Effects of elevated temperature and [CO2] on ...

943

Can. J. For. Res. Downloaded from www.nrcresearchpress.com by UNIVERSITY OF GEORGIA on 01/05/13 For personal use only.

Effects of elevated temperature and [CO2] on photosynthesis, leaf respiration, and biomass accumulation of Pinus taeda seedlings at a cool and a warm site within the species’ current range Timothy M. Wertin, Mary Anne McGuire, Marc van Iersel, John M. Ruter, and Robert O. Teskey

Abstract: We examined the influence of elevated temperature (ambient +2 °C) and atmospheric CO2 concentration ([CO2]) (700 µmol·mol–1), applied singly and in combination, on biomass accumulation and the temperature response of net photosynthesis (Anet) and leaf respiration (Rd) of loblolly pine (Pinus taeda L.) seedlings grown simultaneously at a northern and a southern site within the species’ range. We used this experimental approach to determine if the response to future climate conditions would differ between a warm and cool location within a species’ range. Seedling biomass accumulation and the temperature responses of Anet and Rd were measured throughout the growing season. Biomass accumulation was substantially greater at the warmer site compared with the cooler site regardless of treatment. At each site, biomass accumulation was greater in the elevated temperature treatment compared with the ambient treatment. There was substantial acclimation of Rd, but not Anet, to site and to temperature treatment at each site. Elevated [CO2] increased biomass accumulation and Anet at both sites and in both temperature treatments. Our study provides an indication that future projected increases in [CO2] and air temperature of 700 µmol·mol–1 and +2 °C, respectively, are likely to increase loblolly pine growth in most, if not all, of its current range. Résumé : Nous avons étudié l’influence, individuelle et combinée, d’une augmentation de la température (température ambiante +2 °C) et de la concentration de CO2 atmosphérique (700 µmol·mol–1) sur l’accumulation de biomasse et la réponse à la température de la photosynthèse nette (Anette) et de la respiration foliaire (Rd) de semis de pin à encens (Pinus taeda L.) cultivés simultanément dans des stations méridionale et septentrionale à l’intérieur de l’aire de répartition de l’espèce. Nous avons utilisé cette approche expérimentale pour déterminer si la réaction aux conditions climatiques futures serait différente dans les zones plus chaudes et les zones plus froides à l’intérieur de l’aire de répartition d’une espèce. L’accumulation de biomasse par les semis et les réponses de Anette et de Rd à la température ont été mesurées pendant toute la saison de croissance. L’accumulation de biomasse était nettement plus importante dans la station la plus chaude comparativement à la station la plus froide peu importe le traitement. Dans chaque station, l’accumulation de biomasse était plus importante à température élevée qu’à température ambiante. Il y avait une importante acclimatation de Rd mais pas de Anette à la station et à la température dans chaque station. Une concentration élevée de CO2 a entraîné une augmentation de l’accumulation de biomasse et de Anette dans les deux stations et les deux traitements de température. Notre étude fournit un indice que les augmentations anticipées de la concentration de CO2 et de la température, de respectivement 700 µmol mol–1 et +2 °C, vont probablement entraîner une augmentation de la croissance du pin à encens presque partout, sinon partout, dans son aire de répartition actuelle. [Traduit par la Rédaction]

Introduction Most studies investigating increases in air temperature and tree growth report that elevated temperature increases growth (Way and Oren 2010). The positive effect of an increase in air temperature on tree growth has been demonstrated in growth chambers where a set daytime and nighttime temper-

ature were used (Hoch and Korner 2009; Ghannoum et al. 2010) as well as in closed-top chambers (Peltola et al. 2002; Kuokkanen et al. 2004; Bronson et al. 2009), open-top chambers (Danby and Hik 2007; Yin et al. 2008), and infrared heater (Mäenpää et al. 2011) experiments where elevated air temperature tracked ambient air temperature. The response has been observed in seedlings and mature trees of conifers

Received 23 January 2012. Accepted 15 March 2012. Published at www.nrcresearchpress.com/cjfr on 20 April 2012. T.M. Wertin, M.A. McGuire, and R.O. Teskey. Daniel B. Warnell School of Forestry and Natural Resources, University of Georgia, Athens, GA 30602, USA. M. van Iersel. Department of Horticulture, University of Georgia, Athens, GA 30602, USA. J.M. Ruter. Department of Horticulture, University of Georgia Coastal Plain Station, Tifton, GA 31793, USA. Corresponding author: Timothy M. Wertin (e-mail: [email protected]). Can. J. For. Res. 42: 943–957 (2012)

doi:10.1139/X2012-050

Published by NRC Research Press

and deciduous species (Way and Oren 2010). The increase in growth in response to an increase in temperature reported in these studies has been attributed to an increase in growing season length (Bronson et al. 2009; Hall et al. 2009) and to air temperatures that are closer to the optimum for photosynthesis (Xu et al. 2007). However, a positive growth response to an increase in temperature is not universal. Two studies have reported that an increase in mean growing season air temperature caused a decrease in tree growth (Norby et al. 2000; Wertin et al. 2011). Both studies were at sites near the southern (warmer) geographic limit of the species’ range, suggesting that increased temperatures at these sites may have been supra-optimal for net carbon gain and growth. In one study, seedlings of sugar maple (Acer saccharum Marsh.) had reduced stem height (21%) and stem diameter (16%) with an increase in air temperature of +3.5 °C (Norby et al. 2000). In the other, seedlings of northern red oak (Quercus rubra L.) had 18 and 36% less biomass in +3 and +6 °C elevated air temperature treatments, respectively, compared with seedlings grown at ambient temperature (Wertin et al. 2011). An analysis of Scots pine (Pinus sylvestris L.) provenance trials also indicated that an increase in air temperature of 1–4 °C at the southern edge of its European distribution would cause a significant decrease in height growth and an increase in mortality (Reich and Oleksyn 2008). There have been many studies of the effects of elevated CO2 concentration ([CO2]) on tree species and almost all have shown an increase in carbon gain and growth due to increased [CO2] (Gunderson and Wullschleger 1994; Ainsworth and Rogers 2007). Loblolly pine (Pinus taeda L.) is among the most well-studied species in this regard. Studies have also consistently shown an increase in net photosynthesis (Anet) of P. taeda under elevated [CO2] (e.g., Lewis et al. 1996; Wertin et al. 2010). In open-top chamber studies, with mean elevated [CO2] ranging from 660 to 800 µmol·mol–1, P. taeda seedling biomass was 21%–43% greater than in ambient [CO2] (360–375 µmol·mol–1) (e.g., Tschaplinski et al. 1993; Friend et al. 2000; Gavazzi et al. 2000). In P. taeda trees, elevated [CO2] enhanced biomass growth by an average of 14% over a 9-year period (Moore et al. 2006). Stand basal area was also increased under elevated [CO2] (McCarthy et al. 2010). However, when soil nitrogen was limiting, elevated [CO2] had no effect on P. taeda growth (Oren et al. 2001). Temperature and [CO2] are expected to rise concurrently in the future (Intergovernmental Panel on Climate Change 2007). There have been a number of studies of dry matter production in trees subjected to both elevated [CO2] and temperature. In most of these studies, there was an interaction between [CO2] and temperature. In some instances, the relative growth response to elevated [CO2] decreased with increasing temperature (Delucia et al. 1997; Uselman et al. 2000; Kuokkanen et al. 2004). In others, the relative growth response to elevated [CO2] increased with increasing temperature (Peltola et al. 2002; Ghannoum et al. 2010). Yet other studies reported an enhancement of dry matter production with elevated [CO2] but not with elevated temperature (Wayne et al. 1998; Norby et al. 2000). There are also studies reporting no growth enhancement in either elevated [CO2] or elevated temperature (Rasmussen et al. 2002; Olszyk et al. 2003).

Can. J. For. Res. Vol. 42, 2012 Fig. 1. Hypothetical model of the combined effects of elevated temperature (shaded arrows) and elevated [CO2] (broken arrows) on plant growth at a cool and a warm site within the geographic range of a species. Initial conditions at each site are indicated by the solid circles. The curves represent the hypothetical growth response to mean growing season air temperature over the range. The lower line (A) represents the response in current ambient [CO2] and the upper line (B) represents the response in elevated [CO2]. The effect of the same increase in mean growing season temperature produces an increase in growth at a location where temperature is below the optimum for growth (T1) and a decrease in growth at a location where temperature is above the optimum (T2). The effect of elevated [CO2] (C) on growth is considered additive in the model. It has the same positive effect on growth at both locations (broken arrows), but the combined effects of elevated [CO2] and elevated temperature produce a large positive effect on growth below the temperature optimum (T1 + C) and a much smaller effect above the temperature optimum (T2 + C).

T1 + C

Growth


944

T1 T2 + C

B

T2

A Low

High

Mean Growing Season Temperature

The inconsistent responses to the combination of elevated [CO2] and temperature may be due to species-specific responses to changes in temperature, specific treatment conditions, and (or) even the ambient temperature of the study site within the species’ distribution (Ghannoum and Way 2011). Our objective in this study was to investigate the effects of elevated temperature and elevated [CO2], applied singly and in combination, on biomass accumulation and leaf carbon gain (Anet and respiration (Rd)) of P. taeda seedlings grown simultaneously at a cool and a warm site in the southeastern United States. This study provides a companion data set to previously published work by Wertin et al. (2010), which reported in situ gas exchange measurements throughout the growing season, but not thermal acclimation or biomass accumulation. We hypothesized that the response of P. taeda to elevated temperature and [CO2] would differ at sites with lower and higher ambient temperatures. Specifically, we hyPublished by NRC Research Press

Wertin et al.


pothesized that at the cooler northern site, both elevated [CO2] and elevated temperature would cause an increase in growth, while at the warmer southern site, elevated [CO2] would increase growth but elevated temperature would decrease it (Fig. 1). Implicit in this hypothesis is the concept that air temperatures are sub-optimum for growth in the northern portion of a species’ range and supra-optimum for growth in the southern portion of its range.

945 Table 1. Mean monthly difference (SE) in average daily air temperature (DTemp) between the elevated and the ambient temperature treatments and mean monthly difference (SE) in average daily [CO2] (D[CO2]) between the elevated and the ambient [CO2] treatments at the cool and warm sites; additionally, the average DTemp and average D[CO2] for the experiment for each site are included.

Materials and methods Experimental setup One-year-old P. taeda seedlings were grown at two sites in Georgia, USA, separated by 385 km. The sites were at facilities of the University of Georgia: the Georgia Mountain Research and Education Center at Blairsville (34°87′N, 83°95′ W) (defined as the cool site) and the Coastal Plain Experiment Station at Tifton (31°29′N, 83°32′W) (defined as the warm site). The long-term mean growing season temperature (from February through October) at the cool site (15.2 °C) is similar to, or lower than, the mean temperature along the northern boundary of the P. taeda distribution, even at higher latitudes, including Huntsville, Alabama (19.0 °C), Greensboro, North Carolina (17.5 °C), and Baltimore, Maryland (16.2 °C). Although the warm site was not located at the edge of the species’ range, the mean growing season temperature at the site (21.5 °C) is similar to mean temperatures found at the southernmost parts of the range including Pensacola, Florida (22.5 °C), Gainesville, Florida (22.6 °C), and Hattiesburg, Mississippi (21.7 °C) (National Climatic Data Center, http://www.ncdc.noaa.gov/oa/ncdc.html). At each site, four half-cylindrical treatment chambers measuring 3.6 m long × 3.6 m wide × 2.4 m tall were constructed of wood and PVC pipe and covered with polyfilm (6 mil clear GT Performance Film; Green-Tek, Edgerton, Wisconsin) that allowed 92% transmittance of light equally across the visible spectrum (Boyette and Bilderback 1996). Chambers were built in an open field at each site, oriented facing north–south and spaced 2.5 m apart to prevent shading. At each site, the chambers were randomly assigned one of four treatment combinations: (1) ambient temperature and ambient [CO2] (380 µmol·mol–1), (2) ambient temperature and elevated [CO2] (700 µmol·mol–1), (3) elevated temperature (ambient +2 °C), and ambient [CO2], and (4) elevated temperature and elevated [CO2]. To minimize chamber effects, each chamber was constructed to exactly the same dimensions, air within the chambers was thoroughly mixed with an oscillating fan, seedlings were randomly assigned to one of five blocks within each chamber, and seedlings were rotated within the blocks and blocks within the chambers half way through the study. Environmental conditions (light, temperature, and [CO2]) were monitored continuously in each chamber. In each chamber, temperature was regulated with a differential thermostat (model DSD-2; Kera Technologies, Mississauga, Ontario) that controlled an air conditioner and electric resistance heater. The thermistor used to control the thermostat in each chamber was housed in a ventilated radiation shield (model SRS100; Ambient Weather, Chandler, Arizona) mounted on a pole 1 m above the ground within the chamber. Thermistors were also similarly mounted outdoors 1.5 m south of the chambers to provide measurements of am-

Cool site February March April May June July August September October Average Warm site February March April May June July August September October November Average

DTemp

D[CO2]

0.50 0.82 1.45 2.34 2.09 2.01 1.84 2.60 1.69 1.70

(0.09) (0.09) (0.06) (0.08) (0.08) (0.08) (0.18) (0.08) (0.19)

286.9 214.3 210.7 296.6 245.9 309.1 244.8

(13.8) (21.5) (18.5) (11.4) (23.9) (9.1)

1.33 2.41 2.87 3.09 2.28 1.67 2.04 2.35 2.04 1.51 2.16

(0.23) (0.08) (0.09) (0.07) (0.09) (0.03) (0.05) (0.04) (0.10) (0.10)

281.5 243.8 247.0 285.5 240.2 269.6 248.1 257.6 256.0

(20.6) (23.9) (17.6) (14.3) (18.2) (21.0) (12.9) (15.3)

bient air temperature to the thermostat. Air temperature was monitored with thermocouples mounted in the radiation shields inside each chamber and outdoors. Ambient and elevated [CO2] were maintained at 380 and 700 µmol·mol–1, respectively. Chamber [CO2] was measured and regulated with a nondispersive infrared CO2 sensor (model GMT222; Vaisala Inc., Woburn, Massachusetts), which controlled a solenoid valve connected to a cylinder of compressed CO2. Vapor pressure deficit (kilopascals) inside the chambers was not measured or controlled. Seedlings were watered to saturation four times per day with an automated irrigation system and drip emitters (Supertif-PLASTRO; Kibbutz Gvat D. N. Ha’Amakim, Israel). Photosynthetically active radiation (PAR) (moles per square metre per day) was measured outdoors and inside one chamber at each site with quantum radiation sensors (model LI-190SZ; LI-COR Biosciences, Lincoln, Nebraska). Measurements of air temperature and PAR were logged every 3 min and averaged and recorded every 15 min using a datalogger (model 23X; Cambell Scientific, Logan, Utah). A weed barrier film was placed over the soil in each chamber. One-year-old bare root P. taeda seedlings were planted in early February 2008 in 8 L pots in potting substrate (Nursery Mix; Conrad Fafard, Agawam, Massachusetts). We used a Published by NRC Research Press

0.558 0.600 0.632

0.20 0.93 0.62 0.11 0.16 0.29 0.79 0.81 0.79 0.68 0.89 0.58 0.094 0.0188 0.454

0.100 0.0967 0.626

0.407 0.541 0.611

0.93 0.98 0.86 0.33 0.85 0.43 0.26 0.13 0.60 0.44 0.85 0.43 0.25 0.95 0.79 0.005 0.76 0.89 0.462 0.097 0.700 0.116 0.364 0.282 0.039 0.001 0.007

0.001 0.95 0.94 0.056 0.70 0.74 0.008 0.34 0.57

0.001 0.001 0.66 0.005 0.95 0.64 0.03 0.57 0.001 0.86 0.001 0.001

0.001 0.23 0.044

0.011 0.91 0.16 0.15 0.005 0.76 0.001 0.011 0.005 0.18 0.18 0.90 0.71 0.99 0.004 0.37 0.004 0.85 0.64 0.30 0.29 0.78 0.99 0.81 0.22 0.023 0.008 0.31 0.81 0.48 0.066 0.21 0.001 0.031 0.001 0.57 0.022 0.93 0.045 0.39

[CO2] Temperature Site

0.001 0.14 0.54 0.73

Date × site × temperature × [CO2] Site × temperature × [CO2] Date × temperature × [CO2] Date × site × [CO2] Date × site × tempature Temperature × [CO2] Site × [CO2] Site × temperature Date × [CO2]

Date Gas exchange Anet 0.001 Topt 0.36 Rd 0.001 Anet/Rd 0.001 Growth Biomass 0.001 SLA 0.16 LA 0.001 Allocation Leaf Stem Root

Measurements of gas exchange and biomass To determine the effects of treatments on the temperature response of light-saturated Anet and dark Rd, temperature response curves were developed in June, August, and October. Five seedlings from each treatment (n = 5) were transported from the sites to the laboratory, watered to saturation, and placed in a walk-in growth chamber (model GC-36; Environmental Growth Chambers, Chagrin Falls, Ohio) overnight. Growth chamber conditions for this period were maintained at 0 µmol·m–2·s–1 PAR, 25 °C, 55% relative humidity, and 380 or 700 µmol·mol–1 CO2 (depending on growing treatment). Seedlings were selected at the cool site on 11 June, 13 August, and 18 October 2008 and at the warm site on 16 June, 17 August, and 14 October 2008.

Date × temperature

Environmental parameters Mean air temperature from February to the final harvest (November for the cool site and December for the warm site) followed a similar profile at both sites, with mean air temperature increasing through May and decreasing from mid-September until the final harvest. Averaged across the entire experimental period, mean air temperature was 4.1 °C warmer at the warm site compared with the cool site (20.8 versus 16.7 °C, respectively, P < 0.001). The greatest difference in monthly temperature between the two sites occurred early in the year (D6.4 °C in February and D6.2 °C March), while the smallest difference in temperature occurred in summer (D3.7 °C in July and D3.8 °C in August). The average daily incoming PAR during the experiment at the two sites differed by less than 4%, 37.9 mol·m–2·day–1 at the cool site and 39.4 mol·m–2·day–1 at the warm site, and was not statistically different (P = 0.98). Averaged across the entire experiment, air temperature in the elevated temperature treatments (which were initiated in February at the time of planting) was 1.70 °C at the cool site and 2.16 °C at the warm site above the ambient temperature treatments (P < 0.001 for both sites) (Table 1). Mean [CO2] in the elevated [CO2] treatments (which were initiated at bud burst) was 245 µmol·mol–1 at the cool site and 256 µmol·mol–1 above the ambient [CO2] treatments (P < 0.001 for both sites) (Table 1). The ambient [CO2] across the experiment was 404 µmol·mol–1.

Date × site

genetic mixture of open-pollinated families (provenance) from the Georgia Piedmont region (Georgia Forestry Commission, Atlanta, Georgia). Average stem height and diameter of the seedlings at planting were 0.27 m and 3.9 mm, respectively. Average growth space for each seedling in the growth chamber was 0.15 m2, which allowed for adequate space between seedlings throughout the experiment. In each treatment chamber, the seedlings were randomly assigned to five blocks, with eight seedlings in each block. Each pot was fertilized with approximately 30 g of 15–9–12 12-month extended release fertilizer (Osmocote Plus No. 903286; ScottsSierra Horticultural Products, Marysville, Ohio) in March and August and 4.93 mg of chelated iron (Sprint 138; Becker Underwood Inc., Ames, Iowa) in May and August. In May, approximately 0.04 mL of Imidacloprid was applied topically to the soil in each pot to prevent the possible emergence of pests (Advanced 12 Month Tree and Shrub Insect Control; Bayer, Monheim am Rhein, Germany).

Can. J. For. Res. Vol. 42, 2012 Table 2. Summary of four-way repeated-measures ANOVA that tested effects of date, site, temperature treatment, and [CO2] treatment on parameters of loblolly pine (Pinus taeda) foliar gas exchange including net photosynthesis (Anet), the optimum temperature of Anet (Topt), leaf dark respiration (Rd), the Anet /Rd, and seedling morphological and growth traits including total biomass, specific leaf area (SLA), total leaf area (LA), and biomass allocation; significant P values (P < 0.05) are in bold.


946


Wertin et al.

947

Fig. 2. Mean net photosynthesis (Anet) and stomatal conductance (gs) of loblolly pine (Pinus taeda) seedlings grown in treatment chambers at a cool and a warm site measured at five temperatures three times during the growing season (n = 5). Measurements were made at growth [CO2]. Treatments: ambient [CO2] (CA) (circles), elevated [CO2] (CE) (squares), ambient temperature (TA) (open symbols), and elevated temperature (TE) (solid symbols). Error bars represent 1 SE. Quadratic curves were fitted to Anet measurements in ambient [CO2] treatments (solid lines) and elevated [CO2] treatments (broken lines). 14

Warm Site

June 2008

TA C A TA C E TE C A

10

TE C E

8 6 4 2 0.4 0.3 0.2 0.1

gs (mol·m–2·s–1) A net (µmol·m–2·s–1)


12

Cool Site

0.0 12

August 2008

10 8 6 4 2 0.4 0.3 0.2 0.1 0.0 12

October 2008

10 8 6 4 2 0.4 0.3 0.2 0.1 0.0 15

20

25

30

35

15

20

25

30

35

o

Measurement Temperature ( C) One hour before starting measurements of Anet, temperature and light were adjusted to 15 °C and 500 µmol·m–2·s–1 PAR in the growth chamber. Measurements of Anet and stomatal conductance (gs) were made on one fully developed

three-needle fascicle per plant using a portable photosynthesis system (model LI-6400; LI-COR Biosciences, Lincoln, Nebraska) with a standard red/blue LED broadleaf cuvette and a CO2 mixer. Cuvette conditions were set to mimic Published by NRC Research Press


948

chamber settings with one exception, PAR was set at 1500 µmol·m–2·s–1. Measurements were recorded when values were stable, which took at least 5 min. When measurements were completed, the growth chamber and cuvette temperature was increased by 5 °C and allowed to equilibrate for 1 h (previously determined to be a sufficient length of time for temperature stabilization) and the measurement sequence was repeated. Measurements of Anet were made at 15, 20, 25, 30, and 35 °C. A similar protocol for chamber and cuvette conditions was followed for measurements of leaf Rd, with the exception that growth chamber light was maintained at 0 µmol·m–2·s–1 PAR and a laboratory-constructed cuvette was used. The cuvette consisted of flat top and bottom pieces made from clear polycarbonate (Lexan; General Electric, New York), each measuring 10 cm × 12 cm and fitted around the edge with a closed cell foam gasket to provide a gas-tight seal. The bottom of the cuvette was equipped with inlet and outlet ports and a small fan to facilitate air mixing. Seedlings were maintained in 0 PAR for 1 h before Rd measurements were conducted. Six fully developed three-needle fascicles were removed from the main stem and clamped between the top and bottom cuvette pieces for each Rd measurement. Air with a known [CO2] was passed through the cuvette at 0.3 L·s–1 and CO2 efflux from the foliage was determined using an infrared gas analyzer (IRGA) (model LI-7000; LICOR Biosciences, Lincoln, Nebraska) operated in open configuration using standard procedures (Long and Hallgren 1985). Measurements of [CO2] were recorded when values were stable for at least 5 min. After a measurement was made, needle area enclosed in the cuvette was determined as described in Fites and Teskey (1988). New fascicles were selected for measurements at each temperature. Measurements of Rd were made at 15, 20, 25, 30, and 35 °C. Pinus taeda trees produce numerous distinct flushes of foliage each growing season. In June and August, fully formed mature needles from the first growth flush were measured. In October, to eliminate the potential effect of foliage age or self-shading on measurements, needles from the second flush were measured. Due to the large number of samples, measurements of leaf gas exchange were conducted over the course of 2 consecutive days. Measured gas exchange values for both Anet and Rd were adjusted to needle area enclosed in the cuvette. The temperature response curves of Anet were fitted with a polynomial function (y = a + b × x + c × x2), and the value for the optimum temperature of Anet was calculated as the temperature corresponding to the maximum value of Anet, as described in Ghannoum et al. (2010). The temperature response curves of Rd were fitted with an exponential growth function (y = a × exp(b × x)). The temperature response curves of Anet/Rd were fitted with an exponential decay function (y = c + a × exp(–b × x)), which had the best fit. After each round of gas exchange measurements was complete, the seedlings were harvested to determine biomass accumulation. An additional harvest was made on 15 April 2008 at the cool site and 17 April 2008 at the warm site. On 6 November 2008 at the cool site and 1 December 2008 at the warm site, the remaining seedlings were harvested (n = 20). Specific leaf area (SLA) was measured by randomly selecting three fascicles per flush at each harvest. The leaf area

Can. J. For. Res. Vol. 42, 2012

(LA) of each fascicle was calculated as the sum of the area of each needle (three for this, and most, P. taeda populations) by measuring the needle length (L) and the two inner needle radiuses (R) and applying the formula LAfascicle ¼ S½1=3 2 P ðR1 þ R2 Þ L Fascicles were then dried and weighed and SLA was calculated for each fascicle by dividing LA by dry mass. Total leaf area was estimated by multiplying the SLA for each flush for each tree by the dry mass of the foliage for each flush for each tree and then summing the leaf area across all flushes. For intermediate and final harvests, seedlings were separated into shoot, foliage, and root components. Roots were hand washed to remove all potting material. While root biomass increased across the growing season, root volume was never observed to be restricted by the pots. All processed biomass (stem and branch, foliage, and root) was dried at 60 °C and weighed. Biomass growth was analyzed across sites (n = 2) and by combining ambient or elevated temperature and [CO2] treatments within sites (n = 2). Statistical analysis All statistical analyses were performed using SAS 9.2 (SAS Institute Inc., Cary, North Carolina). Significant differences in total biomass with treatment were tested with repeated-measures ANOVA with blocking using Proc Mixed. Fixed effects were site (two levels), temperature treatment (two levels) or [CO2] treatment (two levels), and date (four levels), with block × chamber (n = 5) as the random factor and block × date as the repeated factor. Significant differences in Anet, gs, Rd, and Anet/Rd were tested with the fixed effects of site, temperature treatment, [CO2] treatment, and date, with factor measurement temperature (five levels) × tree (n = 5) as the random factor and date × block as the repeated factor. When interactions occurred, we performed tests of simple main effects using the SLICE option in the LSMEANS statement (Schabenberger et al. 2000; Littell et al. 2006). To determine if biomass partitioning was affected by treatment (site, temperature treatment or [CO2] treatment), values of leaf, stem, and root biomass for each seedling were natural log transformed and plotted against the natural log of total biomass for that seedling (Bongarten and Teskey 1987). Biomass from all harvest dates was used for this analysis. Significant differences among the slopes of the eight treatment combinations were tested using Proc Mixed with estimate statements. Relative growth rate (RGR) was calculated as the change in natural log of biomass over the change in time. Biomass from sequential harvests, within blocks, was compared and plotted against both day of year and sum of mean daily temperature. Mean daily temperature sum was determined by averaging the temperature for each treatment chamber for each day and then summing the values continuously throughout the season across all harvest dates. Treatment differences in RGR were determined by the same method used for biomass analysis. The relationship between total biomass and the sum of mean daily temperature was analyzed by transforming total biomass by the square root to linearize the data. Significant differences among the slopes of the eight Published by NRC Research Press

Wertin et al.

949

Table 3. Optimum temperature (°C) (SE) for net photosynthesis (Anet) of loblolly pine (Pinus taeda) seedlings grown in treatment chambers at a cool and a warm site.


June Cool site T A CA T A CE T E CA T E CE Warm site T A CA T A CE T E CA T E CE

August

October

23.5 32.0 23.0 32.5

(1.5) (1.0) (1.0) (1.0)

22.0 30.0 23.0 29.0

(1.0) (1.5) (1.0) (1.5)

24.5 28.5 22.5 29.0

(0.5) (1.5) (1.0) (1.5)

23.5 28.5 23.0 30.0

(0.5) (1.5) (0.5) (0.5)

22.5 29.0 25.0 28.0

(1.0) (1.0) (1.0) (2.5)

26.5 28.0 26.5 29.5

(0.5) (1.0) (1.5) (2.0)

Note: Measurements were conducted in June, August, and October 2008. Treatments: ambient temperature and ambient [CO2] (TA CA), ambient temperature and elevated [CO2] (TA CE), elevated temperature and ambient [CO2] (TE CA), and elevated temperature and elevated [CO2] (TE CE).

treatment combinations were tested using Proc Mixed with estimate statements.

Results Gas exchange Seedlings grown at the cool site had higher light-saturated Anet than seedlings grown at the warm site in all treatments in June and September, but not in October (Fig. 2; Table 2). Across the growing season, Anet decreased from June to October at both sites and in all treatments. Averaged across treatments, sites, and measurement temperatures, Anet decreased from 7.8 µmol·m–2·s–1 in June to 4.8 µmol·m–2·s–1 in October. The elevated temperature treatment did not have a substantial effect on mean Anet at either site (Table 2). At the cool site, averaged across both [CO2] treatments and all measurement temperatures, seedlings grown in the elevated temperature treatments did not have significantly different mean Anet than seedlings grown at ambient temperature on any measurement date (June: 6.5 versus 6.5, July: 4.2 versus 4.1, and October: 4.5 versus 4.8 µmol·m–2·s–1). At the warm site, averaged across both [CO2] treatments and all measurement temperatures, mean Anet of seedlings grown in the elevated temperature treatments was lower in June (8.6 versus 9.6 µmol·m–2·s–1, P < 0.001), higher in August (6.4 versus 7.7 µmol·m–2·s–1, P < 0.001), and not significantly different in October (5.1 versus 4.8 µmol·m–2·s–1, P = 0.31) compared with seedlings in the ambient temperature treatments. Seedlings grown in the elevated [CO2] treatments had higher Anet than seedlings grown in the ambient [CO2] treatments when measured at their respective treatment [CO2] (Table 2; Fig. 2). This was particularly evident at measurement temperatures higher than 25 °C. The magnitude of the effect diminished in August and October, causing a date × temperature × [CO2] interaction (Table 2). The optimum temperature (Topt) for Anet was not significantly different between sites, nor did it change significantly over the three measurement dates or in response to the ambi-

ent or elevated temperature treatments (Table 3). Despite a 4.2 °C difference in mean growing season temperature between the sites, mean Topt (averaged across all treatments and measurement dates) was approximately 26.5 °C for both sites. The elevated [CO2] treatment significantly increased Topt for Anet at both sites on every measurement date, except at the warm site in October (Table 3), which caused a significant date × [CO2] interaction (Table 2). During temperature response curve measurements, relative humidity was controlled, but not vapor pressure deficit, which ranged from 0.76 kPa at 15 °C to 2.80 kPa at 35 °C. The gs decreased with increasing measurement temperature (P < 0.001), but measurement temperature did not appear to significantly affect Anet (Fig. 2). For example, plants in elevated [CO2] had the highest Anet at the highest measurement temperatures (30 and 35 °C). The gs was consistently lower in elevated [CO2] than in ambient [CO2] in seedlings grown at the cool site (0.083 versus 0.091 mol·m–2·s–1, P = 0.03), but only in August in seedlings grown at the warm site (0.045 versus 0.070 mol·m–2·s–1, P = 0.005). The gs was unaffected by temperature treatment in June and October and showed an inconsistent response in August when plants grown at the warm and cool sites had higher and lower gs, respectively, in the ambient temperature treatments. The Rd was significantly higher at the cool site than at the warm site (Fig. 3; Table 2). Averaged across all measurement dates, seedlings grown in the elevated temperature treatments had significantly lower Rd than seedlings grown in the ambient temperature treatments (0.364 versus 0.391 µmol·m–2·s–1, P = 0.028). The Rd of seedlings grown and measured in ambient [CO2] did not differ from those grown and measured in elevated [CO2] (Table 2; Fig. 3). There were significant date × [CO2] and site × [CO2] interactions caused by higher Rd in seedlings grown and measured at elevated [CO2] on some measurement dates but not others and an inconsistent pattern between sites (Fig. 3). While Anet was significantly higher at the cool site, Rd was as well. The ratio of Anet/Rd was higher at the warm site compared with the cool site (Fig. 4; Table 2). It was significantly higher in the elevated [CO2] treatments (26.8 versus 21.7, P = 0.005), but not in the elevated temperature treatments (23.8 versus 24.8). It was also significantly affected by measurement date and was higher in June than in October. Biomass Seedlings grown at the warm site had significantly more biomass than seedlings grown at the cool site at every harvest date throughout the growing season, with the exception of April (Fig. 5). Seedlings grown at the warm site accumulated biomass more rapidly at the beginning of the growing season compared with those at the cool site. At the June harvest, seedling biomass was 227% greater (59 versus 18 g, P = 0.029) at the warm site compared with the cool site. By August, the difference had decreased to 75%, and at the end of the growing season, mean seedling biomass was 53% greater at the warm site than at the cool site (441 versus 289 g, P < 0.001). Elevated [CO2] increased biomass accumulation at both sites (Fig. 5). At final harvest, biomass was 11% greater (P = 0.001) in the elevated [CO2] treatments at the warm site and 13% greater (P = 0.041) at the cool site compared Published by NRC Research Press

950


Fig. 3. Mean dark respiration (Rd) of loblolly pine (Pinus taeda) seedlings grown in treatment chambers at a cool and a warm site measured at five temperatures three times during the growing season (n = 5). Measurements were made at growth [CO2]. Treatments: ambient [CO2] (CA) (circles), elevated [CO2] (CE) (squares), ambient temperature (TA) (open symbols), and elevated temperature (TE) (filled symbols). Error bars represent ±1 SE. Exponential growth curves were fitted to Rd measurements of ambient temperature treatments (solid lines) and elevated temperature treatments (broken lines).

Cool Site

1.2

Warm Site

June 2008

TA CA TA CE TE CA TE CE

0.8 0.6 0.4 0.2 0.0

Leaf Rd (µmol·m–2·s–1)


1.0

August 2008

1.0 0.8 0.6 0.4 0.2 0.0

October 2008 1.0 0.8 0.6 0.4 0.2 0.0 10

15

20

25

30

35

10

15

20

25

30

35

40

o

Measurement Temperature ( C) with the ambient [CO2] treatments. The elevated temperature treatments had a similar effect. Seedlings grown in the elevated temperature treatments, averaged across both [CO2] treatments, accrued more biomass compared with seedlings grown in the ambient temperature treatments, although the effect was significant only at the final harvest (12% at the warm site, P = 0.001; 30% at the cool site, P = 0.001).

Biomass partitioning between leaf, stem, and root tissue was not significantly affected by the [CO2] or temperature treatments (data not shown). However, it was significantly affected by site (Table 2). At the warm site, as seedling size increased, seedlings allocated slightly more biomass to foliage and shoots than roots compared with seedlings at the cool site. For the average-sized seedling, the difference was Published by NRC Research Press

Wertin et al.

951

Fig. 4. Ratio of net photosynthesis to dark respiration (Anet/Rd) of loblolly pine (Pinus taeda) seedlings grown in treatment chambers at a cool and a warm site measured at five temperatures three times during the growing season (n = 5). Measurements were made at growth [CO2]. Treatments: ambient [CO2] (CA) (circles), elevated [CO2] (CE) (squares), ambient temperature (TA) (open symbols), and elevated temperature (TE) (solid symbols). Error bars represent ±1 SE. Exponential decay curves were fitted to Anet/Rd measurements (all treatments combined) (line). 150

Warm Site

Cool Site June 2008

TA CA TA CE TE CA TE CE

100 75 50 25

Anet / Rd (µmol m–2·s–1 / µmol m–2·s–1)


125

0

August 2008

125 100 75 50 25 0

October 2008

125 100 75 50 25 0 10

15

20

25

30

35

10

15

20

25

30

35

40

Measurement Temperature ( oC) approximately 3% more biomass allocation aboveground and 3% less belowground. However, differences in the patterns of carbon partitioning between sites, although significant, were small and did not appear to be biologically relevant. SLA, averaged across all dates and treatments, was 0.0164 m2·g–1 at the warm site and 0.0170 m2·g–1 at the cool site. SLA was not affected by date, site, temperature, or

[CO2] treatments (Table 2). Estimated total leaf area per seedling at the final harvest was significantly different between sites, with seedlings at the warm site having more estimated total leaf area (2.14 versus 1.64 m2, P = 0.016) (Table 2). The highest RGRs occurred at the warm site in June and at the cool site in August (Fig. 6A). Across the season, RGR Published by NRC Research Press


Fig. 5. Total biomass of loblolly pine (Pinus taeda) seedlings grown in (A) ambient [CO2] (CA) (open symbols, solid curve) and elevated [CO2] (CE) (crossed symbols, broken curve) averaged across both temperature treatments and (B) ambient temperature (TA) (open symbols, solid curves) and elevated temperature (TE) (solid symbols, broken curves) averaged across both [CO2] treatments. Seedlings grown at the cool site (circles, thin curves) and warm site (squares, thick curves) were harvested five times during the growing season. Exponential curves were fitted to the data. Error bars represent ±1 SE. 500

A 400

CA Cool Site CE Cool Site CA Warm Site

300

CE Warm Site

200

Biomass (g)


952

100

0

B 400

TA Cool Site TE Cool Site TA Warm Site

300

TE Warm Site

200

100

0

100

150

200

250

300

350

Day of Year was not significantly affected by temperature or [CO2] treatments (P > 0.75). However, it was evident that RGR at both sites followed a similar pattern of variation related to the seasonal change in mean daily temperature sum (Fig. 6B). There was no significant difference between sites in the temperature sum at the peak RGR (P = 0.56). Total biomass accumulation was strongly correlated with the sum of mean daily temperature (Fig. 7). Seedlings at the warm site accrued slightly less biomass as the sum of mean daily temperature increased compared with seedlings at the cool site, indicating slightly lower carbon gain efficiency at the warm site. For each 1000° of accumulated temperature, the cool site accrued 5.6% more biomass than the warm site. However, the longer growing season at the warm site produced an increase in the overall temperature sum, which corresponded to greater total biomass growth. There were no significant [CO2] or temperature treatment effects on the relationship between daily temperature sum and growth.

Discussion We hypothesized that at the cooler northern site, both elevated [CO2] and elevated temperature would cause an increase in growth, while at the warmer southern site, elevated [CO2] would increase growth but elevated temperature would decrease it. With regard to the effects of elevated [CO2], our hypothesis was supported: seedlings grown in elevated [CO2] had a higher rate of photosynthesis and accumulated more biomass at both sites compared with seedlings grown in ambient [CO2]. However, contrary to our hypothesis, seedlings grown in elevated temperature accumulated more biomass at both the cool and warm sites. The positive effect of an increase in temperature on growth at the warm site was especially interesting given that the growing season temperature was comparable with, or exceeded, temperatures at many locations along the southern-most edge of the species’ range. This result suggests that temperatures within the current range of P. taeda are lower than the optimum for growth. The strong relationship between temperature sum and growth, regardless of treatment or site, provides additional evidence that we did not reach the thermal limit for growth of this species in this study. A similar relationship between tree growth and thermal sum was observed in boreal tree species in Finland (Kauppi and Posch 1985). Our results are consistent with the majority of studies reporting that elevated temperatures increase growth of tree species (Way and Oren 2010), but are in conflict with those that have reported a reduction in growth with an increase in temperature at southern (warm) locations within species’ ranges (Norby et al. 2000; Peñuelas et al. 2007; Wertin et al. 2011). In our study, the elevated temperature treatment at the warm site increased rather than decreased growth and was comparable with the mean temperature at the southern edge of the species’ range. We speculate that mean temperature at the warm limit of this species’ distribution may be simply correlated with the boundary of the range rather than the factor that determines it. Growth was greater in all treatments at the warm site than at the cool site. The difference in growth between sites may be explained by a combination of factors including thermal acclimation of respiration, the temperature response of Anet, and a longer growing season at the warm site. Based on the temperature response curves measured in June, August, and October, equivalent rates of Rd occurred at temperatures 5– 10 °C lower at the cool site compared with the warm site, indicating that Rd acclimated to the different temperature regimes at the two sites. The adjustment in Rd exceeded the average temperature difference between the sites, suggesting that the process of acclimation may be sensitive to thermal conditions not simply characterized by the mean temperature. Atkin et al. (2005) suggested that Rd may acclimate to daily minimum temperature or nighttime temperature. A study of P. taeda seedlings in which plants were grown at 22/15 and 29/15 °C (day/night temperature) had nearly identical Rd when measured at 22 and 29 °C, respectively (Will 2000), suggesting that Rd acclimation in this species may be more strongly influenced by daytime temperature or maximum temperature than by daily mean temperature. Unlike Rd, Anet was not very sensitive to temperature and changed little from 20 to 30 °C. It also remained relatively high at temperatures of 15 and 35 °C, the highest and lowest Published by NRC Research Press

Wertin et al.

953

Fig. 6. Relative growth rate (RGR), calculated as the change in ln(biomass (g dry mass)) over the change in time, of loblolly pine (Pinus taeda) seedlings grown in treatment chambers at a cool and a warm site plotted against (A) day of year and (B) sum of mean daily temperature. The cool site is indicated by circles and broken line and the warm site by squares and solid line. Treatment combinations indicated by symbol markings: open, ambient temperature (TA); shaded, elevated temperature (TE); not crossed, ambient [CO2] (CA); crossed, elevated [CO2] (CE). Error bars represent ±1 SE. 0.04

A

B

TA CA Cool Site

RGR (D ln(biomass)/(D Day)

TE CA Cool Site TE CE Cool Site

0.03

TA CA Warm Site TA CE Warm Site TE CA Warm Site TE CE Warm Site

0.02

0.01

0.00 100

200

0

300

1000

2000

3000

4000

5000

6000

7000

Sum of Mean Daily Temperature

Day of Year

Fig. 7. Square root of total biomass of loblolly pine (Pinus taeda) seedlings grown in treatment chambers at a cool and a warm site in relation to the sum of mean daily temperature. The cool site is indicated by circles and broken line and the warm site by squares and solid line. Treatment combinations indicated by symbol markings: open, ambient temperature (TA); shaded, elevated temperature (TE); not crossed, ambient [CO2] (CA); crossed, elevated [CO2] (CE). Solid line is fit to data at the cool site (y = 0.0037x – 1809, r2 = 0.98); the broken line is fit to the data at the warm site (y = 0.0033x – 1.2898, r2 = 0.97). 25 TA CA Cool Site TA CE Cool Site TE CA Cool Site TE CE Cool Site

20

TA CA Warm Site

Biomass (g )

TA CE Warm Site

?Total


TA CE Cool Site

TE CA Warm Site TE CE Warm Site

15

10

5

0 0

1500

3000

4500

6000

7500

Sum of Mean Daily Temperature measurement temperatures. Mean Anet was higher at the cool site than at the warm site and decreased from June to October, which was consistent with in situ Anet measurements made during the growing season (Wertin et al. 2010). Since growth was not higher at the cool site, factors other than Anet

must have been responsible. One possible explanation is that the higher Rd at that site contributed by reducing carbon gain. The difference in the length of the growing season between the two sites also may have affected the accumulation of biomass. Bud burst occurred 2 weeks earlier and the end of the Published by NRC Research Press


954

growing season was extended by 3 weeks at the warm site compared with the cool site. The elevated temperature treatments had the greatest effect on later harvests at both sites, suggesting that elevated temperature further postponed the end of growing season. The effect that this postponement had on growth was to provide an additional 5–7 weeks for growth at the warm site, depending on temperature treatment. Hyvönen et al. (2007) concluded that lengthening the growing season was the main influence of higher temperatures on forest growth in temperate and boreal regions. Although we detected a difference in Anet between sites, which we attributed to the difference in thermal conditions, we did not detect a change in Anet due to the +2 °C elevated temperature treatment at either site, which was consistent with previous observations for P. teada (Teskey 1997; Wertin et al. 2010). The response of Anet to elevated growth temperatures (between +1 and +5 °C) has been variable among tree species: similar to our results, Anet did not change in Sydney bluegum (Eucalyptus saligna Sm.) and red ironbark (Eucalyptus sideroxylonA. Cunn. ex Woolls) seedlings (Ghannoum et al. 2010), but it increased in dragon spruce (Picea asperata Mast.) seedlings (Han et al. 2009) and Douglas-fir (Psuedotsuga menziesii (Mirb.) Franco) seedlings (Lewis et al. 2001) and decreased in ponderosa pine (Pinus ponderosa) seedlings (Callaway et al. 1994), dwarf apple (Malus domestica Borkh. cv. Fuji) saplings (Ro et al. 2001), P. sylvestris trees (Wang et al. 1995), and Q. rubra seedlings (Wertin et al. 2011). The growth response to elevated [CO2] at both sites is consistent with a majority of similar studies, which have reported a positive effect of elevated [CO2] on growth and photosynthesis (Hyvönen et al. 2007). Elevated [CO2] stimulated net photosynthesis of P. taeda saplings by 60%–130% (Tissue et al. 1997) and biomass production by 21%–43% (Tschaplinski et al. 1993; Friend et al. 2000; Gavazzi et al. 2000). We observed a similar response, i.e., seedlings grown in elevated [CO2] at both sites had higher rates of Anet and accumulated more biomass compared with seedlings grown in ambient [CO2], although the increase in biomass was significant only at the last harvest. The absence of significant differences in the intermediate harvests was likely due to the small sample size. At the last harvest, biomass was approximately 12% greater in the elevated [CO2] treatment, which is a smaller increase than previously reported in P. taeda seedlings. Of the studies that have investigated the combined effect of elevated temperature and [CO2] on tree growth, some have reported an increase in growth with exposure to both elevated temperature and [CO2] (Peltola et al. 2002; Ghannoum et al. 2010), while others have not (Kellomäki and Wang 2000; Norby et al. 2000; Olszyk et al. 2003). An increase in growth temperature (30/22 versus 26/18 °C day/night) substantially increased the positive effect of elevated [CO2] (640 µmol·mol–1) on biomass accumulation of E. saligna and E. sideroxylon seedlings (Ghannoum et al. 2010). Similarly, elevated temperature (+6 °C in winter, +4 °C in spring and autumn, and +2 °C in summer) and elevated [CO2] (700 µmol·mol–1) applied together increased diameter growth of mature P. sylvestris trees more than either elevated temperature or elevated [CO2] alone (Peltola et al. 2002). In contrast, Kellomäki and Wang (2001) reported that elevated temperature (+3 °C) and elevated [CO2] (700 µmol·mol–1)


applied in combination did not increase biomass accumulation of European white birch (Betula pendula Roth) seedlings more than either elevated temperature or elevated [CO2] alone. Elevated temperature (+3.5 °C) and elevated [CO2] (ambient +180 µmol·mol–1) did not increase biomass accumulation of P. menziesii seedlings compared with those grown in ambient conditions (Olszyk et al. 2003). Elevated [CO2] (ambient +330 µmol·mol–1) increased growth of branches of mature P. taeda trees, but elevated temperature (+2 °C) did not (Teskey 1997). Elevated [CO2] (ambient +300 µmol·mol–1) increased growth of red maple (Acer rubrum L.) and A. saccharium seedlings, but elevated temperature (+3.5 °C) reduced it, with the combination causing a reduction in growth compared with seedlings grown in ambient temperature and [CO2] (Norby et al. 2000). A meta-analysis suggested that C3 plants grown in elevated temperature (ambient + 1.4–6 °C) and elevated [CO2] had higher Anet and photosystem II efficiency compared with plants grown in ambient temperature and elevated [CO2] (Wang et al. 2012). We found that elevated temperature and [CO2] did not interactively affect biomass accumulation at either site. Compared across sites, when we did observe a temperature treatment effect on Anet, we did not observe any interaction between temperature and [CO2] treatment; seedlings responded similarly to elevated [CO2] at both sites. Elevated [CO2] increased the Topt for Anet, consistent with predictions for C3 plants in general (Sage et al. 2008). There appeared to be no acclimation of photosynthesis to elevated temperature within each site, or when comparing temperature responses between sites, despite a 6 °C difference in growth temperature between the ambient temperature treatment at the cool site and the elevated temperature treatment at the warm site. This lack of acclimation is consistent with previous reports in most tree species that have been examined (Tjoelker et al. 1998; Ow et al. 2008; Dillaway and Kruger 2010). This study did not address whether locally adapted genotypes would have performed differently than the seed source that was used or whether older plants would have responded differently than seedlings. The seed source used in the study was from central Georgia, which is in the center of the species’ range and is equidistant with respect to mean temperature (approximately ±2 °C) from both sites used in this study. Schmidtling (1994) analyzed P. taeda growth in trials in which 14 seed source provenances from Georgia and northern Florida were tested at 10 locations throughout Georgia. He examined provenance height growth in relation to temperatures at the trial location and seed source origin and concluded that, on average, moving a seed source either +2 or –2 °C (difference in mean annual temperature between the original seed source location and the planting site) would cause less than a 3% difference in height growth, giving us confidence that the seed source selected for this experiment would respond similarly to that of the average local source yet allow us to interpret physiological responses without added genetic variation. With regard to thermal acclimation of Rd, Teskey and Will (1999) found no significant differences in the abiltiy to acclimate among P. taeda genotyoes from Texas, Arkansas, and Maryland. Bolstad et al. (2003) observed the same result for genotypes of white oak (QuerPublished by NRC Research Press


Wertin et al.

cus alba L.) and Q. rubra from a similarly wide geographic range. However, Arend et al. (2011) reported significant differences in the response of provenances to elevated temperature for four Quercus species, although the reponses did not reflect local adaptation to climate conditions of origin. Seedlings were used in this study because of experimental limitations to applying both elevated [CO2] and temperature to larger individuals. Seedlings are generally more sensitive than mature trees to environmental stresses and changes in atmospheric conditions (Rasmussen et al. 2002), suggesting that they would be more responsive to changes in temperature and [CO2] than mature trees. Way and Oren (2010) reported the same general growth response to temperature regardless of tree age, indicating that the direction of change, if not the magnitude of change, is likely to remain consistent with age. These seedlings were grown in pots with a known substrate to control for variations in soil type and soil fertility. Way and Oren (2010) reported that seedlings respond similarly to an increase in temperature, regardless of whether the trees were grown in pots or in the field. For this study, the seedlings were well watered and supplied with adequate nutrients. It is possible that the responses that we observed could be altered if water or nitrogen were severely limited. The response of growth to elevated [CO2] has been shown to be sensitive to nutrient and moisture availability (McCarthy et al. 2010), while the effect of temperature on growth has been shown to be regulated by soil moisture (D’Arrigo et al. 2004). We found no evidence that pot size limited root growth, as biomass partitioning was not affected by site or treatments. Additionally, Will and Teskey (1997) demonstrated that pot size has only a limited impact on belowground biomass allocation for P. taeda seedlings. We expect that field-grown seedlings with equal access to water and nutrients would respond similarly to the potted seedlings in this study. Conclusions Although responses to elevated [CO2] and temperature treatments used in this study were consistent at both the warm and cool sites, the use of two sites provided valuable insights that would not have been apparent if only one had been used, given our use of only one elevated temperature and [CO2] treatment level at each site. For example, thermal acclimation between the ambient and elevated temperature treatment was not detectable at either site, but was readily apparent between sites due to the larger difference in temperature. Similarly, effects of length of growing season and temperature sum on growth would not have been easily identified with measurements solely at either site, but were distinguishable when results at the two sites were compared. Our findings also suggest that, in nonlimiting water and soil nutrient conditions, predicted future climate conditions are likely to increase growth throughout the range of P. taeda, at least of seedlings, and provide an indication that the species may be growing in conditions below its Topt throughout its current range. Although this conclusion requires additional investigation, and may be limited to P. taeda, the large number of studies that have reported an increase in tree growth in elevated growth temperatures compared with current ambient temperature (Way and Oren 2010) suggest that other species may respond similarly.

955

Acknowledgements We thank Steven Pettis for advice on growing seedlings. We thank Nancy Hand and Bruce Tucker at Tifton and Marta Mead at Blairsville for assistance in maintaining the sites. We thank Conrad Fafard, Inc. for their generous donation of potting medium. We thank Katie Bower for her assistance weighing samples. This work was supported by a grant from the United States Department of Energy NICCR Program (grant 07-SC-NICCR-1060).

References Ainsworth, E.A., and Rogers, A. 2007. The response of photosynthesis and stomatal conductance to rising [CO2]: mechanisms and environmental interactions. Plant Cell Environ. 30(3): 258–270. doi:10.1111/j.1365-3040.2007.01641.x. PMID:17263773. Arend, M., Kuster, T., Günthardt-Goerg, M.S., and Dobbertin, M. 2011. Provenance-specific growth responses to drought and air warming in three European oak species (Quercus robur, Q. petraea and Q. pubescens). Tree Physiol. 31(3): 287–297. doi:10. 1093/treephys/tpr004. PMID:21422189. Atkin, O.K., Bruhn, D., Hurry, V.M., and Tjoelker, M.G. 2005. The hot and the cold: unravelling the variable response of plant respiration to temperature. Funct. Plant Biol. 32(2): 87–105. doi:10.1071/FP03176. Bolstad, P.V., Reich, P., and Lee, T. 2003. Rapid temperature acclimation of leaf respiration rates in Quercus alba and Quercus rubra. Tree Physiol. 23(14): 969–976. doi:10.1093/treephys/23. 14.969. PMID:12952783. Bongarten, B.C., and Teskey, R.O. 1987. Dry weight partitioning and its relationship to productivity in loblolly pine seedlings from 7 sources. For. Sci. 33(2): 255–267. Boyette, M.D., and Bilderback, T.E. 1996. A small backyard greenhouse for the home gardener. North Carolina Cooperative Extension Service AG-426. Revised. 3rd ed. North Carolina Cooperative Extension Service, Greensboro, N.C. pp. 1–4. Available from http://www.bae.ncsu.edu/programs/extension/publicat/postharv/green/small_greenhouse.pdf. Bronson, D.R., Gower, S.T., Tanner, M., and Van Herk, I. 2009. Effect of ecosystem warming on boreal black spruce bud burst and shoot growth. Glob. Change Biol. 15(6): 1534–1543. doi:10.1111/ j.1365-2486.2009.01845.x. Callaway, R.M., Delucia, E.H., Thomas, E.M., and Schlesinger, W.H. 1994. Compensatory responses of CO2 exchange and biomass allocation and their effects on the relative growth-rate of ponderosa pine in different CO2 and temperature regimes. Oecologia (Berl.), 98(2): 159–166. doi:10.1007/BF00341468. Danby, R.K., and Hik, D.S. 2007. Responses of white spruce (Picea glauca) to experimental warming at a subarctic alpine treeline. Glob. Change Biol. 13(2): 437–451. doi:10.1111/j.1365-2486. 2006.01302.x. D’Arrigo, R.D., Kaufmann, R.K., Davi, N., Jacoby, G.C., Laskowski, C., Myneni, R.B., and Cherubini, P. 2004. Thresholds for warming-induced growth decline at elevational tree line in the Yukon Territory, Canada. Global Biogeochem. Cycles, 18: GB3021. doi:10.1029/2004GB002249. Delucia, E.H., Callaway, R.M., Thomas, E.M., and Schlesinger, W.H. 1997. Mechanisms of phosphorus acquisition for ponderosa pine seedlings under high CO2 and temperature. Ann. Bot. (Lond.), 79 (2): 111–120. doi:10.1006/anbo.1996.0320. Dillaway, D.N., and Kruger, E.L. 2010. Thermal acclimation of photosynthesis: a comparison of boreal and temperate tree species along a latitudinal transect. Plant Cell Environ. 33(6): 888–899. doi:10.1111/j.1365-3040.2010.02114.x. PMID:20082671. Published by NRC Research Press


956 Fites, J.A., and Teskey, R.O. 1988. CO2 and water vapor exchange of Pinus taeda in relation to stomatal behavior: test of an optimization hypothesis. Can. J. For. Res. 18(2): 150–157. doi:10.1139/x88-024. Friend, A.L., Jifon, J.L., Berrang, P.C., Seiler, J.R., and Mobley, J.A. 2000. Elevated atmospheric CO2 and species mixture alter N acquisition of trees in stand microcosms. Can. J. For. Res. 30(5): 827–836. doi:10.1139/x00-019. Gavazzi, M., Seiler, J., Aust, W., and Zedaker, S. 2000. The influence of elevated carbon dioxide and water availability on herbaceous weed development and growth of transplanted loblolly pine (Pinus taeda). Environ. Exp. Bot. 44(3): 185–194. doi:10.1016/S00988472(00)00065-4. PMID:11064039. Ghannoum, O., and Way, D.A. 2011. On the role of ecological adaptation and geographic distribution in the response of trees to climate change. Tree Physiol. 31(12): 1273–1276. doi:10.1093/ treephys/tpr115. PMID:22158008. Ghannoum, O., Phillips, N.G., Conroy, J.P., Smith, R.A., Attard, R. D., Woodfield, R., Logan, B.A., Lewis, J.D., and Tissue, D.T. 2010. Exposure to preindustrial, current and future atmospheric CO2 and temperature differentially affects growth and photosynthesis in Eucalyptus. Glob. Change Biol. 16(1): 303–319. doi:10. 1111/j.1365-2486.2009.02003.x. Gunderson, C.A., and Wullschleger, S.D. 1994. Photosynthetic acclimation in trees to rising atmospheric CO2: a broader perspective. Photosynth. Res. 39(3): 369–388. doi:10.1007/ BF00014592. Hall, M., Rantfors, M., Slaney, M., Linder, S., and Wallin, G. 2009. Carbon dioxide exchange of buds and developing shoots of boreal Norway spruce exposed to elevated or ambient CO2 concentration and temperature in whole-tree chambers. Tree Physiol. 29(4): 461– 481. doi:10.1093/treephys/tpn047. PMID:19203983. Han, C., Liu, Q., and Yang, Y. 2009. Short-term effects of experimental warming and enhanced ultraviolet-B radiation on photosynthesis and antioxidant defense of Picea asperata seedlings. Plant Growth Regul. 58(2): 153–162. doi:10.1007/s10725009-9363-2. Hoch, G., and Korner, C. 2009. Growth and carbon relations of tree line forming conifers at constant vs. variable low temperatures. J. Ecol. 97(1): 57–66. doi:10.1111/j.1365-2745.2008.01447.x. Hyvönen, R., Ågren, G.I., Linder, S., Persson, T., Cotrufo, M.F., Ekblad, A., Freeman, M., Grelle, A., Janssens, I.A., Jarvis, P.G., Kellomäki, S., Lindroth, A., Loustau, D., Lundmark, T., Norby, R. J., Oren, R., Pilegaard, K., Ryan, M.G., Sigurdsson, B.D., Strömgren, M., van Oijen, M., and Wallin, G. 2007. The likely impact of elevated [CO2], nitrogen deposition, increased temperature and management on carbon sequestration in temperate and boreal forest ecosystems: a literature review. New Phytol. 173(3): 463–480. doi:10.1111/j.1469-8137.2007.01967.x. PMID: 17244042. Intergovernmental Panel on Climate Change. 2007. Climate Change 2007: Working Group I: The physical science basis: global climate projections. Cambridge University Press, Cambridge, U.K. Kauppi, P., and Posch, M. 1985. Sensitivity of boreal forests to possible climate warming. Clim. Change, 7(1): 45–54. doi:10. 1007/BF00139440. Kellomäki, S., and Wang, K.Y. 2000. Modelling and measuring transpiration from Scots pine with increased temperature and carbon dioxide enrichment. Ann. Bot. (Lond.), 85(2): 263–278. doi:10.1006/anbo.1999.1030. Kellomäki, S., and Wang, K.Y. 2001. Growth and resource use of birch seedlings under elevated carbon dioxide and temperature. Ann. Bot. (Lond.), 87(5): 669–682. doi:10.1006/anbo.2001.1393. Kuokkanen, K., Niemelä, P., Matala, J., Julkunen-Tiitto, R.,

Can. J. For. Res. Vol. 42, 2012 Heinonen, J., Rousi, M., Henttonen, H., Tahvanainen, J., and Kellomäki, S. 2004. The effects of elevated CO2 and temperature on the resistance of winter-dormant birch seedlings (Betula pendula) to hares and voles. Glob. Change Biol. 10(9): 1504– 1512. doi:10.1111/j.1365-2486.2004.00820.x. Lewis, J.D., Tissue, D.T., and Strain, B.R. 1996. Seasonal response of photosynthesis to elevated CO2 in loblolly pine (Pinus taeda L.) over two growing seasons. Glob. Change Biol. 2(2): 103–114. doi:10.1111/j.1365-2486.1996.tb00055.x. Lewis, J.D., Lucash, M., Olszyk, D., and Tingey, D.T. 2001. Seasonal patterns of photosynthesis in Douglas fir seedlings during the third and fourth year of exposure to elevated CO2 and temperature. Plant Cell Environ. 24(5): 539–548. doi:10.1046/j.1365-3040.2001. 00700.x. Littell, R., Milliken, G., Stroup, W., Wolfinger, R., and Schabenberger, O. 2006. SAS for mixed models. SAS Institute Inc., Cary, N. C. Long, S.P., and Hallgren, J.-E. (Editors). 1985. Measurement of CO2 assimilation by plants in the field and the laboratory. In Techniques in bioproductivity and photosynthesis. Pergamon Press, Oxford, U.K. Mäenpää, M., Riikonen, J., Kontunen-Soppela, S., Rousi, M., and Oksanen, E. 2011. Vertical profiles reveal impact of ozone and temperature on carbon assimilation of Betula pendula and Populus tremula. Tree Physiol. 31(8): 808–818. doi:10.1093/treephys/ tpr075. PMID:21856655. McCarthy, H.R., Oren, R., Johnsen, K.H., Gallet-Budynek, A., Pritchard, S.G., Cook, C.W., LaDeau, S.L., Jackson, R.B., and Finzi, A.C. 2010. Re-assessment of plant carbon dynamics at the Duke free-air CO2 enrichment site: interactions of atmospheric [CO2] with nitrogen and water availability over stand development. New Phytol. 185(2): 514–528. doi:10.1111/j.1469-8137.2009. 03078.x. PMID:19895671. Moore, D.J.P., Aref, S., Ho, R.M., Pippen, J.S., Hamilton, J.G., and De Lucia, E.H. 2006. Annual basal area increment and growth duration of Pinus taeda in response to eight years of free-air carbon dioxide enrichment. Glob. Change Biol. 12(8): 1367–1377. doi:10.1111/j.1365-2486.2006.01189.x. Norby, R.J., Long, T.M., Hartz-Rubin, J.S., and O’Neill, E.G. 2000. Nitrogen resorption in senescing tree leaves in a warmer, CO2enriched atmosephere. Plant Soil, 224(1): 15–29. doi:10.1023/ A:1004629231766. Olszyk, D.M., Johnson, M.G., Tingey, D.T., Rygiewicz, P.T., Wise, C., VanEss, E., Benson, A., Storm, M.J., and King, R. 2003. Whole-seedling biomass allocation, leaf area, and tissue chemistry for Douglas-fir exposed to elevated CO2 and temperature for 4 years. Can. J. For. Res. 33(2): 269–278. doi:10.1139/x02-186. Oren, R., Ellsworth, D.S., Johnsen, K.H., Phillips, N., Ewers, B.E., Maier, C., Schafer, K.V.R., McCarthy, H., Hendrey, G., McNulty, S.G., and Katul, G.G. 2001. Soil fertility limits carbon sequestration by forest ecosystems in a CO2-enriched atmosphere. Nature, 411(6836): 469–472. doi:10.1038/35078064. PMID:11373677. Ow, L.F., Griffin, K.L., Whitehead, D., Walcroft, A.S., and Turnbull, M.H. 2008. Thermal acclimation of leaf respiration but not photosynthesis in Populus deltoides × nigra. New Phytol. 178(1): 123–134. doi:10.1111/j.1469-8137.2007.02357.x. PMID: 18221247. Peltola, H., Kilpelainen, A., and Kellomäki, S. 2002. Diameter growth of Scots pine (Pinus sylvestris) trees grown at elevated temperature and carbon dioxide concentration under boreal conditions. Tree Physiol. 22(14): 963–972. doi:10.1093/treephys/ 22.14.963. PMID:12359523. Peñuelas, J., Prieto, P., Beier, C., Cesaraccio, C., de Angelis, P., de Dato, G., Emmett, B.A., Estiarte, M., Garadnai, J., Gorissen, A., Published by NRC Research Press


Wertin et al. Láng, E.K., Kröel-Dulay, G., Llorens, L., Pellizzaro, G., RiisNielsen, T., Schmidt, I.K., Sirca, C., Sowerby, A., Spano, D., and Tietema, A. 2007. Response of plant species richness and primary productivity in shrublands along a north-south gradient in Europe to seven years of experimental warming and drought: reductions in primary productivity in the heat and drought year of 2003. Glob. Change Biol. 13(12): 2563–2581. doi:10.1111/j.1365-2486.2007. 01464.x. Rasmussen, L., Beier, C., and Bergstedt, A. 2002. Experimental manipulations of old pine forest ecosystems to predict the potential tree growth effects of increased CO2 and temperature in a future climate. For. Ecol. Manage. 158(1–3): 179–188. doi:10.1016/ S0378-1127(00)00677-0. Reich, P.B., and Oleksyn, J. 2008. Climate warming will reduce growth and survival of Scots pine except in the far north. Ecol. Lett. 11(6): 588–597. doi:10.1111/j.1461-0248.2008.01172.x. PMID:18363717. Ro, H.M., Kim, P.G., Lee, I.B., Yiem, M.S., and Woo, S.Y. 2001. Photosynthetic characteristics and growth responses of dwarf apple (Malus domestica Borkh. cv. Fuji) saplings after 3 years of exposure to elevated atmospheric carbon dioxide concentration and temperature. Trees-Struct. Funct. 15(4): 195–203. doi:10. 1007/s004680100099. Sage, R.F., Way, D.A., and Kubien, D.S. 2008. Rubisco, Rubisco activase, and global climate change. J. Exp. Bot. 59(7): 1581– 1595. doi:10.1093/jxb/ern053. PMID:18436544. Schabenberger, O., Gregoire, T.G., and Kong, F.Z. 2000. Collections of simple effects and their relationship to main effects and interactions in factorials. Am. Stat. 54(3): 210–214. doi:10.2307/ 2685592. Schmidtling, R.C. 1994. Use of provenance tests to predict response to climate change — loblolly pine and Norway spruce. Tree Physiol. 14(7–9): 805–817. PMID:14967650. Teskey, R.O. 1997. Combined effects of elevated CO2 and air temperature on carbon assimilation of Pinus taeda trees. Plant Cell Environ. 20(3): 373–380. doi:10.1046/j.1365-3040.1997.d01-75. x. Teskey, R.O., and Will, R.E. 1999. Acclimation of loblolly pine (Pinus taeda) seedlings to high temperatures. Tree Physiol. 19(8): 519–525. PMID:12651542. Tissue, D.T., Thomas, R.B., and Strain, B.R. 1997. Atmospheric CO2 enrichment increases growth and photosynthesis of Pinus taeda: a 4 year experiment in the field. Plant Cell Environ. 20(9): 1123– 1134. doi:10.1046/j.1365-3040.1997.d01-140.x. Tjoelker, M.G., Oleksyn, J., and Reich, P.B. 1998. Seedlings of five boreal tree species differ in acclimation of net photosynthesis to elevated CO2 and temperature. Tree Physiol. 18(11): 715–726. PMID:12651406. Tschaplinski, T.J., Norby, R.J., and Wullschleger, S.D. 1993. Responses of loblolly pine seedlings to elevated CO2 and

957 fluctuating water supply. Tree Physiol. 13(3): 283–296. PMID: 14969886. Uselman, S.M., Qualls, R.G., and Thomas, R.B. 2000. Effects of increased atmospheric CO2, temperature, and soil N availability on root exudation of dissolved organic carbon by a N-fixing tree (Robinia pseudoacacia L.). Plant Soil, 222(1–2): 191–202. doi:10. 1023/A:1004705416108. Wang, D., Heckathorn, S.A., Wang, X.Z., and Philpott, S.M. 2012. A meta-analysis of plant physiological and growth responses to temperature and elevated CO2. Oecologia (Berl.). In press. doi:10. 1007/s00442-011-2172-0. Wang, K., Kellomäki, S., and Laitinen, K. 1995. Effects of needle age, long-term temperature and CO2 treatments on the photosynthesis of Scots pine. Tree Physiol. 15(4): 211–218. doi:10. 1093/treephys/15.4.211. PMID:14965960. Way, D.A., and Oren, R. 2010. Differential responses to changes in growth temperature between trees from different functional groups and biomes: a review and synthesis of data. Tree Physiol. 30(6): 669–688. doi:10.1093/treephys/tpq015. PMID:20368338. Wayne, P.M., Reekie, E.G., and Bazzaz, F.A. 1998. Elevated CO2 ameliorates birch response to high temperature and frost stress: implications for modeling climate-induced geographic range shifts. Oecologia (Berl.), 114(3): 335–342. doi:10.1007/ s004420050455. Wertin, T.M., McGuire, M.A., and Teskey, R.O. 2010. The influence of elevated temperature, elevated atmospheric CO2 concentration and water stress on net photosynthesis of loblolly pine (Pinus taeda L.) at northern, central and southern sites in its native range. Glob. Change Biol. 16(7): 2089–2103. doi:10.1111/j.1365-2486. 2009.02053.x. Wertin, T.M., McGuire, M.A., and Teskey, R.O. 2011. Higher growth temperatures decreased net carbon assimilation and biomass accumulation of northern red oak seedlings near the southern limit of the species range. Tree Physiol. 31(12): 1277–1288. doi:10.1093/treephys/tpr091. PMID:21937670. Will, R. 2000. Effect of different daytime and night-time temperature regimes on the foliar respiration of Pinus taeda: predicting the effect of variable temperature on acclimation. J. Exp. Bot. 51(351): 1733–1739. doi:10.1093/jexbot/51.351.1733. PMID:11053463. Will, R.E., and Teskey, R.O. 1997. Effect of elevated carbon dioxide concentration and root restriction on net photosynthesis, water relations and foliar carbohydrate status of loblolly pine seedlings. Tree Physiol. 17(10): 655–661. PMID:14759905. Xu, C.G., Gertner, G.Z., and Scheller, R.M. 2007. Potential effects of interaction between CO2 and temperature on forest landscape response to global warming. Glob. Change Biol. 13(7): 1469– 1483. doi:10.1111/j.1365-2486.2007.01387.x. Yin, H.J., Liu, Q., and Lai, T. 2008. Warming effects on growth and physiology in the seedlings of the two conifers Picea asperata and Abies faxoniana under two contrasting light conditions. Ecol. Res. 23(2): 459–469. doi:10.1007/s11284-007-0404-x.
