Effects of elevated [CO2] on photosynthesis in European forest species: a meta-analysis of model parameters

The effects of elevated atmospheric CO₂ concentration on growth of forest tree species are difficult to predict because practical limitations restrict experiments to much shorter than the average life-span of a tree. Long-term, process-based computer models must be used to extrapolate from shorter-term experiments. A key problem is to ensure a strong flow of information between experiments and models. In this study, meta-analysis techniques were used to summarize a suite of photosynthetic model parameters obtained from 15 field-based elevated [CO₂] experiments on European forest tree species. The parameters studied are commonly used in modelling photosynthesis, and include observed light-saturated photosynthetic rates (A_max), the potential electron transport rate (J_max), the maximum Rubisco activity (V_cmax) and leaf nitrogen concentration on mass (N_m) and area (N_a) bases. Across all experiments, light-saturated photosynthesis was strongly stimulated by growth in elevated [CO₂]. However, significant down-regulation of photosynthesis was also observed; when measured at the same CO₂ concentration, photosynthesis was reduced by 10–20%. The underlying biochemistry of photosynthesis was affected, as shown by a down-regulation of the parameters J_max and V_cmax of the order of 10%. This reduction in J_max and V_cmax was linked to the effects of elevated [CO₂] on leaf nitrogen concentration. It was concluded that the current model is adequate to model photosynthesis in elevated [CO₂]. Tables of model parameter values for different European forest species are given.     

Long-term exposure to elevated [CO2] in a natural Quercus ilex L. community: net photosynthesis and photochemical efficiency of PSII at different levels of water stress

Naturally grown trees of Mediterranean evergreen oak (Quercus ilex L.), representing the climax species of the region, were enclosed in six large open-top chambers and exposed to ambient and elevated CO₂ concentrations during a 3 year period. Maximum daily net photosynthetic rates measured at the two different CO₂ concentrations were from 30 to 100% higher in elevated than in ambient [CO₂] throughout the experimental period. The increase in maximum daily photosynthesis was also accompanied by a 93% rise in the apparent quantum yield of CO₂ assimilation, measured during periods of optimum soil moisture conditions. Hence, no clear evidence of down-regulation of net photosynthetic activity was found. Interactions between atmospheric CO₂ concentration and plant water stress were studied by following the natural evolution of drought in different seasons and years. At each level of water stress, the maximum rate of carbon assimilation was higher in elevated than in ambient [CO₂] by up to 100%. Analysis of in vivo chlorophyll fluorescence parameters in normal (21%) and low (2%) oxygen concentrations provided useful insights into the functioning and stability of the photosynthetic processes. The photochemical efficiency of PSII (F_v/F_m) progressively decreased as drought conditions became more evident; this trend was accentuated under elevated [CO₂]. Thermal de-excitation processes were possibly more significant under elevated than under ambient [CO₂], in a combination of environmental stresses. This research suggests two possible conclusions: (i) a ‘positive’ interaction between elevated [CO₂] and carbon metabolism can be obtained through relief of water stress limitation in the summer months, and (ii) elevated [CO₂], under drought conditions, may also enhance the significance of slow-relaxing quenching.     

Growth in elevated CO2 protects photosynthesis against high-temperature damage

We present evidence that plant growth at elevated atmospheric CO₂ increases the high‐temperature tolerance of photosynthesis in a wide variety of plant species under both greenhouse and field conditions. We grew plants at ambient CO₂ (~ 360 μ mol mol ^{? 1}) and elevated CO₂ (550–1000 μ mol mol ^{? 1}) in three separate growth facilities, including the Nevada Desert Free‐Air Carbon Dioxide Enrichment (FACE) facility. Excised leaves from both the ambient and elevated CO₂ treatments were exposed to temperatures ranging from 28 to 48 °C. In more than half the species examined (4 of 7, 3 of 5, and 3 of 5 species in the three facilities), leaves from elevated CO₂‐grown plants maintained PSII efficiency (F_v/F_m) to significantly higher temperatures than ambient‐grown leaves. This enhanced PSII thermotolerance was found in both woody and herbaceous species and in both monocots and dicots. Detailed experiments conducted with Cucumis sativus showed that the greater F_v/F_m in elevated versus ambient CO₂‐grown leaves following heat stress was due to both a higher F_m and a lower F_o, and that F_v/F_m differences between elevated and ambient CO₂‐grown leaves persisted for at least 20 h following heat shock. Cucumis sativus leaves from elevated CO₂‐grown plants had a critical temperature for the rapid rise in F_o that averaged 2·9 °C higher than leaves from ambient CO₂‐grown plants, and maintained a higher maximal rate of net CO₂ assimilation following heat shock. Given that photosynthesis is considered to be the physiological process most sensitive to high‐temperature damage and that rising atmospheric CO₂ content will drive temperature increases in many already stressful environments, this CO₂‐induced increase in plant high‐temperature tolerance may have a substantial impact on both the productivity and distribution of many plant species in the 21st century.     

CO2 enrichment predisposes foliage of a eucalypt to freezing injury and reduces spring growth

Seedlings of Eucalyptus pauciflora, were grown in open-top chambers fumigated with ambient and elevated [CO₂], and were divided into two populations using 10% light transmittance screens. The aim was to separate the effects of timing of light interception, temperature and [CO₂] on plant growth. The orientation of the screens exposed plants to a similar total irradiance, but incident during either cold mornings (east-facing) or warm afternoons (west-facing). Following the first autumn freezing event elevated CO₂-grown plants had 10 times more necrotic leaf area than ambient CO₂ plants. West-facing plants had significantly greater (25% more) leaf damage and lower photochemical efficiency (F_v/F_m) in comparison with east-facing plants. Following a late spring freezing event east-facing elevated CO₂ plants suffered a greater sustained loss in F_v/F_m than west-facing elevated CO₂- and ambient CO₂-grown plants. Stomatal conductance was lower under elevated CO₂ than ambient CO₂ except during late spring, with the highest leaf temperatures occurring in west-facing plants under elevated CO₂. These higher leaf temperatures apparently interfered with cold acclimation thereby enhancing frost damage and reducing the ability to take advantage of optimal growing conditions under elevated CO₂.     

Exposure to preindustrial, current and future atmospheric CO2 and temperature differentially affects growth and photosynthesis in Eucalyptus

To investigate if Eucalyptus species have responded to industrial-age climate change, and how they may respond to a future climate, we measured growth and physiology of fast- ( E. saligna ) and slow-growing ( E. sideroxylon ) seedlings exposed to preindustrial (290), current (400) or projected (650 μL L⁻¹) CO₂ concentration ([CO₂]) and to current or projected (current +4 °C) temperature. To evaluate maximum potential treatment responses, plants were grown with nonlimiting soil moisture. We found that: (1) E. sideroxylon responded more strongly to elevated [CO₂] than to elevated temperature, while E. saligna responded similarly to elevated [CO₂] and elevated temperature; (2) the transition from preindustrial to current [CO₂] did not enhance eucalypt plant growth under ambient temperature, despite enhancing photosynthesis; (3) the transition from current to future [CO₂] stimulated both photosynthesis and growth of eucalypts, independent of temperature; and (4) warming enhanced eucalypt growth, independent of future [CO₂], despite not affecting photosynthesis. These results suggest large potential carbon sequestration by eucalypts in a future world, and highlight the need to evaluate how future water availability may affect such responses.     

Stomatal behaviour, photosynthesis and transpiration under rising CO2

Definitions of the variables used and the units are given in Table 1

The literature reports enormous variation between species in the extent of stomatal responses to rising CO₂. This paper attempts to provide a framework within which some of this diversity can be explained. We describe the role of stomata in the short-term response of leaf gas exchange to increases in ambient CO₂ concentration by developing the recently proposed stomatal model of Jarvis & Davies (1998 ). In this model stomatal conductance is correlated with the functioning of the photosynthetic system so that the effects of increases in CO₂ on stomata are experienced through changes in the rate of photosynthesis in a simple and mechanistically transparent way. This model also allows us to consider the effects of evaporative demand and soil moisture availability on stomatal responses to photosynthesis and therefore provides a means of considering these additional sources of variation. We emphasize that the relationship between the rate of photosynthesis and the internal CO₂ concentration and also drought will have important effects on the relative gains to be achieved under rising CO₂.  

  Table 1 . Abbreviations     

7.

Effects of long-term elevated [CO₂] from natural CO₂ springs on Nardus stricta: photosynthesis, biochemistry, growth and phenology   总被引：2，自引：1，他引：2  

A. C. Cook  D. T. Tissue  S. W. Roberts  & W. C. Oechel 《Plant, cell & environment》1998,21(4):417-425

Plants of Nardus stricta growing near a cold, naturally emitting CO₂ spring in Iceland were used to investigate the long-term (> 100 years) effects of elevated [CO₂] on photosynthesis, biochemistry, growth and phenology in a northern grassland ecosystem. Comparisons were made between plants growing in an atmosphere naturally enriched with CO₂ (≈ 790 μ mol mol^–1) near the CO₂ spring and plants of the same species growing in adjacent areas exposed to ambient CO₂ concentrations (≈360 μ mol mol^–1). Nardus stricta growing near the spring exhibited earlier senescence and reductions in photosynthetic capacity (≈25%), Rubisco content (≈26%), Rubisco activity (≈40%), Rubisco activation state (≈23%), chlorophyll content (≈33%) and leaf area index (≈22%) compared with plants growing away from the spring. The potential positive effects of elevated [CO₂] on grassland ecosystems in Iceland are likely to be reduced by strong down-regulation in the photosynthetic apparatus of the abundant N. stricta species.     

8.

Gas exchange and chlorophyll fluorescence responses of three south-western Yucca species to elevated CO₂ and high temperature     

T. E. Huxman  E. P. Hamerlynck  M. E. Loik  & S. D. Smith 《Plant, cell & environment》1998,21(12):1275-1283

The ability of seedlings to tolerate temperature extremes is important in determining the distribution of perennial plants in the arid south-western USA, and the manner in which elevated CO₂ impacts the ability of plants to tolerate high temperatures is relatively unknown. Whereas the effects of chronic high temperature (30–38°C) and elevated CO₂ are comparatively well understood, little research has assessed plant performance in elevated CO₂ during extreme (> 45 °C) temperature events. We exposed three species of Yucca to 360 and 700 μmol CO₂ mol^–1 for 8 months, then 9 d of high temperature (up to 53 °C) to evaluate the impacts of elevated CO₂ on the potential for photosynthetic function during external high temperature. Seedlings of a coastal C₃ species (Yucca whipplei), a desert C₃ species (Yucca brevifolia), and a desert CAM species (Yucca schidigera), were used to test for differences among functional groups. In general, Yuccas exposed to elevated CO₂ showed decreases in carboxylation efficiency as compared with plants grown at ambient before the initiation of high temperature. The coastal species (Y. whipplei) showed significant reductions (33%) in CO₂ saturated maximum assimilation rate (A_max), but the desert species (Y. brevifolia and Y. schidigera) showed no such reductions in A_max. Stomatal conductance was lower in elevated CO₂ as compared with ambient throughout the temperature event; however, there were species-specific differences over time. Elevated CO₂ enhanced photosynthesis in Y. whipplei at high temperatures for a period of 4 d, but not for Y. brevifolia or Y. schidigera. Elevated CO₂ offset photoinhibition (measured as F_v/F_m) in Y. whipplei as compared with ambient CO₂, depending on exposure time to high temperature. Stable F_v/F_m in Y. whipplei occurred in parallel with increases in the quantum yield of photosystem II (ΦPSII) at high temperatures in elevated CO₂. The value of ΦPSII remained constant or decreased with increasing temperature in all other treatment and species combinations. This suggests that the reductions in F_v/F_m resulted from thermal energy dissipation in the pigment bed for Y. brevifolia and Y. schidigera. The greater efficiency of photosystem II in Y. whipplei helped to maintain photosynthetic function at high temperatures in elevated CO₂. These patterns are in contrast to the hypothesis that high temperatures in elevated CO₂ would increase the potential for photoinhibition. Our results suggest that elevated CO₂ may offset high-temperature stress in coastal Yucca, but not in those species native to drier systems. Therefore, in the case of Y. whipplei, elevated CO₂ may allow plants to survive extreme temperature events, potentially relaxing the effects of high temperature on the establishment in novel habitats.     

9.

Rice carbon balance under elevated CO₂   总被引：2，自引：1，他引：1  

Hidemitsu Sakai  Kazuyuki Yagi  Kazuhiko Kobayashi  Shigeto Kawashima 《The New phytologist》2001,150(2):241-249

     

10.

Elevated atmospheric [CO₂] promotes frost damage in evergreen tree seedlings   总被引：1，自引：1，他引：0  

J. L. Lutze  J. S. Roden  C. J. Holly  J. Wolfe  J. J. G. Egerton  & M. C. Ball 《Plant, cell & environment》1998,21(6):631-635

Growth under elevated [CO₂] promoted spring frost damage in field grown seedlings of snow gum ( Eucalyptus pauciflora Sieb. ex Spreng.), one of the most frost tolerant of eucalypts. Freezing began in the leaf midvein, consistent with it being a major site of frost damage under field conditions. The average ice nucleation temperature was higher in leaves grown under elevated [CO₂] (– 5·7 °C versus – 4·3 °C), consistent with the greater incidence of frost damage in these leaves (34% versus 68% of leaves damaged). These results have major implications for agriculture, forestry and vegetation dynamics, as an increase in frost susceptibility may reduce potential gains in productivity from CO₂ fertilization and may affect predictions of vegetation change based on increasing temperature.     

11.

Seedling performance of pasture species under elevated CO₂     

G. R. Edwards  P. C. D. Newton  J. C. Tilbrook  H. Clark 《The New phytologist》2001,150(2):359-369

     

12.

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

Lewis H. Ziska  James A. Bunce 《Physiologia plantarum》1994,91(2):183-190

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.     

13.

The relationship between leaf composition and morphology at elevated CO₂ concentrations     

MICHAEL L. RODERICK  SANDRA L. BERRY  & IAN R. NOBLE 《The New phytologist》1999,143(1):63-72

The composition and morphology of leaves exposed to elevated [CO₂] usually change so that the leaf nitrogen (N) per unit dry mass decreases and the leaf dry mass per unit area increases. However, at ambient [CO₂], leaves with a high leaf dry mass per unit area usually have low leaf N per unit dry mass. Whether the changes in leaf properties induced by elevated [CO₂] follow the same overall pattern as that at ambient [CO₂] has not previously been addressed. Here we address this issue by using leaf measurements made at ambient [CO₂] to develop an empirical model of the composition and morphology of leaves. Predictions from that model are then compared with a global database of leaf measurements made at ambient [CO₂]. Those predictions are also compared with measurements showing the impact of elevated [CO₂]. In the empirical model both the leaf dry mass and liquid mass per unit area are positively correlated with leaf thickness, whereas the mass of C per unit dry mass and the mass of N per unit liquid mass are constant. Consequently, both the N:C ratio and the surface area:volume ratio of leaves are positively correlated with the liquid content. Predictions from that model were consistent with measurements of leaf properties made at ambient [CO₂] from around the world. The changes induced by elevated [CO₂] follow the same overall trajectory. It is concluded that elevated [CO₂] enhances the rate at which dry matter is accumulated but the overall trajectory of leaf development is conserved.     

14.

Modelling photosynthesis of cotton grown in elevated CO₂   总被引：22，自引：16，他引：6  

P. C. HARLEY  R. B. THOMAS  †  J. F. REYNOLDS  B. R. STRAIN† 《Plant, cell & environment》1992,15(3):271-282

     

15.

Regenerating temperate forests under elevated CO₂ and nitrogen deposition: comparing biochemical and stomatal limitation of photosynthesis   总被引：1，自引：0，他引：1  

G. A. Bauer  G. M. Berntson  F. A. Bazzaz 《The New phytologist》2001,152(2):249-266

     

16.

Grassland species effects on soil CO₂ flux track the effects of elevated CO₂ and nitrogen     

Joseph M. Craine  David A. Wedin  Peter B. Reich 《The New phytologist》2001,150(2):425-434

     

17.

N₂ fixation by Acacia species increases under elevated atmospheric CO₂   总被引：1，自引：0，他引：1  

M. Schortemeyer  O. K. Atkin  N. McFarlane  & J. R. Evans 《Plant, cell & environment》2002,25(4):567-579

In the present study the effect of elevated CO₂ on growth and nitrogen fixation of seven Australian Acacia species was investigated. Two species from semi‐arid environments in central Australia (Acacia aneura and A. tetragonophylla) and five species from temperate south‐eastern Australia (Acacia irrorata, A. mearnsii, A. dealbata, A. implexa and A. melanoxylon) were grown for up to 148 d in controlled greenhouse conditions at either ambient (350 µmol mol^?1) or elevated (700 µmol mol^?1) CO₂ concentrations. After establishment of nodules, the plants were completely dependent on symbiotic nitrogen fixation. Six out of seven species had greater relative growth rates and lower whole plant nitrogen concentrations under elevated versus normal CO₂. Enhanced growth resulted in an increase in the amount of nitrogen fixed symbiotically for five of the species. In general, this was the consequence of lower whole‐plant nitrogen concentrations, which equate to a larger plant and greater nodule mass for a given amount of nitrogen. Since the average amount of nitrogen fixed per unit nodule mass was unaltered by atmospheric CO₂, more nitrogen could be fixed for a given amount of plant nitrogen. For three of the species, elevated CO₂ increased the rate of nitrogen fixation per unit nodule mass and time, but this was completely offset by a reduction in nodule mass per unit plant mass.     

18.

The impact of elevated CO₂ and global climate change on arbuscular mycorrhizas: a mycocentric approach   总被引：2，自引：2，他引：0  

A. H. FITTER  A. HEINEMEYER  & P. L. STADDON 《The New phytologist》2000,147(1):179-187

Arbuscular mycorrhizal (AM) symbioses are a potentially important link in the chain of response of ecosystems to elevated atmospheric [CO₂]. By promoting plant phosphorus uptake and acting as a sink for plant carbon, they can alleviate photosynthetic down-regulation. Because hyphal turnover is likely to be fast, especially in warmer soils, they can also act as a rapid pathway for the return of carbon to the atmosphere. However, most experiments on AM responses to [CO₂] have failed to take into account the difference in growth of mycorrhizal and non- mycorrhizal plants; those that have done so suggest that AM colonization of roots is little altered by [CO₂], although this issue remains to be resolved. Very little is known about the effects of other factors of global environmental change on mycorrhizas. These issues need urgent attention. It is also necessary to understand the potential for the various AM fungal taxa to respond differentially to environmental changes, including carbon supply and soil temperature and moisture, especially because of the differential abilities of plant and fungal species to migrate in response to changing environments. Indeed, there is a need for a new approach to the study of mycorrhizal associations, which has been too plant-centred. It is essential to regard the fungus as an organism itself, and to understand its biology both as an entity and as part of a symbiosis.     

19.

Stomatal conductance of forest species after long-term exposure to elevated CO₂ concentration: a synthesis   总被引：13，自引：4，他引：9  

B. E. Medlyn  C. V. M. Barton  M. S. J. Broadmeadow  R. Ceulemans  P. De Angelis  M. Forstreuter  M. Freeman  S. B. Jackson  S. Kellomäki  E. Laitat  A. Rey  P. Roberntz  B. D. Sigurdsson  J. Strassemeyer  K. Wang  P. S. Curtis  P. G. Jarvis 《The New phytologist》2001,149(2):247-264

     

20.

Soil CO₂ efflux of two silver birch clones exposed to elevated CO₂ and O₃ levels during three growing seasons     

Anne Kasurinen  Paula Kokko-Gonzales  Johanna Riikonen †  Elina Vapaavuori†  Toini Holopainen 《Global Change Biology》2004,10(10):1654-1665

In the present open‐top chamber experiment, two silver birch clones (Betula pendula Roth, clone 4 and clone 80) were exposed to elevated levels of carbon dioxide (CO₂) and ozone (O₃), singly and in combination, and soil CO₂ efflux was measured 14 times during three consecutive growing seasons (1999–2001). In the beginning of the experiment, all experimental trees were 7 years old and during the experiment the trees were growing in sandy field soil and fertilized regularly. In general, elevated O₃ caused soil CO₂ efflux stimulation during most measurement days and this stimulation enhanced towards the end of the experiment. The overall soil respiration response to CO₂ was dependent on the genotype, as the soil CO₂ efflux below clone 80 trees was enhanced and below clone 4 trees was decreased under elevated CO₂ treatments. Like the O₃ impact, this clonal difference in soil respiration response to CO₂ increased as the experiment progressed. Although the O₃ impact did not differ significantly between clones, a significant time × clone × CO₂× O₃ interaction revealed that the O₃‐induced stimulation of soil respiration was counteracted by elevated CO₂ in clone 4 on most measurement days, whereas in clone 80, the effect of elevated CO₂ and O₃ in combination was almost constantly additive during the 3‐year experiment. Altogether, the root or above‐ground biomass results were only partly parallel with the observed soil CO₂ efflux responses. In conclusion, our data show that O₃ impacts may appear first in the below‐ground processes and that relatively long‐term O₃ exposure had a cumulative effect on soil CO₂ efflux. Although the soil respiration response to elevated CO₂ depended on the tree genotype as a result of which the O₃ stress response might vary considerably within a single tree species under elevated CO₂, the present experiment nonetheless indicates that O₃ stress is a significant factor affecting the carbon cycling in northern forest ecosystems.     

Effects of long-term elevated [CO2] from natural CO2 springs on Nardus stricta: photosynthesis, biochemistry, growth and phenology

Plants of Nardus stricta growing near a cold, naturally emitting CO₂ spring in Iceland were used to investigate the long-term (> 100 years) effects of elevated [CO₂] on photosynthesis, biochemistry, growth and phenology in a northern grassland ecosystem. Comparisons were made between plants growing in an atmosphere naturally enriched with CO₂ (≈ 790 μ mol mol^–1) near the CO₂ spring and plants of the same species growing in adjacent areas exposed to ambient CO₂ concentrations (≈360 μ mol mol^–1). Nardus stricta growing near the spring exhibited earlier senescence and reductions in photosynthetic capacity (≈25%), Rubisco content (≈26%), Rubisco activity (≈40%), Rubisco activation state (≈23%), chlorophyll content (≈33%) and leaf area index (≈22%) compared with plants growing away from the spring. The potential positive effects of elevated [CO₂] on grassland ecosystems in Iceland are likely to be reduced by strong down-regulation in the photosynthetic apparatus of the abundant N. stricta species.     

Gas exchange and chlorophyll fluorescence responses of three south-western Yucca species to elevated CO2 and high temperature

The ability of seedlings to tolerate temperature extremes is important in determining the distribution of perennial plants in the arid south-western USA, and the manner in which elevated CO₂ impacts the ability of plants to tolerate high temperatures is relatively unknown. Whereas the effects of chronic high temperature (30–38°C) and elevated CO₂ are comparatively well understood, little research has assessed plant performance in elevated CO₂ during extreme (> 45 °C) temperature events. We exposed three species of Yucca to 360 and 700 μmol CO₂ mol^–1 for 8 months, then 9 d of high temperature (up to 53 °C) to evaluate the impacts of elevated CO₂ on the potential for photosynthetic function during external high temperature. Seedlings of a coastal C₃ species (Yucca whipplei), a desert C₃ species (Yucca brevifolia), and a desert CAM species (Yucca schidigera), were used to test for differences among functional groups. In general, Yuccas exposed to elevated CO₂ showed decreases in carboxylation efficiency as compared with plants grown at ambient before the initiation of high temperature. The coastal species (Y. whipplei) showed significant reductions (33%) in CO₂ saturated maximum assimilation rate (A_max), but the desert species (Y. brevifolia and Y. schidigera) showed no such reductions in A_max. Stomatal conductance was lower in elevated CO₂ as compared with ambient throughout the temperature event; however, there were species-specific differences over time. Elevated CO₂ enhanced photosynthesis in Y. whipplei at high temperatures for a period of 4 d, but not for Y. brevifolia or Y. schidigera. Elevated CO₂ offset photoinhibition (measured as F_v/F_m) in Y. whipplei as compared with ambient CO₂, depending on exposure time to high temperature. Stable F_v/F_m in Y. whipplei occurred in parallel with increases in the quantum yield of photosystem II (ΦPSII) at high temperatures in elevated CO₂. The value of ΦPSII remained constant or decreased with increasing temperature in all other treatment and species combinations. This suggests that the reductions in F_v/F_m resulted from thermal energy dissipation in the pigment bed for Y. brevifolia and Y. schidigera. The greater efficiency of photosystem II in Y. whipplei helped to maintain photosynthetic function at high temperatures in elevated CO₂. These patterns are in contrast to the hypothesis that high temperatures in elevated CO₂ would increase the potential for photoinhibition. Our results suggest that elevated CO₂ may offset high-temperature stress in coastal Yucca, but not in those species native to drier systems. Therefore, in the case of Y. whipplei, elevated CO₂ may allow plants to survive extreme temperature events, potentially relaxing the effects of high temperature on the establishment in novel habitats.     

Elevated atmospheric [CO2] promotes frost damage in evergreen tree seedlings

Growth under elevated [CO₂] promoted spring frost damage in field grown seedlings of snow gum ( Eucalyptus pauciflora Sieb. ex Spreng.), one of the most frost tolerant of eucalypts. Freezing began in the leaf midvein, consistent with it being a major site of frost damage under field conditions. The average ice nucleation temperature was higher in leaves grown under elevated [CO₂] (– 5·7 °C versus – 4·3 °C), consistent with the greater incidence of frost damage in these leaves (34% versus 68% of leaves damaged). These results have major implications for agriculture, forestry and vegetation dynamics, as an increase in frost susceptibility may reduce potential gains in productivity from CO₂ fertilization and may affect predictions of vegetation change based on increasing temperature.     

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 relationship between leaf composition and morphology at elevated CO2 concentrations

The composition and morphology of leaves exposed to elevated [CO₂] usually change so that the leaf nitrogen (N) per unit dry mass decreases and the leaf dry mass per unit area increases. However, at ambient [CO₂], leaves with a high leaf dry mass per unit area usually have low leaf N per unit dry mass. Whether the changes in leaf properties induced by elevated [CO₂] follow the same overall pattern as that at ambient [CO₂] has not previously been addressed. Here we address this issue by using leaf measurements made at ambient [CO₂] to develop an empirical model of the composition and morphology of leaves. Predictions from that model are then compared with a global database of leaf measurements made at ambient [CO₂]. Those predictions are also compared with measurements showing the impact of elevated [CO₂]. In the empirical model both the leaf dry mass and liquid mass per unit area are positively correlated with leaf thickness, whereas the mass of C per unit dry mass and the mass of N per unit liquid mass are constant. Consequently, both the N:C ratio and the surface area:volume ratio of leaves are positively correlated with the liquid content. Predictions from that model were consistent with measurements of leaf properties made at ambient [CO₂] from around the world. The changes induced by elevated [CO₂] follow the same overall trajectory. It is concluded that elevated [CO₂] enhances the rate at which dry matter is accumulated but the overall trajectory of leaf development is conserved.     

N2 fixation by Acacia species increases under elevated atmospheric CO2

In the present study the effect of elevated CO₂ on growth and nitrogen fixation of seven Australian Acacia species was investigated. Two species from semi‐arid environments in central Australia (Acacia aneura and A. tetragonophylla) and five species from temperate south‐eastern Australia (Acacia irrorata, A. mearnsii, A. dealbata, A. implexa and A. melanoxylon) were grown for up to 148 d in controlled greenhouse conditions at either ambient (350 µmol mol^?1) or elevated (700 µmol mol^?1) CO₂ concentrations. After establishment of nodules, the plants were completely dependent on symbiotic nitrogen fixation. Six out of seven species had greater relative growth rates and lower whole plant nitrogen concentrations under elevated versus normal CO₂. Enhanced growth resulted in an increase in the amount of nitrogen fixed symbiotically for five of the species. In general, this was the consequence of lower whole‐plant nitrogen concentrations, which equate to a larger plant and greater nodule mass for a given amount of nitrogen. Since the average amount of nitrogen fixed per unit nodule mass was unaltered by atmospheric CO₂, more nitrogen could be fixed for a given amount of plant nitrogen. For three of the species, elevated CO₂ increased the rate of nitrogen fixation per unit nodule mass and time, but this was completely offset by a reduction in nodule mass per unit plant mass.     

The impact of elevated CO2 and global climate change on arbuscular mycorrhizas: a mycocentric approach

Arbuscular mycorrhizal (AM) symbioses are a potentially important link in the chain of response of ecosystems to elevated atmospheric [CO₂]. By promoting plant phosphorus uptake and acting as a sink for plant carbon, they can alleviate photosynthetic down-regulation. Because hyphal turnover is likely to be fast, especially in warmer soils, they can also act as a rapid pathway for the return of carbon to the atmosphere. However, most experiments on AM responses to [CO₂] have failed to take into account the difference in growth of mycorrhizal and non- mycorrhizal plants; those that have done so suggest that AM colonization of roots is little altered by [CO₂], although this issue remains to be resolved. Very little is known about the effects of other factors of global environmental change on mycorrhizas. These issues need urgent attention. It is also necessary to understand the potential for the various AM fungal taxa to respond differentially to environmental changes, including carbon supply and soil temperature and moisture, especially because of the differential abilities of plant and fungal species to migrate in response to changing environments. Indeed, there is a need for a new approach to the study of mycorrhizal associations, which has been too plant-centred. It is essential to regard the fungus as an organism itself, and to understand its biology both as an entity and as part of a symbiosis.     

Soil CO2 efflux of two silver birch clones exposed to elevated CO2 and O3 levels during three growing seasons

In the present open‐top chamber experiment, two silver birch clones (Betula pendula Roth, clone 4 and clone 80) were exposed to elevated levels of carbon dioxide (CO₂) and ozone (O₃), singly and in combination, and soil CO₂ efflux was measured 14 times during three consecutive growing seasons (1999–2001). In the beginning of the experiment, all experimental trees were 7 years old and during the experiment the trees were growing in sandy field soil and fertilized regularly. In general, elevated O₃ caused soil CO₂ efflux stimulation during most measurement days and this stimulation enhanced towards the end of the experiment. The overall soil respiration response to CO₂ was dependent on the genotype, as the soil CO₂ efflux below clone 80 trees was enhanced and below clone 4 trees was decreased under elevated CO₂ treatments. Like the O₃ impact, this clonal difference in soil respiration response to CO₂ increased as the experiment progressed. Although the O₃ impact did not differ significantly between clones, a significant time × clone × CO₂× O₃ interaction revealed that the O₃‐induced stimulation of soil respiration was counteracted by elevated CO₂ in clone 4 on most measurement days, whereas in clone 80, the effect of elevated CO₂ and O₃ in combination was almost constantly additive during the 3‐year experiment. Altogether, the root or above‐ground biomass results were only partly parallel with the observed soil CO₂ efflux responses. In conclusion, our data show that O₃ impacts may appear first in the below‐ground processes and that relatively long‐term O₃ exposure had a cumulative effect on soil CO₂ efflux. Although the soil respiration response to elevated CO₂ depended on the tree genotype as a result of which the O₃ stress response might vary considerably within a single tree species under elevated CO₂, the present experiment nonetheless indicates that O₃ stress is a significant factor affecting the carbon cycling in northern forest ecosystems.