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Intraspecific variation in response to warming acrosslevels of organization: a test with Solidago altissima

LARA SOUZA,1,� DAVE J. WESTON,2 NATHAN J. SANDERS,1 ABHIJIT KARVE,2 GREGORY M. CRUTSINGER,1,3

AND AIMEE T. CLASSEN1

1Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, Tennessee 37996 USA2Biosciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831 USA

3Department of Zoology, University of British Columbia, Vancouver, British Columbia V6T 1Z4 Canada

Citation: Souza, L., D. J. Weston, N. J. Sanders, A. Karve, G. M. Crutsinger, and A. T. Classen. 2011. Intraspecific

variation in response to warming across levels of organization: a test with Solidago altissima. Ecosphere 2(12):132. doi:10.

1890/ES11-00283.1

Abstract. Plant species, and the traits associated with them, can help buffer ecosystems to

environmental perturbations. Few studies have examined whether within species variation, both among

and within populations, can similarly buffer ecosystems to environmental perturbations, such as climatic

warming, across levels of organization. Using a dominant plant species in the eastern US, Solidago altissima,

we examined whether genotypes of the same species from both southern and northern latitude populations

exhibited differential short-term responses to temperature at the cell, leaf, and plant level. At the cell level

we quantified the production of reactive oxygen species (by-product of temperature stress) and total

oxygen radical antioxidant capacity (which ameliorates temperature stress by-products). At the leaf and

plant levels, we measured CO2 assimilation. Increasing temperatures had strong negative impacts on plant-

level carbon gain, but weak impacts on cell-level antioxidant capacity. Southern latitude genotypes had

greater total antioxidant capacity, but lower leaf-level carbon gain, than did northern genotypes under

elevated temperature. At the plant level, northern and southern genotypes exhibited similar declines in

carbon gain under elevated temperature, likely because total plant leaf area was higher for southern

genotypes than northern genotypes, which compensated for their lower per unit area leaf-level carbon

gain. Overall, short-term temperature-induced declines in carbon gain at the plant level may scale to

reduce within species variation, both across and within populations, potentially altering ecosystem carbon

cycling.

Key words: carbon gain; cell level; leaf level; northern genotypes; plant level; population; Solidago altissima; southern

genotypes; temperature; warming.

Received 3 October 2011; revised and accepted 2 November 2011; published 13 December 2011. Corresponding Editor:

D. P. C. Peters.

Copyright: � 2011 Souza et al. This is an open-access article distributed under the terms of the Creative Commons

Attribution License, which permits restricted use, distribution, and reproduction in any medium, provided the original

author and sources are credited.

� E-mail: [email protected]

INTRODUCTION

Predicting the responses of species to climate

change is a fundamental challenge, especially

near range boundaries where the effects of

warming are likely to be most dramatic (Angert

2011, De Frenne 2011a). Plant species can vary

considerably in morphological and physiological

traits both within and across latitudinal gradients

and such variation may influence that species’

responses to climate change along with associat-

ed ecosystem processes such as net primary

productivity (NPP) and carbon (C) cycling

(Hooper et al. 2005). Studies on plants along

v www.esajournals.org 1 December 2011 v Volume 2(12) v Article 132

temperature gradients show there is extensivetrait variation among and within species, espe-cially between northern and southern popula-tions (Diaz et al. 2004, De Frenne et al. 2011b).Linking plant traits to ecosystem processeswithin and across species along latitudinalgradients may lead to a better understanding ofthe underlying processes shaping C cyclinglocally and globally in the context of climaticchange.

Variation in morphological and physiologicaltraits within species, like variation across species,has been shown to influence individual perfor-mance, community dynamics and ecosystemprocesses (Booth and Grime 2003, Schweitzer etal. 2005, Crutsinger et al. 2006, Johnson et al.2006, Classen et al. 2007, Hughes et al. 2008,Cianciaruso et al. 2009, Crutsinger et al. 2009,Albert et al. 2010, Jung et al. 2010). Crutsinger etal. (2006) documented that plots with greaterintra-specific diversity in a dominant old-fieldspecies (Solidago altissima) led to greater above-ground net primary productivity (ANPP) andhigher arthropod diversity than plots with lowerintra-specific diversity. One interpretation of thisresult is that variance in physiological andmorphological traits among Solidago altissimagenotypes promoted greater resource comple-mentarity in diversity treatments generatinggreater ANPP and greater diversity acrosstrophic levels (Hooper et al. 2005). Few studies,however, have examined the influence of within-species variation, especially comparing individ-uals from range boundaries (De Frenne et al.2011a), in terms of how such trait variationinfluences C cycling under warming.

To date, research addressing the impact ofclimatic warming on individual species largelyfocused on a single level of organization withoutexamining whether the observed responses towarming scale across levels of organization(Petchey et al. 2004, Bailey et al. 2009, Purdy etal. 2010, Woodward et al. 2010). Scaling bio-chemical and physiological constraints to elevat-ed temperature, from cells to leaves toindividuals may enable scientists to better predictthe underlying processes associated with atmo-spheric and climatic change impacts at local andglobal scales. At the lowest level of organization,the cell level, studies have documented anincrease in the production of reactive oxygen

species (ROS) H2O2 (Chen and Hartman 1995,Kim and Portis 2004) under elevated temperaturelinked to reduced photosynthetic capacity bothwithin and across species (Snider et al. 2010,Weston et al. 2011). The ability of species and/orgenotypes to scavenge ROS therefore will deter-mine leaf and potentially plant-level photosyn-thetic capacity under warming (e.g., reviewed inBerry and Bjorkman 1980). Recently a number ofstudies have implicated increased cell-level anti-oxidant activity with enhanced thermal toleranceamong plant genotypes (Larkindale and Huang2004, Snider et al. 2010). Furthermore, linkingbiochemical and physiological constraints tomorphological trait variation will elucidate po-tential trade-offs in temperature responses bothwithin and across species (Bailey et al. 2009).

Our study addressed differences in tempera-ture responses at the cell (ROS production andtotal antioxidant capacity), leaf, and plant level(CO2 assimilation) among genotypes from pop-ulations near the range boundaries of a dominantold-field plant species distributed along a latitu-dinal gradient. We assessed whether constraintsto heat stress scaled across levels of organizationamong genotypes in the same species andwhether temperature responses vary in northernvs. southern Solidago genotypes. To examinewhether temperature may differentially alter theresponses of Solidago genotypes from southernand northern populations, we conducted agrowth chamber experiment. We measuredtemperature response curves, across 8 tempera-ture levels, on southern and northern latitudegenotypes of Solidago altissima and quantifiedcell, leaf, and plant level responses. We predictedthat: (1) Solidago temperature responses would begreatest at the lowest level organization, the celllevel, because of fewer levels below to counteractthe overall effect of temperature. Solidago tem-perature responses would be the lowest at thehighest level of organization, the plant level,because of several levels below that would likelycounteract the overall effect of temperature.Thus, structure and functional complexity athigher levels of organization may buffer warm-ing effects, (2) greater cell-level antioxidantcapacity of southern than northern populationsunder heat stress would promote greater leaf andplant-level C gain due to the reduction of heatstress by-products that may be detrimental to


SOUZA ET AL.

photosynthetic capacity, and (3) variation inmorphological traits across southern and north-ern populations will likely shape temperatureresponses as bigger individuals from southernpopulations may outperform smaller individualsfrom northern populations simply by havinggreater C reserves for allocation toward heatstress defense and C gain than northern popula-tions.

METHODS

Solidago altissima (hereafter Solidago) is adominant plant species in old-field ecosystems(Schmitz 2003, Abrahamson et al. 2005, Schmitz2008, Wise and Abrahamson 2008, Souza et al.2011; Souza et al., in press), across a range ofthermal zones in eastern North America, fromFlorida (average January and July temperatures¼26.68C and 32.38C, respectively) to Maine (aver-age January and July temperatures ¼ 4.28C and26.68C, respectively), and has significant mor-phological trait variation shaping ecosystemprocesses (Crutsinger et al. 2006, Crutsinger etal. 2008, Crutsinger et al. 2009). Additionally,predicted increases in global temperatures rangefrom 1.5–1.98C to 3.4–6.18C, which would raisethe average growing season temperature ofSolidago up to ;378C in Florida and ;318C inMaine.

We chose genotypes from near the northernand southern ends of Solidago’s geographicdistribution because we were interested indocumenting temperature responses of ‘warm-adapted’ (southern individuals) and ‘cool-adapt-ed’ (northern individuals) populations. We col-lected 35 Solidago genotypes across northern (15genotypes from three old-field sites near Stafford,Connecticut) and southern (20 genotypes fromthree old-field sites near Oak Ridge, Tennessee)latitude populations in 2009. Solidago genotypesfrom northern populations experience a meanannual temperature of 15.58C (winter ¼�0.48C,summer¼ 23.28C), while Solidago genotypes fromsouthern populations experienced a mean annualtemperature of 22.58C (winter¼ 2.48C, summer¼24.88C). Each old-field site we surveyed, innorthern and southern locations, represented acontinuous Solidago population. We excavatedrhizomes from each Solidago genotype fromnatural patches growing 50–150 m apart. Given

the majority of Solidago’s reproduction takesplace vegetatively rather than sexually, suchspatial separation (50–150 m) between patchesensured that we were obtaining different indi-viduals (i.e., genotypes) (Crutsinger et al. 2006).Rhizomes were cut into 3-cm sections, placed inflats containing sterilized potting soil (Pro-MixBX, Premier Brands, New Rochelle, NY) and rootstimulator (Roots 2, Roots OSIA, Independence,MO). They were grown in a greenhouse (at 258C)for 12 weeks and fertilized as needed. Northernand southern populations did not differ inrhizome volume (F¼ 0.0004, P¼ 0.98). Similarly,within southern populations rhizome volumedid not differ among genotypes (F ¼ 1.26, P ¼0.37), but at northern populations there was asignificant difference in rhizome volumesamongst genotypes (F¼ 6.72, P¼ 0.01). Previousstudies addressing temperature responses withinand across species have also grown species and/or genotypes at 258C prior to conductingtemperature response curves (Snider et al. 2010,Weston et al. 2011).

In June and July of 2009, we selected tenSolidago genotypes that significantly diverged inmorphological traits (i.e., represented a full rangeof morphological traits within northern andwithin southern genotypes) from across northernand southern populations (southern n ¼ 5,northern n ¼ 5). The morphological traits weused to select genotypes included: leaf width,leaf length, leaf area, and internode space. Theselected genotypes originated from all threesampled populations in southern and northernlocations, and for each genotype we had threereplicate individuals. We then placed individualsacross genotypes in a growth chamber, acclimat-ed the plants to 228C over 24 hours so genotypescould better adjust to the temperature curves (inparticular the low temperature ends). We thenexposed all of the genotypes to the followingtemperatures: 148C, 188C, 228C, 268C, 308C, 348C,388C, 428C to generate photosynthetic tempera-ture response curves (Berry and Bjorkman 1980).At each of the eight temperature levels, plantswere allowed to acclimate for approximately 30minutes. Cell (ROS and total antioxidant capac-ity) and plant-level C gain were measured atsuboptimum (148C), optimum (268C), and inhi-bition (388C) temperatures (Weston et al. 2011);leaf-level C gain was measured at each of the 8


SOUZA ET AL.

temperature levels. Growth chamber and cuvette(leaf and plant) were maintained at 55% relativehumidity (average leaf-level VPD ¼ 1.6). Tem-perature responses at the cell, leaf, and plantlevels were measured between 9:30 and 11:30 amand between 11:30 am and 2:00 pm which is themost photosynthetically active time period. Wealso measured two morphological traits that canshape short-term temperature responses at thecell, leaf and plant levels but are independent ofthe manipulations (Crutsinger et al. 2006, Crut-singer et al. 2008): total plant leaf area (cm2) andtotal aboveground biomass. For total leaf area,we clipped all leaves of each individual and ranthem through a leaf area scanner (WinFolia,Ottawa, Canada). We then clipped the stems andplaced both leaves and stems into paper bagsand oven dried them at 658C for approximately48 hours to obtain total aboveground biomass.

To determine the short-term cell-level responseto temperature, we measured the production ofreactive oxygen species (ROS) and oxygenradical antioxidant capacity (ORAC). We esti-mated ROS production using the total H2O2

assay where we extracted 500 l cold leafextraction buffer (10 mM Tris-HCL pH 7.3, 1mM EDTA, 20% glycerol, 2 mM DTT and 1 mMPMSF, all from EMD, Gibbstown, NJ) andcentrifuged the extract at 13,000 rpm for 10 minat 48C. The oxidation of H2DCF-DA to fluores-cent 2’, 7’-dichlorofluoriscein (DCF) was mea-sured using a Fluoroscan Ascent microplatefluorometer (Volkov et al. 2006). Total antioxi-dant capacity was estimated using the ORACassay using fluorescein (Sigma-Aldrich). Decayarea curves were generated from a microplatefluorometer and total protein concentration wasestimated with Coomassie Blue assay usingQuick start Bradford reagent (Bio-Rad, Hercules,CA) (Gillespie et al. 2007). We determined totalprotein concentration with Coomassie Blue assayusing Quick start Bradford reagent (Bio-Rad,Hercules, CA), but we found no differencesbetween northern and southern genotypes intotal protein concentration (P ¼ 0.49, F ¼ 0.52).

To examine leaf-level CO2 assimilation (i.e., Cgain), we took instantaneous measurements ofleaf net photosynthetic rate using a LI-6400 gasexchange system (Li-Cor, NE, USA). We placedone attached Solidago leaf per genotype in acuvette and allowed it to acclimate (between 5–

15 minutes) to saturating PPFD (1500 lmol m�2

s�1), temperature, relative humidity, and ambient[CO2]. We recorded leaf-level CO2 assimilation ateach temperature level; data are expressed as therate of C gain per unit leaf area (m2) per second.

We measured plant-level CO2 assimilation(hereafter C gain), using an LI-7500 infrared gasanalyzer (IRGA; Li-Cor). It was housed in a plantcuvette (0.75 m wide3 0.75 m long3 0.75 m tall)attached to a laptop with two fans mixing the airwithin the plant cuvette. The cuvette was coveredwith tightly sewn polyethylene sheet (ShelterSystems, CA) (Arnone and Obrist 2003, Huxmanet al. 2004, Potts et al. 2006). Specifically, in eachplant-level measurement, we placed a singleindividual Solidago genotype at the center of thelarge cuvette, and adjacent to the plant we placedthe IRGAwith fans mounted on the plant cuvettemetal frame. At the base of the cuvette, weinstalled foam that was impermeable to [CO2] toensure the cuvette system was well-sealedminimizing CO2 leaks. CO2 concentrations wereallowed to draw down overtime, and from suchdraw down CO2 flux was determined. We didnot record CO2 flux data for the first 20 secondsthat the plant cuvette was put in place to allow acomplete mix of air within the chamber. We thenrecorded the concentration of CO2 overtime forapproximately 90 seconds in order to calculatethe slope of change in CO2 concentration acrosstime. We corrected the slope of CO2 depletionovertime with the chamber volume and correctedthese values with total plant leaf area in order toobtain a plant-level C gain value.

In order to compare the responses of cells,leaves, and whole plants, we estimated the effectsizes of warming at the cell, leaf, and plant levelas the log-response ratio (ln R)

lnR ¼ Xe

Xa

� �

where Xe is the mean of the response variable foreach northern and each southern Solidago geno-type (n ¼ 5) under elevated temperature (388C)and Xa is the mean of the response variables oreach northern and each southern Solidago geno-type (n ¼ 5) under ambient conditions (268C)(Hedges et al. 1999). Positive or negative valuesindicate a positive or negative effect of warmingon cell-, leaf-, and plant-level responses. A two-way analysis of variance (ANOVA) followed by a


SOUZA ET AL.

Tukey HSD mean separation test was used todetermine if the log-response ratio differedamong the cell, leaf, or plant level, betweennorthern and southern populations, and if therewas an interaction between population sourceand level of organization.

The impacts of warming on northern andsouthern genotypes were investigated usingone-way ANOVA models with populationsource (northern, southern) as the main effect ateach temperature (148C, 268C, 388C) and totalantioxidant capacity (cell level) and C gain (leafand plant level) as the response variables. Tobetter understand how trait variation within aspecies shapes short-term temperature responses,we used linear regression to examine whethervariation in total aboveground biomass, acrosssouthern and northern genotypes, predictedtemperature responses at the cell (total antioxi-dant capacity) and leaf level (C gain).

RESULTS

We found the standardized effect of warmingto be the greatest at the highest level oforganization (plant level) and to be smallest atthe lowest levels of organization (leaf and celllevels) (F ¼ 8.95, P ¼ 0.0012). In other words,warming did not have a negative impact on cell-level antioxidant capacity, but it had a strongnegative impact on plant-level C gain. We foundan interaction between level of organization (cell,leaf, plant) and population source (northern vs.southern population) on the standardized effectof warming (Fig. 1, F ¼ 5.05, P ¼ 0.0026). Thestandardized effect of warming, for the northernpopulation, was significantly more negative atplant-level C gain than at both leaf-level C gainand cell-level antioxidant capacity. The standard-ized effect of warming for the southern popula-tion, however, was significantly more negative atthe plant level (C gain) than cell level (antioxi-dant capacity), but did not differ from leaf-levelC gain.

Solidago genotypes from northern and southernpopulations varied in their response to increasingtemperature at the cell and leaf, but not at theplant level. At the cell level, southern genotypeshad 32% greater production of reactive oxygenspecies and antioxidants under elevated temper-ature than did northern genotypes (Fig. 2, Table

1; Appendix A: Fig. A1, Appendix B: Fig. B1). Atthe leaf level, southern genotypes had 25% lowerleaf-level C gain than northern genotypes indi-cating a potential tradeoff between defense at thecell level (higher ORAC production) and invest-ment towards leaf growth (lower C uptake)under warming (Fig. 2, Table 1). Nonetheless,plant-level C gain significantly declined withincreasing temperature across northern andsouthern genotypes (Fig. 2, Table 1). Finally, wefound that both leaf and plant-level C gain didnot differ within northern (F¼ 0.195, P¼ 0.93; F¼0.110, P ¼ 0.972) and southern (F ¼ 0.792, P ¼0.566; F ¼ 0.858, P ¼ 0.557) populations. Thisresult indicates that warming will negativelyimpact individual genotypes in northern andsouthern ranges equally (Appendix C: Fig. C1,Appendix D: Fig. D1).

Trait variation across northern and southerngenotypes, at the plant level, predicted temper-ature responses at the cell and leaf level.Southern genotypes had 30% more leaf area (P¼ 0.02, F ¼ 8.13, df ¼ 1, 8) and 40% greateraboveground biomass (P¼ 0.01, F¼ 11.62, df¼ 1,8) than northern genotypes. Variation in leaf area

Fig. 1. Mean (6 95% CI) standardized effect size of

temperature on carbon processes between populations

(northern and southern) at the cell, leaf, and plant

level. Temperature effects were greatest at the highest

level of organization (plant level) and lowest at the

smallest level of organization (cell level).


SOUZA ET AL.

and biomass ranged from 505–726 cm2 and 5–10g in southern populations and 295–533 cm2 and3–5 g in northern populations. Solidago genotypeswith more biomass, generally from southernpopulations, tended to have both higher cell-level antioxidant capacity (Fig. 3, R2 ¼ 0.36, P ¼0.065) and lower leaf-level C gain than northerngenotypes with less biomass (R2¼0.52, P¼0.028)(Fig. 3).

DISCUSSION

Temperature effects on Solidago individuals

were greatest at the plant level and weakest at

the cell level. Even though we predicted temper-

ature responses to be the weakest at the highest

level of organization, the plant level, levels below

it likely responded similarly without counteract-

Fig. 2. Mean (6 SE) temperature response at the cell, reactive oxygen species (ROS) and total antioxidant

capacity (ORAC); and at the leaf and plant level (CO2 assimilation) between northern and southern populations.

Cell- and leaf-level responses to temperature differed between northern and southern populations, yet plant-level

responses to temperature were similar between northern and southern populations.


SOUZA ET AL.

ing overall temperature effects. Elevated temper-ature appears to have greater negative effects athigher levels of organization despite increasingstructural and functional complexity. Previousstudies have addressed temperature effects at thecell and leaf levels, not at the plant level wherewe found temperature effects to play the stron-gest role. Thus adding a level of complexity(plant level) might explain the divergence ofresults found in previous studies (Petchey et al.2004, Bailey et al. 2009, Purdy et al. 2010,Woodward et al. 2010). Although our studydemonstrated that short-term temperature effectswere stronger at higher levels of organization(i.e., plant level), it is clear that future studiesshould address both short- and long-term effectsof temperature at even higher (community andecosystem) levels of organization as in De Frenne(2011a).

Southern Solidago genotypes had higher anti-oxidant capacity and lower leaf-level C gain thannorthern genotypes indicating a potential trade-off between defense at the cell level (higherORAC production) and investment towardsgrowth at the leaf level (lower C gain). Thissuggests that southern genotypes, which experi-ence longer and warmer growing seasons thannorthern genotypes, have greater antioxidantcapacity to combat the by-products of warming.These results support previous findings docu-menting both inter- and intra-specific variation inantioxidant capacity under heat stress generated

from local adaptation (Snider et al. 2010).

Likewise, recent studies show species from

southern ranges outperform northern range

species under climatic warming (Parmesan

1996, Parmesan et al. 1999, Parmesan and Yohe

2003, Walther 2010). Our results suggest that if

cell-level antioxidant capacity promotes thermal

tolerance within Solidago, then southern popula-

tions may persist while northern populations

may not, potentially leading towards a reduction

of geographic range, assuming dispersal is

minimal.

Our findings contradict previous work indi-

Table 1. Impacts of warming (ANOVA) on southern

and northern genotypes at the cell (ROS, ORAC),

leaf, and plant level (C gain). Significant P-values are

bold (a ¼ 0.10).

Metric df MS F P

148CPlant-level C gain 1, 7 2.2 3 10�4 0.01 0.93Leaf-level C gain 1, 7 0.44 0.31 0.59Cell-level ROS 1, 8 4.19 3 1012 3.81 0.09Cell-level ORAC 1, 8 2649.6 1.89 0.21

268CPlant-level C gain 1, 7 0.02 1.45 0.27Leaf-level C gain 1, 7 4.4 3.57 0.1Cell-level ROS 1, 8 5.29 3 1012 7.25 0.03Cell-level ORAC 1, 8 4305.58 9.42 0.02

388CPlant-level C gain 1, 7 0.001 0.86 0.38Leaf-level C gain 1, 7 10.97 15.4 0.01Cell-level ROS 1, 8 5.35 3 1012 5.84 0.04Cell-level ORAC 1, 8 6179 2.9 0.12

Fig. 3. Intra-specific variation in aboveground

biomass across northern and southern Solidago indi-

viduals vs. cell-level ORAC (bottom panel) and vs.

leaf-level CO2 assimilation (top panel). The square

point has not been considered in the analysis, but is

shown in the figure, because it was identified as an

outlier. Overall, variation in aboveground biomass

between northern and southern populations influ-

enced cell- and leaf-level responses to temperature.


SOUZA ET AL.

cating that thermal tolerance at the cell levelmaintains C gain at the leaf level. This contra-diction may be due to the lack of temperatureacclimation of fully expanded leaves in our studyrelative to measurements made on newly devel-oped leaves in other studies (Atkin and Tjoelker2003). Unlike leaf-level responses, plant C gaindid not differ between northern and southernpopulations because southern genotypes hadgreater leaf area than northern genotypes therebycompensating for lower C gain at the leaf level.In fact, total plant leaf area predicted total plantC gain (R2¼ 0.4, P , 0.005) whereby increases intotal plant leaf area equaled increased rates ofCO2 assimilation. Overall, our findings suggestthat short-term temperature effects can negative-ly impact C gain at the individual level in thesame ways among northern and southern Soli-dago genotypes. Further our results indicate thata reduction in population size and loss ofgenotypic variation in this dominant plantspecies could occur with increasing atmospherictemperatures - an idea that needs to be tested inlarge-scale, long-term field manipulations. Whileshort-term temperature exposure provides uswith valuable information on responses withinand across populations to climatic warming,long-term studies and experiments could providea more realistic picture of plant populationresponses to changing temperature.

Intraspecific variation in morphological traits,such as plant leaf area or aboveground biomass,can shape both short-and long-term plant re-sponses to global change. We found that intra-specific variation in Solidago abovegroundbiomass across northern and southern genotypesshaped both cell- and leaf-level responses toelevated temperature. Understanding the role oftrait variation and its relationship to variation infunction is key to understanding key physiolog-ical processes in the context of global change andpredict the consequences of such changes (Re-usch et al. 2005, Hughes et al. 2009).

CONCLUSIONS

The most salient results of our experiments arethat (1) temperature effects are more detrimentalto C processes at the plant level leading to thedecline in C gain across Solidago populations; (2)greater cell-level antioxidant capacity in southern

genotypes than northern genotypes does notinfer greater leaf and plant-level C gain; and (3)intra-specific variation in temperature responsesbetween northern and southern genotypes likelyresults from tradeoffs between higher cell de-fense at a cost of lower leaf-level C uptake underwarming. Overall, the negative impact of short-term warming on plant-level C gain acrossnorthern and southern genotypes could lead toa reduction in individual (i.e., genotypic) varia-tion both within and between populations atrange boundaries. The consequences for thesereductions for other key ecosystem processesremain an open, but important, question to beaddressed. To test the generality of our findingsfuture studies should address long-term effects ofwarming within and across species and acrosslevels of biological organization including alarger sample of genotypes and several species,thereby increasing ecological inference of gener-ated results. Long-term growth chamber studieswould allow plants to be grown under differenttemperature regimes providing a more realisticscenario of warming effects.

ACKNOWLEDGMENTS

Thanks to L. Breza, M. Cregger, O. Schmitz and H.Smith for assisting with plant collections and to S.Allen, L. Gunter and H. Tran for assisting with growthchamber experiments. UT Science Alliance Program(Joint Directed Research and Development) and theLaboratory Directed Research and Development Pro-gram of ORNL, managed by UT-Battelle, LLC, for theU. S. Department of Energy under contract DE-AC05-000R22725 sponsored the research. LS was supportedby an American Fellowship from AAUW.

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SUPPLEMENTAL MATERIAL

APPENDIX A

Fig. A1. Reactive oxygen species production at the cell level in northern (top panel) and southern (bottom

panel) genotypes at three temperatures (148C, 268C, and 388C).


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APPENDIX B

Fig. B1. Antioxidant capacity production at the cell level in northern (top panel) and southern (bottom panel)

genotypes at three temperatures (148C, 268C, and 388C).


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APPENDIX C

Fig. C1. Mean CO2 assimilation and SE at the leaf level in northern (top panel) and southern (bottom panel)



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APPENDIX D

Fig. D1. Mean CO2 assimilation and SE at the plant level in northern (top panel) and southern (bottom panel)



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intraspecific variation in response to warming across ...web.utk.edu/~aclassen/lab_news_files/souza...

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