLEpsteinHEEuskirchenESHinzmanLD JiaGPingC-LTapeKDThompson CDC Walker DA Welker JM (2005) Role of land-surfacechanges in Arctic summer warming Science 310 657ndash660 doi101126science1117368
Covey-Crump EM Attwood RG Atkin OK (2002) Regulation of rootrespiration in two species of Plantago that differ in relative growthrate the effect of short and long term changes in temperature PlantCell amp Environment 25 1501ndash1513 doi101046j1365-3040200200932x
Crous KY Zaragoza-Castells J Ellsworth DS Duursma RA LowM TissueDT Atkin OK (2012) Light inhibition of leaf respiration in field-grownEucalyptus saligna in whole-tree chambers under elevated atmosphericCO2 and summer drought Plant Cell amp Environment 35 966ndash981doi101111j1365-3040201102465x
Dungan RJ Whitehead D Duncan RP (2003) Seasonal and temperaturedependence of photosynthesis and respiration for two co-occurringbroad-leaved tree species with contrasting leaf phenology TreePhysiology 23 561ndash568 doi101093treephys238561
Elmendorf SC Henry GHR Hollister RD Bjoumlrk RG Bjorkman ADCallaghan TV Collier LS Cooper EJ Cornelissen JHC Day TAFosaa AM Gould WA Greacutetarsdoacutettir J Harte J Hermanutz L Hik DSHofgaard A Jarrad F Joacutensdoacutettir IS Keuper F Klanderud K Klein JAKoh S Kudo G Lang SI Loewen V May JL Mercado J Michelsen AMolau U Myers-Smith IH Oberbauer SF Pieper S Post E Rixen CRobinson CH Schmidt NM Shaver GR Stenstroumlm A Tolvanen ATotland Oslash Troxler T Wahren C-H Webber PJ Welker JM WookeyPA (2012) Global assessment of experimental climate warming ontundra vegetation heterogeneity over space and time Ecology Letters15 164ndash175 doi101111j1461-0248201101716x
EnvironmentalDataCenterTeam (2011)Meteorologicalmonitoring programat Toolik Alaska Toolik Field Station Institute of Arctic BiologyUniversity of Alaska Fairbanks Fairbanks AK 99775 Available athttptoolikalaskaeduedcabiotic_monitoringdata_queryphp
L Functional Plant Biology M A Heskel et al
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
respiration rates (P lt 00001) with individual leaf values rangingwidely Rlight Rdark was less in E vaginatum thanB nana underboth growth conditions (P lt 0001) and decreased further underwarming inE vaginatum (P lt 001) For both species and growthconditions Rlight Rdark decreased over the growing season(Fig 5) Warming decreased rates in only E vaginatum forarea- and N-based rates (Plt 005 for both) but not mass-basedrates which found warming to significantly lower rates whenconsidering all measurements (P lt 001) Furthermore the effecta speciesmeasurement round interaction (Plt 005) wasevident in the different trends of respiratory release over thecourse of the season (Fig 2endashf Fig S4) The ratio of carbon lossvia respiration in the light to gross photosynthesis (Rlight Agross)which describes the relative carbon loss from respirationcompared with the total carbon cycled in a leaf (as opposed tothe difference between the fluxes) decreased through the seasonin both species (Fig 6) though at different rates Theseproportional respiratory losses were significantly greater inB nana than in E vaginatum and warming significantlydecreased these values considering all measurements acrossthe season (Table 2) No clear relationships were foundbetween the degree of inhibition and foliar fluxes or traits inthese species regression analysis between Rlight Rdark andNAsat and Vcmax produced r2lt 010 (data not shown)
Intraseasonal ambient temperature influenceson gas exchange
Toevaluate the influenceof ambient temperature ongas exchangerates we compared multivariate linear models that incorporatedtemperature values from the day of andmean temperature values
from the week before gas exchange measurement Table S1shows the results of models incorporating the effect of specieswarming treatment and day-of and week-before minimummaximum and average temperatures on area-based Asat RdarkRlight and Rlight Rdark evaluating by comparing relative AICcweights The best model for photosynthesis (measured at 20C)incorporates only species with the next best only incorporatingspecies andwarming treatment suggestingno significant effect ofshort-term ambient air temperature on rates of Asat In contrastboth Rdark and Rlight (also measured at 20C) were more sensitiveto short-term ambient air temperatures with the best modelsincorporating the effect of species growth conditions andeither the prior weekrsquos average temperature (Rdark) or themeasurement dayrsquos minimum temperature (Rlight Fig S5) Thetop three models for Rlight Rdark incorporated minimumtemperature values When Rlight is considered alone a clearpredictive response is not obvious (Fig S5) though significantrelationships were observed between Rlight and day-of minimumtemperature (P lt 001) day-of maximum temperature(P lt 001) day-of average temperature (P lt 001) and week-prior minimum temperature (Plt 001) though not with theprior weekrsquos average or maximum temperature suggestinggreater sensitivity to short-term conditions colder conditionsor both
Discussion
The primary objective of this study was to evaluate themechanistic physiological responses of foliar photosynthesisand respiration to long-term warming conditions and naturalvariations in ambient temperature through the growing season
Table 2 Results from three-way repeated-measuresANOVAanalysing foliar gas exchange variables including light-saturating photosynthesis (Asat)dark respiration (Rdark) and respiration in the light (Rlight) expressed on an area mass and nitrogen basis the ratio of Rlight to Rdark (Rlight RDark)the ratio of photosynthetic carbon assimilation to respiratory carbon release (Rlight Agross) and the maximum carboxylation rate of Rubisco (Vcmax)and the electron transport rate (J) both measured at ambient leaf temperature and corrected for a constant temperature of 258C in Betula nana and
Eriophorum vaginatum Plt 005 Plt 001 Plt 0001 S species R round W warming df degrees of freedom
Sdf = 1 158
Rdf = 9 150
Wdf = 1 158
SRdf = 9 140
SWdf = 1 156
RWdf = 9 140
SRWdf= 9 120
F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P F-stat P
in two dominant Arctic plants We hypothesised that the within-growing season rates of photosynthesis respiration and theinhibition of respiration will relate to growth-induced energydemand These rates will acclimate to long-term warming and be
responsive to short-term temperature fluctuations Our resultsshow stronger seasonal and thermal responses in respiration thanin photosynthesis (when both are measured at 20C) which wasfurther influenced by species differences and growth under long-
15
Betula Eriophorum
10
5
0
15
10
5
0
6
4
2
0
6
4
2
0
64
3
2
1
0
4
2
0
Asa
t (microm
ol C
O2
mndash2
sndash1
)R
dark
(microm
ol C
O2
mndash2
sndash1
)R
light
(microm
ol C
O2
mndash2
sndash1
)
160 170 180 190 200 210160 170 180 190 200
Julian day
(a)
(c)
(e)
(b)
(d)
(f )
Fig 2 (a b)Saturating photosynthesis (Asat measured at PAR=1500mmolmndash2 sndash1 and 400 parts per million (ppm)CO2) (c d) foliar dark respiration (Rdark measured at PAR=0mmolmndash2 sndash1 and 400 ppmCO2) and (e f) respiration inthe light (Rlight estimated via the Kok method at low PAR levels) through the growing season for leaves of (a c e)Betula nana (circles) and (bd f)Eriophorumvaginatum (triangles) grown in ambient (unfilled symbols) andpassivelywarmed (filled symbols) conditions All variables were measured at 20 C in laboratory conditions (n= 4)
Seasonal carbon exchange under long-term warming Functional Plant Biology G
termwarmingThis studyalsoprovided theopportunity toexpanduponexisting knowledge about biotic and environmental controlson foliar Rlight and how seasonal timing and temperaturevariability may mediate Rlight Rdark Together these findingsprovide a more detailed mechanistic understanding of thecontrol of carbon exchange in tundra ecosystems
Species-mediated influences of seasonal and environmentaleffects on carbon exchange
In our study the rates of photosynthesis and respiration ofB nana when measured at a common temperature tended tobe higher than those of E vaginatum during the Arctic tundragrowing season (Fig 2 Table 2) grown under control conditionsand long-term passive warming The higher rates of carbonexchange in B nana both when measured at 20C (Fig 2)and over the 5ndash35C temperature range (Fig 3) wereconsistent with previous studies that sampled plants grownunder warming at midseason (Chapin and Shaver 1996) andwith the addition of soil nitrogen and phosphorus (Heskel et al2012) The difference in Asat between B nana and E vaginatumwas greatest directly after leaf out (Fig 2) which emphasises theimpact of growth formon foliar physiological processesB nanaa deciduouswoody shrub is likely to have a relatively high energydemand immediately after snowmelt to accommodate efficientaboveground and belowground growth during the short growingseason and thismay be further enhanced under warming (Chapinet al 1995 Sullivan et al 2007) By contrast E vaginatum atiller-producing graminoid can retain leaves for multiple years(Fetcher and Shaver 1983) and for this reason may act lessopportunistically upon snowmelt in the spring in terms ofnutrient acquisition via fine root growth (Sullivan et al 2007)andquick leaf production These trends are supported byprevious
work on deciduous and evergreen species where seasonalphotosynthesis and respiration were consistently significantlylower in the evergreen (Ow et al 2010) The species contrastis underscored by the respective leaf architectures andcomposition interspecies comparisons show a less dense(greater SLA) leaf with higher N content in B nana than inE vaginatum (Table 1 Figs S2 S3)
We found a general though not significant decline in Asat inleaves sampled from long-term warming plots suggesting apotential duration-dependent warming effect at the leaf levelthat may contribute to a similar lack of photosyntheticstimulation reported at the ecosystem level (Shaver et al2000 Elmendorf et al 2012) Also these tundra species maybe pushed beyond their photosynthetic thermal optimum underelevated temperatures (Sage and Kubien 2007) which could bea likely scenario in Arctic populations that may be locallyadapted for colder growth temperatures We observed noapparent seasonal arch in the Asat Vcmax and J rates which cangenerally characterise growing season carbon assimilation ofleaves measured at ambient leaf temperatures as previouslyreported (for Asat) in these species (Starr et al 2008) At theleaf level the highest rates of Asat Vcmax and J can occur in theearly growing season (Dungan et al 2003 Xu and Baldocchi2003) This discrepancy may be explained by the highly variableambient temperature and precipitation conditions (Fig 1) thoughmodel evaluation found no strong relationship betweenphotosynthetic variables (measured at 20C) and short-termtemperature values (Table S1) Also as we measured allleaves at a common temperature (20 C) we controlled for theinfluence of ambient temperature on leaf temperature which canaffect photosynthetic rates and allowed for comparison acrosschanges in the ambient temperature to assess thermal acclimation
Respiration rates of both species were generally highest in thefirst few weeks after snowmelt (measured at 20C Fig 2endashf)which is more apparent in Rlight than in Rdark and are similar topreviously reported values (Heskel et al 2012 Heskel et al2013) In both B nana and E vaginatum higher energy demandand potentially respiratory rates in the early season relative to themid- and late season may be attributable to new leaf growth anddevelopment (Vose and Ryan 2002 Xu et al 2007 Ow et al2010) and possibly a higher density of mitochondria in youngerleaves (Armstrong et al 2006) This respiratory release can befurther enhanced by the colder ambient temperatures experiencedby plants in the early growing season due to a short-term coldacclimation (Atkin and Tjoelker 2003 Armstrong et al 2006)The thermal response of respiration is nonlinear and variesbetween species in response curve shape and slope (Fig 3Table 3) higher respiration in B nana across the range oftemperatures experienced during the growing season (Fig 1)and the lack of thermal acclimation may favour its expansion inthe tundra under current conditions (Heskel et al 2013) Furtherthe observed warm acclimation in leaves ofE vaginatum (Fig 3)suggests lowered metabolic rates that may limit growth despitereducing respiratory C loss under a future climate
It should be noted that similar rates of respiration rates acrossspecies may not necessary equate to similar energy efficiencyB nana is reported to exhibit greater respiratory efficiency thanE vaginatum through the differential use of the alternativeand cytochrome pathways potentially lending a competitive
05 10 15 20 25 30 35
10
20
30
40
50
60
70
Betula CT
Betula WG
Eriophorum CT
Eriophorum WG
Temperature (degC)
Rda
rk (
nmol
CO
2 gndash1
sndash1
)
Fig 3 The mean short-term thermal responses of foliar dark respirationfrom 5C to 35C modelled from a polynomial fit from raw data in Betulanana (circles) and Eriophorum vaginatum (triangles) sampled from control(CT open) andwarmed (WG closed) growth environments Temperature (T)response curves of respiration (R) were fit to second-order polynomialequations as follows B nana control r= 0027T2 + 0082T+ 660 B nanawarmed r= 0026T2 + 0103T+ 677 E vaginatum control r = 0045T2 ndash
0106T+ 310 and E vaginatum warmed r = 0018T2 + 0176T+ 2633 Forall thermal response curve fits R2 gt 0990
H Functional Plant Biology M A Heskel et al
advantage in growth and development (Kornfeld et al 2013)In addition Rlight Agross decreased over the growing season(Fig 6) driven by lower values of Rlight as the season progressedrather than any significant change in photosynthesis which is
discordant with the idea that the processes would increase in acoupled manner based on carbohydrate substrate supply anddemand Kornfeld et al (2013) found no difference in starchand sugar content between control and warmed leaves within
0
20
40
60
80Betula
0
20
40
60Eriophorum
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20
40
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80
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40
80
120
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1
2
3
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1
2
3
4
Julian day
160 170 180 190 200 210160 170 180 190 200
(a)
(c)
(e)
(b)
(d)
(f )
Vcm
ax (micro
mol
mndash2
sndash1
)J
Vcm
ax
J (micro
mol
mndash2
sndash1
)
Fig 4 (a b) Maximal carboxylation rate (Vcmax) and (c d) electron transport rate (J) both measured in laboratoryconditions and corrected to a constant 25C and (e f) the ratio of these variables (J Vcmax) over the growing season inleaves of (a c e) Betula nana and (b d f) Eriophorum vaginatum grown under ambient (open) and warmed (filled)conditions Values shown are means with se
Seasonal carbon exchange under long-term warming Functional Plant Biology I
species when measured at a single sampling point during thegrowing season though leaves of B nana contained a greaterconcentration of both sugar and starch than E vaginatum Wewere unable to quantify foliar carbohydrate values in thesespecies through the season though future studies couldinclude this informative measurement to further relate themechanistic links between these processes
Both species exhibited the lowest values of Rlight Rdark
towards the end of the season in late July (Fig 5) suggestinga potential developmental or energy demand-related control onthe inhibition of respiration by light that allows for greater
respiratory energy production in the early season during leafexpansionTheonlyother knownstudy that explored the seasonalresponse of Rlight Rdark observed the lowest level of inhibitionearlier in the growing season before the warmest month (Crouset al 2012) This response similar to our study despite a starklydifferent ecological system supports the idea of shared seasonaltiming-mediated biochemical controls on the light inhibition ofrespiration across species The degree of inhibition is known toshow a relaxed response under environmental conditions thatcan stimulate growth due to increased demand for energy and Cskeletons as exhibited under elevated CO2 (Wang et al 2001Shapiro et al 2004) and increased soil nutrient availability(Heskel et al 2012) though this is not always the case (Tissueet al 2002 Ayub et al 2011) Long-termwarming alsomediatedRlight Rdark with lower values observed in warm-grownleaves though measurement temperature can also impact theserelationships (Atkin et al 2006)
Mechanistic explanations for the relationships amonggrowth demand cellular energy status and the degree of lightinhibition of respiration have been previously reported Theenvironmental challenges of a short growing season and ~24-hdiel light exposure in the Arctic tundra control growth ratesthat may then influence values of Rlight Rdark (Bret-Harteet al 2001 Bret-Harte et al 2002) In the light adenylatesupply from photosynthesis may decrease the energy demandfrom mitochondrial respiration Additionally photorespirationis associated with the inactivation of pyruvate dehydrogenasea precursor to the tri-carboxylic acid (TCA) cycle (Budde andRandall 1990) though previous estimations of photorespirationin these species didnotfinda strong correlationwith the inhibitionof Rlight (Heskel et al 2013) The TCA cycle can also besignificantly altered in the light to support N assimilationwhich effectively reduces CO2 release (Tcherkez et al 2005Tcherkez et al 2008 Tcherkez et al 2009) This effect may beenhanced in tundra plantswhen soil N is less limiting under theseconditions growth rates increase (Bret-Harte et al 2002) Theinhibition ofRlight is observed to be slightly increased in fertilisedsoils (Heskel et al 2013) Additionally the oxidative pentosephosphate pathway (OPPP) which contributes the necessaryreductant for the synthesis of multiple metabolites in darknessmay be relaxed in the light when reductants are provided byphotosynthesis diminishing CO2 release by the OPPP Thoughthe OPPP comprises a proportionally smaller CO2 flux comparedwith that from the TCA cycle in the light this decrease maycontribute to lower overall CO2 release causing Rlight ltRdark
(Buckley and Adams 2011) In our current study the apparentrelaxation of the light inhibition of respiration in leaves of bothstudy species (Fig 5) resulting in the greatest proportional CO2
losses (Fig 6) suggesting that high growth demand for ATP andcarbon skeletons outweigh potential carbon losses in the earlyseason
Influences of variable ambient temperature on gas exchange
In addition to quantifying the effects of long-termwarming on thefoliar gas exchange during the growing season this studyevaluated the influence of intraseasonal short-term temperaturevariability on these physiological processes Arctic tundragrowing seasons can exhibit dramatic shifts in temperature and
Table 3 Parameters describing the respiratory response tomeasurement temperature in leaves of Betula nana and Eriophorumvaginatum grown under control (CT) and long-term warming (WG)conditions including Q10 (the temperature sensitivity of respiration(estimated over 5ndash358C)) R10 (the basal respiration rate at 108C(mmol CO2 gndash1 sndash1)) and Eo (kJmolndash1) a variable related to the energyof activation calculated from values of respiration at 208C and R10
Values presented are means and se Statistical differences between speciesand treatments are denoted alphabetically
Fig 5 Means and se of the ratio of respiration in the light (Rlight) estimatedvia the Kok method to dark respiration (Rdark) both variables measured at20C and 400 parts per million CO2 through the growing season in Betulanana (circles) and Eriophorum vaginatum (triangles) under ambient controlfield conditions (CT open symbols) and under a long-term passive warmingtreatment (WGfilled symbols n= 4)A linear regression (y= ndash0009x+ 219R2 = 047) depicts the declining Rlight Rdark values with the progressinggrowing season
J Functional Plant Biology M A Heskel et al
precipitation within days as was observed in this study duringsummer 2010 in Toolik Lake Alaska (Fig 1) and this variabilityis likely to influence the temperature-sensitive processescontrolling leaf carbon cycling Respiration in both speciescan be highly temperature-sensitive especially at warmertemperatures which may become more common in a futureArctic (Fig 3 Table 3) When modelled against thetemperature of the measurement day the average temperatureof the prior week accounted for species and treatment effectsand some patterns emerged that help to characterise the nature ofthe temperature sensitivity of foliar gas exchange (Table S1)However photosynthesis which is known to respond to short-term changes in temperature in field-grown plants (Berry andBjoumlrkman 1980 Poyatos et al 2012) did not show strongrelationships to daily or weekly variation in temperatureinstead species effects explained most variation (Table S1)The underlying photosynthetic machinery maintains consistentrates of carbon assimilation (measured at a constant temperature)throughout the growing season though daily fluctuations intemperature and precipitation experiences in the Arctic mayinfluence in vivo rates Plant species from alpine and Arcticecosystems can exhibit local adaptation to the extremevariability experienced in those locations (Korner and Larcher1988) which may translate to a limited acclimation (Atkin et al2006) Further because the ambient temperature conditionsfrom this region fluctuate so unpredictably in short periodsthrough the season any analysis using temperature averages ofprevious time windows might not be as useful for estimatingthermal acclimation as they are in regions with more predictableseasonal temperature patterns (Ow et al 2010 Searle et al 2011)
In contrast respiratory rates in the light and dark whenmeasured at a constant temperature appeared to be morecorrelated with short-term ambient temperature variation TheAICc analysis shows that the strongest model for Rdark includedspecies and warming effects and the prior weekrsquos average
temperature in contrast the strongest model for Rlight includedthe former parameters and the minimum temperature from themeasurement day (Table S1) The model relationships maysuggest a short-term thermal sensitivity of respiration that maybe more responsive to temperature minimums though this isnot clearly reflected in regressions of Rlight with the ambienttemperature values (Fig S5) This sensitivity of respiration totemperature minimums has been observed previously inroots as well and suggests that thermal acclimation in coldgrowth temperatures may regulate metabolic activity to meetthe demands of growth and maintenance at the expense ofgreater carbon loss (Covey-Crump et al 2002) Low ambienttemperatures did not clearly relate to the highest rates ofrespiration when measured at a common temperature aswould be expected under respiratory cold acclimation (Atkinand Tjoelker 2003) However it is possible that developmentaltiming may also play into the leavesrsquo respiratory rates and abilityto acclimate especially in B nana in the early season when itsleaves are still expanding (Armstrong et al 2006) Through arelatively simple modelling exercise we found some evidenceof differential temperature controls on photosynthesis andrespiration in B nana and E vaginatum we suggest thatfuture work should include more detailed environmental andbiotic factors including soil temperature daily photosyntheticradiation and leaf carbohydrate data to more accurately informthe seasonal temperature sensitivity of foliar carbon exchange
Conclusions and implications
The physiological measurements collected in this study allowedfor the quantification of mechanistic responses in the nightlessand highly variable growing season of the arctic tundra andprovided new insights into foliar carbon regulation We presentnew information on the seasonal trends of foliar carbon cyclingin two dominant tundra species and relate these fluxes to short-
0
015
045
030
0
015
045
030
Julian day
Betula Eriophorum
160 170 180 190 200 210 160 170 180 190 200 210
Rlig
ht
Agr
oss
Fig 6 The proportional carbon loss through respiration in the light to gross photosynthesis in (a) Betula nana and(b) Eriophorum vaginatum over the growing season (Rlight Agross both variables measured at a common temperatureof 20C and 400 parts per million CO2) Linear regressions across growth conditions depict the relationship ofRlight Agross and measurement date during the growing season (B nana y= ndash0003 + 088x R2 = 030 E vaginatumy= ndash0005 + 117x R2 = 065) Values represent variable means and se error bars are not visible for many values dueto their small value
Seasonal carbon exchange under long-term warming Functional Plant Biology K
term temperature variability and long-term warming Our studypresents the first published values of the photosyntheticparameters Vcmax and J at multiple points during the season inthese species which can inform and refine parameterisation ofvegetationmodels atmany scalesAlsowe present thefirst reportof the seasonal flexibility of the degree of the inhibition of Rlight
under long-term warmed growth conditions in these or anyspecies which will enable more accurate calculations ofecosystem respiration Based on these measurements aseasonal estimate (not accounting for potential temperaturesensitivity as discussed above see Eqn S3) shows thatneglecting the inhibition of foliar Rlight in Arctic tundravegetation could lead to overestimations of foliar carbon lossof~4molmndash2 leaf (~176 g)over thegrowingseason or evenmore(~42molmndash2) when considering plants grown under warmedconditionsAlthough the temperature effect on respiratory carbonloss in the light may beminimal the overall discrepancy in fluxesbetween Rdark and Rlight must be considered for realisticdescriptions of Arctic carbon cycling Our study contributesimportant data that allow for an increased understanding ofdominant tundra speciesrsquo responses to environmental changeboth long- and short-term and will allow for more predictivepower in models estimating ecosystem carbon storage
Acknowledgements
We are grateful to the researchers and support staff at the Arctic LTER andToolik Field Station for their help in this study as well as Greg PendergastThis study was made possible through funding from the NSF 0732664OKA acknowledges grant support from the Australian Research Council(DP0986823)
References
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Armstrong AF Logan DC Tobin AK OrsquoToole P Atkin OK (2006b)Heterogeneity of plant mitochondrial responses underpinningrespiratory acclimation to the cold in Arabidopsis thaliana leavesPlant Cell amp Environment 29 940ndash949 doi101111j1365-3040200501475x
Armstrong AF Badger MR Day DA Barthet MM Smith P Millar AHWhelan J Atkin OK (2008) Dynamic changes in the mitochondrialelectron transport chain underpinning cold acclimation of leafrespiration Plant Cell amp Environment 31 1156ndash1169 doi101111j1365-3040200801830x
Atkin OK Tjoelker MG (2003) Thermal acclimation and the dynamicresponse of plant respiration to temperature Trends in Plant Science 8343ndash351 doi101016S1360-1385(03)00136-5
Atkin OK Bruhn D Tjoelker MG (2005) Response of plant respiration tochanges in temperature mechanisms and consequences of variations inQ10 values and acclimation In lsquoPlant respirationrsquo (Eds H Lambers ampM Ribas-Carbo) pp 95ndash135 (Springer Dordrecht The Netherlands)
Atkin OK Scheurwater I Pons TL (2006)High thermal acclimation potentialof both photosynthesis and respiration in two lowlandPlantago species incontrast to an alpine congeneric Global Change Biology 12 500ndash515doi101111j1365-2486200601114x
Ayub G Smith RA Tissue DT Atkin OK (2011) Impacts of drought onleaf respiration in darkness and light in Eucalyptus saligna exposed toindustrial-age atmosphericCO2 and growth temperatureNewPhytologist190 1003ndash1018 doi101111j1469-8137201103673x
Berry J Bjoumlrkman O (1980) Photosynthetic response and adaptation totemperature in higher plants Annual Review of Plant Physiology andPlant Molecular Biology 31 491ndash543 doi101146annurevpp31060180002423
Bret-HarteMSShaverGRZoerner JP JohnstoneJFWagner JLChavezASGunkelman RF IV Lippert SC Laundre JA (2001) Developmentalplasticity allows Betula nana to dominate tundra subjected to analtered environment Ecology 82 18ndash32
Bret-Harte MS Shaver GR Chapin FS III (2002) Primary and secondarystem growth in Arctic shrubs implications for community response toenvironmental change Journal of Ecology 90 251ndash267 doi101046j1365-2745200100657x
BuckleyTNAdamsMA(2011)An analyticalmodel of non-photorespiratoryCO2 release in the light and dark in leaves of C3 species based onstoichiometric flux balance Plant Cell amp Environment 34 89ndash112doi101111j1365-3040201002228x
Budde RJ Randall DD (1990) Pea leaf mitochondrial pyruvatedehydrogenase complex is inactivated in vivo in a light-dependentmanner Proceedings of the National Academy of Sciences of theUnited States of America 87 673ndash676 doi101073pnas872673
CahoonSMPSullivanPF ShaverGRWelker JM PostE (2012) Interactionsamong shrub cover and the soil microclimatemay determine futureArcticcarbon budgets Ecology Letters 15 1415ndash1422 doi101111j1461-0248201201865x
Chapin FS Shaver GR (1996) Physiological and growth responses of Arcticplants to a field experiment simulating climatic change Ecology 77822ndash840 doi1023072265504
Chapin FS Shaver GR Giblin AE Nadelhoffer KJ Laundre JA (1995)Responses of Arctic tundra to experimental and observed changes inclimate Ecology 76 694ndash711 doi1023071939337
Chapin FS Sturm M Serreze MC McFadden JP Key JR Lloyd AHMcGuire AD Rupp TS Lynch AH Schimel JP Beringer J ChapmanWLEpsteinHEEuskirchenESHinzmanLD JiaGPingC-LTapeKDThompson CDC Walker DA Welker JM (2005) Role of land-surfacechanges in Arctic summer warming Science 310 657ndash660 doi101126science1117368
Covey-Crump EM Attwood RG Atkin OK (2002) Regulation of rootrespiration in two species of Plantago that differ in relative growthrate the effect of short and long term changes in temperature PlantCell amp Environment 25 1501ndash1513 doi101046j1365-3040200200932x
Crous KY Zaragoza-Castells J Ellsworth DS Duursma RA LowM TissueDT Atkin OK (2012) Light inhibition of leaf respiration in field-grownEucalyptus saligna in whole-tree chambers under elevated atmosphericCO2 and summer drought Plant Cell amp Environment 35 966ndash981doi101111j1365-3040201102465x
Dungan RJ Whitehead D Duncan RP (2003) Seasonal and temperaturedependence of photosynthesis and respiration for two co-occurringbroad-leaved tree species with contrasting leaf phenology TreePhysiology 23 561ndash568 doi101093treephys238561
Elmendorf SC Henry GHR Hollister RD Bjoumlrk RG Bjorkman ADCallaghan TV Collier LS Cooper EJ Cornelissen JHC Day TAFosaa AM Gould WA Greacutetarsdoacutettir J Harte J Hermanutz L Hik DSHofgaard A Jarrad F Joacutensdoacutettir IS Keuper F Klanderud K Klein JAKoh S Kudo G Lang SI Loewen V May JL Mercado J Michelsen AMolau U Myers-Smith IH Oberbauer SF Pieper S Post E Rixen CRobinson CH Schmidt NM Shaver GR Stenstroumlm A Tolvanen ATotland Oslash Troxler T Wahren C-H Webber PJ Welker JM WookeyPA (2012) Global assessment of experimental climate warming ontundra vegetation heterogeneity over space and time Ecology Letters15 164ndash175 doi101111j1461-0248201101716x
EnvironmentalDataCenterTeam (2011)Meteorologicalmonitoring programat Toolik Alaska Toolik Field Station Institute of Arctic BiologyUniversity of Alaska Fairbanks Fairbanks AK 99775 Available athttptoolikalaskaeduedcabiotic_monitoringdata_queryphp
L Functional Plant Biology M A Heskel et al
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
in two dominant Arctic plants We hypothesised that the within-growing season rates of photosynthesis respiration and theinhibition of respiration will relate to growth-induced energydemand These rates will acclimate to long-term warming and be
responsive to short-term temperature fluctuations Our resultsshow stronger seasonal and thermal responses in respiration thanin photosynthesis (when both are measured at 20C) which wasfurther influenced by species differences and growth under long-
15
Betula Eriophorum
10
5
0
15
10
5
0
6
4
2
0
6
4
2
0
64
3
2
1
0
4
2
0
Asa
t (microm
ol C
O2
mndash2
sndash1
)R
dark
(microm
ol C
O2
mndash2
sndash1
)R
light
(microm
ol C
O2
mndash2
sndash1
)
160 170 180 190 200 210160 170 180 190 200
Julian day
(a)
(c)
(e)
(b)
(d)
(f )
Fig 2 (a b)Saturating photosynthesis (Asat measured at PAR=1500mmolmndash2 sndash1 and 400 parts per million (ppm)CO2) (c d) foliar dark respiration (Rdark measured at PAR=0mmolmndash2 sndash1 and 400 ppmCO2) and (e f) respiration inthe light (Rlight estimated via the Kok method at low PAR levels) through the growing season for leaves of (a c e)Betula nana (circles) and (bd f)Eriophorumvaginatum (triangles) grown in ambient (unfilled symbols) andpassivelywarmed (filled symbols) conditions All variables were measured at 20 C in laboratory conditions (n= 4)
Seasonal carbon exchange under long-term warming Functional Plant Biology G
termwarmingThis studyalsoprovided theopportunity toexpanduponexisting knowledge about biotic and environmental controlson foliar Rlight and how seasonal timing and temperaturevariability may mediate Rlight Rdark Together these findingsprovide a more detailed mechanistic understanding of thecontrol of carbon exchange in tundra ecosystems
Species-mediated influences of seasonal and environmentaleffects on carbon exchange
In our study the rates of photosynthesis and respiration ofB nana when measured at a common temperature tended tobe higher than those of E vaginatum during the Arctic tundragrowing season (Fig 2 Table 2) grown under control conditionsand long-term passive warming The higher rates of carbonexchange in B nana both when measured at 20C (Fig 2)and over the 5ndash35C temperature range (Fig 3) wereconsistent with previous studies that sampled plants grownunder warming at midseason (Chapin and Shaver 1996) andwith the addition of soil nitrogen and phosphorus (Heskel et al2012) The difference in Asat between B nana and E vaginatumwas greatest directly after leaf out (Fig 2) which emphasises theimpact of growth formon foliar physiological processesB nanaa deciduouswoody shrub is likely to have a relatively high energydemand immediately after snowmelt to accommodate efficientaboveground and belowground growth during the short growingseason and thismay be further enhanced under warming (Chapinet al 1995 Sullivan et al 2007) By contrast E vaginatum atiller-producing graminoid can retain leaves for multiple years(Fetcher and Shaver 1983) and for this reason may act lessopportunistically upon snowmelt in the spring in terms ofnutrient acquisition via fine root growth (Sullivan et al 2007)andquick leaf production These trends are supported byprevious
work on deciduous and evergreen species where seasonalphotosynthesis and respiration were consistently significantlylower in the evergreen (Ow et al 2010) The species contrastis underscored by the respective leaf architectures andcomposition interspecies comparisons show a less dense(greater SLA) leaf with higher N content in B nana than inE vaginatum (Table 1 Figs S2 S3)
We found a general though not significant decline in Asat inleaves sampled from long-term warming plots suggesting apotential duration-dependent warming effect at the leaf levelthat may contribute to a similar lack of photosyntheticstimulation reported at the ecosystem level (Shaver et al2000 Elmendorf et al 2012) Also these tundra species maybe pushed beyond their photosynthetic thermal optimum underelevated temperatures (Sage and Kubien 2007) which could bea likely scenario in Arctic populations that may be locallyadapted for colder growth temperatures We observed noapparent seasonal arch in the Asat Vcmax and J rates which cangenerally characterise growing season carbon assimilation ofleaves measured at ambient leaf temperatures as previouslyreported (for Asat) in these species (Starr et al 2008) At theleaf level the highest rates of Asat Vcmax and J can occur in theearly growing season (Dungan et al 2003 Xu and Baldocchi2003) This discrepancy may be explained by the highly variableambient temperature and precipitation conditions (Fig 1) thoughmodel evaluation found no strong relationship betweenphotosynthetic variables (measured at 20C) and short-termtemperature values (Table S1) Also as we measured allleaves at a common temperature (20 C) we controlled for theinfluence of ambient temperature on leaf temperature which canaffect photosynthetic rates and allowed for comparison acrosschanges in the ambient temperature to assess thermal acclimation
Respiration rates of both species were generally highest in thefirst few weeks after snowmelt (measured at 20C Fig 2endashf)which is more apparent in Rlight than in Rdark and are similar topreviously reported values (Heskel et al 2012 Heskel et al2013) In both B nana and E vaginatum higher energy demandand potentially respiratory rates in the early season relative to themid- and late season may be attributable to new leaf growth anddevelopment (Vose and Ryan 2002 Xu et al 2007 Ow et al2010) and possibly a higher density of mitochondria in youngerleaves (Armstrong et al 2006) This respiratory release can befurther enhanced by the colder ambient temperatures experiencedby plants in the early growing season due to a short-term coldacclimation (Atkin and Tjoelker 2003 Armstrong et al 2006)The thermal response of respiration is nonlinear and variesbetween species in response curve shape and slope (Fig 3Table 3) higher respiration in B nana across the range oftemperatures experienced during the growing season (Fig 1)and the lack of thermal acclimation may favour its expansion inthe tundra under current conditions (Heskel et al 2013) Furtherthe observed warm acclimation in leaves ofE vaginatum (Fig 3)suggests lowered metabolic rates that may limit growth despitereducing respiratory C loss under a future climate
It should be noted that similar rates of respiration rates acrossspecies may not necessary equate to similar energy efficiencyB nana is reported to exhibit greater respiratory efficiency thanE vaginatum through the differential use of the alternativeand cytochrome pathways potentially lending a competitive
05 10 15 20 25 30 35
10
20
30
40
50
60
70
Betula CT
Betula WG
Eriophorum CT
Eriophorum WG
Temperature (degC)
Rda
rk (
nmol
CO
2 gndash1
sndash1
)
Fig 3 The mean short-term thermal responses of foliar dark respirationfrom 5C to 35C modelled from a polynomial fit from raw data in Betulanana (circles) and Eriophorum vaginatum (triangles) sampled from control(CT open) andwarmed (WG closed) growth environments Temperature (T)response curves of respiration (R) were fit to second-order polynomialequations as follows B nana control r= 0027T2 + 0082T+ 660 B nanawarmed r= 0026T2 + 0103T+ 677 E vaginatum control r = 0045T2 ndash
0106T+ 310 and E vaginatum warmed r = 0018T2 + 0176T+ 2633 Forall thermal response curve fits R2 gt 0990
H Functional Plant Biology M A Heskel et al
advantage in growth and development (Kornfeld et al 2013)In addition Rlight Agross decreased over the growing season(Fig 6) driven by lower values of Rlight as the season progressedrather than any significant change in photosynthesis which is
discordant with the idea that the processes would increase in acoupled manner based on carbohydrate substrate supply anddemand Kornfeld et al (2013) found no difference in starchand sugar content between control and warmed leaves within
0
20
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80Betula
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(a)
(c)
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(f )
Vcm
ax (micro
mol
mndash2
sndash1
)J
Vcm
ax
J (micro
mol
mndash2
sndash1
)
Fig 4 (a b) Maximal carboxylation rate (Vcmax) and (c d) electron transport rate (J) both measured in laboratoryconditions and corrected to a constant 25C and (e f) the ratio of these variables (J Vcmax) over the growing season inleaves of (a c e) Betula nana and (b d f) Eriophorum vaginatum grown under ambient (open) and warmed (filled)conditions Values shown are means with se
Seasonal carbon exchange under long-term warming Functional Plant Biology I
species when measured at a single sampling point during thegrowing season though leaves of B nana contained a greaterconcentration of both sugar and starch than E vaginatum Wewere unable to quantify foliar carbohydrate values in thesespecies through the season though future studies couldinclude this informative measurement to further relate themechanistic links between these processes
Both species exhibited the lowest values of Rlight Rdark
towards the end of the season in late July (Fig 5) suggestinga potential developmental or energy demand-related control onthe inhibition of respiration by light that allows for greater
respiratory energy production in the early season during leafexpansionTheonlyother knownstudy that explored the seasonalresponse of Rlight Rdark observed the lowest level of inhibitionearlier in the growing season before the warmest month (Crouset al 2012) This response similar to our study despite a starklydifferent ecological system supports the idea of shared seasonaltiming-mediated biochemical controls on the light inhibition ofrespiration across species The degree of inhibition is known toshow a relaxed response under environmental conditions thatcan stimulate growth due to increased demand for energy and Cskeletons as exhibited under elevated CO2 (Wang et al 2001Shapiro et al 2004) and increased soil nutrient availability(Heskel et al 2012) though this is not always the case (Tissueet al 2002 Ayub et al 2011) Long-termwarming alsomediatedRlight Rdark with lower values observed in warm-grownleaves though measurement temperature can also impact theserelationships (Atkin et al 2006)
Mechanistic explanations for the relationships amonggrowth demand cellular energy status and the degree of lightinhibition of respiration have been previously reported Theenvironmental challenges of a short growing season and ~24-hdiel light exposure in the Arctic tundra control growth ratesthat may then influence values of Rlight Rdark (Bret-Harteet al 2001 Bret-Harte et al 2002) In the light adenylatesupply from photosynthesis may decrease the energy demandfrom mitochondrial respiration Additionally photorespirationis associated with the inactivation of pyruvate dehydrogenasea precursor to the tri-carboxylic acid (TCA) cycle (Budde andRandall 1990) though previous estimations of photorespirationin these species didnotfinda strong correlationwith the inhibitionof Rlight (Heskel et al 2013) The TCA cycle can also besignificantly altered in the light to support N assimilationwhich effectively reduces CO2 release (Tcherkez et al 2005Tcherkez et al 2008 Tcherkez et al 2009) This effect may beenhanced in tundra plantswhen soil N is less limiting under theseconditions growth rates increase (Bret-Harte et al 2002) Theinhibition ofRlight is observed to be slightly increased in fertilisedsoils (Heskel et al 2013) Additionally the oxidative pentosephosphate pathway (OPPP) which contributes the necessaryreductant for the synthesis of multiple metabolites in darknessmay be relaxed in the light when reductants are provided byphotosynthesis diminishing CO2 release by the OPPP Thoughthe OPPP comprises a proportionally smaller CO2 flux comparedwith that from the TCA cycle in the light this decrease maycontribute to lower overall CO2 release causing Rlight ltRdark
(Buckley and Adams 2011) In our current study the apparentrelaxation of the light inhibition of respiration in leaves of bothstudy species (Fig 5) resulting in the greatest proportional CO2
losses (Fig 6) suggesting that high growth demand for ATP andcarbon skeletons outweigh potential carbon losses in the earlyseason
Influences of variable ambient temperature on gas exchange
In addition to quantifying the effects of long-termwarming on thefoliar gas exchange during the growing season this studyevaluated the influence of intraseasonal short-term temperaturevariability on these physiological processes Arctic tundragrowing seasons can exhibit dramatic shifts in temperature and
Table 3 Parameters describing the respiratory response tomeasurement temperature in leaves of Betula nana and Eriophorumvaginatum grown under control (CT) and long-term warming (WG)conditions including Q10 (the temperature sensitivity of respiration(estimated over 5ndash358C)) R10 (the basal respiration rate at 108C(mmol CO2 gndash1 sndash1)) and Eo (kJmolndash1) a variable related to the energyof activation calculated from values of respiration at 208C and R10
Values presented are means and se Statistical differences between speciesand treatments are denoted alphabetically
Fig 5 Means and se of the ratio of respiration in the light (Rlight) estimatedvia the Kok method to dark respiration (Rdark) both variables measured at20C and 400 parts per million CO2 through the growing season in Betulanana (circles) and Eriophorum vaginatum (triangles) under ambient controlfield conditions (CT open symbols) and under a long-term passive warmingtreatment (WGfilled symbols n= 4)A linear regression (y= ndash0009x+ 219R2 = 047) depicts the declining Rlight Rdark values with the progressinggrowing season
J Functional Plant Biology M A Heskel et al
precipitation within days as was observed in this study duringsummer 2010 in Toolik Lake Alaska (Fig 1) and this variabilityis likely to influence the temperature-sensitive processescontrolling leaf carbon cycling Respiration in both speciescan be highly temperature-sensitive especially at warmertemperatures which may become more common in a futureArctic (Fig 3 Table 3) When modelled against thetemperature of the measurement day the average temperatureof the prior week accounted for species and treatment effectsand some patterns emerged that help to characterise the nature ofthe temperature sensitivity of foliar gas exchange (Table S1)However photosynthesis which is known to respond to short-term changes in temperature in field-grown plants (Berry andBjoumlrkman 1980 Poyatos et al 2012) did not show strongrelationships to daily or weekly variation in temperatureinstead species effects explained most variation (Table S1)The underlying photosynthetic machinery maintains consistentrates of carbon assimilation (measured at a constant temperature)throughout the growing season though daily fluctuations intemperature and precipitation experiences in the Arctic mayinfluence in vivo rates Plant species from alpine and Arcticecosystems can exhibit local adaptation to the extremevariability experienced in those locations (Korner and Larcher1988) which may translate to a limited acclimation (Atkin et al2006) Further because the ambient temperature conditionsfrom this region fluctuate so unpredictably in short periodsthrough the season any analysis using temperature averages ofprevious time windows might not be as useful for estimatingthermal acclimation as they are in regions with more predictableseasonal temperature patterns (Ow et al 2010 Searle et al 2011)
In contrast respiratory rates in the light and dark whenmeasured at a constant temperature appeared to be morecorrelated with short-term ambient temperature variation TheAICc analysis shows that the strongest model for Rdark includedspecies and warming effects and the prior weekrsquos average
temperature in contrast the strongest model for Rlight includedthe former parameters and the minimum temperature from themeasurement day (Table S1) The model relationships maysuggest a short-term thermal sensitivity of respiration that maybe more responsive to temperature minimums though this isnot clearly reflected in regressions of Rlight with the ambienttemperature values (Fig S5) This sensitivity of respiration totemperature minimums has been observed previously inroots as well and suggests that thermal acclimation in coldgrowth temperatures may regulate metabolic activity to meetthe demands of growth and maintenance at the expense ofgreater carbon loss (Covey-Crump et al 2002) Low ambienttemperatures did not clearly relate to the highest rates ofrespiration when measured at a common temperature aswould be expected under respiratory cold acclimation (Atkinand Tjoelker 2003) However it is possible that developmentaltiming may also play into the leavesrsquo respiratory rates and abilityto acclimate especially in B nana in the early season when itsleaves are still expanding (Armstrong et al 2006) Through arelatively simple modelling exercise we found some evidenceof differential temperature controls on photosynthesis andrespiration in B nana and E vaginatum we suggest thatfuture work should include more detailed environmental andbiotic factors including soil temperature daily photosyntheticradiation and leaf carbohydrate data to more accurately informthe seasonal temperature sensitivity of foliar carbon exchange
Conclusions and implications
The physiological measurements collected in this study allowedfor the quantification of mechanistic responses in the nightlessand highly variable growing season of the arctic tundra andprovided new insights into foliar carbon regulation We presentnew information on the seasonal trends of foliar carbon cyclingin two dominant tundra species and relate these fluxes to short-
0
015
045
030
0
015
045
030
Julian day
Betula Eriophorum
160 170 180 190 200 210 160 170 180 190 200 210
Rlig
ht
Agr
oss
Fig 6 The proportional carbon loss through respiration in the light to gross photosynthesis in (a) Betula nana and(b) Eriophorum vaginatum over the growing season (Rlight Agross both variables measured at a common temperatureof 20C and 400 parts per million CO2) Linear regressions across growth conditions depict the relationship ofRlight Agross and measurement date during the growing season (B nana y= ndash0003 + 088x R2 = 030 E vaginatumy= ndash0005 + 117x R2 = 065) Values represent variable means and se error bars are not visible for many values dueto their small value
Seasonal carbon exchange under long-term warming Functional Plant Biology K
term temperature variability and long-term warming Our studypresents the first published values of the photosyntheticparameters Vcmax and J at multiple points during the season inthese species which can inform and refine parameterisation ofvegetationmodels atmany scalesAlsowe present thefirst reportof the seasonal flexibility of the degree of the inhibition of Rlight
under long-term warmed growth conditions in these or anyspecies which will enable more accurate calculations ofecosystem respiration Based on these measurements aseasonal estimate (not accounting for potential temperaturesensitivity as discussed above see Eqn S3) shows thatneglecting the inhibition of foliar Rlight in Arctic tundravegetation could lead to overestimations of foliar carbon lossof~4molmndash2 leaf (~176 g)over thegrowingseason or evenmore(~42molmndash2) when considering plants grown under warmedconditionsAlthough the temperature effect on respiratory carbonloss in the light may beminimal the overall discrepancy in fluxesbetween Rdark and Rlight must be considered for realisticdescriptions of Arctic carbon cycling Our study contributesimportant data that allow for an increased understanding ofdominant tundra speciesrsquo responses to environmental changeboth long- and short-term and will allow for more predictivepower in models estimating ecosystem carbon storage
Acknowledgements
We are grateful to the researchers and support staff at the Arctic LTER andToolik Field Station for their help in this study as well as Greg PendergastThis study was made possible through funding from the NSF 0732664OKA acknowledges grant support from the Australian Research Council(DP0986823)
References
Armstrong AF Logan DC Atkin OK (2006a) On the developmentaldependence of leaf respiration responses to short and long-termchanges in growth temperature American Journal of Botany 931633ndash1639 doi103732ajb93111633
Armstrong AF Logan DC Tobin AK OrsquoToole P Atkin OK (2006b)Heterogeneity of plant mitochondrial responses underpinningrespiratory acclimation to the cold in Arabidopsis thaliana leavesPlant Cell amp Environment 29 940ndash949 doi101111j1365-3040200501475x
Armstrong AF Badger MR Day DA Barthet MM Smith P Millar AHWhelan J Atkin OK (2008) Dynamic changes in the mitochondrialelectron transport chain underpinning cold acclimation of leafrespiration Plant Cell amp Environment 31 1156ndash1169 doi101111j1365-3040200801830x
Atkin OK Tjoelker MG (2003) Thermal acclimation and the dynamicresponse of plant respiration to temperature Trends in Plant Science 8343ndash351 doi101016S1360-1385(03)00136-5
Atkin OK Bruhn D Tjoelker MG (2005) Response of plant respiration tochanges in temperature mechanisms and consequences of variations inQ10 values and acclimation In lsquoPlant respirationrsquo (Eds H Lambers ampM Ribas-Carbo) pp 95ndash135 (Springer Dordrecht The Netherlands)
Atkin OK Scheurwater I Pons TL (2006)High thermal acclimation potentialof both photosynthesis and respiration in two lowlandPlantago species incontrast to an alpine congeneric Global Change Biology 12 500ndash515doi101111j1365-2486200601114x
Ayub G Smith RA Tissue DT Atkin OK (2011) Impacts of drought onleaf respiration in darkness and light in Eucalyptus saligna exposed toindustrial-age atmosphericCO2 and growth temperatureNewPhytologist190 1003ndash1018 doi101111j1469-8137201103673x
Berry J Bjoumlrkman O (1980) Photosynthetic response and adaptation totemperature in higher plants Annual Review of Plant Physiology andPlant Molecular Biology 31 491ndash543 doi101146annurevpp31060180002423
Bret-HarteMSShaverGRZoerner JP JohnstoneJFWagner JLChavezASGunkelman RF IV Lippert SC Laundre JA (2001) Developmentalplasticity allows Betula nana to dominate tundra subjected to analtered environment Ecology 82 18ndash32
Bret-Harte MS Shaver GR Chapin FS III (2002) Primary and secondarystem growth in Arctic shrubs implications for community response toenvironmental change Journal of Ecology 90 251ndash267 doi101046j1365-2745200100657x
BuckleyTNAdamsMA(2011)An analyticalmodel of non-photorespiratoryCO2 release in the light and dark in leaves of C3 species based onstoichiometric flux balance Plant Cell amp Environment 34 89ndash112doi101111j1365-3040201002228x
Budde RJ Randall DD (1990) Pea leaf mitochondrial pyruvatedehydrogenase complex is inactivated in vivo in a light-dependentmanner Proceedings of the National Academy of Sciences of theUnited States of America 87 673ndash676 doi101073pnas872673
CahoonSMPSullivanPF ShaverGRWelker JM PostE (2012) Interactionsamong shrub cover and the soil microclimatemay determine futureArcticcarbon budgets Ecology Letters 15 1415ndash1422 doi101111j1461-0248201201865x
Chapin FS Shaver GR (1996) Physiological and growth responses of Arcticplants to a field experiment simulating climatic change Ecology 77822ndash840 doi1023072265504
Chapin FS Shaver GR Giblin AE Nadelhoffer KJ Laundre JA (1995)Responses of Arctic tundra to experimental and observed changes inclimate Ecology 76 694ndash711 doi1023071939337
Chapin FS Sturm M Serreze MC McFadden JP Key JR Lloyd AHMcGuire AD Rupp TS Lynch AH Schimel JP Beringer J ChapmanWLEpsteinHEEuskirchenESHinzmanLD JiaGPingC-LTapeKDThompson CDC Walker DA Welker JM (2005) Role of land-surfacechanges in Arctic summer warming Science 310 657ndash660 doi101126science1117368
Covey-Crump EM Attwood RG Atkin OK (2002) Regulation of rootrespiration in two species of Plantago that differ in relative growthrate the effect of short and long term changes in temperature PlantCell amp Environment 25 1501ndash1513 doi101046j1365-3040200200932x
Crous KY Zaragoza-Castells J Ellsworth DS Duursma RA LowM TissueDT Atkin OK (2012) Light inhibition of leaf respiration in field-grownEucalyptus saligna in whole-tree chambers under elevated atmosphericCO2 and summer drought Plant Cell amp Environment 35 966ndash981doi101111j1365-3040201102465x
Dungan RJ Whitehead D Duncan RP (2003) Seasonal and temperaturedependence of photosynthesis and respiration for two co-occurringbroad-leaved tree species with contrasting leaf phenology TreePhysiology 23 561ndash568 doi101093treephys238561
Elmendorf SC Henry GHR Hollister RD Bjoumlrk RG Bjorkman ADCallaghan TV Collier LS Cooper EJ Cornelissen JHC Day TAFosaa AM Gould WA Greacutetarsdoacutettir J Harte J Hermanutz L Hik DSHofgaard A Jarrad F Joacutensdoacutettir IS Keuper F Klanderud K Klein JAKoh S Kudo G Lang SI Loewen V May JL Mercado J Michelsen AMolau U Myers-Smith IH Oberbauer SF Pieper S Post E Rixen CRobinson CH Schmidt NM Shaver GR Stenstroumlm A Tolvanen ATotland Oslash Troxler T Wahren C-H Webber PJ Welker JM WookeyPA (2012) Global assessment of experimental climate warming ontundra vegetation heterogeneity over space and time Ecology Letters15 164ndash175 doi101111j1461-0248201101716x
EnvironmentalDataCenterTeam (2011)Meteorologicalmonitoring programat Toolik Alaska Toolik Field Station Institute of Arctic BiologyUniversity of Alaska Fairbanks Fairbanks AK 99775 Available athttptoolikalaskaeduedcabiotic_monitoringdata_queryphp
L Functional Plant Biology M A Heskel et al
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
termwarmingThis studyalsoprovided theopportunity toexpanduponexisting knowledge about biotic and environmental controlson foliar Rlight and how seasonal timing and temperaturevariability may mediate Rlight Rdark Together these findingsprovide a more detailed mechanistic understanding of thecontrol of carbon exchange in tundra ecosystems
Species-mediated influences of seasonal and environmentaleffects on carbon exchange
In our study the rates of photosynthesis and respiration ofB nana when measured at a common temperature tended tobe higher than those of E vaginatum during the Arctic tundragrowing season (Fig 2 Table 2) grown under control conditionsand long-term passive warming The higher rates of carbonexchange in B nana both when measured at 20C (Fig 2)and over the 5ndash35C temperature range (Fig 3) wereconsistent with previous studies that sampled plants grownunder warming at midseason (Chapin and Shaver 1996) andwith the addition of soil nitrogen and phosphorus (Heskel et al2012) The difference in Asat between B nana and E vaginatumwas greatest directly after leaf out (Fig 2) which emphasises theimpact of growth formon foliar physiological processesB nanaa deciduouswoody shrub is likely to have a relatively high energydemand immediately after snowmelt to accommodate efficientaboveground and belowground growth during the short growingseason and thismay be further enhanced under warming (Chapinet al 1995 Sullivan et al 2007) By contrast E vaginatum atiller-producing graminoid can retain leaves for multiple years(Fetcher and Shaver 1983) and for this reason may act lessopportunistically upon snowmelt in the spring in terms ofnutrient acquisition via fine root growth (Sullivan et al 2007)andquick leaf production These trends are supported byprevious
work on deciduous and evergreen species where seasonalphotosynthesis and respiration were consistently significantlylower in the evergreen (Ow et al 2010) The species contrastis underscored by the respective leaf architectures andcomposition interspecies comparisons show a less dense(greater SLA) leaf with higher N content in B nana than inE vaginatum (Table 1 Figs S2 S3)
We found a general though not significant decline in Asat inleaves sampled from long-term warming plots suggesting apotential duration-dependent warming effect at the leaf levelthat may contribute to a similar lack of photosyntheticstimulation reported at the ecosystem level (Shaver et al2000 Elmendorf et al 2012) Also these tundra species maybe pushed beyond their photosynthetic thermal optimum underelevated temperatures (Sage and Kubien 2007) which could bea likely scenario in Arctic populations that may be locallyadapted for colder growth temperatures We observed noapparent seasonal arch in the Asat Vcmax and J rates which cangenerally characterise growing season carbon assimilation ofleaves measured at ambient leaf temperatures as previouslyreported (for Asat) in these species (Starr et al 2008) At theleaf level the highest rates of Asat Vcmax and J can occur in theearly growing season (Dungan et al 2003 Xu and Baldocchi2003) This discrepancy may be explained by the highly variableambient temperature and precipitation conditions (Fig 1) thoughmodel evaluation found no strong relationship betweenphotosynthetic variables (measured at 20C) and short-termtemperature values (Table S1) Also as we measured allleaves at a common temperature (20 C) we controlled for theinfluence of ambient temperature on leaf temperature which canaffect photosynthetic rates and allowed for comparison acrosschanges in the ambient temperature to assess thermal acclimation
Respiration rates of both species were generally highest in thefirst few weeks after snowmelt (measured at 20C Fig 2endashf)which is more apparent in Rlight than in Rdark and are similar topreviously reported values (Heskel et al 2012 Heskel et al2013) In both B nana and E vaginatum higher energy demandand potentially respiratory rates in the early season relative to themid- and late season may be attributable to new leaf growth anddevelopment (Vose and Ryan 2002 Xu et al 2007 Ow et al2010) and possibly a higher density of mitochondria in youngerleaves (Armstrong et al 2006) This respiratory release can befurther enhanced by the colder ambient temperatures experiencedby plants in the early growing season due to a short-term coldacclimation (Atkin and Tjoelker 2003 Armstrong et al 2006)The thermal response of respiration is nonlinear and variesbetween species in response curve shape and slope (Fig 3Table 3) higher respiration in B nana across the range oftemperatures experienced during the growing season (Fig 1)and the lack of thermal acclimation may favour its expansion inthe tundra under current conditions (Heskel et al 2013) Furtherthe observed warm acclimation in leaves ofE vaginatum (Fig 3)suggests lowered metabolic rates that may limit growth despitereducing respiratory C loss under a future climate
It should be noted that similar rates of respiration rates acrossspecies may not necessary equate to similar energy efficiencyB nana is reported to exhibit greater respiratory efficiency thanE vaginatum through the differential use of the alternativeand cytochrome pathways potentially lending a competitive
05 10 15 20 25 30 35
10
20
30
40
50
60
70
Betula CT
Betula WG
Eriophorum CT
Eriophorum WG
Temperature (degC)
Rda
rk (
nmol
CO
2 gndash1
sndash1
)
Fig 3 The mean short-term thermal responses of foliar dark respirationfrom 5C to 35C modelled from a polynomial fit from raw data in Betulanana (circles) and Eriophorum vaginatum (triangles) sampled from control(CT open) andwarmed (WG closed) growth environments Temperature (T)response curves of respiration (R) were fit to second-order polynomialequations as follows B nana control r= 0027T2 + 0082T+ 660 B nanawarmed r= 0026T2 + 0103T+ 677 E vaginatum control r = 0045T2 ndash
0106T+ 310 and E vaginatum warmed r = 0018T2 + 0176T+ 2633 Forall thermal response curve fits R2 gt 0990
H Functional Plant Biology M A Heskel et al
advantage in growth and development (Kornfeld et al 2013)In addition Rlight Agross decreased over the growing season(Fig 6) driven by lower values of Rlight as the season progressedrather than any significant change in photosynthesis which is
discordant with the idea that the processes would increase in acoupled manner based on carbohydrate substrate supply anddemand Kornfeld et al (2013) found no difference in starchand sugar content between control and warmed leaves within
0
20
40
60
80Betula
0
20
40
60Eriophorum
0
20
40
60
80
0
40
80
120
0
1
2
3
0
1
2
3
4
Julian day
160 170 180 190 200 210160 170 180 190 200
(a)
(c)
(e)
(b)
(d)
(f )
Vcm
ax (micro
mol
mndash2
sndash1
)J
Vcm
ax
J (micro
mol
mndash2
sndash1
)
Fig 4 (a b) Maximal carboxylation rate (Vcmax) and (c d) electron transport rate (J) both measured in laboratoryconditions and corrected to a constant 25C and (e f) the ratio of these variables (J Vcmax) over the growing season inleaves of (a c e) Betula nana and (b d f) Eriophorum vaginatum grown under ambient (open) and warmed (filled)conditions Values shown are means with se
Seasonal carbon exchange under long-term warming Functional Plant Biology I
species when measured at a single sampling point during thegrowing season though leaves of B nana contained a greaterconcentration of both sugar and starch than E vaginatum Wewere unable to quantify foliar carbohydrate values in thesespecies through the season though future studies couldinclude this informative measurement to further relate themechanistic links between these processes
Both species exhibited the lowest values of Rlight Rdark
towards the end of the season in late July (Fig 5) suggestinga potential developmental or energy demand-related control onthe inhibition of respiration by light that allows for greater
respiratory energy production in the early season during leafexpansionTheonlyother knownstudy that explored the seasonalresponse of Rlight Rdark observed the lowest level of inhibitionearlier in the growing season before the warmest month (Crouset al 2012) This response similar to our study despite a starklydifferent ecological system supports the idea of shared seasonaltiming-mediated biochemical controls on the light inhibition ofrespiration across species The degree of inhibition is known toshow a relaxed response under environmental conditions thatcan stimulate growth due to increased demand for energy and Cskeletons as exhibited under elevated CO2 (Wang et al 2001Shapiro et al 2004) and increased soil nutrient availability(Heskel et al 2012) though this is not always the case (Tissueet al 2002 Ayub et al 2011) Long-termwarming alsomediatedRlight Rdark with lower values observed in warm-grownleaves though measurement temperature can also impact theserelationships (Atkin et al 2006)
Mechanistic explanations for the relationships amonggrowth demand cellular energy status and the degree of lightinhibition of respiration have been previously reported Theenvironmental challenges of a short growing season and ~24-hdiel light exposure in the Arctic tundra control growth ratesthat may then influence values of Rlight Rdark (Bret-Harteet al 2001 Bret-Harte et al 2002) In the light adenylatesupply from photosynthesis may decrease the energy demandfrom mitochondrial respiration Additionally photorespirationis associated with the inactivation of pyruvate dehydrogenasea precursor to the tri-carboxylic acid (TCA) cycle (Budde andRandall 1990) though previous estimations of photorespirationin these species didnotfinda strong correlationwith the inhibitionof Rlight (Heskel et al 2013) The TCA cycle can also besignificantly altered in the light to support N assimilationwhich effectively reduces CO2 release (Tcherkez et al 2005Tcherkez et al 2008 Tcherkez et al 2009) This effect may beenhanced in tundra plantswhen soil N is less limiting under theseconditions growth rates increase (Bret-Harte et al 2002) Theinhibition ofRlight is observed to be slightly increased in fertilisedsoils (Heskel et al 2013) Additionally the oxidative pentosephosphate pathway (OPPP) which contributes the necessaryreductant for the synthesis of multiple metabolites in darknessmay be relaxed in the light when reductants are provided byphotosynthesis diminishing CO2 release by the OPPP Thoughthe OPPP comprises a proportionally smaller CO2 flux comparedwith that from the TCA cycle in the light this decrease maycontribute to lower overall CO2 release causing Rlight ltRdark
(Buckley and Adams 2011) In our current study the apparentrelaxation of the light inhibition of respiration in leaves of bothstudy species (Fig 5) resulting in the greatest proportional CO2
losses (Fig 6) suggesting that high growth demand for ATP andcarbon skeletons outweigh potential carbon losses in the earlyseason
Influences of variable ambient temperature on gas exchange
In addition to quantifying the effects of long-termwarming on thefoliar gas exchange during the growing season this studyevaluated the influence of intraseasonal short-term temperaturevariability on these physiological processes Arctic tundragrowing seasons can exhibit dramatic shifts in temperature and
Table 3 Parameters describing the respiratory response tomeasurement temperature in leaves of Betula nana and Eriophorumvaginatum grown under control (CT) and long-term warming (WG)conditions including Q10 (the temperature sensitivity of respiration(estimated over 5ndash358C)) R10 (the basal respiration rate at 108C(mmol CO2 gndash1 sndash1)) and Eo (kJmolndash1) a variable related to the energyof activation calculated from values of respiration at 208C and R10
Values presented are means and se Statistical differences between speciesand treatments are denoted alphabetically
Fig 5 Means and se of the ratio of respiration in the light (Rlight) estimatedvia the Kok method to dark respiration (Rdark) both variables measured at20C and 400 parts per million CO2 through the growing season in Betulanana (circles) and Eriophorum vaginatum (triangles) under ambient controlfield conditions (CT open symbols) and under a long-term passive warmingtreatment (WGfilled symbols n= 4)A linear regression (y= ndash0009x+ 219R2 = 047) depicts the declining Rlight Rdark values with the progressinggrowing season
J Functional Plant Biology M A Heskel et al
precipitation within days as was observed in this study duringsummer 2010 in Toolik Lake Alaska (Fig 1) and this variabilityis likely to influence the temperature-sensitive processescontrolling leaf carbon cycling Respiration in both speciescan be highly temperature-sensitive especially at warmertemperatures which may become more common in a futureArctic (Fig 3 Table 3) When modelled against thetemperature of the measurement day the average temperatureof the prior week accounted for species and treatment effectsand some patterns emerged that help to characterise the nature ofthe temperature sensitivity of foliar gas exchange (Table S1)However photosynthesis which is known to respond to short-term changes in temperature in field-grown plants (Berry andBjoumlrkman 1980 Poyatos et al 2012) did not show strongrelationships to daily or weekly variation in temperatureinstead species effects explained most variation (Table S1)The underlying photosynthetic machinery maintains consistentrates of carbon assimilation (measured at a constant temperature)throughout the growing season though daily fluctuations intemperature and precipitation experiences in the Arctic mayinfluence in vivo rates Plant species from alpine and Arcticecosystems can exhibit local adaptation to the extremevariability experienced in those locations (Korner and Larcher1988) which may translate to a limited acclimation (Atkin et al2006) Further because the ambient temperature conditionsfrom this region fluctuate so unpredictably in short periodsthrough the season any analysis using temperature averages ofprevious time windows might not be as useful for estimatingthermal acclimation as they are in regions with more predictableseasonal temperature patterns (Ow et al 2010 Searle et al 2011)
In contrast respiratory rates in the light and dark whenmeasured at a constant temperature appeared to be morecorrelated with short-term ambient temperature variation TheAICc analysis shows that the strongest model for Rdark includedspecies and warming effects and the prior weekrsquos average
temperature in contrast the strongest model for Rlight includedthe former parameters and the minimum temperature from themeasurement day (Table S1) The model relationships maysuggest a short-term thermal sensitivity of respiration that maybe more responsive to temperature minimums though this isnot clearly reflected in regressions of Rlight with the ambienttemperature values (Fig S5) This sensitivity of respiration totemperature minimums has been observed previously inroots as well and suggests that thermal acclimation in coldgrowth temperatures may regulate metabolic activity to meetthe demands of growth and maintenance at the expense ofgreater carbon loss (Covey-Crump et al 2002) Low ambienttemperatures did not clearly relate to the highest rates ofrespiration when measured at a common temperature aswould be expected under respiratory cold acclimation (Atkinand Tjoelker 2003) However it is possible that developmentaltiming may also play into the leavesrsquo respiratory rates and abilityto acclimate especially in B nana in the early season when itsleaves are still expanding (Armstrong et al 2006) Through arelatively simple modelling exercise we found some evidenceof differential temperature controls on photosynthesis andrespiration in B nana and E vaginatum we suggest thatfuture work should include more detailed environmental andbiotic factors including soil temperature daily photosyntheticradiation and leaf carbohydrate data to more accurately informthe seasonal temperature sensitivity of foliar carbon exchange
Conclusions and implications
The physiological measurements collected in this study allowedfor the quantification of mechanistic responses in the nightlessand highly variable growing season of the arctic tundra andprovided new insights into foliar carbon regulation We presentnew information on the seasonal trends of foliar carbon cyclingin two dominant tundra species and relate these fluxes to short-
0
015
045
030
0
015
045
030
Julian day
Betula Eriophorum
160 170 180 190 200 210 160 170 180 190 200 210
Rlig
ht
Agr
oss
Fig 6 The proportional carbon loss through respiration in the light to gross photosynthesis in (a) Betula nana and(b) Eriophorum vaginatum over the growing season (Rlight Agross both variables measured at a common temperatureof 20C and 400 parts per million CO2) Linear regressions across growth conditions depict the relationship ofRlight Agross and measurement date during the growing season (B nana y= ndash0003 + 088x R2 = 030 E vaginatumy= ndash0005 + 117x R2 = 065) Values represent variable means and se error bars are not visible for many values dueto their small value
Seasonal carbon exchange under long-term warming Functional Plant Biology K
term temperature variability and long-term warming Our studypresents the first published values of the photosyntheticparameters Vcmax and J at multiple points during the season inthese species which can inform and refine parameterisation ofvegetationmodels atmany scalesAlsowe present thefirst reportof the seasonal flexibility of the degree of the inhibition of Rlight
under long-term warmed growth conditions in these or anyspecies which will enable more accurate calculations ofecosystem respiration Based on these measurements aseasonal estimate (not accounting for potential temperaturesensitivity as discussed above see Eqn S3) shows thatneglecting the inhibition of foliar Rlight in Arctic tundravegetation could lead to overestimations of foliar carbon lossof~4molmndash2 leaf (~176 g)over thegrowingseason or evenmore(~42molmndash2) when considering plants grown under warmedconditionsAlthough the temperature effect on respiratory carbonloss in the light may beminimal the overall discrepancy in fluxesbetween Rdark and Rlight must be considered for realisticdescriptions of Arctic carbon cycling Our study contributesimportant data that allow for an increased understanding ofdominant tundra speciesrsquo responses to environmental changeboth long- and short-term and will allow for more predictivepower in models estimating ecosystem carbon storage
Acknowledgements
We are grateful to the researchers and support staff at the Arctic LTER andToolik Field Station for their help in this study as well as Greg PendergastThis study was made possible through funding from the NSF 0732664OKA acknowledges grant support from the Australian Research Council(DP0986823)
References
Armstrong AF Logan DC Atkin OK (2006a) On the developmentaldependence of leaf respiration responses to short and long-termchanges in growth temperature American Journal of Botany 931633ndash1639 doi103732ajb93111633
Armstrong AF Logan DC Tobin AK OrsquoToole P Atkin OK (2006b)Heterogeneity of plant mitochondrial responses underpinningrespiratory acclimation to the cold in Arabidopsis thaliana leavesPlant Cell amp Environment 29 940ndash949 doi101111j1365-3040200501475x
Armstrong AF Badger MR Day DA Barthet MM Smith P Millar AHWhelan J Atkin OK (2008) Dynamic changes in the mitochondrialelectron transport chain underpinning cold acclimation of leafrespiration Plant Cell amp Environment 31 1156ndash1169 doi101111j1365-3040200801830x
Atkin OK Tjoelker MG (2003) Thermal acclimation and the dynamicresponse of plant respiration to temperature Trends in Plant Science 8343ndash351 doi101016S1360-1385(03)00136-5
Atkin OK Bruhn D Tjoelker MG (2005) Response of plant respiration tochanges in temperature mechanisms and consequences of variations inQ10 values and acclimation In lsquoPlant respirationrsquo (Eds H Lambers ampM Ribas-Carbo) pp 95ndash135 (Springer Dordrecht The Netherlands)
Atkin OK Scheurwater I Pons TL (2006)High thermal acclimation potentialof both photosynthesis and respiration in two lowlandPlantago species incontrast to an alpine congeneric Global Change Biology 12 500ndash515doi101111j1365-2486200601114x
Ayub G Smith RA Tissue DT Atkin OK (2011) Impacts of drought onleaf respiration in darkness and light in Eucalyptus saligna exposed toindustrial-age atmosphericCO2 and growth temperatureNewPhytologist190 1003ndash1018 doi101111j1469-8137201103673x
Berry J Bjoumlrkman O (1980) Photosynthetic response and adaptation totemperature in higher plants Annual Review of Plant Physiology andPlant Molecular Biology 31 491ndash543 doi101146annurevpp31060180002423
Bret-HarteMSShaverGRZoerner JP JohnstoneJFWagner JLChavezASGunkelman RF IV Lippert SC Laundre JA (2001) Developmentalplasticity allows Betula nana to dominate tundra subjected to analtered environment Ecology 82 18ndash32
Bret-Harte MS Shaver GR Chapin FS III (2002) Primary and secondarystem growth in Arctic shrubs implications for community response toenvironmental change Journal of Ecology 90 251ndash267 doi101046j1365-2745200100657x
BuckleyTNAdamsMA(2011)An analyticalmodel of non-photorespiratoryCO2 release in the light and dark in leaves of C3 species based onstoichiometric flux balance Plant Cell amp Environment 34 89ndash112doi101111j1365-3040201002228x
Budde RJ Randall DD (1990) Pea leaf mitochondrial pyruvatedehydrogenase complex is inactivated in vivo in a light-dependentmanner Proceedings of the National Academy of Sciences of theUnited States of America 87 673ndash676 doi101073pnas872673
CahoonSMPSullivanPF ShaverGRWelker JM PostE (2012) Interactionsamong shrub cover and the soil microclimatemay determine futureArcticcarbon budgets Ecology Letters 15 1415ndash1422 doi101111j1461-0248201201865x
Chapin FS Shaver GR (1996) Physiological and growth responses of Arcticplants to a field experiment simulating climatic change Ecology 77822ndash840 doi1023072265504
Chapin FS Shaver GR Giblin AE Nadelhoffer KJ Laundre JA (1995)Responses of Arctic tundra to experimental and observed changes inclimate Ecology 76 694ndash711 doi1023071939337
Chapin FS Sturm M Serreze MC McFadden JP Key JR Lloyd AHMcGuire AD Rupp TS Lynch AH Schimel JP Beringer J ChapmanWLEpsteinHEEuskirchenESHinzmanLD JiaGPingC-LTapeKDThompson CDC Walker DA Welker JM (2005) Role of land-surfacechanges in Arctic summer warming Science 310 657ndash660 doi101126science1117368
Covey-Crump EM Attwood RG Atkin OK (2002) Regulation of rootrespiration in two species of Plantago that differ in relative growthrate the effect of short and long term changes in temperature PlantCell amp Environment 25 1501ndash1513 doi101046j1365-3040200200932x
Crous KY Zaragoza-Castells J Ellsworth DS Duursma RA LowM TissueDT Atkin OK (2012) Light inhibition of leaf respiration in field-grownEucalyptus saligna in whole-tree chambers under elevated atmosphericCO2 and summer drought Plant Cell amp Environment 35 966ndash981doi101111j1365-3040201102465x
Dungan RJ Whitehead D Duncan RP (2003) Seasonal and temperaturedependence of photosynthesis and respiration for two co-occurringbroad-leaved tree species with contrasting leaf phenology TreePhysiology 23 561ndash568 doi101093treephys238561
Elmendorf SC Henry GHR Hollister RD Bjoumlrk RG Bjorkman ADCallaghan TV Collier LS Cooper EJ Cornelissen JHC Day TAFosaa AM Gould WA Greacutetarsdoacutettir J Harte J Hermanutz L Hik DSHofgaard A Jarrad F Joacutensdoacutettir IS Keuper F Klanderud K Klein JAKoh S Kudo G Lang SI Loewen V May JL Mercado J Michelsen AMolau U Myers-Smith IH Oberbauer SF Pieper S Post E Rixen CRobinson CH Schmidt NM Shaver GR Stenstroumlm A Tolvanen ATotland Oslash Troxler T Wahren C-H Webber PJ Welker JM WookeyPA (2012) Global assessment of experimental climate warming ontundra vegetation heterogeneity over space and time Ecology Letters15 164ndash175 doi101111j1461-0248201101716x
EnvironmentalDataCenterTeam (2011)Meteorologicalmonitoring programat Toolik Alaska Toolik Field Station Institute of Arctic BiologyUniversity of Alaska Fairbanks Fairbanks AK 99775 Available athttptoolikalaskaeduedcabiotic_monitoringdata_queryphp
L Functional Plant Biology M A Heskel et al
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
advantage in growth and development (Kornfeld et al 2013)In addition Rlight Agross decreased over the growing season(Fig 6) driven by lower values of Rlight as the season progressedrather than any significant change in photosynthesis which is
discordant with the idea that the processes would increase in acoupled manner based on carbohydrate substrate supply anddemand Kornfeld et al (2013) found no difference in starchand sugar content between control and warmed leaves within
0
20
40
60
80Betula
0
20
40
60Eriophorum
0
20
40
60
80
0
40
80
120
0
1
2
3
0
1
2
3
4
Julian day
160 170 180 190 200 210160 170 180 190 200
(a)
(c)
(e)
(b)
(d)
(f )
Vcm
ax (micro
mol
mndash2
sndash1
)J
Vcm
ax
J (micro
mol
mndash2
sndash1
)
Fig 4 (a b) Maximal carboxylation rate (Vcmax) and (c d) electron transport rate (J) both measured in laboratoryconditions and corrected to a constant 25C and (e f) the ratio of these variables (J Vcmax) over the growing season inleaves of (a c e) Betula nana and (b d f) Eriophorum vaginatum grown under ambient (open) and warmed (filled)conditions Values shown are means with se
Seasonal carbon exchange under long-term warming Functional Plant Biology I
species when measured at a single sampling point during thegrowing season though leaves of B nana contained a greaterconcentration of both sugar and starch than E vaginatum Wewere unable to quantify foliar carbohydrate values in thesespecies through the season though future studies couldinclude this informative measurement to further relate themechanistic links between these processes
Both species exhibited the lowest values of Rlight Rdark
towards the end of the season in late July (Fig 5) suggestinga potential developmental or energy demand-related control onthe inhibition of respiration by light that allows for greater
respiratory energy production in the early season during leafexpansionTheonlyother knownstudy that explored the seasonalresponse of Rlight Rdark observed the lowest level of inhibitionearlier in the growing season before the warmest month (Crouset al 2012) This response similar to our study despite a starklydifferent ecological system supports the idea of shared seasonaltiming-mediated biochemical controls on the light inhibition ofrespiration across species The degree of inhibition is known toshow a relaxed response under environmental conditions thatcan stimulate growth due to increased demand for energy and Cskeletons as exhibited under elevated CO2 (Wang et al 2001Shapiro et al 2004) and increased soil nutrient availability(Heskel et al 2012) though this is not always the case (Tissueet al 2002 Ayub et al 2011) Long-termwarming alsomediatedRlight Rdark with lower values observed in warm-grownleaves though measurement temperature can also impact theserelationships (Atkin et al 2006)
Mechanistic explanations for the relationships amonggrowth demand cellular energy status and the degree of lightinhibition of respiration have been previously reported Theenvironmental challenges of a short growing season and ~24-hdiel light exposure in the Arctic tundra control growth ratesthat may then influence values of Rlight Rdark (Bret-Harteet al 2001 Bret-Harte et al 2002) In the light adenylatesupply from photosynthesis may decrease the energy demandfrom mitochondrial respiration Additionally photorespirationis associated with the inactivation of pyruvate dehydrogenasea precursor to the tri-carboxylic acid (TCA) cycle (Budde andRandall 1990) though previous estimations of photorespirationin these species didnotfinda strong correlationwith the inhibitionof Rlight (Heskel et al 2013) The TCA cycle can also besignificantly altered in the light to support N assimilationwhich effectively reduces CO2 release (Tcherkez et al 2005Tcherkez et al 2008 Tcherkez et al 2009) This effect may beenhanced in tundra plantswhen soil N is less limiting under theseconditions growth rates increase (Bret-Harte et al 2002) Theinhibition ofRlight is observed to be slightly increased in fertilisedsoils (Heskel et al 2013) Additionally the oxidative pentosephosphate pathway (OPPP) which contributes the necessaryreductant for the synthesis of multiple metabolites in darknessmay be relaxed in the light when reductants are provided byphotosynthesis diminishing CO2 release by the OPPP Thoughthe OPPP comprises a proportionally smaller CO2 flux comparedwith that from the TCA cycle in the light this decrease maycontribute to lower overall CO2 release causing Rlight ltRdark
(Buckley and Adams 2011) In our current study the apparentrelaxation of the light inhibition of respiration in leaves of bothstudy species (Fig 5) resulting in the greatest proportional CO2
losses (Fig 6) suggesting that high growth demand for ATP andcarbon skeletons outweigh potential carbon losses in the earlyseason
Influences of variable ambient temperature on gas exchange
In addition to quantifying the effects of long-termwarming on thefoliar gas exchange during the growing season this studyevaluated the influence of intraseasonal short-term temperaturevariability on these physiological processes Arctic tundragrowing seasons can exhibit dramatic shifts in temperature and
Table 3 Parameters describing the respiratory response tomeasurement temperature in leaves of Betula nana and Eriophorumvaginatum grown under control (CT) and long-term warming (WG)conditions including Q10 (the temperature sensitivity of respiration(estimated over 5ndash358C)) R10 (the basal respiration rate at 108C(mmol CO2 gndash1 sndash1)) and Eo (kJmolndash1) a variable related to the energyof activation calculated from values of respiration at 208C and R10
Values presented are means and se Statistical differences between speciesand treatments are denoted alphabetically
Fig 5 Means and se of the ratio of respiration in the light (Rlight) estimatedvia the Kok method to dark respiration (Rdark) both variables measured at20C and 400 parts per million CO2 through the growing season in Betulanana (circles) and Eriophorum vaginatum (triangles) under ambient controlfield conditions (CT open symbols) and under a long-term passive warmingtreatment (WGfilled symbols n= 4)A linear regression (y= ndash0009x+ 219R2 = 047) depicts the declining Rlight Rdark values with the progressinggrowing season
J Functional Plant Biology M A Heskel et al
precipitation within days as was observed in this study duringsummer 2010 in Toolik Lake Alaska (Fig 1) and this variabilityis likely to influence the temperature-sensitive processescontrolling leaf carbon cycling Respiration in both speciescan be highly temperature-sensitive especially at warmertemperatures which may become more common in a futureArctic (Fig 3 Table 3) When modelled against thetemperature of the measurement day the average temperatureof the prior week accounted for species and treatment effectsand some patterns emerged that help to characterise the nature ofthe temperature sensitivity of foliar gas exchange (Table S1)However photosynthesis which is known to respond to short-term changes in temperature in field-grown plants (Berry andBjoumlrkman 1980 Poyatos et al 2012) did not show strongrelationships to daily or weekly variation in temperatureinstead species effects explained most variation (Table S1)The underlying photosynthetic machinery maintains consistentrates of carbon assimilation (measured at a constant temperature)throughout the growing season though daily fluctuations intemperature and precipitation experiences in the Arctic mayinfluence in vivo rates Plant species from alpine and Arcticecosystems can exhibit local adaptation to the extremevariability experienced in those locations (Korner and Larcher1988) which may translate to a limited acclimation (Atkin et al2006) Further because the ambient temperature conditionsfrom this region fluctuate so unpredictably in short periodsthrough the season any analysis using temperature averages ofprevious time windows might not be as useful for estimatingthermal acclimation as they are in regions with more predictableseasonal temperature patterns (Ow et al 2010 Searle et al 2011)
In contrast respiratory rates in the light and dark whenmeasured at a constant temperature appeared to be morecorrelated with short-term ambient temperature variation TheAICc analysis shows that the strongest model for Rdark includedspecies and warming effects and the prior weekrsquos average
temperature in contrast the strongest model for Rlight includedthe former parameters and the minimum temperature from themeasurement day (Table S1) The model relationships maysuggest a short-term thermal sensitivity of respiration that maybe more responsive to temperature minimums though this isnot clearly reflected in regressions of Rlight with the ambienttemperature values (Fig S5) This sensitivity of respiration totemperature minimums has been observed previously inroots as well and suggests that thermal acclimation in coldgrowth temperatures may regulate metabolic activity to meetthe demands of growth and maintenance at the expense ofgreater carbon loss (Covey-Crump et al 2002) Low ambienttemperatures did not clearly relate to the highest rates ofrespiration when measured at a common temperature aswould be expected under respiratory cold acclimation (Atkinand Tjoelker 2003) However it is possible that developmentaltiming may also play into the leavesrsquo respiratory rates and abilityto acclimate especially in B nana in the early season when itsleaves are still expanding (Armstrong et al 2006) Through arelatively simple modelling exercise we found some evidenceof differential temperature controls on photosynthesis andrespiration in B nana and E vaginatum we suggest thatfuture work should include more detailed environmental andbiotic factors including soil temperature daily photosyntheticradiation and leaf carbohydrate data to more accurately informthe seasonal temperature sensitivity of foliar carbon exchange
Conclusions and implications
The physiological measurements collected in this study allowedfor the quantification of mechanistic responses in the nightlessand highly variable growing season of the arctic tundra andprovided new insights into foliar carbon regulation We presentnew information on the seasonal trends of foliar carbon cyclingin two dominant tundra species and relate these fluxes to short-
0
015
045
030
0
015
045
030
Julian day
Betula Eriophorum
160 170 180 190 200 210 160 170 180 190 200 210
Rlig
ht
Agr
oss
Fig 6 The proportional carbon loss through respiration in the light to gross photosynthesis in (a) Betula nana and(b) Eriophorum vaginatum over the growing season (Rlight Agross both variables measured at a common temperatureof 20C and 400 parts per million CO2) Linear regressions across growth conditions depict the relationship ofRlight Agross and measurement date during the growing season (B nana y= ndash0003 + 088x R2 = 030 E vaginatumy= ndash0005 + 117x R2 = 065) Values represent variable means and se error bars are not visible for many values dueto their small value
Seasonal carbon exchange under long-term warming Functional Plant Biology K
term temperature variability and long-term warming Our studypresents the first published values of the photosyntheticparameters Vcmax and J at multiple points during the season inthese species which can inform and refine parameterisation ofvegetationmodels atmany scalesAlsowe present thefirst reportof the seasonal flexibility of the degree of the inhibition of Rlight
under long-term warmed growth conditions in these or anyspecies which will enable more accurate calculations ofecosystem respiration Based on these measurements aseasonal estimate (not accounting for potential temperaturesensitivity as discussed above see Eqn S3) shows thatneglecting the inhibition of foliar Rlight in Arctic tundravegetation could lead to overestimations of foliar carbon lossof~4molmndash2 leaf (~176 g)over thegrowingseason or evenmore(~42molmndash2) when considering plants grown under warmedconditionsAlthough the temperature effect on respiratory carbonloss in the light may beminimal the overall discrepancy in fluxesbetween Rdark and Rlight must be considered for realisticdescriptions of Arctic carbon cycling Our study contributesimportant data that allow for an increased understanding ofdominant tundra speciesrsquo responses to environmental changeboth long- and short-term and will allow for more predictivepower in models estimating ecosystem carbon storage
Acknowledgements
We are grateful to the researchers and support staff at the Arctic LTER andToolik Field Station for their help in this study as well as Greg PendergastThis study was made possible through funding from the NSF 0732664OKA acknowledges grant support from the Australian Research Council(DP0986823)
References
Armstrong AF Logan DC Atkin OK (2006a) On the developmentaldependence of leaf respiration responses to short and long-termchanges in growth temperature American Journal of Botany 931633ndash1639 doi103732ajb93111633
Armstrong AF Logan DC Tobin AK OrsquoToole P Atkin OK (2006b)Heterogeneity of plant mitochondrial responses underpinningrespiratory acclimation to the cold in Arabidopsis thaliana leavesPlant Cell amp Environment 29 940ndash949 doi101111j1365-3040200501475x
Armstrong AF Badger MR Day DA Barthet MM Smith P Millar AHWhelan J Atkin OK (2008) Dynamic changes in the mitochondrialelectron transport chain underpinning cold acclimation of leafrespiration Plant Cell amp Environment 31 1156ndash1169 doi101111j1365-3040200801830x
Atkin OK Tjoelker MG (2003) Thermal acclimation and the dynamicresponse of plant respiration to temperature Trends in Plant Science 8343ndash351 doi101016S1360-1385(03)00136-5
Atkin OK Bruhn D Tjoelker MG (2005) Response of plant respiration tochanges in temperature mechanisms and consequences of variations inQ10 values and acclimation In lsquoPlant respirationrsquo (Eds H Lambers ampM Ribas-Carbo) pp 95ndash135 (Springer Dordrecht The Netherlands)
Atkin OK Scheurwater I Pons TL (2006)High thermal acclimation potentialof both photosynthesis and respiration in two lowlandPlantago species incontrast to an alpine congeneric Global Change Biology 12 500ndash515doi101111j1365-2486200601114x
Ayub G Smith RA Tissue DT Atkin OK (2011) Impacts of drought onleaf respiration in darkness and light in Eucalyptus saligna exposed toindustrial-age atmosphericCO2 and growth temperatureNewPhytologist190 1003ndash1018 doi101111j1469-8137201103673x
Berry J Bjoumlrkman O (1980) Photosynthetic response and adaptation totemperature in higher plants Annual Review of Plant Physiology andPlant Molecular Biology 31 491ndash543 doi101146annurevpp31060180002423
Bret-HarteMSShaverGRZoerner JP JohnstoneJFWagner JLChavezASGunkelman RF IV Lippert SC Laundre JA (2001) Developmentalplasticity allows Betula nana to dominate tundra subjected to analtered environment Ecology 82 18ndash32
Bret-Harte MS Shaver GR Chapin FS III (2002) Primary and secondarystem growth in Arctic shrubs implications for community response toenvironmental change Journal of Ecology 90 251ndash267 doi101046j1365-2745200100657x
BuckleyTNAdamsMA(2011)An analyticalmodel of non-photorespiratoryCO2 release in the light and dark in leaves of C3 species based onstoichiometric flux balance Plant Cell amp Environment 34 89ndash112doi101111j1365-3040201002228x
Budde RJ Randall DD (1990) Pea leaf mitochondrial pyruvatedehydrogenase complex is inactivated in vivo in a light-dependentmanner Proceedings of the National Academy of Sciences of theUnited States of America 87 673ndash676 doi101073pnas872673
CahoonSMPSullivanPF ShaverGRWelker JM PostE (2012) Interactionsamong shrub cover and the soil microclimatemay determine futureArcticcarbon budgets Ecology Letters 15 1415ndash1422 doi101111j1461-0248201201865x
Chapin FS Shaver GR (1996) Physiological and growth responses of Arcticplants to a field experiment simulating climatic change Ecology 77822ndash840 doi1023072265504
Chapin FS Shaver GR Giblin AE Nadelhoffer KJ Laundre JA (1995)Responses of Arctic tundra to experimental and observed changes inclimate Ecology 76 694ndash711 doi1023071939337
Chapin FS Sturm M Serreze MC McFadden JP Key JR Lloyd AHMcGuire AD Rupp TS Lynch AH Schimel JP Beringer J ChapmanWLEpsteinHEEuskirchenESHinzmanLD JiaGPingC-LTapeKDThompson CDC Walker DA Welker JM (2005) Role of land-surfacechanges in Arctic summer warming Science 310 657ndash660 doi101126science1117368
Covey-Crump EM Attwood RG Atkin OK (2002) Regulation of rootrespiration in two species of Plantago that differ in relative growthrate the effect of short and long term changes in temperature PlantCell amp Environment 25 1501ndash1513 doi101046j1365-3040200200932x
Crous KY Zaragoza-Castells J Ellsworth DS Duursma RA LowM TissueDT Atkin OK (2012) Light inhibition of leaf respiration in field-grownEucalyptus saligna in whole-tree chambers under elevated atmosphericCO2 and summer drought Plant Cell amp Environment 35 966ndash981doi101111j1365-3040201102465x
Dungan RJ Whitehead D Duncan RP (2003) Seasonal and temperaturedependence of photosynthesis and respiration for two co-occurringbroad-leaved tree species with contrasting leaf phenology TreePhysiology 23 561ndash568 doi101093treephys238561
Elmendorf SC Henry GHR Hollister RD Bjoumlrk RG Bjorkman ADCallaghan TV Collier LS Cooper EJ Cornelissen JHC Day TAFosaa AM Gould WA Greacutetarsdoacutettir J Harte J Hermanutz L Hik DSHofgaard A Jarrad F Joacutensdoacutettir IS Keuper F Klanderud K Klein JAKoh S Kudo G Lang SI Loewen V May JL Mercado J Michelsen AMolau U Myers-Smith IH Oberbauer SF Pieper S Post E Rixen CRobinson CH Schmidt NM Shaver GR Stenstroumlm A Tolvanen ATotland Oslash Troxler T Wahren C-H Webber PJ Welker JM WookeyPA (2012) Global assessment of experimental climate warming ontundra vegetation heterogeneity over space and time Ecology Letters15 164ndash175 doi101111j1461-0248201101716x
EnvironmentalDataCenterTeam (2011)Meteorologicalmonitoring programat Toolik Alaska Toolik Field Station Institute of Arctic BiologyUniversity of Alaska Fairbanks Fairbanks AK 99775 Available athttptoolikalaskaeduedcabiotic_monitoringdata_queryphp
L Functional Plant Biology M A Heskel et al
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
species when measured at a single sampling point during thegrowing season though leaves of B nana contained a greaterconcentration of both sugar and starch than E vaginatum Wewere unable to quantify foliar carbohydrate values in thesespecies through the season though future studies couldinclude this informative measurement to further relate themechanistic links between these processes
Both species exhibited the lowest values of Rlight Rdark
towards the end of the season in late July (Fig 5) suggestinga potential developmental or energy demand-related control onthe inhibition of respiration by light that allows for greater
respiratory energy production in the early season during leafexpansionTheonlyother knownstudy that explored the seasonalresponse of Rlight Rdark observed the lowest level of inhibitionearlier in the growing season before the warmest month (Crouset al 2012) This response similar to our study despite a starklydifferent ecological system supports the idea of shared seasonaltiming-mediated biochemical controls on the light inhibition ofrespiration across species The degree of inhibition is known toshow a relaxed response under environmental conditions thatcan stimulate growth due to increased demand for energy and Cskeletons as exhibited under elevated CO2 (Wang et al 2001Shapiro et al 2004) and increased soil nutrient availability(Heskel et al 2012) though this is not always the case (Tissueet al 2002 Ayub et al 2011) Long-termwarming alsomediatedRlight Rdark with lower values observed in warm-grownleaves though measurement temperature can also impact theserelationships (Atkin et al 2006)
Mechanistic explanations for the relationships amonggrowth demand cellular energy status and the degree of lightinhibition of respiration have been previously reported Theenvironmental challenges of a short growing season and ~24-hdiel light exposure in the Arctic tundra control growth ratesthat may then influence values of Rlight Rdark (Bret-Harteet al 2001 Bret-Harte et al 2002) In the light adenylatesupply from photosynthesis may decrease the energy demandfrom mitochondrial respiration Additionally photorespirationis associated with the inactivation of pyruvate dehydrogenasea precursor to the tri-carboxylic acid (TCA) cycle (Budde andRandall 1990) though previous estimations of photorespirationin these species didnotfinda strong correlationwith the inhibitionof Rlight (Heskel et al 2013) The TCA cycle can also besignificantly altered in the light to support N assimilationwhich effectively reduces CO2 release (Tcherkez et al 2005Tcherkez et al 2008 Tcherkez et al 2009) This effect may beenhanced in tundra plantswhen soil N is less limiting under theseconditions growth rates increase (Bret-Harte et al 2002) Theinhibition ofRlight is observed to be slightly increased in fertilisedsoils (Heskel et al 2013) Additionally the oxidative pentosephosphate pathway (OPPP) which contributes the necessaryreductant for the synthesis of multiple metabolites in darknessmay be relaxed in the light when reductants are provided byphotosynthesis diminishing CO2 release by the OPPP Thoughthe OPPP comprises a proportionally smaller CO2 flux comparedwith that from the TCA cycle in the light this decrease maycontribute to lower overall CO2 release causing Rlight ltRdark
(Buckley and Adams 2011) In our current study the apparentrelaxation of the light inhibition of respiration in leaves of bothstudy species (Fig 5) resulting in the greatest proportional CO2
losses (Fig 6) suggesting that high growth demand for ATP andcarbon skeletons outweigh potential carbon losses in the earlyseason
Influences of variable ambient temperature on gas exchange
In addition to quantifying the effects of long-termwarming on thefoliar gas exchange during the growing season this studyevaluated the influence of intraseasonal short-term temperaturevariability on these physiological processes Arctic tundragrowing seasons can exhibit dramatic shifts in temperature and
Table 3 Parameters describing the respiratory response tomeasurement temperature in leaves of Betula nana and Eriophorumvaginatum grown under control (CT) and long-term warming (WG)conditions including Q10 (the temperature sensitivity of respiration(estimated over 5ndash358C)) R10 (the basal respiration rate at 108C(mmol CO2 gndash1 sndash1)) and Eo (kJmolndash1) a variable related to the energyof activation calculated from values of respiration at 208C and R10
Values presented are means and se Statistical differences between speciesand treatments are denoted alphabetically
Fig 5 Means and se of the ratio of respiration in the light (Rlight) estimatedvia the Kok method to dark respiration (Rdark) both variables measured at20C and 400 parts per million CO2 through the growing season in Betulanana (circles) and Eriophorum vaginatum (triangles) under ambient controlfield conditions (CT open symbols) and under a long-term passive warmingtreatment (WGfilled symbols n= 4)A linear regression (y= ndash0009x+ 219R2 = 047) depicts the declining Rlight Rdark values with the progressinggrowing season
J Functional Plant Biology M A Heskel et al
precipitation within days as was observed in this study duringsummer 2010 in Toolik Lake Alaska (Fig 1) and this variabilityis likely to influence the temperature-sensitive processescontrolling leaf carbon cycling Respiration in both speciescan be highly temperature-sensitive especially at warmertemperatures which may become more common in a futureArctic (Fig 3 Table 3) When modelled against thetemperature of the measurement day the average temperatureof the prior week accounted for species and treatment effectsand some patterns emerged that help to characterise the nature ofthe temperature sensitivity of foliar gas exchange (Table S1)However photosynthesis which is known to respond to short-term changes in temperature in field-grown plants (Berry andBjoumlrkman 1980 Poyatos et al 2012) did not show strongrelationships to daily or weekly variation in temperatureinstead species effects explained most variation (Table S1)The underlying photosynthetic machinery maintains consistentrates of carbon assimilation (measured at a constant temperature)throughout the growing season though daily fluctuations intemperature and precipitation experiences in the Arctic mayinfluence in vivo rates Plant species from alpine and Arcticecosystems can exhibit local adaptation to the extremevariability experienced in those locations (Korner and Larcher1988) which may translate to a limited acclimation (Atkin et al2006) Further because the ambient temperature conditionsfrom this region fluctuate so unpredictably in short periodsthrough the season any analysis using temperature averages ofprevious time windows might not be as useful for estimatingthermal acclimation as they are in regions with more predictableseasonal temperature patterns (Ow et al 2010 Searle et al 2011)
In contrast respiratory rates in the light and dark whenmeasured at a constant temperature appeared to be morecorrelated with short-term ambient temperature variation TheAICc analysis shows that the strongest model for Rdark includedspecies and warming effects and the prior weekrsquos average
temperature in contrast the strongest model for Rlight includedthe former parameters and the minimum temperature from themeasurement day (Table S1) The model relationships maysuggest a short-term thermal sensitivity of respiration that maybe more responsive to temperature minimums though this isnot clearly reflected in regressions of Rlight with the ambienttemperature values (Fig S5) This sensitivity of respiration totemperature minimums has been observed previously inroots as well and suggests that thermal acclimation in coldgrowth temperatures may regulate metabolic activity to meetthe demands of growth and maintenance at the expense ofgreater carbon loss (Covey-Crump et al 2002) Low ambienttemperatures did not clearly relate to the highest rates ofrespiration when measured at a common temperature aswould be expected under respiratory cold acclimation (Atkinand Tjoelker 2003) However it is possible that developmentaltiming may also play into the leavesrsquo respiratory rates and abilityto acclimate especially in B nana in the early season when itsleaves are still expanding (Armstrong et al 2006) Through arelatively simple modelling exercise we found some evidenceof differential temperature controls on photosynthesis andrespiration in B nana and E vaginatum we suggest thatfuture work should include more detailed environmental andbiotic factors including soil temperature daily photosyntheticradiation and leaf carbohydrate data to more accurately informthe seasonal temperature sensitivity of foliar carbon exchange
Conclusions and implications
The physiological measurements collected in this study allowedfor the quantification of mechanistic responses in the nightlessand highly variable growing season of the arctic tundra andprovided new insights into foliar carbon regulation We presentnew information on the seasonal trends of foliar carbon cyclingin two dominant tundra species and relate these fluxes to short-
0
015
045
030
0
015
045
030
Julian day
Betula Eriophorum
160 170 180 190 200 210 160 170 180 190 200 210
Rlig
ht
Agr
oss
Fig 6 The proportional carbon loss through respiration in the light to gross photosynthesis in (a) Betula nana and(b) Eriophorum vaginatum over the growing season (Rlight Agross both variables measured at a common temperatureof 20C and 400 parts per million CO2) Linear regressions across growth conditions depict the relationship ofRlight Agross and measurement date during the growing season (B nana y= ndash0003 + 088x R2 = 030 E vaginatumy= ndash0005 + 117x R2 = 065) Values represent variable means and se error bars are not visible for many values dueto their small value
Seasonal carbon exchange under long-term warming Functional Plant Biology K
term temperature variability and long-term warming Our studypresents the first published values of the photosyntheticparameters Vcmax and J at multiple points during the season inthese species which can inform and refine parameterisation ofvegetationmodels atmany scalesAlsowe present thefirst reportof the seasonal flexibility of the degree of the inhibition of Rlight
under long-term warmed growth conditions in these or anyspecies which will enable more accurate calculations ofecosystem respiration Based on these measurements aseasonal estimate (not accounting for potential temperaturesensitivity as discussed above see Eqn S3) shows thatneglecting the inhibition of foliar Rlight in Arctic tundravegetation could lead to overestimations of foliar carbon lossof~4molmndash2 leaf (~176 g)over thegrowingseason or evenmore(~42molmndash2) when considering plants grown under warmedconditionsAlthough the temperature effect on respiratory carbonloss in the light may beminimal the overall discrepancy in fluxesbetween Rdark and Rlight must be considered for realisticdescriptions of Arctic carbon cycling Our study contributesimportant data that allow for an increased understanding ofdominant tundra speciesrsquo responses to environmental changeboth long- and short-term and will allow for more predictivepower in models estimating ecosystem carbon storage
Acknowledgements
We are grateful to the researchers and support staff at the Arctic LTER andToolik Field Station for their help in this study as well as Greg PendergastThis study was made possible through funding from the NSF 0732664OKA acknowledges grant support from the Australian Research Council(DP0986823)
References
Armstrong AF Logan DC Atkin OK (2006a) On the developmentaldependence of leaf respiration responses to short and long-termchanges in growth temperature American Journal of Botany 931633ndash1639 doi103732ajb93111633
Armstrong AF Logan DC Tobin AK OrsquoToole P Atkin OK (2006b)Heterogeneity of plant mitochondrial responses underpinningrespiratory acclimation to the cold in Arabidopsis thaliana leavesPlant Cell amp Environment 29 940ndash949 doi101111j1365-3040200501475x
Armstrong AF Badger MR Day DA Barthet MM Smith P Millar AHWhelan J Atkin OK (2008) Dynamic changes in the mitochondrialelectron transport chain underpinning cold acclimation of leafrespiration Plant Cell amp Environment 31 1156ndash1169 doi101111j1365-3040200801830x
Atkin OK Tjoelker MG (2003) Thermal acclimation and the dynamicresponse of plant respiration to temperature Trends in Plant Science 8343ndash351 doi101016S1360-1385(03)00136-5
Atkin OK Bruhn D Tjoelker MG (2005) Response of plant respiration tochanges in temperature mechanisms and consequences of variations inQ10 values and acclimation In lsquoPlant respirationrsquo (Eds H Lambers ampM Ribas-Carbo) pp 95ndash135 (Springer Dordrecht The Netherlands)
Atkin OK Scheurwater I Pons TL (2006)High thermal acclimation potentialof both photosynthesis and respiration in two lowlandPlantago species incontrast to an alpine congeneric Global Change Biology 12 500ndash515doi101111j1365-2486200601114x
Ayub G Smith RA Tissue DT Atkin OK (2011) Impacts of drought onleaf respiration in darkness and light in Eucalyptus saligna exposed toindustrial-age atmosphericCO2 and growth temperatureNewPhytologist190 1003ndash1018 doi101111j1469-8137201103673x
Berry J Bjoumlrkman O (1980) Photosynthetic response and adaptation totemperature in higher plants Annual Review of Plant Physiology andPlant Molecular Biology 31 491ndash543 doi101146annurevpp31060180002423
Bret-HarteMSShaverGRZoerner JP JohnstoneJFWagner JLChavezASGunkelman RF IV Lippert SC Laundre JA (2001) Developmentalplasticity allows Betula nana to dominate tundra subjected to analtered environment Ecology 82 18ndash32
Bret-Harte MS Shaver GR Chapin FS III (2002) Primary and secondarystem growth in Arctic shrubs implications for community response toenvironmental change Journal of Ecology 90 251ndash267 doi101046j1365-2745200100657x
BuckleyTNAdamsMA(2011)An analyticalmodel of non-photorespiratoryCO2 release in the light and dark in leaves of C3 species based onstoichiometric flux balance Plant Cell amp Environment 34 89ndash112doi101111j1365-3040201002228x
Budde RJ Randall DD (1990) Pea leaf mitochondrial pyruvatedehydrogenase complex is inactivated in vivo in a light-dependentmanner Proceedings of the National Academy of Sciences of theUnited States of America 87 673ndash676 doi101073pnas872673
CahoonSMPSullivanPF ShaverGRWelker JM PostE (2012) Interactionsamong shrub cover and the soil microclimatemay determine futureArcticcarbon budgets Ecology Letters 15 1415ndash1422 doi101111j1461-0248201201865x
Chapin FS Shaver GR (1996) Physiological and growth responses of Arcticplants to a field experiment simulating climatic change Ecology 77822ndash840 doi1023072265504
Chapin FS Shaver GR Giblin AE Nadelhoffer KJ Laundre JA (1995)Responses of Arctic tundra to experimental and observed changes inclimate Ecology 76 694ndash711 doi1023071939337
Chapin FS Sturm M Serreze MC McFadden JP Key JR Lloyd AHMcGuire AD Rupp TS Lynch AH Schimel JP Beringer J ChapmanWLEpsteinHEEuskirchenESHinzmanLD JiaGPingC-LTapeKDThompson CDC Walker DA Welker JM (2005) Role of land-surfacechanges in Arctic summer warming Science 310 657ndash660 doi101126science1117368
Covey-Crump EM Attwood RG Atkin OK (2002) Regulation of rootrespiration in two species of Plantago that differ in relative growthrate the effect of short and long term changes in temperature PlantCell amp Environment 25 1501ndash1513 doi101046j1365-3040200200932x
Crous KY Zaragoza-Castells J Ellsworth DS Duursma RA LowM TissueDT Atkin OK (2012) Light inhibition of leaf respiration in field-grownEucalyptus saligna in whole-tree chambers under elevated atmosphericCO2 and summer drought Plant Cell amp Environment 35 966ndash981doi101111j1365-3040201102465x
Dungan RJ Whitehead D Duncan RP (2003) Seasonal and temperaturedependence of photosynthesis and respiration for two co-occurringbroad-leaved tree species with contrasting leaf phenology TreePhysiology 23 561ndash568 doi101093treephys238561
Elmendorf SC Henry GHR Hollister RD Bjoumlrk RG Bjorkman ADCallaghan TV Collier LS Cooper EJ Cornelissen JHC Day TAFosaa AM Gould WA Greacutetarsdoacutettir J Harte J Hermanutz L Hik DSHofgaard A Jarrad F Joacutensdoacutettir IS Keuper F Klanderud K Klein JAKoh S Kudo G Lang SI Loewen V May JL Mercado J Michelsen AMolau U Myers-Smith IH Oberbauer SF Pieper S Post E Rixen CRobinson CH Schmidt NM Shaver GR Stenstroumlm A Tolvanen ATotland Oslash Troxler T Wahren C-H Webber PJ Welker JM WookeyPA (2012) Global assessment of experimental climate warming ontundra vegetation heterogeneity over space and time Ecology Letters15 164ndash175 doi101111j1461-0248201101716x
EnvironmentalDataCenterTeam (2011)Meteorologicalmonitoring programat Toolik Alaska Toolik Field Station Institute of Arctic BiologyUniversity of Alaska Fairbanks Fairbanks AK 99775 Available athttptoolikalaskaeduedcabiotic_monitoringdata_queryphp
L Functional Plant Biology M A Heskel et al
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
precipitation within days as was observed in this study duringsummer 2010 in Toolik Lake Alaska (Fig 1) and this variabilityis likely to influence the temperature-sensitive processescontrolling leaf carbon cycling Respiration in both speciescan be highly temperature-sensitive especially at warmertemperatures which may become more common in a futureArctic (Fig 3 Table 3) When modelled against thetemperature of the measurement day the average temperatureof the prior week accounted for species and treatment effectsand some patterns emerged that help to characterise the nature ofthe temperature sensitivity of foliar gas exchange (Table S1)However photosynthesis which is known to respond to short-term changes in temperature in field-grown plants (Berry andBjoumlrkman 1980 Poyatos et al 2012) did not show strongrelationships to daily or weekly variation in temperatureinstead species effects explained most variation (Table S1)The underlying photosynthetic machinery maintains consistentrates of carbon assimilation (measured at a constant temperature)throughout the growing season though daily fluctuations intemperature and precipitation experiences in the Arctic mayinfluence in vivo rates Plant species from alpine and Arcticecosystems can exhibit local adaptation to the extremevariability experienced in those locations (Korner and Larcher1988) which may translate to a limited acclimation (Atkin et al2006) Further because the ambient temperature conditionsfrom this region fluctuate so unpredictably in short periodsthrough the season any analysis using temperature averages ofprevious time windows might not be as useful for estimatingthermal acclimation as they are in regions with more predictableseasonal temperature patterns (Ow et al 2010 Searle et al 2011)
In contrast respiratory rates in the light and dark whenmeasured at a constant temperature appeared to be morecorrelated with short-term ambient temperature variation TheAICc analysis shows that the strongest model for Rdark includedspecies and warming effects and the prior weekrsquos average
temperature in contrast the strongest model for Rlight includedthe former parameters and the minimum temperature from themeasurement day (Table S1) The model relationships maysuggest a short-term thermal sensitivity of respiration that maybe more responsive to temperature minimums though this isnot clearly reflected in regressions of Rlight with the ambienttemperature values (Fig S5) This sensitivity of respiration totemperature minimums has been observed previously inroots as well and suggests that thermal acclimation in coldgrowth temperatures may regulate metabolic activity to meetthe demands of growth and maintenance at the expense ofgreater carbon loss (Covey-Crump et al 2002) Low ambienttemperatures did not clearly relate to the highest rates ofrespiration when measured at a common temperature aswould be expected under respiratory cold acclimation (Atkinand Tjoelker 2003) However it is possible that developmentaltiming may also play into the leavesrsquo respiratory rates and abilityto acclimate especially in B nana in the early season when itsleaves are still expanding (Armstrong et al 2006) Through arelatively simple modelling exercise we found some evidenceof differential temperature controls on photosynthesis andrespiration in B nana and E vaginatum we suggest thatfuture work should include more detailed environmental andbiotic factors including soil temperature daily photosyntheticradiation and leaf carbohydrate data to more accurately informthe seasonal temperature sensitivity of foliar carbon exchange
Conclusions and implications
The physiological measurements collected in this study allowedfor the quantification of mechanistic responses in the nightlessand highly variable growing season of the arctic tundra andprovided new insights into foliar carbon regulation We presentnew information on the seasonal trends of foliar carbon cyclingin two dominant tundra species and relate these fluxes to short-
0
015
045
030
0
015
045
030
Julian day
Betula Eriophorum
160 170 180 190 200 210 160 170 180 190 200 210
Rlig
ht
Agr
oss
Fig 6 The proportional carbon loss through respiration in the light to gross photosynthesis in (a) Betula nana and(b) Eriophorum vaginatum over the growing season (Rlight Agross both variables measured at a common temperatureof 20C and 400 parts per million CO2) Linear regressions across growth conditions depict the relationship ofRlight Agross and measurement date during the growing season (B nana y= ndash0003 + 088x R2 = 030 E vaginatumy= ndash0005 + 117x R2 = 065) Values represent variable means and se error bars are not visible for many values dueto their small value
Seasonal carbon exchange under long-term warming Functional Plant Biology K
term temperature variability and long-term warming Our studypresents the first published values of the photosyntheticparameters Vcmax and J at multiple points during the season inthese species which can inform and refine parameterisation ofvegetationmodels atmany scalesAlsowe present thefirst reportof the seasonal flexibility of the degree of the inhibition of Rlight
under long-term warmed growth conditions in these or anyspecies which will enable more accurate calculations ofecosystem respiration Based on these measurements aseasonal estimate (not accounting for potential temperaturesensitivity as discussed above see Eqn S3) shows thatneglecting the inhibition of foliar Rlight in Arctic tundravegetation could lead to overestimations of foliar carbon lossof~4molmndash2 leaf (~176 g)over thegrowingseason or evenmore(~42molmndash2) when considering plants grown under warmedconditionsAlthough the temperature effect on respiratory carbonloss in the light may beminimal the overall discrepancy in fluxesbetween Rdark and Rlight must be considered for realisticdescriptions of Arctic carbon cycling Our study contributesimportant data that allow for an increased understanding ofdominant tundra speciesrsquo responses to environmental changeboth long- and short-term and will allow for more predictivepower in models estimating ecosystem carbon storage
Acknowledgements
We are grateful to the researchers and support staff at the Arctic LTER andToolik Field Station for their help in this study as well as Greg PendergastThis study was made possible through funding from the NSF 0732664OKA acknowledges grant support from the Australian Research Council(DP0986823)
References
Armstrong AF Logan DC Atkin OK (2006a) On the developmentaldependence of leaf respiration responses to short and long-termchanges in growth temperature American Journal of Botany 931633ndash1639 doi103732ajb93111633
Armstrong AF Logan DC Tobin AK OrsquoToole P Atkin OK (2006b)Heterogeneity of plant mitochondrial responses underpinningrespiratory acclimation to the cold in Arabidopsis thaliana leavesPlant Cell amp Environment 29 940ndash949 doi101111j1365-3040200501475x
Armstrong AF Badger MR Day DA Barthet MM Smith P Millar AHWhelan J Atkin OK (2008) Dynamic changes in the mitochondrialelectron transport chain underpinning cold acclimation of leafrespiration Plant Cell amp Environment 31 1156ndash1169 doi101111j1365-3040200801830x
Atkin OK Tjoelker MG (2003) Thermal acclimation and the dynamicresponse of plant respiration to temperature Trends in Plant Science 8343ndash351 doi101016S1360-1385(03)00136-5
Atkin OK Bruhn D Tjoelker MG (2005) Response of plant respiration tochanges in temperature mechanisms and consequences of variations inQ10 values and acclimation In lsquoPlant respirationrsquo (Eds H Lambers ampM Ribas-Carbo) pp 95ndash135 (Springer Dordrecht The Netherlands)
Atkin OK Scheurwater I Pons TL (2006)High thermal acclimation potentialof both photosynthesis and respiration in two lowlandPlantago species incontrast to an alpine congeneric Global Change Biology 12 500ndash515doi101111j1365-2486200601114x
Ayub G Smith RA Tissue DT Atkin OK (2011) Impacts of drought onleaf respiration in darkness and light in Eucalyptus saligna exposed toindustrial-age atmosphericCO2 and growth temperatureNewPhytologist190 1003ndash1018 doi101111j1469-8137201103673x
Berry J Bjoumlrkman O (1980) Photosynthetic response and adaptation totemperature in higher plants Annual Review of Plant Physiology andPlant Molecular Biology 31 491ndash543 doi101146annurevpp31060180002423
Bret-HarteMSShaverGRZoerner JP JohnstoneJFWagner JLChavezASGunkelman RF IV Lippert SC Laundre JA (2001) Developmentalplasticity allows Betula nana to dominate tundra subjected to analtered environment Ecology 82 18ndash32
Bret-Harte MS Shaver GR Chapin FS III (2002) Primary and secondarystem growth in Arctic shrubs implications for community response toenvironmental change Journal of Ecology 90 251ndash267 doi101046j1365-2745200100657x
BuckleyTNAdamsMA(2011)An analyticalmodel of non-photorespiratoryCO2 release in the light and dark in leaves of C3 species based onstoichiometric flux balance Plant Cell amp Environment 34 89ndash112doi101111j1365-3040201002228x
Budde RJ Randall DD (1990) Pea leaf mitochondrial pyruvatedehydrogenase complex is inactivated in vivo in a light-dependentmanner Proceedings of the National Academy of Sciences of theUnited States of America 87 673ndash676 doi101073pnas872673
CahoonSMPSullivanPF ShaverGRWelker JM PostE (2012) Interactionsamong shrub cover and the soil microclimatemay determine futureArcticcarbon budgets Ecology Letters 15 1415ndash1422 doi101111j1461-0248201201865x
Chapin FS Shaver GR (1996) Physiological and growth responses of Arcticplants to a field experiment simulating climatic change Ecology 77822ndash840 doi1023072265504
Chapin FS Shaver GR Giblin AE Nadelhoffer KJ Laundre JA (1995)Responses of Arctic tundra to experimental and observed changes inclimate Ecology 76 694ndash711 doi1023071939337
Chapin FS Sturm M Serreze MC McFadden JP Key JR Lloyd AHMcGuire AD Rupp TS Lynch AH Schimel JP Beringer J ChapmanWLEpsteinHEEuskirchenESHinzmanLD JiaGPingC-LTapeKDThompson CDC Walker DA Welker JM (2005) Role of land-surfacechanges in Arctic summer warming Science 310 657ndash660 doi101126science1117368
Covey-Crump EM Attwood RG Atkin OK (2002) Regulation of rootrespiration in two species of Plantago that differ in relative growthrate the effect of short and long term changes in temperature PlantCell amp Environment 25 1501ndash1513 doi101046j1365-3040200200932x
Crous KY Zaragoza-Castells J Ellsworth DS Duursma RA LowM TissueDT Atkin OK (2012) Light inhibition of leaf respiration in field-grownEucalyptus saligna in whole-tree chambers under elevated atmosphericCO2 and summer drought Plant Cell amp Environment 35 966ndash981doi101111j1365-3040201102465x
Dungan RJ Whitehead D Duncan RP (2003) Seasonal and temperaturedependence of photosynthesis and respiration for two co-occurringbroad-leaved tree species with contrasting leaf phenology TreePhysiology 23 561ndash568 doi101093treephys238561
Elmendorf SC Henry GHR Hollister RD Bjoumlrk RG Bjorkman ADCallaghan TV Collier LS Cooper EJ Cornelissen JHC Day TAFosaa AM Gould WA Greacutetarsdoacutettir J Harte J Hermanutz L Hik DSHofgaard A Jarrad F Joacutensdoacutettir IS Keuper F Klanderud K Klein JAKoh S Kudo G Lang SI Loewen V May JL Mercado J Michelsen AMolau U Myers-Smith IH Oberbauer SF Pieper S Post E Rixen CRobinson CH Schmidt NM Shaver GR Stenstroumlm A Tolvanen ATotland Oslash Troxler T Wahren C-H Webber PJ Welker JM WookeyPA (2012) Global assessment of experimental climate warming ontundra vegetation heterogeneity over space and time Ecology Letters15 164ndash175 doi101111j1461-0248201101716x
EnvironmentalDataCenterTeam (2011)Meteorologicalmonitoring programat Toolik Alaska Toolik Field Station Institute of Arctic BiologyUniversity of Alaska Fairbanks Fairbanks AK 99775 Available athttptoolikalaskaeduedcabiotic_monitoringdata_queryphp
L Functional Plant Biology M A Heskel et al
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
term temperature variability and long-term warming Our studypresents the first published values of the photosyntheticparameters Vcmax and J at multiple points during the season inthese species which can inform and refine parameterisation ofvegetationmodels atmany scalesAlsowe present thefirst reportof the seasonal flexibility of the degree of the inhibition of Rlight
under long-term warmed growth conditions in these or anyspecies which will enable more accurate calculations ofecosystem respiration Based on these measurements aseasonal estimate (not accounting for potential temperaturesensitivity as discussed above see Eqn S3) shows thatneglecting the inhibition of foliar Rlight in Arctic tundravegetation could lead to overestimations of foliar carbon lossof~4molmndash2 leaf (~176 g)over thegrowingseason or evenmore(~42molmndash2) when considering plants grown under warmedconditionsAlthough the temperature effect on respiratory carbonloss in the light may beminimal the overall discrepancy in fluxesbetween Rdark and Rlight must be considered for realisticdescriptions of Arctic carbon cycling Our study contributesimportant data that allow for an increased understanding ofdominant tundra speciesrsquo responses to environmental changeboth long- and short-term and will allow for more predictivepower in models estimating ecosystem carbon storage
Acknowledgements
We are grateful to the researchers and support staff at the Arctic LTER andToolik Field Station for their help in this study as well as Greg PendergastThis study was made possible through funding from the NSF 0732664OKA acknowledges grant support from the Australian Research Council(DP0986823)
References
Armstrong AF Logan DC Atkin OK (2006a) On the developmentaldependence of leaf respiration responses to short and long-termchanges in growth temperature American Journal of Botany 931633ndash1639 doi103732ajb93111633
Armstrong AF Logan DC Tobin AK OrsquoToole P Atkin OK (2006b)Heterogeneity of plant mitochondrial responses underpinningrespiratory acclimation to the cold in Arabidopsis thaliana leavesPlant Cell amp Environment 29 940ndash949 doi101111j1365-3040200501475x
Armstrong AF Badger MR Day DA Barthet MM Smith P Millar AHWhelan J Atkin OK (2008) Dynamic changes in the mitochondrialelectron transport chain underpinning cold acclimation of leafrespiration Plant Cell amp Environment 31 1156ndash1169 doi101111j1365-3040200801830x
Atkin OK Tjoelker MG (2003) Thermal acclimation and the dynamicresponse of plant respiration to temperature Trends in Plant Science 8343ndash351 doi101016S1360-1385(03)00136-5
Atkin OK Bruhn D Tjoelker MG (2005) Response of plant respiration tochanges in temperature mechanisms and consequences of variations inQ10 values and acclimation In lsquoPlant respirationrsquo (Eds H Lambers ampM Ribas-Carbo) pp 95ndash135 (Springer Dordrecht The Netherlands)
Atkin OK Scheurwater I Pons TL (2006)High thermal acclimation potentialof both photosynthesis and respiration in two lowlandPlantago species incontrast to an alpine congeneric Global Change Biology 12 500ndash515doi101111j1365-2486200601114x
Ayub G Smith RA Tissue DT Atkin OK (2011) Impacts of drought onleaf respiration in darkness and light in Eucalyptus saligna exposed toindustrial-age atmosphericCO2 and growth temperatureNewPhytologist190 1003ndash1018 doi101111j1469-8137201103673x
Berry J Bjoumlrkman O (1980) Photosynthetic response and adaptation totemperature in higher plants Annual Review of Plant Physiology andPlant Molecular Biology 31 491ndash543 doi101146annurevpp31060180002423
Bret-HarteMSShaverGRZoerner JP JohnstoneJFWagner JLChavezASGunkelman RF IV Lippert SC Laundre JA (2001) Developmentalplasticity allows Betula nana to dominate tundra subjected to analtered environment Ecology 82 18ndash32
Bret-Harte MS Shaver GR Chapin FS III (2002) Primary and secondarystem growth in Arctic shrubs implications for community response toenvironmental change Journal of Ecology 90 251ndash267 doi101046j1365-2745200100657x
BuckleyTNAdamsMA(2011)An analyticalmodel of non-photorespiratoryCO2 release in the light and dark in leaves of C3 species based onstoichiometric flux balance Plant Cell amp Environment 34 89ndash112doi101111j1365-3040201002228x
Budde RJ Randall DD (1990) Pea leaf mitochondrial pyruvatedehydrogenase complex is inactivated in vivo in a light-dependentmanner Proceedings of the National Academy of Sciences of theUnited States of America 87 673ndash676 doi101073pnas872673
CahoonSMPSullivanPF ShaverGRWelker JM PostE (2012) Interactionsamong shrub cover and the soil microclimatemay determine futureArcticcarbon budgets Ecology Letters 15 1415ndash1422 doi101111j1461-0248201201865x
Chapin FS Shaver GR (1996) Physiological and growth responses of Arcticplants to a field experiment simulating climatic change Ecology 77822ndash840 doi1023072265504
Chapin FS Shaver GR Giblin AE Nadelhoffer KJ Laundre JA (1995)Responses of Arctic tundra to experimental and observed changes inclimate Ecology 76 694ndash711 doi1023071939337
Chapin FS Sturm M Serreze MC McFadden JP Key JR Lloyd AHMcGuire AD Rupp TS Lynch AH Schimel JP Beringer J ChapmanWLEpsteinHEEuskirchenESHinzmanLD JiaGPingC-LTapeKDThompson CDC Walker DA Welker JM (2005) Role of land-surfacechanges in Arctic summer warming Science 310 657ndash660 doi101126science1117368
Covey-Crump EM Attwood RG Atkin OK (2002) Regulation of rootrespiration in two species of Plantago that differ in relative growthrate the effect of short and long term changes in temperature PlantCell amp Environment 25 1501ndash1513 doi101046j1365-3040200200932x
Crous KY Zaragoza-Castells J Ellsworth DS Duursma RA LowM TissueDT Atkin OK (2012) Light inhibition of leaf respiration in field-grownEucalyptus saligna in whole-tree chambers under elevated atmosphericCO2 and summer drought Plant Cell amp Environment 35 966ndash981doi101111j1365-3040201102465x
Dungan RJ Whitehead D Duncan RP (2003) Seasonal and temperaturedependence of photosynthesis and respiration for two co-occurringbroad-leaved tree species with contrasting leaf phenology TreePhysiology 23 561ndash568 doi101093treephys238561
Elmendorf SC Henry GHR Hollister RD Bjoumlrk RG Bjorkman ADCallaghan TV Collier LS Cooper EJ Cornelissen JHC Day TAFosaa AM Gould WA Greacutetarsdoacutettir J Harte J Hermanutz L Hik DSHofgaard A Jarrad F Joacutensdoacutettir IS Keuper F Klanderud K Klein JAKoh S Kudo G Lang SI Loewen V May JL Mercado J Michelsen AMolau U Myers-Smith IH Oberbauer SF Pieper S Post E Rixen CRobinson CH Schmidt NM Shaver GR Stenstroumlm A Tolvanen ATotland Oslash Troxler T Wahren C-H Webber PJ Welker JM WookeyPA (2012) Global assessment of experimental climate warming ontundra vegetation heterogeneity over space and time Ecology Letters15 164ndash175 doi101111j1461-0248201101716x
EnvironmentalDataCenterTeam (2011)Meteorologicalmonitoring programat Toolik Alaska Toolik Field Station Institute of Arctic BiologyUniversity of Alaska Fairbanks Fairbanks AK 99775 Available athttptoolikalaskaeduedcabiotic_monitoringdata_queryphp
L Functional Plant Biology M A Heskel et al
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
FetcherNShaverGR(1983)Lifehistoriesof tillers ofEriophorumvaginatumin relation to tundra disturbance Journal of Ecology 71 131ndash147doi1023072259967
Gough L Hobbie SE (2003) Responses of moist non-acidic tundra to alteredenvironment productivity biomass and species richness Oikos 103204ndash216 doi101034j1600-0706200312363x
HeskelMAAndersonORAtkinOK TurnbullMHGriffinKL (2012) Leaf-and cell-level carbon cycling responses to a nitrogen and phosphorusgradient in two Arctic tundra species American Journal of Botany 991702ndash1714 doi103732ajb1200251
Heskel MA Greaves HE Kornfeld A Gough L Atkin OK Turnbull MHShaver GR Griffin KL (2013) Differential physiological responses toenvironmental change promote woody shrub expansion Ecology andEvolution 3 1149ndash1162 doi101002ece3525
Huumlve K Bichele I Ivanova H Keerberg O Paumlrnik T Rasulov B Tobias MNiinemetsUuml (2012)Temperature responses of dark respiration in relationsto leaf sugar concentration Physiologia Plantarum 144 320ndash334doi101111j1399-3054201101562x
KirschbaumMUF FarquharGD (1987) Investigation of the CO2 dependenceof quantum yield and respiration in Eucalyptus pauciflora PlantPhysiology 83 1032ndash1036
Kok B (1948) A critical consideration of the quantum yield of Chlorella-photosynthesis Enzymologia 13 1ndash56
KornerCLarcherW(1988)Plant life in coldclimatesSymposiaof theSocietyfor Experimental Biology 42 25ndash57
KornfeldAHeskelMAtkinOKGoughLGriffinKLHortonTWTurnbullMH (2013) Respiratory flexibility and efficiency are affected bysimulated global change in Arctic plants New Phytologist 1971161ndash1172 doi101111nph12083
LorantyMGoetz S Rastetter E RochaA ShaverGHumphreys E Lafleur P(2011) Scaling an instantaneous model of tundra NEE to the Arcticlandscape Ecosystems 14 76ndash93 doi101007s10021-010-9396-4
Mitchell KA Bolstad PV Vose JM (1999) Interspecific and environmentallyinduced variation in foliar dark respiration among eighteen southeasterndeciduous tree species Tree Physiology 19 861ndash870 doi101093treephys1913861
Natali SM Schuur EAG Trucco C Hicks-Pries CE Crummer KG Baron-Lopez AF (2011) Effects of experimental warming of air soil andpermafrost on carbon balance in Alaskan tundra Global ChangeBiology 17 1394ndash1407 doi101111j1365-2486201002303x
OrsquoSullivan OS Weerasinghe KWLK Evans JR Egerton JJG Tjoelker MGAtkin OK (2013) High-resolution temperature responses of leafrespiration in snow gum (Eucalyptus pauciflora) reveal high-temperature limits to respiratory function Plant Cell amp Environment36 1268ndash1284 doi101111pce12057
Oberbauer SF Starr G Pop EW (1998) Effects of extended growing seasonand soil warming on carbon dioxide and methane exchange of tussocktundra in Alaska Journal of Geophysical Research 103 29 075ndash29 082doi10102998JD00522
OberbauerSFTweedieCEWelker JMFahnestock JTHenryGHRWebberPJ Hollister RDWalkerMDKuchyA Elmore E Starr G (2007) TundraCO2 fluxes in response to experimental warming across latitudinal andmoisture gradients Ecological Monographs 77 221ndash238 doi10189006-0649
Oechel W Vourlitis G Hastings S Zulueta R Hinzman L Kane D (2000)Acclimationof ecosystemCO2exchange in theAlaskanArctic in responseto decadal climate warming Nature 406 978ndash981 doi10103835023137
OwLFWhiteheadDWalcroftASTurnbullMH(2010)Seasonalvariation infoliar carbonexchange inPinus radiata andPopulusdeltoides respirationacclimates fully to changes in temperature but photosynthesis does notGlobal Change Biology 16 288ndash302 doi101111j1365-2486200901892x
Pomeroy JW Bewley DS Essery RLH Hedstrom NR Link T Granger RJSicart JE Ellis CR Janowicz JR (2006) Shrub tundra snowmeltHydrological Processes 20 923ndash941 doi101002hyp6124
Post E Forchhammer MC Bret-Harte MS Callaghan TV Christensen TRElberling B Fox AD Gilg O Hik DS Hoye TT Ims RA Jeppesen EKleinDRMadsen JMcGuireAD Rysgaard Sr Schindler DE Stirling ITamstorfMP Tyler NJC van derWal RWelker JWookey PA SchmidtNM Aastrup P (2009) Ecological dynamics across the Arctic associatedwith recent climate change Science 325 1355ndash1358 doi101126science1173113
Poyatos R Gornall J Mencuccini M Huntley B Baxter R (2012) Seasonalcontrols on net branch CO2 assimilation in sub-Arctic mountain birch(Betula pubescens ssp czerepanovii (Orlova) Hamet-Ahti) Agriculturaland Forest Meteorology 158ndash159 90ndash100 doi101016jagrformet201202009
Searle SY Thomas S Griffin KL Horton T Kornfeld A Yakir D Hurry VTurnbull MH (2011) Leaf respiration and alternative oxidase infield-grown alpine grasses respond to natural changes in temperatureand light New Phytologist 189 1027ndash1039 doi101111j1469-8137201003557x
Serreze MC Walsh JE Chapin FS Osterkamp T Dyurgerov MRomanovsky V Oechel WC Morison J Zhang T Barry RG (2000)Observational evidence of recent change in the northern high-latitudeenvironment Climatic Change 46 159ndash207 doi101023A1005504031923
Shapiro JB Griffin KL Lewis JD Tissue DT (2004) Response of Xanthiumstrumarium leaf respiration in the light to elevated CO2 concentrationnitrogen availability and temperature New Phytologist 162 377ndash386doi101111j1469-8137200401046x
Sharkey TD Bernacchi CJ Farquhar GD Singaas EL (2007) Fittingphotosynthetic carbon dioxide response curves for C3 leaves PlantCell and Environment 30 1035ndash1040 doi101111j1365-3040200701710x
ShaverGRBillingsWDChapinFSGiblinAENadelhofferKJOechelWCRastetter EB (1992) Global change and the carbon balance of arcticecosystems Bioscience 42 433ndash441 doi1023071311862
Shaver GR Johnson LC Cades DH Murray G Laundre JA Rastetter EBNadelhoffer KJ Giblin AE (1998) Biomass and CO2 flux in wet sedgetundras responses to nutrients temperature and light EcologicalMonographs 68 75ndash97
Shaver GR Canadell J Chapin FS Gurevitch J Harte J Henry G Ineson PJonasson S Melillo J Pitelka L Rustad L (2000) Global warming andterrestrial ecosystems a conceptual framework for analysis Bioscience50 871ndash882 doi1016410006-3568(2000)050[0871GWATEA]20CO2
Starr G Oberbauer SF Ahlquist LE (2008) The photosynthetic response ofAlaskan tundra plants to increased season length and soilwarmingArcticAntarctic andAlpineResearch40 181ndash191 doi1016571523-0430(06-015)[STARR]20CO2
Stone RS Dutton EG Harris JM Longenecker D (2002) Earlier springsnowmelt in northern Alaska as an indicator of climate changeJournal of Geophysical Research Atmospheres 107 4089 doi1010292000JD000286
Sullivan P Sommerkorn M Rueth H Nadelhoffer K Shaver G Welker J(2007) Climate and species affect fine root production with long-termfertilization in acidic tussock tundra near Toolik Lake AlaskaOecologia153 643ndash652 doi101007s00442-007-0753-8
Seasonal carbon exchange under long-term warming Functional Plant Biology M
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
N Functional Plant Biology M A Heskel et al
wwwpublishcsiroaujournalsfpb
Sullivan P Arens S Chimner R Welker J (2008) Temperature andmicrotopography interact to control carbon cycling in a high Arcticfen Ecosystems 11 61ndash76 doi101007s10021-007-9107-y
Tcherkez G Cornic G Bligny R Gout E Ghashghaie J (2005) In vivorespiratory metabolism of illuminated leaves Plant Physiology 1381596ndash1606 doi101104pp105062141
Tcherkez G Bligny R Gout E Mahe A Hodges M Cornic G (2008)Respiratory metabolism of illuminated leaves depends on CO2 and O2
conditions Proceedings of the National Academy of Sciences of theUnited States of America 105 797ndash802 doi101073pnas0708947105
Tcherkez G Mahe A Gauthier P Mauve C Gout E Bligny R Cornic GHodgesM (2009) In folio respiratory fluxomics revealed by 13C isotopiclabeling and HD isotope effects highlight the noncyclic nature of thetricarboxylic acid ldquocyclerdquo in illuminated leaves Plant Physiology 151620ndash630 doi101104pp109142976
Tissue DT Lewis JD Wullschleger SD Amthor JS Griffin KL AndersonOR (2002) Leaf respiration at different canopy positions in sweetgum(Liquidambar styraciflua) grown in ambient and elevated concentrationsof carbon dioxide in the field Tree Physiology 22 1157ndash1166doi101093treephys2215-161157
TurnbullMHWhiteheadDTissueDTSchusterWSFBrownKJGriffinKL(2003) Scaling foliar respiration in two contrasting forest canopiesFunctional Ecology 17 101ndash114 doi101046j1365-2435200300713x
Vose J RyanM (2002) Seasonal respiration of foliage fine roots and woodytissues in relation to growth tissueN and photosynthesisGlobal ChangeBiology 8 182ndash193 doi101046j1365-2486200200464x
Vourlitis GL Oechel WC (1999) Eddy covariance measurements of CO2
and energy fluxes of an Alaskan tussock tundra ecosystem Ecology 80686ndash701 doi1018900012-9658(1999)080[0686ECMOCA]20CO2
Vourlitis GL Harazono Y Oechel WC Yoshimoto M Mano M (2000)Spatial and temporal variations in hectare-scale net CO2 flux respiration
and gross primary production of Arctic tundra ecosystems FunctionalEcology 14 203ndash214 doi101046j1365-2435200000419x
Wang X Lewis JD Tissue DT Seemann JR Griffin KL (2001) Effects ofelevated atmospheric CO2 concentration on leaf dark respiration ofXanthium strumarium in light and in darkness Proceedings of theNational Academy of Sciences of the United States of America 982479ndash2484 doi101073pnas051622998
Welker JM Fahnestock JT Jones MH (2000) Annual CO2 flux in dry andmoist Arctic tundra field responses to increases in summer temperaturesand winter snow depth Climatic Change 44 139ndash150 doi101023A1005555012742
Welker JM Fahnestock JT Henry GHR OrsquoDea KW Chimner RA (2004)CO2 exchange in three Canadian high Arctic ecosystems response tolong-term experimental warming Global Change Biology 101981ndash1995 doi101111j1365-2486200400857x
Wookey PA Aerts R Bardgett RD Baptist F BrathenKA Cornelissen JHCGough L Hartley IP Hopkins DW Lavorel S Shaver GR (2009)Ecosystem feedbacks and cascade processes understanding their rolein the responses ofArctic and alpine ecosystems to environmental changeGlobal Change Biology 15 1153ndash1172 doi101111j1365-2486200801801x
Xu LK Baldocchi DD (2003) Seasonal trends in photosynthetic parametersand stomatal conductance of blue oak (Quercus douglasii) underprolonged summer drought and high temperature Tree Physiology 23865ndash877 doi101093treephys2313865
Xu C Schuster W Griffin KL (2007) Seasonal variation of temperatureresponse of respiration in invasive Berberis thunbergii (Japanesebarberry) and two co-occurring native understory shrubs in anortheasternUS deciduous forestOecologia 153 809ndash819 doi101007s00442-007-0790-3
