Convergence in the relationship of CO2 and N2O exchanges between soil and atmosphere within terrestrial ecosystems

Ecosystem CO₂ and N₂O exchanges between soils and the atmosphere play an important role in climate warming and global carbon and nitrogen cycling; however, it is still not clear whether the fluxes of these two greenhouse gases are correlated at the ecosystem scale. We collected 143 pairs of ecosystem CO₂ and N₂O exchanges between soils and the atmosphere measured simultaneously in eight ecosystems around the world and developed relationships between soil CO₂ and N₂O fluxes. Significant linear regressions of soil CO₂ and N₂O fluxes were found for all eight ecosystems; the highest slope occurred in rice paddies and the lowest in temperate grasslands. We also found the dominant role of growing season on the relationship of annual CO₂ and N₂O fluxes. No significant relationship between soil CO₂ and N₂O fluxes was found across all eight ecosystem types. The estimated annual global N₂O emission based on our findings is 13.31 Tg N yr⁻¹ with a range of 8.19–18.43 Tg N yr⁻¹ for 1980–2000, of which cropland contributes nearly 30%. Our findings demonstrated that stoichiometric relationships may work on ecological functions at the ecosystem level. The relationship of soil N₂O and CO₂ fluxes developed here could be helpful in biogeochemical modeling and large-scale estimations of soil CO₂ and N₂O fluxes.     

Large annual net ecosystem CO2 uptake of a Mojave Desert ecosystem

The net ecosystem CO₂ exchange (NEE) between a Mojave Desert ecosystem and the atmosphere was measured over the course of 2 years at the Mojave Global Change Facility (MGCF, Nevada, USA) using the eddy covariance method. The investigated desert ecosystem was a sink for CO₂, taking up 102±67 and 110±70 g C m⁻² during 2005 and 2006, respectively. A comprehensive uncertainty analysis showed that most of the uncertainty of the inferred sink strength was due to the need to account for the effects of air density fluctuations on CO₂ densities measured with an open-path infrared gas analyser. In order to keep this uncertainty within acceptable bounds, highest standards with regard to maintenance of instrumentation and flux measurement postprocessing have to be met. Most of the variability in half-hourly NEE was explained by the amount of incident photosynthetically active radiation (PAR). On a seasonal scale, PAR and soil water content were the most important determinants of NEE. Precipitation events resulted in an initial pulse of CO₂ to the atmosphere, temporarily reducing NEE or even causing it to switch sign. During summer, when soil moisture was low, a lag of 3–4 days was observed before the correlation between NEE and precipitation switched from positive to negative, as opposed to conditions of high soil water availability in spring, when this transition occurred within the same day the rain took place. Our results indicate that desert ecosystem CO₂ exchange may be playing a much larger role in global carbon cycling and in modulating atmospheric CO₂ levels than previously assumed – especially since arid and semiarid biomes make up >30% of Earth's land surface.     

Sensing of atmospheric CO2 by plants

Abstract. Despite recent interest in the effects of high CO₂ on plant growth and physiology, very little is known about the mechanisms by which plants sense changes in the concentration of this gas. Because atmospheric CO₂ concentration is relatively constant and because the conductance of the cuticle to CO₂ is low, sensory mechanisms are likely to exist only for intercellular CO₂ concentration. Therefore, responses of plants to changes in atmospheric CO₂ will depend on the effect of these changes on intercellular CO₂ concentration. Although a variety of plant responses to atmospheric CO₂ concentration have been reported, most of these can be attributed to the effects of intercellular CO₂ on photosynthesis or stomatal conductance. Short-term and long-term effects of CO₂ on photosynthesis and stomatal conductance are discussed as sensory mechanisms for responses of plants to atmospheric CO₂. Available data suggest that plants do not fully realize the potential increases in productivity associated with increased atmospheric CO₂. This may be because of genetic and environmental limitations to productivity or because plant responses to CO₂ have evolved to cope with variations in intercellular CO₂ caused by factors other than changes in atmospheric CO₂.     

A review of nitrogen enrichment effects on three biogenic GHGs: the CO2 sink may be largely offset by stimulated N2O and CH4 emission

Anthropogenic nitrogen (N) enrichment of ecosystems, mainly from fuel combustion and fertilizer application, alters biogeochemical cycling of ecosystems in a way that leads to altered flux of biogenic greenhouse gases (GHGs). Our meta-analysis of 313 observations across 109 studies evaluated the effect of N addition on the flux of three major GHGs: CO₂, CH₄ and N₂O. The objective was to quantitatively synthesize data from agricultural and non-agricultural terrestrial ecosystems across the globe and examine whether factors, such as ecosystem type, N addition level and chemical form of N addition influence the direction and magnitude of GHG fluxes. Results indicate that N addition increased ecosystem carbon content of forests by 6%, marginally increased soil organic carbon of agricultural systems by 2%, but had no significant effect on net ecosystem CO₂ exchange for non-forest natural ecosystems. Across all ecosystems, N addition increased CH₄ emission by 97%, reduced CH₄ uptake by 38% and increased N₂O emission by 216%. The net effect of N on the global GHG budget is calculated and this topic is reviewed. Most often N addition is considered to increase forest C sequestration without consideration of N stimulation of GHG production in other ecosystems. However, our study indicated that although N addition increased the global terrestrial C sink, the CO₂ reduction could be largely offset (53–76%) by N stimulation of global CH₄ and N₂O emission from multiple ecosystems.     

Canopy photosynthesis of crops and native plant communities exposed to long-term elevated CO2

Abstract. There have been seven studies of canopy photosynthesis of plants grown in elevated atmospheric CO₂: three of seed crops, two of forage crops and two of native plant ecosystems. Growth in elevated CO₂ increased canopy photosynthesis in all cases. The relative effect of CO₂ was correlated with increasing temperature: the least stimulation occurred in tundra vegetation grown at an average temperature near 10°C and the greatest in rice grown at 43°C. In soybean, effects of CO₂ were greater during leaf expansion and pod fill than at other stages of crop maturation. In the longest running experiment with elevated CO₂ treatment to date, monospecific stands of a C₃ sedge, Scirpus olneyi (Grey), and a C₄ grass, Spartina patens (Ait.) Muhl., have been exposed to twice normal ambient CO₂ concentrations for four growing seasons, in open top chambers on a Chesapeake Bay salt marsh. Net ecosystem CO₂ exchange per unit green biomass (NCE_b) increased by an average of 48% throughout the growing season of 1988, the second year of treatment. Elevated CO₂ increased net ecosystem carbon assimilation by 88% in the Scirpus olneyi community and 40% in the Spartina patens community.     

Respiration in a future, higher-CO2 world

Abstract. Apart from its impact on global warming, the annually increasing atmospheric [CO₂] is of interest to plant scientists primarily because of its direct influence on photosynthesis and photorespiration in C₃ species. But in addition, 'dark' respiration, another major component of the carbon budget of higher plants, may be affected by a change in [CO₂] independent of an increase in temperature. Literature pertaining to an impact of [CO₂] on respiration rate is reviewed. With an increase in [CO₂], respiration rate is increased in some cases, but decreased in others. The effects of [CO₂] on respiration rate may be direct or indirect. Mechanisms responsible for various observations are proposed. These proposed mechanisms relate to changes in: (1) levels of nonstructural carbohydrates, (2) growth rate and structural phytomass accumulation, (3) composition of phytomass, (4) direct chemical interactions between CO₂ and respiratory enzymes, (5) direct chemical interactions between CO₂ and other cellular components, (6) dark CO₂ fixation rate, and (7) ethylene biosynthesis rate. Because a range-of (possibly interactive) effects exist, and present knowledge is limited, the impact of future [CO₂] on respiration rate cannot be predicted. Theoretical considerations and types of experiments that can lead to an increase in the understanding of this issue are outlined.     

A new method to measure carbon isotope composition of CO2 respired by trees: stem CO2 equilibration

1.  Applying Keeling plot techniques to derive δ¹³C of respiratory input in a closed non-equilibrated chamber can lead to large errors because steady-state diffusion rules are violated in a non-steady-state environment. To avoid these errors, respiratory δ¹³C can be derived using equilibrated closed chambers.
2.  We introduce a new method to obtain stem respired CO₂δ¹³C (δ_{st - r}) with closed equilibrated stem chambers (E-SC). We present a theoretical model describing the equilibration process, test the model against field data and find excellent agreement. The method is further tested by comparing it with closed non-equilibrated stem chambers (NE-SC); we found no difference between these methods.
3.  Our theoretical model to describe CO₂ diffusion from the respiratory pool into the chamber and the equation to derive the δ¹³C of the efflux are general. They could be applied to other ecosystem components (e.g. soils).
4.  Our method is easy to implement, cost effective, minimizes sources of error and allows for rigorous leak detection. One major limitation is its inability to detect rapid change; the equilibration process requires 15 ± 2 h. A second limitation is that it cannot be used for species that produce abundant pitch at sites of stem wounding (e.g. Pseudotsuga menziesii ).
5.  Investigating δ¹³C of CO₂ respired by different ecosystem components is necessary to interpret δ¹³C of ecosystem respiration. This parameter has major implications with respect to global carbon cycle science.     

Elevated CO 2and water supply interactions in grasslands: a pastures and rangelands management perspective

Water is a key variable driving the composition and productivity of pastures and rangelands, and many of the ecosystems in these grasslands are highly sensitive to changes in water supply. The possibility that elevated CO₂ concentrations may alter plant water relations is therefore particularly relevant to pastures and rangelands, and may have important consequences for grassland ecosystem function, water use, carbon storage and nutrient cycling. The planning of effective research to better understand these changes requires attention to both: (i) gaps in knowledge about CO₂ and water interactions, and (ii) knowledge of how precisely the effects of CO₂ must be understood in relation to other factors, in order to predict changes in grassland structure and production. A recent microcosm experiment illustrates that non-linear effects of CO₂ and water stress could perturb primary production by triggering changes in grassland community composition. The magnitudes of the effects of CO₂ on key grassland ecosystems remain to be precisely determined through ecosystem-level experiments. A simplified simulation of the impact of different levels of productivity change in a water-limited Australian rangeland system was conducted by varying effects of CO₂ on radiation and water use efficiency. The results indicate that direct effects of CO₂ may be moderated at the enterprise scale by accompanying changes in adaptive management by farmers. We conclude that future research should aim to construct quantitative relationships and identify thresholds of response for different grassland systems. The sensitivity of these systems to management (such as grazing pressure) should also be considered when developing integrated predictions of future effects of CO₂ on water supply to grassland ecosystems.     

Influence of soil O2 and CO2 on root respiration for Agave deserti

Respiration measured as CO₂ efflux was determined at various soil O₂ and CO₂ concentrations for individual, attached roots of a succulent perennial from the Sonoran Desert, Agave deserti Engelm. The respiration rate increased with increasing O₂ concentration up to about 16% O₂ for established roots and 5% O₂ for rain roots (fine branch roots on established roots induced by wetting of the soil) and then remained fairly constant up to 21% O₂. When O₂ was decreased from 21 to 0%, the respiration rates were similar to those obtained with increasing O₂ concentration. The CO₂ concentration in the root zone, which for the shallow-rooted A. deserti in the field was about 1 000 μl l^-1, did not affect root respiration at concentrations up to 2 000 μl l^-1, but higher concentrations reduced it, respiration being abolished at 20 000 μl l^-1 (2%) CO₂ for both established and rain roots. Upon lowering CO₂ to 1 000 μl l^-1 after exposure to concentrations up to 10000 μl l^-1 CO₂, inhibition of respiration was reversible. Uptake of the vital stain neutral red by root cortical cells was reduced to zero, indicating cell death, in about 4 h at 2% CO₂, substantiating the detrimental effects of high soil CO₂ concentrations on roots of A. deserti . This CO₂ response may explain why roots of desert succulents tend to occur in porous, well-aerated soils.     

Affinity for CO2 in relation to the ability of freshwater macrophytes to use HCO–3

1. The affinity of photosynthesis for CO₂ is calculated here as the initial slope of net-photosynthetic rate against concentration of CO₂. The affinity for CO₂ for pairs of freshwater macrophytes with similar leaf morphology but able or unable to use HCO^–₃ as a carbon source was compared.
2. Species restricted to CO₂ had a higher affinity for CO₂ than species that were also able to use HCO^–₃ when rates were expressed on the basis of area, dry mass and content of chlorophyll a .
3. Published values for the affinity for CO₂ and the concentration of CO₂ which half-saturated rate of photosynthesis were compiled and compared. Despite a large range of values, affinity for CO₂ was greater for species restricted to CO₂ than for those also able to use HCO^–₃ and statistically different when the slope was expressed on the basis of dry mass and chlorophyll a content.
4. The difference in affinity is consistent with predicted benefits of a high permeability to CO₂ for species relying on passive diffusion of CO₂ and a lower permeability for species able to use HCO^–₃ in order to reduce efflux of CO₂ from a high internal concentration generated by active transport.
5. The implications of the different affinities are discussed in terms of species distribution.     

Two mutants of Arabidopsis thaliana that become chlorotic in atmospheres enriched with CO2

Abstract. Two nonallelic, nuclear recessive mutants of Arabidopsis thaliana (L.) Heynh. which become chlorotic when grown in an atmosphere enriched to 20000 cm³ CO₂ m^-3 have been isolated. For one of the mutants, chlorosis begins at the veins and gradually spreads to the interveinal regions. A minimum photon flux density of ca 50 μmol m^-2 s^-1 is required for this response. For the other mutant, the yellowing is independent of the light intensity and begins at the basal regions of the leaves and spreads to the tips. The injurious effects of CO₂ seem to be restricted to photosynthetic tissues, since root elongation and callus growth were not inhibited by a high atmospheric CO₂ concentration for either mutant. Neither mutant became chlorotic in a low O₂ atmosphere that suppressed photorespiration as effectively as the elevated CO₂ does. Thus, the mutations do not impose a requirement for photorespiration. The possibilities that the high CO₂-sensitive phenotypes are caused by an effect of CO₂ in stomata, on ethylene synthesis, or on mineral uptake are discussed but are considered unlikely.     

Productivity and community structure of ectomycorrhizal fungal sporocarps under increased atmospheric CO2 and O3

Sporocarp production is essential for ectomycorrhizal fungal recombination and dispersal, which influences fungal community dynamics. Increasing atmospheric carbon dioxide (CO₂) and ozone (O₃) affect host plant carbon gain and allocation, which may in turn influence ectomycorrhizal sporocarp production if the carbon available to the ectomycorrhizal fungus is dependant upon the quantity of carbon assimilated by the host. We measured sporocarp production of ectomycorrhizal fungi over 4 years at the Aspen FACE (free air CO₂ enrichment) site, which corresponded to stand ages seven to 10 years. Total mean sporocarp biomass was greatest under elevated CO₂, regardless of O₃ concentration, while it was generally lowest under elevated O₃ with ambient CO₂. Community composition differed significantly among the treatments, with less difference in the final year of the study. Whether this convergence was due to succession or environmental factors is uncertain. CO₂ and O₃ affect ectomycorrhizal sporocarp productivity and community composition, with likely effects on dispersal, colonization and sporocarp-dependent food webs.     

Carbon and water fluxes in a calcareous grassland under elevated CO2

1. As part of a long-term study of the effects of elevated CO₂ on biodiversity and ecosystem function in a calcareous grassland, we measured ecosystem carbon dioxide and water-vapour fluxes over 24-h periods during the 1994 and 1995 growing seasons. Data were used to derive CO₂ and H₂O gas-exchange response functions to quantum flux density (QFD).
2. The relative increase in net ecosystem CO₂ flux (NEC) owing to CO₂ enrichment increased as QFD rose. Daytime NEC at high QFD under elevated CO₂ increased by 25% to 60%, with the greatest increases in the spring and after mowing in June when above-ground biomass was lowest. There was much less stimulation of NEC in early June and again in October when the canopy was fully developed. Night-time NEC was not significantly altered under elevated CO₂.
3. Short-term reversal of CO₂ concentrations between treatments after two seasons of CO₂ exposure provided evidence for a 50% downward adjustment of NEC expressed per unit above-ground plant dry weight. However, when expressed on a land area basis, this difference disappeared because of a c. 20% increase in above-ground biomass under elevated CO₂.
4. Ecosystem evapotranspiration (ET) was not significantly altered by elevated CO₂ when averaged over all measurement dates and positions. However, ET was reduced 3–18% at high QFD in plots at the top of the slope at our study site. In summary, CO₂ enrichment resulted in a large stimulation of ecosystem CO₂ capture, especially during periods of a large demand of carbon in relationship to its supply, and resulted in a relatively small and variable effect on ecosystem water consumption.     

Increasing growth temperature reduces the stimulatory effect of elevated CO2 on photosynthesis or biomass in two perennial species

We examined how anticipated changes in CO₂ concentration and temperature interacted to alter plant growth, harvest characteristics and photosynthesis in two cold-adapted herbaceous perennials, alfalfa ( Medicago sativa L. cv. Arc) and orchard grass ( Dactylis glomerata L. cv. Potomac). Plants were grown at two CO₂ concentrations (362 [ambient] and 717 [elevated] μmol mol⁻¹ CO₂) and four constant day/night temperatures of 15, 20, 25 and 30°C in controlled environmental chambers. Elevated CO₂ significantly increased total plant biomass and protein over a wide range of temperatures in both species. Stimulation of photosynthetic rate, however, was eliminated at the highest growth temperature in M. sativa and relative stimulation of plant biomass and protein at high CO₂ declined as temperature increased in both species. Lack of a synergistic effect between temperature and CO₂ was unexpected since elevated CO₂ reduces the amount of carbon lost via photorespiration and photorespiration increases with temperature. Differences between anticipated stimulatory effects of CO₂ and temperature and whole plant single and leaf measurements are discussed. Data from this study suggest that stimulatory effects of atmospheric CO₂ on growth and photosynthesis may decline with anticipated increases in global temperature, limiting the degree of carbon storage in these two perennial species.     

The cellular basis of guard cell sensing of rising CO2

Numerous studies conducted on both whole plants and isolated epidermes have documented stomatal sensitivity to CO₂. In general, CO₂ concentrations below ambient stimulate stomatal opening, or an inhibition of stomatal closure, while CO₂ concentrations above ambient have the opposite effect. The rise in atmospheric CO₂ concentrations which has occurred since the industrial revolution, and which is predicted to continue, will therefore alter rates of transpirational water loss and CO₂ uptake in terrestrial plants. An understanding of the cellular basis for guard cell CO₂ sensing could allow us to better predict, and perhaps ultimately to manipulate, such vegetation responses to climate change. However, the mechanisms by which guard cells sense and respond to the CO₂ signal remain unknown. It has been hypothesized that cytosolic pH and malate levels, cytosolic Ca²⁺ levels, chloroplastic zeaxanthin levels, or plasma-membrane anion channel regulation by apoplastic malate are involved in guard cell perception and response to CO₂. In this review, these hypotheses are discussed, and the evidence for guard cell acclimation to prevailing CO₂ concentrations is also considered.     

Comparison of gas use efficiency and treatment uniformity in a forest ecosystem exposed to elevated [CO2] using pure and prediluted free-air CO2 enrichment technology

A direct comparison of treatment uniformity and CO₂ use of pure and prediluted free-air CO₂ enrichment (FACE) systems was conducted in a forest ecosystem. A vertical release pure CO₂ fumigation system was superimposed on an existing prediluted CO₂ fumigation system and operated on alternate days. The FACE system using prediluted CO₂ fumigation technology exhibited less temporal and spatial variability than the pure CO₂ fumigation system. The pure CO₂ fumigation system tended to over-fumigate the upwind portions of the plot and used 25% more CO₂ than the prediluted CO₂ fumigation system. The increased CO₂ use by the pure CO₂ system was exacerbated at low wind speeds. It is not clear if this phenomenon will also be observed in plots with smaller diameters and low-stature vegetation.     

Artificial drainage and associated carbon fluxes (CO2/CH4) in a tundra ecosystem

Ecosystem flux measurements using the eddy covariance (EC) technique were undertaken in 4 subsequent years during summer for a total of 562 days in an arctic wet tundra ecosystem, located near Cherskii, Far-Eastern Federal District, Russia. Methane (CH₄) emissions were measured using permanent chambers. The experimental field is characterized by late thawing of permafrost soils in June and periodic spring floods. A stagnant water table below the grass canopy is fed by melting of the active layer of permafrost and by flood water. Following 3 years of EC measurements, the site was drained by building a 3 m wide drainage channel surrounding the EC tower to examine possible future effects of global change on the tundra tussock ecosystem. Cumulative summertime net carbon fluxes before experimental alteration were estimated to be about +15 g C m⁻² (i.e. an ecosystem C loss) and +8 g C m⁻² after draining the study site. When taking CH₄ as another important greenhouse gas into account and considering the global warming potential (GWP) of CH₄ vs. CO₂, the ecosystem had a positive GWP during all summers. However CH₄ emissions after drainage decreased significantly and therefore the carbon related greenhouse gas flux was much smaller than beforehand (475 ± 253 g C-CO₂-e m⁻² before drainage in 2003 vs. 23 ± 26 g C-CO₂-e m⁻² after drainage in 2005).     

Effect of elevated CO2 on the stomatal distribution and leaf physiology of Alnus glutinosa

Variation in stomatal development and physiology of mature leaves from Alnus glutinosa plants grown under reference (current ambient, 360 μmol mol⁻¹ CO₂) and double ambient (720 μmol mol⁻¹ CO₂) carbon dioxide (CO₂) mole fractions is assessed in terms of relative plant growth, stomatal characters (i.e. stomatal index and density) and leaf photosynthetic characters. This is the first study to consider the effects of elevated CO₂ concentration on the distribution of stomata and epidermal cells across the whole leaf and to try to ascertain the cause of intraleaf variation. In general, a doubling of the atmospheric CO₂ concentration enhanced plant growth and significantly increased stomatal index. However, there was no significant change in relative stomatal density. Under elevated CO₂ concentration there was a significant decrease in stomatal conductance and an increase in assimilation rate. However, no significant differences were found for the maximum rate of carboxylation ( V _cmax) and the light saturated rate of electron transport ( J _max) between the control and elevated CO₂ treatment.     

Soil CO2 efflux in a boreal pine forest under atmospheric CO2 enrichment and air warming

The response of forest soil CO₂ efflux to the elevation of two climatic factors, the atmospheric concentration of CO₂ (↑CO₂ of 700 μmol mol⁻¹) and air temperature (↑ T with average annual increase of 5°C), and their combination (↑CO₂+↑ T ) was investigated in a 4-year, full-factorial field experiment consisting of closed chambers built around 20-year-old Scots pines ( Pinus sylvestris L.) in the boreal zone of Finland. Mean soil CO₂ efflux in May–October increased with elevated CO₂ by 23–37%, with elevated temperature by 27–43%, and with the combined treatment by 35–59%. Temperature elevation was a significant factor in the combined 4-year efflux data, whereas the effect of elevated CO₂ was not as evident. Elevated temperature had the most pronounced impact early and late in the season, while the influence of elevated CO₂ alone was especially notable late in the season. Needle area was found to be a significant predictor of soil CO₂ efflux, particularly in August, a month of high root growth, thus supporting the assumption of a close link between whole-tree physiology and soil CO₂ emissions. The decrease in the temperature sensitivity of soil CO₂ efflux observed in the elevated temperature treatments in the second year nevertheless suggests the existence of soil response mechanisms that may be independent of the assimilating component of the forest ecosystem. In conclusion, elevated atmospheric CO₂ and air temperature consistently increased forest soil CO₂ efflux over the 4-year period, their combined effect being additive, with no apparent interaction.     

Gas exchange and carbon isotope composition of Ananas comosus in response to elevated CO2 and temperature

Ananas comosus L. (Merr.) (pineapple) was grown at three day/night temperatures and 350 (ambient) and 700 (elevated) μ mol mol^–1 CO₂ to examine the interactive effects of these factors on leaf gas exchange and stable carbon isotope discrimination ( Δ ,‰). All data were collected on the youngest mature leaf for 24 h every 6 weeks. CO₂ uptake (mmol m^–2 d^–1) at ambient and elevated CO₂, respectively, were 306 and 352 at 30/20 °C, 175 and 346 at 30/25 °C and 187 and 343 at 35/25 °C. CO₂ enrichment enhanced CO₂ uptake substantially in the day in all environments. Uptake at night at elevated CO₂, relative to that at ambient CO₂, was unchanged at 30/20 °C, but was 80% higher at 30/25 °C and 44% higher at 35/25 °C suggesting that phosphoenolpyruvate carboxylase was not CO₂-saturated at ambient CO₂ levels and a 25 °C night temperature. Photosynthetic water use efficiency (WUE) was higher at elevated than at ambient CO₂. Leaf Δ -values were higher at elevated than at ambient CO₂ due to relatively higher assimilation in the light. Leaf Δ was significantly and linearly related to the fraction of total CO₂ assimilated at night. The data suggest that a simultaneous increase in CO₂ level and temperature associated with global warming would enhance carbon assimilation, increase WUE, and reduce the temperature dependence of CO₂ uptake by A. comosus .