Quantifying the response of photosynthesis to changes in leaf nitrogen content and leaf mass per area in plants grown under atmospheric CO2 enrichment

Previous modelling exercises and conceptual arguments have predicted that a reduction in biochemical capacity for photosynthesis (A_area) at elevated CO₂ may be compensated by an increase in mesophyll tissue growth if the total amount of photosynthetic machinery per unit leaf area is maintained (i.e. morphological upregulation). The model prediction was based on modelling photosynthesis as a function of leaf N per unit leaf area (N_area), where N_area = N_mass×LMA. Here, N_mass is percentage leaf N and is used to estimate biochemical capacity and LMA is leaf mass per unit leaf area and is an index of leaf morphology. To assess the relative importance of changes in biochemical capacity versus leaf morphology we need to control for multiple correlations that are known, or that are likely to exist between CO₂ concentration, N_area, N_mass, LMA and A_area. Although this is impractical experimentally, we can control for these correlations statistically using systems of linear multiple-regression equations. We developed a linear model to partition the response of A_area to elevated CO₂ into components representing the independent and interactive effects of changes in indexes of biochemical capacity, leaf morphology and CO₂ limitation of photosynthesis. The model was fitted to data from three pine and seven deciduous tree species grown in separate chamber-based field experiments. Photosynthetic enhancement at elevated CO₂ due to morphological upregulation was negligible for most species. The response of A_area in these species was dominated by the reduction in CO₂ limitation occurring at higher CO₂ concentration. However, some species displayed a significant reduction in potential photosynthesis at elevated CO₂ due to an increase in LMA that was independent of any changes in N_area. This morphologically based inhibition of A_area combined additively with a reduction in biochemical capacity to significantly offset the direct enhancement of A_area caused by reduced CO₂ limitation in two species. This offset was 100% for Acer rubrum, resulting in no net effect of elevated CO₂ on A_area for this species, and 44% for Betula pendula. This analysis shows that interactions between biochemical and morphological responses to elevated CO₂ can have important effects on photosynthesis.     

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

We investigated the effect of elevated [CO₂] (700 μmol mol^?1), elevated temperature (+2 °C above ambient) and decreased soil water availability on net photosynthesis (A_net) and water relations of one‐year old potted loblolly pine (Pinus taeda L.) seedlings grown in treatment chambers with high fertility at three sites along a north‐south transect covering a large portion of the species native range. At each location (Blairsville, Athens and Tifton, GA) we constructed four treatment chambers and randomly assigned each chamber one of four treatments: ambient [CO₂] and ambient temperature, elevated [CO₂] and ambient temperature, ambient [CO₂] and elevated temperature, or elevated [CO₂] and elevated temperature. Within each chamber half of the seedlings were well watered and half received much less water (1/4 that of the well watered). Measurements of net photosynthesis (A_net), stomatal conductance (g_s), leaf water potential and leaf fluorescence were made in June and September, 2008. We observed a significant increase in A_net in response to elevated [CO₂] regardless of site or temperature treatment in June and September. An increase in air temperature of over 2 °C had no significant effect on A_net at any of the sites in June or September despite over a 6 °C difference in mean annual temperature between the sites. Decreased water availability significantly reduced A_net in all treatments at each site in June. The effects of elevated [CO₂] and temperature on g_s followed a similar trend. The temperature, [CO₂] and water treatments did not significantly affect leaf water potential or chlorophyll fluorescence. Our findings suggest that predicted increases in [CO₂] will significantly increase A_net, while predicted increases in air temperature will have little effect on A_net across the native range of loblolly pine. Potential decreases in precipitation will likely cause a significant reduction in A_net, though this may be mitigated by increased [CO₂].     

Direct and indirect effects of elevated atmospheric CO2 on net ecosystem production in a Chesapeake Bay tidal wetland

The rapid increase in atmospheric CO₂ concentrations (C_a) has resulted in extensive research efforts to understand its impact on terrestrial ecosystems, especially carbon balance. Despite these efforts, there are relatively few data comparing net ecosystem exchange of CO₂ between the atmosphere and the biosphere (NEE), under both ambient and elevated C_a. Here we report data on annual sums of CO₂ (NEE_net) for 19 years on a Chesapeake Bay tidal wetland for Scirpus olneyi (C₃ photosynthetic pathway)‐ and Spartina patens (C₄ photosynthetic pathway)‐dominated high marsh communities exposed to ambient and elevated C_a (ambient + 340 ppm). Our objectives were to (i) quantify effects of elevated C_a on seasonally integrated CO₂ assimilation (NEE_net = NEE_day + NEE_night, kg C m^?2 y^?1) for the two communities; and (ii) quantify effects of altered canopy N content on ecosystem photosynthesis and respiration. Across all years, NEE_net averaged 1.9 kg m^?2 y^?1 in ambient C_a and 2.5 kg m^?2 y^?1 in elevated C_a, for the C₃‐dominated community. Similarly, elevated C_a significantly (P < 0.01) increased carbon uptake in the C₄‐dominated community, as NEE_net averaged 1.5 kg m^?2 y^?1 in ambient C_a and 1.7 kg m^?2 y^?1 in elevated C_a. This resulted in an average CO₂ stimulation of 32% and 13% of seasonally integrated NEE_net for the C₃‐ and C₄‐dominated communities, respectively. Increased NEE_day was correlated with increased efficiencies of light and nitrogen use for net carbon assimilation under elevated C_a, while decreased NEE_night was associated with lower canopy nitrogen content. These results suggest that rising C_a may increase carbon assimilation in both C₃‐ and C₄‐dominated wetland communities. The challenge remains to identify the fate of the assimilated carbon.     

CO2 enrichment in a maturing pine forest: are CO2 exchange and water status in the canopy affected?

A _net, leaf net CO₂ assimilation
c_a, CO₂ concentration of air surrounding a leaf
c_i, leaf intercellular CO₂ concentration
Δ, ¹³C isotope discrimination
δ¹³C, relative stable carbon isotope content
?, ratio of A_net at c_a = 560μmol mol^–1 to A_net at c_a = 360 μmol mol^–1
FACE, free-air CO₂ enrichment
g_w, stomatal conductance to water vapour
Π_i, initial leaf osmotic potential
R_t, relative water content at incipient turgor loss
Ψ_l, xylem water potential of leaves
Ψ_m, soil matric potential

Elevated CO₂ is expected to reduce forest water use as a result of CO₂-induced stomatal closure, which has implications for ecosystem-scale phenomena controlled by water availability. Leaf-level CO₂ and H₂O exchange responses and plant and soil water relations were examined in a maturing loblolly pine (Pinus taeda L.) stand in a free-air CO₂ enrichment (FACE) experiment in North Carolina, USA to test if these parameters were affected by elevated CO₂. Current-year foliage in the canopy was continuously exposed to elevated CO₂ (ambient CO₂+200μmol mol^–1) in free-air during needle growth and development for up to 400 d. Photosynthesis in upper canopy foliage was stimulated by 50–60% by elevated CO₂ compared with ambient controls. This enhancement was similar in current-year, ambient-grown foliage temporarily measured at elevated CO₂ compared with long-term elevated CO₂ grown foliage. Significant photosynthetic enhancement by CO₂ was maintained over a range of conditions except during peak drought. There was no evidence of water savings in elevated CO₂ plots in FACE compared to ambient plots under drought and non-drought conditions. This was supported by evidence from three independent measures. First, stomatal conductance was not significantly different in elevated CO₂ versus ambient trees of P. taeda. Calculations of time-integrated c_i/c_a ratios from analysis of foliar δ¹³C showed that these ratios were maintained in foliage under elevated CO₂. Second, soil moisture was not significantly different between ambient and elevated CO₂ plots during drought. Third, pre-dawn and mid-day leaf water potentials were also unaffected by the seasonal CO₂ exposure, as were tissue osmotic potentials and turgor loss points. Together the results strongly support the hypothesis that maturing P. taeda trees have low stomatal responsiveness to elevated CO₂. Elevated CO₂ effects on water relations in loblolly pine-dominated forest ecosystems may be absent or small apart from those mediated by leaf area. Large photosynthetic enhancements in the upper canopy of P. taeda by elevated CO₂ indicate that this maturing forest may have a large carbon sequestration capacity with limiting water supply.     

Effects of elevated CO2 and temperature on photosynthesis and leaf traits of an understory dwarf bamboo in subalpine forest zone,China

The dwarf bamboo (Fargesia rufa Yi), growing understory in subalpine dark coniferous forest, is one of the main foods for giant panda, and it influences the regeneration of subalpine coniferous forests in southwestern China. To investigate the effects of elevated CO₂, temperature and their combination, the dwarf bamboo plantlets were exposed to two CO₂ regimes (ambient and double ambient CO₂ concentration) and two temperatures (ambient and +2.2°C) in growth chambers. Gas exchange, leaf traits and carbohydrates concentration were measured after the 150‐day experiment. Elevated CO₂ significantly increased the net photosynthetic rate (A_net), intrinsic water‐use efficiency (WUE_i) and carbon isotope composition (δ¹³C) and decreased stomatal conductance (g_s) and total chlorophyll concentration based on mass (Chl_m) and area (Chl_a). On the other hand, elevated CO₂ decreased specific leaf area (SLA), which was increased by elevated temperature. Elevated CO₂ also increased foliar carbon concentration based on mass (C_m) and area (C_a), nitrogen concentration based on area (N_a), carbohydrates concentration (i.e. sucrose, sugar, starch and non‐structural carbohydrates) and the slope of the A_net–N_a relationship. However, elevated temperature decreased C_m, C_a and N_a. The combination of elevated CO₂ and temperature hardly affected SLA, C_m, C_a, N_m, N_a, Chl_m and Chl_a. Variables A_net and N_a had positive linear relationships in all treatments. Our results showed that photosynthetic acclimation did not occur in dwarf bamboo at elevated CO₂ and it could adjust physiology and morphology to enable the capture of more light, to increase WUE and improve nutritional conditions.     

The effect of heat waves,elevated [CO2] and low soil water availability on northern red oak (Quercus rubra L.) seedlings

The frequency and intensity of heat waves are predicted to increase. This study investigates whether heat waves would have the same impact as a constant increase in temperature with the same heat sum, and whether there would be any interactive effects of elevated [CO₂] and soil moisture content. We grew Quercus rubra seedlings in treatment chambers maintained at either ambient or elevated [CO₂] (380 or 700 μmol CO₂ mol^?1) with temperature treatments of ambient, ambient +3 °C, moderate heat wave (+6 °C every other week) or severe heat wave (+12 °C every fourth week) temperatures. Averaged over a 4‐week period, and the entire growing season, the three elevated temperature treatments had the same average temperature and heat sum. Half the seedlings were watered to a soil water content near field capacity, half to about 50% of this value. Foliar gas exchange measurements were performed morning and afternoon (9:00 and 15:00 hours) before, during and after an applied heat wave in August 2010. Biomass accumulation was measured after five heat wave cycles. Under ambient [CO₂] and well‐watered conditions, biomass accumulation was highest in the +3 °C treatment, intermediate in the +6 °C heat wave and lowest in the +12 °C heat wave treatment. This response was mitigated by elevated [CO₂]. Low soil moisture significantly decreased net photosynthesis (A_net) and biomass in all [CO₂] and temperature treatments. The +12 °C heat wave reduced afternoon A_net by 23% in ambient [CO₂]. Although this reduction was relatively greater under elevated [CO₂], A_net values during this heat wave were still 34% higher than under ambient [CO₂]. We concluded that heat waves affected biomass growth differently than the same amount of heat applied uniformly over the growing season, and that the plant response to heat waves also depends on [CO₂] and soil moisture conditions.     

Photosynthetic capacity of loblolly pine (Pinus taeda L.) trees during the first year of carbon dioxide enrichment in a forest ecosystem

Our objective was to assess the photosynthetic responses of loblolly pine trees (Pinus taeda L.) during the first full growth season (1997) at the Brookhaven National Lab/Duke University Free Air CO₂ Enrichment (FACE) experiment. Gas exchange, fluorescence characteristics, and leaf biochemistry of ambient CO₂ (control) needles and ambient + 20 Pa CO₂ (elevated) needles were examined five times during the year. The enhancement of photosynthesis by elevated CO₂ in mature loblolly pine trees varied across the season and was influenced by abiotic and biotic factors. Photosynthetic enhancement by elevated CO₂ was strongly correlated with leaf temperature. The magnitude of photosynthetic enhancement was zero in March but was as great as 52% later in the season. In March, reduced sink demand and lower temperatures resulted in lower net photosynthesis, lower carboxylation rates and higher excess energy dissipation from the elevated CO₂ needles than from control needles. The greatest photosynthetic enhancement by CO₂ enrichment was observed in July during a period of high temperature and low precipitation, and in September during recovery from this period of low precipitation. In July, loblolly pine trees in the control rings exhibited lower net photosynthetic rates, lower maximum rates of photosynthesis at saturating CO₂ and light, lower values of carboxylation and electron transport rates (modelled from A–C_i curves), lower total Rubisco activity, and lower photochemical quenching of fluorescence in comparison to other measurement periods. During this period of low precipitation trees in the elevated CO₂ rings exhibited reduced net photosynthesis and photochemical quenching of fluorescence, but there was little effect on light- and CO₂-saturated rates of photosynthesis, modelled rates of carboxylation or electron transport, or Rubisco activity. These first-year data will be used to compare with similar measurements from subsequent years of the FACE experiment in order to determine whether photosynthetic acclimation to CO₂ occurs in these canopy loblolly pine trees growing in a forest ecosystem.     

Leaf and canopy responses to elevated CO2 in a pine forest under free-air CO2 enrichment

Physiological responses to elevated CO₂ at the leaf and canopy-level were studied in an intact pine (Pinus taeda) forest ecosystem exposed to elevated CO₂ using a free-air CO₂ enrichment (FACE) technique. Normalized canopy water-use of trees exposed to elevated CO₂ over an 8-day exposure period was similar to that of trees exposed to current ambient CO₂ under sunny conditions. During a portion of the exposure period when sky conditions were cloudy, CO₂-exposed trees showed minor (7%) but significant reductions in relative sap flux density compared to trees under ambient CO₂ conditions. Short-term (minutes) direct stomatal responses to elevated CO₂ were also relatively weak (5% reduction in stomatal aperture in response to high CO₂ concentrations). We observed no evidence of adjustment in stomatal conductance in foliage grown under elevated CO₂ for nearly 80 days compared to foliage grown under current ambient CO₂, so intrinsic leaf water-use efficiency at elevated CO₂ was enhanced primarily by direct responses of photosynthesis to CO₂. We did not detect statistical differences in parameters from photosynthetic responses to intercellular CO₂ (A _net-C _i curves) for Pinus taeda foliage grown under elevated CO₂ (550 mol mol^–1) for 50–80 days compared to those for foliage grown under current ambient CO₂ from similar-sized reference trees nearby. In both cases, leaf net photosynthetic rate at 550 mol mol^–1 CO₂ was enhanced by approximately 65% compared to the rate at ambient CO₂ (350 mol mol^–1). A similar level of enhancement under elevated CO₂ was observed for daily photosynthesis under field conditions on a sunny day. While enhancement of photosynthesis by elevated CO₂ during the study period appears to be primarily attributable to direct photosynthetic responses to CO₂ in the pine forest, longer-term CO₂ responses and feedbacks remain to be evaluated.     

Effects of elevated carbon dioxide and nitrogen fertilization on nitrate reductase activity in sweetgum and loblolly pine trees in two temperate forests

Nitrogen (N) availability is a major factor limiting plant production in many terrestrial ecosystems and is a key regulator of plant response to elevated CO₂. Plant N status is a function of both soil N availability and plant N uptake and assimilation capacity. As a rate-limiting step in nitrate assimilation, the reduction of nitrate is an important component of plant physiological response to elevated CO₂ and terrestrial carbon sequestration. We examine the effects of elevated CO₂ and N availability on the activity of nitrate reductase, the enzyme catalyzing the reduction of nitrate to nitrite, in two temperate forests—a closed canopy sweetgum (Liquidambar styraciflua) plantation in Tennessee (Oak Ridge National Laboratory (ORNL)) and a loblolly pine (Pinus taeda) stand in North Carolina (Duke). Both CO₂ and N enrichment had species specific impacts on nitrate reductase activity (NaR). Elevated CO₂ and N fertilization decreased foliar NaR in P. taeda, but there were no treatment effects on L. styraciflua NaR at ORNL or Duke. NaR in 1-year P. taeda needles was significantly greater than in 0-year old needles across treatments. P. taeda NaR was negatively correlated with bio-available molybdenum concentrations in soils, suggesting that CO₂ and N-mediated changes in soil nutrient status may be altering soil-plant N-dynamics. The variation in response among species may reflect different strategies for acquiring N and suggests that elevated CO₂ may alter plant N dynamics through changes in NaR.     

Infection with the parasitic angiosperm Striga hermonthica influences the response of the C3 cereal Oryza sativa to elevated CO2

Upland rice (Oryza sativa L.) was grown at both ambient (350 μmol mol^?1) and elevated (700 μmol mol^?1) CO₂ in either the presence or absence of the root hemi‐parasitic angiosperm Striga hermonthica (Del) Benth. Elevated CO₂ alleviated the impact of the parasite on host growth: biomass of infected rice grown at ambient CO₂ was 35% that of uninfected, control plants, while at elevated CO₂, biomass of infected plants was 73% that of controls. This amelioration occurred despite the fact that O. sativa grown at elevated CO₂ supported both greater numbers and a higher biomass of parasites per host than plants grown at ambient CO₂. The impact of infection on host leaf area, leaf mass, root mass and reproductive tissue mass was significantly lower in plants grown at elevated as compared with ambient CO₂. There were significant CO₂ and Striga effects on photosynthetic metabolism and instantaneous water‐use efficiency of O. sativa. The response of photosynthesis to internal [CO₂] (A/C_i curves) indicated that, at 45 days after sowing (DAS), prior to emergence of the parasites, uninfected plants grown at elevated CO₂ had significantly lower CO₂ saturated rates of photosynthesis, carboxylation efficiencies and ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Rubisco; EC 4.1.1.39) contents than uninfected, ambient CO₂‐grown O. sativa. In contrast, infection with S. hermonthica prevented down‐regulation of photosynthesis in O. sativa grown at elevated CO₂, but had no impact on photosynthesis of hosts grown at ambient CO₂. At 76 DAS (after parasites had emerged), however, infected plants grown at both elevated and ambient CO₂ had lower carboxylation efficiencies and Rubisco contents than uninfected O. sativa grown at ambient CO₂. The reductions in carboxylation efficiency (and Rubisco content) were accompanied by similar reductions in nitrogen concentration of O. sativa leaves, both before and after parasite emergence. There were no significant CO₂ or infection effects on the concentrations of soluble sugars in leaves of O. sativa, but starch concentration was significantly lower in infected plants at both CO₂ concentrations. These results demonstrate that elevated CO₂ concentrations can alleviate the impact of infection with Striga on the growth of C₃ hosts such as rice and also that infection can delay the onset of photosynthetic down‐regulation in rice grown at elevated CO₂.     

Photosynthesis, carboxylation and leaf nitrogen responses of 16 species to elevated pCO2 across four free-air CO2 enrichment experiments in forest, grassland and desert

The magnitude of changes in carboxylation capacity in dominant plant species under long‐term elevated CO₂ exposure (elevated pC_a) directly impacts ecosystem CO₂ assimilation from the atmosphere. We analyzed field CO₂ response curves of 16 C₃ species of different plant growth forms in favorable growth conditions in four free‐air CO₂ enrichment (FACE) experiments in a pine and deciduous forest, a grassland and a desert. Among species and across herb, tree and shrub growth forms there were significant enhancements in CO₂ assimilation (A) by +40±5% in elevated pC_a (49.5–57.1 Pa), although there were also significant reductions in photosynthetic capacity in elevated pC_a in some species. Photosynthesis at a common pC_a (A_a) was significantly reduced in five species growing under elevated pC_a, while leaf carboxylation capacity (V_cmax) was significantly reduced by elevated pC_a in seven species (change of ?19±3% among these species) across different growth forms and FACE sites. Adjustments in V_cmax with elevated pC_a were associated with changes in leaf N among species, and occurred in species with the highest leaf N. Elevated pC_a treatment did not affect the mass‐based relationships between A or V_cmax and N, which differed among herbs, trees and shrubs. Thus, effects of elevated pC_a on leaf C assimilation and carboxylation capacity occurred largely through changes in leaf N, rather than through elevated pC_a effects on the relationships themselves. Maintenance of leaf carboxylation capacity among species in elevated pC_a at these sites depends on maintenance of canopy N stocks, with leaf N depletion associated with photosynthetic capacity adjustments. Since CO₂ responses can only be measured experimentally on a small number of species, understanding elevated CO₂ effects on canopy N_m and N_a will greatly contribute to an ability to model responses of leaf photosynthesis to atmospheric CO₂ in different species and plant growth forms.     

Direct and indirect effects of elevated CO2 on leaf respiration in a forest ecosystem

We measured the short‐term direct and long‐term indirect effects of elevated CO₂ on leaf dark respiration of loblolly pine (Pinus taeda) and sweetgum (Liquidambar styraciflua) in an intact forest ecosystem. Trees were exposed to ambient or ambient + 200 µmol mol^?1 atmospheric CO₂ using free‐air carbon dioxide enrichment (FACE) technology. After correcting for measurement artefacts, a short‐term 200 µmol mol^?1 increase in CO₂ reduced leaf respiration by 7–14% for sweetgum and had essentially no effect on loblolly pine. This direct suppression of respiration was independent of the CO₂ concentration under which the trees were grown. Growth under elevated CO₂ did not appear to have any long‐term indirect effects on leaf maintenance respiration rates or the response of respiration to changes in temperature (Q₁₀, R₀). Also, we found no relationship between mass‐based respiration rates and leaf total nitrogen concentrations. Leaf construction costs were unaffected by growth CO₂ concentration, although leaf construction respiration decreased at elevated CO₂ in both species for leaves at the top of the canopy. We conclude that elevated CO₂ has little effect on leaf tissue respiration, and that the influence of elevated CO₂ on plant respiratory carbon flux is primarily through increased biomass.     

Modelling assimilation and intercellular CO2 from measured conductance: a synthesis of approaches

A spectrum of models that estimate assimilation rate A from intercellular carbon dioxide concentration (C_i) and measured stomatal conductance to CO₂ (g_c) were investigated using leaf‐level gas exchange measurements. The gas exchange measurements were performed in a uniform loblolly pine stand (Pinus taeda L.) using the Free Air CO₂ Enrichment (FACE) facility under ambient and elevated atmospheric CO₂ for 3 years. These measurements were also used to test a newly proposed framework that combines basic properties of the A–C_i curve with a Fickian diffusion transport model to predict the relationship between C_i/C_a and g_c, where C_a is atmospheric carbon dioxide concentration. The widely used Ball–Berry model and five other models as well as the biochemical model proposed by Farquhar et al. (1980) were also reformulated to express variations in C_i/C_a as a function of their corresponding driving mechanisms. To assess the predictive capabilities of these approaches, their respective parameters were estimated from independent measurements of long‐term stable carbon isotope determinations (δ¹³C), meteorological variables, and ensemble A–C_i curves. All eight approaches reproduced the measured A reasonably well, in an ensemble sense, from measured water vapour conductance and modeled C_i/C_a. However, the scatter in the instantaneous A estimates was sufficiently large for both ambient and elevated C_a to suggest that other transient processes were not explicitly resolved by all eight parameterizations. An important finding from our analysis is that added physiological complexity in modeling C_i/C_a (when g_c is known) need not always translate to increased accuracy in predicting A. Finally, the broader utility of these approaches to estimate assimilation and net ecosystem exchange is discussed in relation to elevated atmospheric CO₂.     

Acclimation of photosynthesis to elevated CO2 in onion (Allium cepa) grown at a range of temperatures

Onion (Allium cepa) was grown in the field within temperature gradient tunnels (providing about ‐2.5°C to +2.5°C from outside temperatures) maintained at either 374 or 532 μmol mol^?1 CO₂. Plant leaf area was determined non‐destructively at 7 day intervals until the time of bulbing in 12 combinations of temperature and CO₂ concentration. Gas exchange was measured in each plot at the time of bulbing, and the carbohydrate content of the leaf (source) and bulb (sink) was determined. Maximum rate of leaf area expansion increased with mean temperature. Leaf area duration and maximum rate of leaf area expansion were not significantly affected by CO₂. The light‐saturated rates of leaf photosynthesis (A_sat) were greater in plants grown at normal than at elevated CO₂ concentrations at the same measurement CO₂ concentration. Acclimation of photosynthesis decreased with an increase in growth temperature, and with an increase in leaf nitrogen content at elevated CO₂. The ratio of intercellular to atmospheric CO₂ (C₁/C₃ ratio) was 7.4% less for plants grown at elevated compared with normal CO₂. A_sat in plants grown at elevated CO₂ was less than in plants grown at normal CO₂ when compared at the same C₁. Hence, acclimation of photosynthesis was due both to stomatal acclimation and to limitations to biochemical CO₂ fixation. Carbohydrate content of the onion bulbs was greater at elevated than at normal CO₂. In contrast, carbohydrate content was less at elevated compared with normal CO₂ in the leaf sections in which CO₂ exchange was measured at the same developmental stage. Therefore, acclimation of photosynthesis in fully expanded onion leaves was detected despite the absence of localised carbohydrate accumulation in these field‐grown crops.     

Leaf senescence of Quercus myrtifolia as affected by long-term CO2 enrichment in its native environment

The long‐term effects of elevated (ambient plus 350 μmol mol^?1) atmospheric CO₂ concentration (C_a) on the leaf senescence of Quercus myrtifolia Willd was studied in a scrub‐oak community during the transition from autumn (December 1997) to spring (April 1998). Plants were grown in large open‐top chambers at the Smithsonian CO₂ Research Site, Merritt Island Wildlife Refuge, Cape Canaveral, Florida. Chlorophyll (a + b) concentration, Rubisco activity and N concentration decreased by 75%, 82%, and 52%, respectively, from December (1997) to April (1998) in the leaves grown at ambient C_a. In contrast, the leaves of plants grown at elevated C_a showed no significant decrease in chlorophyll (a + b) concentration or Rubisco activity, and only a 25% reduction in nitrogen. These results indicate that leaf senescence was delayed during this period at elevated C_a. Delayed leaf senescence in elevated C_a had important consequences for leaf photosynthesis. In elevated C_a the net photosynthetic rate of leaves that flushed in Spring 1997 (last year's leaves) and were 13 months old was not different from fully‐expanded leaves that flushed in 1998, and were approximately 1 month old (current year's leaves). In ambient C_a the net photosynthetic rate of last year's leaves was 54% lower than for current year's leaves. When leaves were fully senesced, nitrogen concentration decreased to about 40% of the concentration in non‐senesced leaves, in both CO₂ treatments. In April, net photosynthesis was 97% greater in leaves grown in elevated C_a than in those grown at ambient. During the period when elevated C_a delayed leaf senescence, more leaves operating at higher photosynthetic rate would allow the ecosystem dominated by Q. myrtifolia to gain more carbon at elevated C_a than at ambient C_a.     

Sex‐specific responses of Populus yunnanensis exposed to elevated CO2 and salinity

Populus yunnanensis Dode., a native dioecious woody plant in southwestern China, was employed as a model species to study sex‐specific morphological, physiological and biochemical responses to elevated CO₂ and salinity. To investigate the effects of elevated CO₂, salinity and their combination, the cuttings were exposed to two CO₂ regimes (ambient CO₂ and double ambient CO₂) and two salt treatments in growth chambers. Males exhibited greater downregulation of net photosynthesis rate (A_net) and carboxylation efficiency (CE) than females at elevated CO₂, whereas these sexual differences were lessened under salt stress. On the other hand, salinity induced a higher decrease in A_net and CE, more growth inhibition and leaf Cl^? accumulation and more damage to cell organelles in females than in males, whereas the sexual differences in photosynthesis and growth were lessened at elevated CO₂. Moreover, elevated CO₂ exacerbated membrane lipid peroxidation and organelle damage in females but not in males under salt stress. Our results indicated that: (1) females are more sensitive and suffer from greater negative effects than do males under salt stress, and elevated CO₂ lessens the sexual differences in photosynthesis and growth under salt stress; (2) elevated CO₂ tends to aggravate the negative effects of salinity in females; and (3) sex‐specific reactions under the combination of elevated CO₂ and salinity are distinct from single‐stress responses. Therefore, these results provide evidence for different adaptive responses between plants of different sexes exposed to elevated CO₂ and salinity.     

No photosynthetic down-regulation in sweetgum trees (Liquidambar styraciflua L.) after three years of CO2 enrichment at the Duke Forest FACE experiment

Photosynthetic capacity and leaf properties of sun and shade leaves of overstorey sweetgum trees (Liquidambar styraciflua L.) were compared over the first 3 years of growth in ambient or ambient + 200 μL L^?1 CO₂ at the Duke Forest Free Air CO₂ Enrichment (FACE) experiment. We were interested in whether photosynthetic down‐regulation to CO₂ occurred in sweetgum trees growing in a forest ecosystem, whether shade leaves down‐regulated to a greater extent than sun leaves, and if there was a seasonal component to photosynthetic down‐regulation. During June and September of each year, we measured net photosynthesis (A) versus the calculated intercellular CO₂ concentration (C_i) in situ and analysed these response curves using a biochemical model that described the limitations imposed by the amount and activity of ribulose‐1,5‐bisphosphate carboxylase/oxygenase (Vc_max) and by the rate of ribulose‐1,5‐bisphosphate (RuBP) regeneration mediated by electron transport (J_max). There was no evidence of photosynthetic down‐regulation to CO₂ in either sun or shade leaves of sweetgum trees over the 3 years of measurements. Elevated CO₂ did not significantly affect Vc_max or J_max. The ratio of Vc_max to J_max was relatively constant, averaging 2·12, and was not affected by CO₂ treatment, position in the canopy, or measurement period. Furthermore, CO₂ enrichment did not affect leaf nitrogen per unit leaf area (N_a), chlorophyll or total non‐structural carbohydrates of sun or shade leaves. We did, however, find a strong relationship between N_a and the modelled components of photosynthetic capacity, Vc_max and J_max. Our data over the first 3 years of this experiment corroborate observations that trees rooted in the ground may not exhibit symptoms of photosynthetic down‐regulation as quickly as tree seedlings growing in pots. There was a strong sustained enhancement of photosynthesis by CO₂ enrichment whereby light‐saturated net photosynthesis of sun leaves was stimulated by 63% and light‐saturated net photosynthesis of shade leaves was stimulated by 48% when averaged over the 3 years. This study suggests that this CO₂ enhancement of photosynthesis will be sustained in the Duke Forest FACE experiment as long as soil N availability keeps pace with photosynthetic and growth processes.     

Consequences of CO2 and light interactions for leaf phenology, growth, and senescence in Quercus rubra

We investigated how light and CO₂ levels interact to influence growth, phenology, and the physiological processes involved in leaf senescence in red oak (Quercus rubra) seedlings. We grew plants in high and low light and in elevated and ambient CO₂. At the end of three years of growth, shade plants showed greater biomass enhancement under elevated CO₂ than sun plants. We attribute this difference to an increase in leaf area ratio (LAR) in shade plants relative to sun plants, as well as to an ontogenetic effect: as plants increased in size, the LAR declined concomitant with a decline in biomass enhancement under elevated CO₂ Elevated CO₂ prolonged the carbon gain capacity of shade‐grown plants during autumnal senescence, thus increasing their functional leaf lifespan. The prolongation of carbon assimilation, however, did not account for the increased growth enhancement in shade plants under elevated CO₂. Elevated CO₂ did not significantly alter leaf phenology. Nitrogen concentrations in both green and senesced leaves were lower under elevated CO₂ and declined more rapidly in sun leaves than in shade leaves. Similar to nitrogen concentration, the initial slope of A/C_i curves indicated that Rubisco activity declined more rapidly in sun plants than in shade plants, particularly under elevated CO₂. Absolute levels of chlorophyll were affected by the interaction of CO₂ and light, and chlorophyll content declined to a minimal level in sun plants sooner than in shade plants. These declines in N concentration, in the initial slope of A/C_i curves, and in chlorophyll content were consistent with declining photosynthesis, such that elevated CO₂ accelerated senescence in sun plants and prolonged leaf function in shade plants. These results have implications for the carbon economy of seedlings and the regeneration of red oak under global change conditions.     

Effects of climate change on labile and structural carbon in Douglas-fir needles as estimated by δ13C and Carea measurements

Models of photosynthesis, respiration, and export predict that foliar labile carbon (C) should increase with elevated CO₂ but decrease with elevated temperature. Sugars, starch, and protein can be compared between treatments, but these compounds make up only a fraction of the total labile pool. Moreover, it is difficult to assess the turnover of labile carbon between years for evergreen foliage. Here, we combined changes in foliar C_area (C concentration on an areal basis) as needles aged with changes in foliar isotopic composition (δ¹³C) caused by inputs of ¹³C‐depleted CO₂ to estimate labile and structural C in needles of different ages in a four‐year, closed‐chamber mesocosm experiment in which Douglas‐fir (Pseudotsuga menziesii (Mirb.) Franco) seedlings were exposed to elevated temperature (ambient + 3.5 °C) and CO₂ (ambient + 179 ppm). Declines in δ ¹³C of needle cohorts as they aged indicated incorporation of newly fixed labile or structural carbon. The δ ¹³C calculations showed that new C was 41 ± 2% and 28 ± 3% of total needle carbon in second‐ and third‐year needles, respectively, with higher proportions of new C in elevated than ambient CO₂ chambers (e.g. 42 ± 2% vs. 37 ± 6%, respectively, for second‐year needles). Relative to ambient CO₂, elevated CO₂ increased labile C in both first‐ and second‐year needles. Relative to ambient temperature, elevated temperature diminished labile C in second‐year needles but not in first‐year needles, perhaps because of differences in sink strength between the two needle age classes. We hypothesize that plant‐soil feedbacks on nitrogen supply contributed to higher photosynthetic rates under elevated temperatures that partly compensated for higher turnover rates of labile C. Strong positive correlations between labile C and sugar concentrations suggested that labile C was primarily determined by carbohydrates. Labile C was negatively correlated with concentrations of cellulose and protein. Elevated temperature increased foliar %C, possibly due to a shift of labile constituents from low %C carbohydrates to relatively high %C protein. Decreased sugar concentrations and increased nitrogen concentrations with elevated temperature were consistent with this explanation. Because foliar constituents that vary in isotopic signature also vary in concentrations with leaf age or environmental conditions, inferences of c_i/c_a values from δ ¹³C of bulk leaf tissue should be done cautiously. Tracing of ¹³C through foliar carbon pools may provide new insight into foliar C constituents and turnover.     

Fine root responses to temporal nutrient heterogeneity and competition in seedlings of two tree species with different rooting strategies

There is little direct evidence for effects of soil heterogeneity and root plasticity on the competitive interactions among plants. In this study, we experimentally examined the impacts of temporal nutrient heterogeneity on root growth and interactions between two plant species with very different rooting strategies: Liquidambar styraciflua (sweet gum), which shows high root plasticity in response to soil nutrient heterogeneity, and Pinus taeda (loblolly pine), a species with less plastic roots. Seedlings of the two species were grown in sandboxes in inter‐ and intraspecific combinations. Nutrients were applied in a patch either in a stable (slow‐release) or in a variable (pulse) manner. Plant aboveground biomass, fine root mass, root allocation between nutrient patch and outside the patch, and root vertical distribution were measured. L. styraciflua grew more aboveground (40% and 27% in stable and variable nutrient treatment, respectively) and fine roots (41% and 8% in stable and variable nutrient treatment, respectively) when competing with P. taeda than when competing with a conspecific individual, but the growth of P. taeda was not changed by competition from L. styraciflua. Temporal variation in patch nutrient level had little effect on the species’ competitive interactions. The more flexible L. styraciflua changed its vertical distribution of fine roots in response to competition from P. taeda, growing more roots in deeper soil layers compared to its roots in conspecific competition, leading to niche differentiation between the species, while the fine root distribution of P. taeda remained unchanged across all treatments. Synthesis. L. styraciflua showed greater flexibility in root growth by changing its root vertical distribution and occupying space of not occupied by P. taeda. This flexibility gave L. styraciflua an advantage in interspecific competition.