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Concentrations of atmospheric CO(2) and tropospheric ozone (O(3)) are rising concurrently in the atmosphere, with potentially antagonistic effects on forest net primary production (NPP) and implications for terrestrial carbon sequestration. Using free-air CO(2) enrichment (FACE) technology, we exposed north-temperate forest communities to concentrations of CO(2) and O(3) predicted for the year 2050 for the first 7 yr of stand development. Site-specific allometric equations were applied to annual nondestructive growth measurements to estimate above- and below-ground biomass and NPP for each year of the experiment. Relative to the control, elevated CO(2) increased total biomass 25, 45 and 60% in the aspen, aspen-birch and aspen-maple communities, respectively. Tropospheric O(3) caused 23, 13 and 14% reductions in total biomass relative to the control in the respective communities. Combined fumigation resulted in total biomass response of -7.8, +8.4 and +24.3% relative to the control in the aspen, aspen-birch and aspen-sugar maple communities, respectively. These results indicate that exposure to even moderate levels of O(3) significantly reduce the capacity of NPP to respond to elevated CO(2) in some forests.

• Concentrations of atmospheric CO2 and tropospheric ozone (O3) are rising concurrently in the atmosphere, with potentially antagonistic effects on forest net primary production (NPP) and implications for terrestrial carbon sequestration. • Using free-air CO2 enrichment (FACE) technology, we exposed north-temperate forest communities to concentrations of CO2 and O3 predicted for the year 2050 for the first 7 yr of stand development. Site-specific allometric equations were applied to annual nondestructive growth measurements to estimate above- and below-ground biomass and NPP for each year of the experiment. • Relative to the control, elevated CO2 increased total biomass 25, 45 and 60% in the aspen, aspen–birch and aspen–maple communities, respectively. Tropospheric O3 caused 23, 13 and 14% reductions in total biomass relative to the control in the respective communities. Combined fumigation resulted in total biomass response of −7.8, +8.4 and +24.3% relative to the control in the aspen, aspen–birch and aspen–sugar maple communities, respectively. • These results indicate that exposure to even moderate levels of O3 significantly reduce the capacity of NPP to respond to elevated CO2 in some forests. [ABSTRACT FROM AUTHOR]

The ratio of leaf internal (c i ) to ambient (c a ) partial pressure of CO 2 , defined here as χ, is an index of adjustments in both leaf stomatal conductance and photosynthetic rate to environmental conditions. Measurements and proxies of this ratio can be used to constrain vegetation model uncertainties for predicting terrestrial carbon uptake and water use. We test a theory based on the least-cost optimality hypothesis for modelling historical changes in χ over the 1951-2014 period, across different tree species and environmental conditions, as reconstructed from stable carbon isotopic measurements across a global network of 103 absolutely dated tree-ring chronologies. The theory predicts optimal χ as a function of air temperature, vapour pressure deficit, c a and atmospheric pressure. The theoretical model predicts 39% of the variance in χ values across sites and years, but underestimates the intersite variability in the reconstructed χ trends, resulting in only 8% of the variance in χ trends across years explained by the model. Overall, our results support theoretical predictions that variations in χ are tightly regulated by the four environmental drivers. They also suggest that explicitly accounting for the effects of plant-available soil water and other site-specific characteristics might improve the predictions., (© 2019 The Authors. New Phytologist © 2019 New Phytologist Trust.)

• Concentrations of atmospheric CO2 and tropospheric ozone (O3) are rising concurrently in the atmosphere, with potentially antagonistic effects on forest net primary production (NPP) and implications for terrestrial carbon sequestration. • Using free-air CO2 enrichment (FACE) technology, we exposed north-temperate forest communities to concentrations of CO2 and O3 predicted for the year 2050 for the first 7 yr of stand development. Site-specific allometric equations were applied to annual nondestructive growth measurements to estimate above- and below-ground biomass and NPP for each year of the experiment. • Relative to the control, elevated CO2 increased total biomass 25, 45 and 60% in the aspen, aspen–birch and aspen–maple communities, respectively. Tropospheric O3 caused 23, 13 and 14% reductions in total biomass relative to the control in the respective communities. Combined fumigation resulted in total biomass response of −7.8, +8.4 and +24.3% relative to the control in the aspen, aspen–birch and aspen–sugar maple communities, respectively. • These results indicate that exposure to even moderate levels of O3 significantly reduce the capacity of NPP to respond to elevated CO2 in some forests. [ABSTRACT FROM AUTHOR]

Elevated atmospheric CO2 concentrations can change chemistry and input rate of plant tissue to soil, potentially influencing above- and below-ground biogeochemical cycles. Given the important role played by leaf and root litter chemistry in controlling ecosystem function and vulnerability to environmental stresses, we investigated the hydrolyzable amino acid distribution and concentration in leaf and fine root litter among control and elevated CO2 treatments at the Rhinelander free air CO2 enrichment (FACE) experiment (WI, USA). We extracted hydrolyzable amino acids from leaf litter and fine (< 2 mm) roots at three depths for both control and elevated CO2 plots. We found that elevated CO2 decreased the proportion of total leaf amino acid carbon (C), but had no effect on total leaf amino acid nitrogen (N). There was no treatment effect for total root amino acid N or amino acid C for any depth. The decrease in leaf amino acids is probably a result of the shift of protein compounds to more structural compounds. Despite the decrease in leaf amino acid C concentrations, the overall increase in annual plant production under elevated CO2 would result in an increase in plant amino acids to the soil., (© 2014 The Authors. New Phytologist © 2014 New Phytologist Trust.)

The Rhinelander free-air CO(2) enrichment (FACE) experiment is designed to understand ecosystem response to elevated atmospheric carbon dioxide (+CO(2)) and elevated tropospheric ozone (+O(3)). The objectives of this study were: to understand how soil respiration responded to the experimental treatments; to determine whether fine-root biomass was correlated to rates of soil respiration; and to measure rates of fine-root turnover in aspen (Populus tremuloides) forests and determine whether root turnover might be driving patterns in soil respiration. Soil respiration was measured, root biomass was determined, and estimates of root production, mortality and biomass turnover were made. Soil respiration was greatest in the +CO(2) and +CO(2) +O(3) treatments across all three plant communities. Soil respiration was correlated with increases in fine-root biomass. In the aspen community, annual fine-root production and mortality (g m(-2)) were positively affected by +O(3). After 10 yr of exposure, +CO(2) +O(3)-induced increases in belowground carbon allocation suggest that the positive effects of elevated CO(2) on belowground net primary productivity (NPP) may not be offset by negative effects of O(3). For the aspen community, fine-root biomass is actually stimulated by +O(3), and especially +CO(2) +O(3).

Details a study which examined the accumulation of H2O2 within the leaf mesophyll of Populus tremuloides aspen genotypes and seedlings of paper birch Betula papyrifera. Gene expression of catalase; Effects of ozone and carbon dioxide on the structure of ozone-tolerant aspen; Correlation between aspen clones, elevated ozone, birch and peroxisomes levels.

Effects of elevated atmospheric [CO2] on plant isoprene emissions are controversial. Relying on leaf-scale measurements, most models simulating isoprene emissions in future higher [CO2] atmospheres suggest reduced emission fluxes. However, combined effects of elevated [CO2] on leaf area growth, net assimilation and isoprene emission rates have rarely been studied on the canopy scale, but stimulation of leaf area growth may largely compensate for possible [CO2] inhibition reported at the leaf scale. This study tests the hypothesis that stimulated leaf area growth leads to increased canopy isoprene emission rates. We studied the dynamics of canopy growth, and net assimilation and isoprene emission rates in hybrid aspen (Populus tremula × Populus tremuloides) grown under 380 and 780 μmol mol(-1) [CO2]. A theoretical framework based on the Chapman-Richards function to model canopy growth and numerically compare the growth dynamics among ambient and elevated atmospheric [CO2]-grown plants was developed. Plants grown under elevated [CO2] had higher C : N ratio, and greater total leaf area, and canopy net assimilation and isoprene emission rates. During ontogeny, these key canopy characteristics developed faster and stabilized earlier under elevated [CO2]. However, on a leaf area basis, foliage physiological traits remained in a transient state over the whole experiment. These results demonstrate that canopy-scale dynamics importantly complements the leaf-scale processes, and that isoprene emissions may actually increase under higher [CO2] as a result of enhanced leaf area production., (© 2013 The Authors. New Phytologist © 2013 New Phytologist Trust.)

The world's forests are currently exposed to increasing concentrations of carbon dioxide (CO2) and ozone (O3). Both pollutants can potentially exert a selective effect on plant populations. This, in turn, may lead to changes in ecosystem properties, such as carbon sequestration. Here, we report how elevated CO2 and O3 affect the genetic composition of a woody plant population via altered survival. Using data from the Aspen free-air CO2 enrichment (FACE) experiment (in which aspen clones were grown in factorial combinations of CO2 and O3), we develop a hierarchical Bayesian model of survival. We also examine how survival differences between clones could affect pollutant responses in the next generation. Our model predicts that the relative abundance of the tested clones, given equal initial abundance, would shift under either elevated CO2 or O3 as a result of changing survival rates. Survival was strongly affected by between-clone differences in growth responses. Selection could noticeably decrease O3 sensitivity in the next generation, depending on the heritability of growth responses and the distribution of seed production. The response to selection by CO2, however, is likely to be small. Our results suggest that the changing atmospheric composition could shift the genotypic composition and average pollutant responses of tree populations over moderate timescales., (© 2013 The Authors. New Phytologist © 2013 New Phytologist Trust.)

We conducted the most extensive meta-analysis of plant and animal responses to elevated CO(2) to date. We analysed > 5000 data points extracted from 270 papers published between 1979 and 2009. We examined the changes in 19 animal response variables to the main effect of elevated CO(2). We found strong evidence for significant variation among arthropod orders and feeding guilds, including interactions in the direction of response. We also examined the main effects of elevated CO(2) on: six plant growth and allocation responses, seven primary metabolite responses, eight secondary metabolite responses, and four physical defence responses. We examined these response variable changes under two-way and three-way interactions between CO(2) and: soil nitrogen, ambient temperature, drought, light availability, photosynthetic pathway, reproductive system, plant growth rate, plant growth form, tissue type, and nitrogen fixation. In general we found smaller effect sizes for many response variables than have been previously reported. We also found that many of the oft-reported main effects of CO(2) obscure the presence of significant two- and three-way interactions, which may help better explain the relationships between the response variables and elevated CO(2)., (© 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.)

This review uses proxies of past temperature and atmospheric CO(2) composition based on fossil leaves to illustrate the uncertainties in biologically based proxies of past environments. Most leaf-based proxies are geographically local or genetically restricted and therefore can be confounded by evolution, extinction, changes in local environment or immigration of species. Stomatal frequency proxies illustrate how genetically restricted proxies can be particularly vulnerable to evolutionary change. High predictive power in the modern world resulting from the use of a very narrow calibration cannot be confidently extrapolated into the past (the Ginkgo paradox). Many foliar physiognomic proxies of climate are geographically local and use traits that are more or less fixed for individual species. Such proxies can therefore be confounded by floristic turnover and biome shifts in the region of calibration. Uncertainty in proxies tends to be greater for more ancient fossils. I present a set of questions that should be considered before using a proxy. Good proxies should be relatively protected from environmental and genetic change, particularly through having high information content and being founded on biomechanical or biochemical principles. Some current and potential developments are discussed, including those that involve more mechanistically sound proxies and better use of multivariate approaches., (© 2011 The Author. New Phytologist © 2011 New Phytologist Trust.)

• The CO(2) efflux of adult trees is supplied by recent photosynthates and carbon (C) stores. The extent to which these C pools contribute to growth and maintenance respiration (R(G) and R(M), respectively) remains obscure. • Recent photosynthates of adult beech (Fagus sylvatica) and spruce (Picea abies) trees were labeled by exposing whole-tree canopies to (13) C-depleted CO(2). Label was applied three times during the year (in spring, early summer and late summer) and changes in the stable C isotope composition (δ(13) C) of trunk and coarse-root CO(2) efflux were quantified. • Seasonal patterns in C translocation rate (CTR) and fractional contribution of label to CO(2) efflux (F(Label-Max)) were found. CTR was fastest during early summer. In beech, F(Label-Max) was lowest in spring and peaked in trunks during late summer (0.6 ± 0.1, mean ± SE), whereas no trend was observed in coarse roots. No seasonal dynamics in F(Label-Max) were found in spruce. • During spring, the R(G) of beech trunks was largely supplied by C stores. Recent photosynthates supplied growth in early summer and refilled C stores in late summer. In spruce, CO(2) efflux was constantly supplied by a mixture of stored (c. 75%) and recent (c. 25%) C. The hypothesis that R(G) is exclusively supplied by recent photosynthates was rejected for both species., (© 2011 The Authors. New Phytologist © 2011 New Phytologist Trust.)

This study compared the leaf transcription profiles, physiological characteristics and primary metabolites of two Populus tremuloides genotypes (clones 216 and 271) known to differ in their responses to long-term elevated [CO2] (e[CO2]) at the Aspen free-air CO2 enrichment site near Rhinelander, WI, USA. The physiological responses of these clones were similar in terms of photosynthesis, stomatal conductance and leaf area index under e[CO2], yet very different in terms of growth enhancement (0-10% in clone 216; 40-50% in clone 271). Although few genes responded to long-term exposure to e[CO2], the transcriptional activity of leaf e[CO2]-responsive genes was distinctly different between the clones, differentially impacting multiple pathways during both early and late growing seasons. An analysis of transcript abundance and carbon/nitrogen biochemistry suggested that the CO2-responsive clone (271) partitions carbon into pathways associated with active defense/response to stress, carbohydrate/starch biosynthesis and subsequent growth. The CO2-unresponsive clone (216) partitions carbon into pathways associated with passive defense (e.g. lignin, phenylpropanoid) and cell wall thickening. This study indicates that there is significant variation in expression patterns between different tree genotypes in response to long-term exposure to e[CO2]. Consequently, future efforts to improve productivity or other advantageous traits for carbon sequestration should include an examination of genetic variability in CO2 responsiveness.

A poplar short rotation coppice (SRC) grown for the production of bioenergy can combine carbon (C) storage with fossil fuel substitution. Here, we summarize the responses of a poplar (Populus) plantation to 6 yr of free air CO(2) enrichment (POP/EUROFACE consisting of two rotation cycles). We show that a poplar plantation growing in nonlimiting light, nutrient and water conditions will significantly increase its productivity in elevated CO(2) concentrations ([CO(2)]). Increased biomass yield resulted from an early growth enhancement and photosynthesis did not acclimate to elevated [CO(2)]. Sufficient nutrient availability, increased nitrogen use efficiency (NUE) and the large sink capacity of poplars contributed to the sustained increase in C uptake over 6 yr. Additional C taken up in high [CO(2)] was mainly invested into woody biomass pools. Coppicing increased yield by 66% and partly shifted the extra C uptake in elevated [CO(2)] to above-ground pools, as fine root biomass declined and its [CO(2)] stimulation disappeared. Mineral soil C increased equally in ambient and elevated [CO(2)] during the 6 yr experiment. However, elevated [CO(2)] increased the stabilization of C in the mineral soil. Increased productivity of a poplar SRC in elevated [CO(2)] may allow shorter rotation cycles, enhancing the viability of SRC for biofuel production.

Species differ in their responses to global changes such as rising CO(2) and temperature, meaning that global changes are likely to change the structure of plant communities. Such alterations in community composition must be underlain by changes in the population dynamics of component species. Here, the impact of elevated CO(2) (550 micromol mol(-1)) and warming (+2 degrees C) on the population growth of four plant species important in Australian temperate grasslands is reported. Data collected from the Tasmanian free-air CO(2) enrichment (TasFACE) experiment between 2003 and 2006 were analysed using population matrix models. Population growth of Themeda triandra, a perennial C(4) grass, was largely unaffected by either factor but population growth of Austrodanthonia caespitosa, a perennial C(3) grass, was reduced substantially in elevated CO(2) plots. Warming and elevated CO(2) had antagonistic effects on population growth of two invasive weeds, Hypochaeris radicata and Leontodon taraxacoides, with warming causing population decline. Analysis of life cycle stages showed that seed production, seedling emergence and establishment were important factors in the responses of the species to global changes. These results show that the demographic approach is very useful in understanding the variable responses of plants to global changes and in elucidating the life cycle stages that are most responsive.

Temperate and boreal forest ecosystems contain a large part of the carbon stored on land, in the form of both biomass and soil organic matter. Increasing atmospheric [CO2], increasing temperature, elevated nitrogen deposition and intensified management will change this C store. Well documented single-factor responses of net primary production are: higher photosynthetic rate (the main [CO2] response); increasing length of growing season (the main temperature response); and higher leaf-area index (the main N deposition and partly [CO2] response). Soil organic matter will increase with increasing litter input, although priming may decrease the soil C stock initially, but litter quality effects should be minimal (response to [CO2], N deposition, and temperature); will decrease because of increasing temperature; and will increase because of retardation of decomposition with N deposition, although the rate of decomposition of high-quality litter can be increased and that of low-quality litter decreased. Single-factor responses can be misleading because of interactions between factors, in particular those between N and other factors, and indirect effects such as increased N availability from temperature-induced decomposition. In the long term the strength of feedbacks, for example the increasing demand for N from increased growth, will dominate over short-term responses to single factors. However, management has considerable potential for controlling the C store.

Despite the abundance of experimental data, understanding of forest responses to elevated CO2 is limited. Here I show that a key to previously unexplained production and leaf area responses lies in the interplay between whole-plant nitrogen (N) allocation and leaf photosynthesis. A simple tree growth model, controlled by net growth maximization through optimization of leaf area index (LAI) and plant N, is used to analyse CO2 responses in both young, expanding and closed, steady-state canopies. The responses are sensitive to only two independent parameters, the photosynthetic capacity per leaf N (a) and the fine-root N:leaf N ratio. The model explains observed CO2 responses of photosynthesis, production and LAI in four forest free air CO2 enrichment (FACE) experiments. Insensitivity of LAI except at low LAI, increase in light-use efficiency, and photosynthetic down-regulation (as a result of reduced leaf N per area) at elevated CO2 are all explained through the combined effects on a and leaf quantum efficiency. The model bridges the gap between the understanding of leaf-level and plant-level responses and provides a transparent framework for interpreting and linking structural (LAI) and functional (net primary production (NPP):gross primary production (GPP) ratio, light-use efficiency, photosynthetic down-regulation) responses to elevated CO2.

Leaves with stomata on both upper and lower surfaces, termed amphistomatous, are relatively rare compared with hypostomatous leaves with stomata only on the lower surface. Amphistomaty occurs predominantly in fast-growing herbaceous annuals and in slow-growing perennial shrubs and trees. In this paper, we present the current understanding and hypotheses on the costs and benefits of amphistomaty related to water and CO 2 transport in contrasting leaf morphologies. First, there is no evidence that amphistomatous species achieve higher stomatal densities on a projected leaf area basis than hypostomatous species, but two-sided gas exchange is less limited by boundary layer effects. Second, amphistomaty may provide a specific advantage in thick leaves by shortening the pathway for CO 2 transport between the atmosphere and the chloroplasts. In thin leaves of fast-growing herbaceous annuals, in which both the adaxial and abaxial pathways are already short, amphistomaty enhances leaf-atmosphere gas-exchange capacity. Third, amphistomaty may help to optimise the leaf-interior water status for CO 2 transport by reducing temperature gradients and so preventing the condensation of water that could limit CO 2 diffusion. Fourth, a potential cost of amphistomaty is the need for additional investments in leaf water transport tissue to balance the water loss through the adaxial surface., (© 2018 The Authors. New Phytologist © 2018 New Phytologist Trust.)

Summary: Leaves with stomata on both upper and lower surfaces, termed amphistomatous, are relatively rare compared with hypostomatous leaves with stomata only on the lower surface. Amphistomaty occurs predominantly in fast‐growing herbaceous annuals and in slow‐growing perennial shrubs and trees. In this paper, we present the current understanding and hypotheses on the costs and benefits of amphistomaty related to water and CO2 transport in contrasting leaf morphologies. First, there is no evidence that amphistomatous species achieve higher stomatal densities on a projected leaf area basis than hypostomatous species, but two‐sided gas exchange is less limited by boundary layer effects. Second, amphistomaty may provide a specific advantage in thick leaves by shortening the pathway for CO2 transport between the atmosphere and the chloroplasts. In thin leaves of fast‐growing herbaceous annuals, in which both the adaxial and abaxial pathways are already short, amphistomaty enhances leaf–atmosphere gas‐exchange capacity. Third, amphistomaty may help to optimise the leaf‐interior water status for CO2 transport by reducing temperature gradients and so preventing the condensation of water that could limit CO2 diffusion. Fourth, a potential cost of amphistomaty is the need for additional investments in leaf water transport tissue to balance the water loss through the adaxial surface. [ABSTRACT FROM AUTHOR]

Ventilated Solardomes (hemispherical glasshouses) have been used for 20 yr for studying the effects of gaseous pollutants on plants. This paper describes a computer-operated facility for studying the effects of CO2 × temperature regimes on plants. The eight chambers were set up for factorial design experiments - with two levels of CO2 (ambient and ambient+ 340 ppmv), two levels of temperature (ambient and 3 °C tracked continuously above ambient) and two replicates of each CO2 × temperature treatment. Monitoring of environmental conditions within the chambers over a 2 yr period has shown highly effective control of CO2 and temperature regimes. Even with high-quality and u.v.-B transmitting glass, the irradiance in the PAR region was reduced by 18% within the domes. Variation in temperature across the radii of the domes increased with higher photosynthetic photon flux density (PPFD). Vapour pressure deficits (VPDs) in the ambient temperature domes compared well with outside conditions but were higher in the elevated temperature domes. The watering regime within the domes affected intermittently the relationship between 'dome' and 'outside' VPDs. The Solardome facility has been used extensively for studies of the impacts of climate change within the UK Programme on Terrestrial Initiative on Global Environmental Research (TIGER). [ABSTRACT FROM AUTHOR]

* A previously published model of crop nitrogen (N) status based on intercepted photosynthetically active radiation (R(i), mol per plant) suggested that plant organic N accumulation is related to R(i) by a constant ratio, defined hereafter as the radiation use efficiency for N (NRUE). The aim of this paper was to compare the effects of N nutrition and CO2 enrichment on NRUE and RUE (radiation use efficiency for biomass accumulation). * In three unrelated glasshouse experiments, tomato plants (Solanum lycopersicum) grown in hydroponics were fed for 28 d (exponential growth) with full solutions containing constant NO3(-) concentrations ([NO3(-)]) ranging from 0.05 to 15 mol m(-3), both under ambient or CO2-enriched (1000 microl l(-1)) air. * Each experiment comprised five harvests. Low [NO3(-)] (< 0.3 mol m(-3)) limited growth via leaf area (LA) restriction and decreased light interception. CO2 enrichment enhanced dry weight and LA. RUE was not affected by [NO3(-)], but increased under CO2-enriched air. By contrast, NRUE was not affected by [NO3(-)] or CO2 enrichment. * It is suggested that the radiation efficiency for organic N acquisition (NRUE) did not depend on C or N nutrition for young plants grown under unstressed conditions.

This paper presents the results of 2 yr of CO(2) flux measurements on grassland communities of varying species richness, exposed to either the current or a warmer climate. We grew experimental plant communities containing one, three or nine grassland species in 12 sunlit, climate-controlled chambers. Half of these chambers were exposed to ambient air temperatures, while the other half were warmed by 3 degrees C. Equal amounts of water were added to heated and unheated communities, implying drier soils if warming increased evapotranspiration. Three main CO(2) fluxes (gross photosynthesis, above-ground and below-ground respiration) were measured multiple times per year and reconstructed hourly or half-hourly by relating them to their most important environmental driver. While CO(2) outputs through respiration were largely unchanged under warming, CO(2) inputs through photosynthesis were lowered, especially in summer, when heat and drought stress were higher. Above-ground CO(2) fluxes were significantly increased in multispecies communities, as more complementary resource use stimulated productivity. Finally, effects of warming appeared to be smallest in monocultures. This study shows that in a future warmer climate the CO(2) sink capacity of temperate grasslands could decline, and that such adverse effects are not likely to be mitigated by efforts to maintain or increase species richness.

• A previously published model of crop nitrogen (N) status based on intercepted photosynthetically active radiation ( Ri, mol per plant) suggested that plant organic N accumulation is related to Ri by a constant ratio, defined hereafter as the radiation use efficiency for N (NRUE). The aim of this paper was to compare the effects of N nutrition and CO2 enrichment on NRUE and RUE (radiation use efficiency for biomass accumulation). • In three unrelated glasshouse experiments, tomato plants ( Solanum lycopersicum) grown in hydroponics were fed for 28 d (exponential growth) with full solutions containing constant concentrations ([ ]) ranging from 0.05 to 15 mol m−3, both under ambient or CO2-enriched (1000 µl l−1) air. • Each experiment comprised five harvests. Low [ ] (< 0.3 mol m−3) limited growth via leaf area (LA) restriction and decreased light interception. CO2 enrichment enhanced dry weight and LA. RUE was not affected by [ ], but increased under CO2-enriched air. By contrast, NRUE was not affected by [ ] or CO2 enrichment. • It is suggested that the radiation efficiency for organic N acquisition (NRUE) did not depend on C or N nutrition for young plants grown under unstressed conditions. [ABSTRACT FROM AUTHOR]

• This paper presents the results of 2 yr of CO2 flux measurements on grassland communities of varying species richness, exposed to either the current or a warmer climate. • We grew experimental plant communities containing one, three or nine grassland species in 12 sunlit, climate-controlled chambers. Half of these chambers were exposed to ambient air temperatures, while the other half were warmed by 3°C. Equal amounts of water were added to heated and unheated communities, implying drier soils if warming increased evapotranspiration. Three main CO2 fluxes (gross photosynthesis, above-ground and below-ground respiration) were measured multiple times per year and reconstructed hourly or half-hourly by relating them to their most important environmental driver. • While CO2 outputs through respiration were largely unchanged under warming, CO2 inputs through photosynthesis were lowered, especially in summer, when heat and drought stress were higher. Above-ground CO2 fluxes were significantly increased in multispecies communities, as more complementary resource use stimulated productivity. Finally, effects of warming appeared to be smallest in monocultures. • This study shows that in a future warmer climate the CO2 sink capacity of temperate grasslands could decline, and that such adverse effects are not likely to be mitigated by efforts to maintain or increase species richness. [ABSTRACT FROM AUTHOR]

Summary • Reproductive traits are key characteristics for predicting the response of communities and ecosystems to global change. • We used meta-analysis to integrate data on eight reproductive traits from 159 CO 2 enrichment papers that provided information on 79 species. • Across all species, CO 2 enrichment (500–800 µl l -1 ) resulted in more flowers (+19%), more fruits (+18%), more seeds (+16%), greater individual seed mass (+4%), greater total seed mass (+25%), and lower seed nitrogen concentration, (N) (-14%). Crops and undomesticated (wild) species did not differ in total mass response to elevated CO 2 (+31%), but crops allocated more mass to reproduction and produced more fruits (+28% vs +4%) and seeds (+21% vs +4%) than did wild species when grown at high CO 2 . Seed [N] was not affected by high CO 2 concentrations in legumes, but declined significantly in most nonlegumes. • Our results provide robust estimates of average plant reproductive responses to CO 2 enrichment and demonstrate important differences among individual taxa and among functional groups. In particular, crops were more responsive to elevated CO 2 than were wild species. These differences and the substantial decline in seed [N] in many species have broad implications for the functioning of future natural and agro-ecosystems. [ABSTRACT FROM AUTHOR]

Branch bags were used to expose branches on mature Sitka spruce trees to either ambient [CO2] (A) or elevated [CO2] (E) for 4 yr. This paper reports the effects of this treatment on the growth, development and phenology of the branches, including shoot expansion, shoot numbers, needle dimensions, needle numbers and stomatal density. The effect of elevated [CO2] on the relationship between leaf area and sapwood area was investigated Exposure to elevated [CO2] doubled photosynthetic rates in current-year shoots and, despite sonic down- regulation, 1-yr-old E shoots also had higher rates of photosynthesis than their A counterpart. Thus the amount of assimilate fixed by E branches was substantially more than that fixed by A branches: however, this increase in the local production of assimilate did not lead to an increase in non -structural carbohydrate or stimulate growth or meristematic activity within the E branches. There was a very consistent relationship between leaf area and stem cross-sectional area that was not influenced by [CO2]. However, unbagged branches had thicker stems than bagged branches, resulting in a slightly lower ratio of leaf area to cross-sectional area. The implications of the results tot the modelling of growth and allocation and the potential utility of the branch bag technique are discussed. [ABSTRACT FROM AUTHOR]

This paper reports general patterns of relative growth rate and related traits in response to elevated atmospheric CO2 in eight woody species ranging widely in life form, leaf habit, taxonomy and ecology. Young plants of these species, all of comparable ontogenetic phases, were grown simultaneously in large containers with favourable nutrient and water availability in transparent outdoor chambers at 350 and 700 μl l-1 CO2 for one growing season. We found the following consistent responses. (1) All species grew faster at elevated CO2, whereas the following leaf and allocation traits were consistently lower in CO2-enriched environments: specific leaf area (quotient of leaf area and leaf weight), leaf area ratio (quotient of total leaf area and plant weight), weight-based foliar N concentration and, to a smaller extent, leaf weight fraction (quotient of leaf weight and plant weight). (2) There was important interspecific variation in the magnitude of the response of relative growth rate to CO2. Specific leaf area at ambient CO2 explained 88% of the variation in relative growth rate to CO2 among the eight species. At ambient CO2, relative growth rate itself, was significantly correlated with the relative growth rate response to CO2 only if the leafless species Ulex gallii was excluded from analysis. (3) The four deciduous species had a significantly stronger relative growth rate response to CO2 than the four evergreens. This corresponded with their generally higher specific leaf area. (4) Specific leaf area and leaf habit might be useful for scaling up exercises, as easy-to-measure substitutes for growth responses of (woody) vegetation to elevated CO2. However, the usefulness of such traits in this context needs to be tested in realistic, longer-term manipulative experiments in real ecosystems. [ABSTRACT FROM AUTHOR]

Theoretical developments in our understanding of leaf gas exchange processes and carbon isotope composition (δ13C) mean that it should now be possible to model their responses to global environmental change. Such a model would be of use for process-based interpretations of historical changes in leaf δ13C and for understanding the global stable carbon isotope balance. This paper describes the development and validation of a model towards this aim. The resulting model is used to simulate changes in leaf photosynthesis, stomatal conductance and δ13C of limber pine (Pinus flexilis) in response to the past 30000 y of global environmental change. The predictions of needle δ13C are in line with reported measurements of δ13C from fossilized Pinus flexilis needles preserved in packrat middens in western USA. Leaf gas exchange predictions show that the increased water use efficiency (WUE) of these trees growing in present-day environments, relative to the past, was brought about through an increase in photosynthetic rates and a decrease in stomatal conductance. This contrasts with the explanation of the recent (past 200 y) increase in the WUE of temperate and Mediterranean ecosystems inferred from δ13C measurements which are predicted by the model to have arisen largely by a decrease in stomatal conductance in response to increases in the concentration of atmospheric CO2 since the pre-industrial era. The model as described offers the potential to contribute to our understanding of vegetation effects on the global carbon isotope balance during the glacial periods, and therefore to provide a further constraint on the carbon cycle models used to explain the low concentrations of atmospheric CO2 at these times. [ABSTRACT FROM AUTHOR]

Because of their prominent role in the global carbon balance and their possible carbon sequestration, trees are very important organisms in relation to global climatic changes. Knowledge of these processes is the key to understanding the functioning of the whole forest ecosystem which can be modelled and predicted based on the physiological process information. This paper reviews the major methods and techniques used to examine the likely effects of elevated CO2 on woody plants, as well as the major physiological responses of trees to elevated CO2. The available exposure techniques and approaches are described. An overview table with all relevant literature data over the period 1989-93 summarizes the percent changes in biomass. root/shoot ratio, photosynthesis, leaf area and water use efficiency under elevated CO2. Interaction between growth, photosynthesis and nutrition is discussed with a special emphasis on downward regulation of photosynthesis. The stimulation or reduction found in the respiratory processes of woody plants are reviewed, as well as the effect of elevated CO2 on stomatal density, conductance and water use efficiency. Changes in plant quality and their consequences are examined. Changes in underground processes under elevated CO2 are especially emphasized and related to the functioning of the ecosystem. Some directions for future research arc put forward. [ABSTRACT FROM AUTHOR]

When 8-day seedlings were exposed to 14CO2 in light, approximately 2 hours were required for 14C to reach the apices of the roots. One method used to detect the progress of 14C down the root was to scan the root with a radiochromatogram strip scanner. At this stage most in aqueous ethanol extracts of roots was present as sucrose, equally labelled in glucose and fructose. This was converted only slowly to other components of the soluble fraction so that after 12 hours almost 50% of 14C was still retained in sucrose. 14C exuded from intact roots into surrounding solutions was present in both volatile and non-volatile compounds. In the non-volatile fraction glutamine, glutamic and aspartic acids were the main 14C-amino acids. Exudation of 14C was not affected by changing the inorganic composition of the collecting solution. A change in pH of collecting solution from 6.4 to 5.9 increased exudation. Exudation also increased markedly when 2,4-dinitrophenol or potassium cyanide was present in the collecting solution. [ABSTRACT FROM AUTHOR]

Summary: We present a robust estimation of the CO2 concentration at the surface of photosynthetic mesophyll cells (cw), applicable under reasonable assumptions of assimilation distribution within the leaf. We used Capsicum annuum, Helianthus annuus and Gossypium hirsutumas model plants for our experiments.We introduce calculations to estimate cw using independent adaxial and abaxial gas exchange measurements, and accounting for the mesophyll airspace resistances.The cw was lower than adaxial and abaxial estimated intercellular CO2 concentrations (ci). Differences between cw and the ci of each surface were usually larger than 10 μmol mol−1. Differences between adaxial and abaxial ci ranged from a few μmol mol−1 to almost 50 μmol mol−1, where the largest differences were found at high air saturation deficits (ASD). Differences between adaxial and abaxial ci and the ci estimated by mixing both fluxes ranged from −30 to +20 μmol mol−1, where the largest differences were found under high ASD or high ambient CO2 concentrations.Accounting for cw improves the information that can be extracted from gas exchange experiments, allowing a more detailed description of the CO2 and water vapor gradients within the leaf. [ABSTRACT FROM AUTHOR]

Summary: Generalised dose–response curves are essential to understand how plants acclimate to atmospheric CO2. We carried out a meta‐analysis of 630 experiments in which C3 plants were experimentally grown at different [CO2] under relatively benign conditions, and derived dose–response curves for 85 phenotypic traits. These curves were characterised by form, plasticity, consistency and reliability. Considered over a range of 200–1200 µmol mol−1 CO2, some traits more than doubled (e.g. area‐based photosynthesis; intrinsic water‐use efficiency), whereas others more than halved (area‐based transpiration). At current atmospheric [CO2], 64% of the total stimulation in biomass over the 200–1200 µmol mol−1 range has already been realised. We also mapped the trait responses of plants to [CO2] against those we have quantified before for light intensity. For most traits, CO2 and light responses were of similar direction. However, some traits (such as reproductive effort) only responded to light, others (such as plant height) only to [CO2], and some traits (such as area‐based transpiration) responded in opposite directions. This synthesis provides a comprehensive picture of plant responses to [CO2] at different integration levels and offers the quantitative dose–response curves that can be used to improve global change simulation models. [ABSTRACT FROM AUTHOR]

Summary: Meta‐analyses enable synthesis of results from globally distributed experiments to draw general conclusions about the impacts of global change factors on ecosystem function. Traditional meta‐analyses, however, are challenged by the complexity and diversity of experimental results. We illustrate how several key issues can be addressed by a multivariate, hierarchical Bayesian meta‐analysis (MHBM) approach applied to information extracted from published studies.We applied an MHBM to log‐response ratios for aboveground biomass (AB, n = 300), belowground biomass (BB, n = 205) and soil CO2 exchange (SCE, n = 544), representing 100 studies. The MHBM accounted for study duration, climate effects and covariation among the AB, BB and SCE responses to elevated CO2 (eCO2) and/or warming.The MHBM revealed significant among‐study covariation in the AB and BB responses to experimental treatments. The MHBM imputed missing duration (4.2%) and climate (6%) data, and revealed that climate context governs how eCO2 and warming impact ecosystem function. Predictions identified biomes that may be particularly sensitive to eCO2 or warming, but that are under‐represented in global change experiments.The MHBM approach offers a flexible and powerful tool for synthesising disparate experimental results reported across multiple studies, sites and response variables. [ABSTRACT FROM AUTHOR]

Focuses on paleoatmospheric reconstructions of carbon dioxide concentration that have utilized the observed physiological relationship between atmospheric carbon dioxide and stomatal frequency. Details of the physically based model developed for the study; Nonlinear response of gaseous diffusion through stomatal pores; Fundamental laws governing diffusion.

Summary: Atmospheric carbon dioxide concentration ([CO2]) is increasing, which increases leaf‐scale photosynthesis and intrinsic water‐use efficiency. These direct responses have the potential to increase plant growth, vegetation biomass, and soil organic matter; transferring carbon from the atmosphere into terrestrial ecosystems (a carbon sink). A substantial global terrestrial carbon sink would slow the rate of [CO2] increase and thus climate change. However, ecosystem CO2 responses are complex or confounded by concurrent changes in multiple agents of global change and evidence for a [CO2]‐driven terrestrial carbon sink can appear contradictory. Here we synthesize theory and broad, multidisciplinary evidence for the effects of increasing [CO2] (iCO2) on the global terrestrial carbon sink. Evidence suggests a substantial increase in global photosynthesis since pre‐industrial times. Established theory, supported by experiments, indicates that iCO2 is likely responsible for about half of the increase. Global carbon budgeting, atmospheric data, and forest inventories indicate a historical carbon sink, and these apparent iCO2 responses are high in comparison to experiments and predictions from theory. Plant mortality and soil carbon iCO2 responses are highly uncertain. In conclusion, a range of evidence supports a positive terrestrial carbon sink in response to iCO2, albeit with uncertain magnitude and strong suggestion of a role for additional agents of global change. See also the Commentary on this article by Way et al., 229: 2383–2385. [ABSTRACT FROM AUTHOR]

Infestation with the bird cherry-oat aphid considerably reduced the dry weight yield, the leaf area, the number of tillers and the number of leaves of barley plants. Growth analysis revealed that the major effect of infestation was to reduce unit leaf rate. However, infested plants compensated partially for this by allocating a greater proportion of their dry weight to the production of leaf laminae than did the control plants. Rates of net photosynthesis in individual, attached leaves were measured using the carbon dioxide exchange method; infested and control plants were found to have very similar rates of photosynthesis. Thus the reduction of unit leaf rate did not result from an effect of photosynthesis. It is suggested that the decreased growth of the barley plants is explicable in terms of cumulative assimilate loss due to aphid feeding. [ABSTRACT FROM AUTHOR]

Young plants of the wilty tomato mutant, flacea, in which the stomata resist closure, were sprayed with abscisic acid eight times over a period of 24 hours in light and darkness. The hormone induced closure of stomata similarly in leaves treated either in light or in darkness. Abscisic add does not appear to induce stomatal closure in darkness by increasing the internal CO2 concentration in the leaf, since this concentration should already be very high in darkness. A considerable and rapid decrease of CO2 assimilation and increase of CO2 evolution were demonstrated in mutant as well as in normal plants transferred from light to darkness. [ABSTRACT FROM AUTHOR]

The state of knowledge of the transition of sapwood into heartwood in trees is reviewed, and attention is drawn to the composition of the gas mixture available to such cells as remain alive in wood. The probable effect of such a mixture is postulated and experiments indicate that the high concentrations of carbon dioxide known to exist inside wood are conducive to formation of compounds known to be typical of the heartwood of Acacia mearnsii. [ABSTRACT FROM AUTHOR]

Summary: Rising atmospheric CO2 (ca) is expected to promote tree growth and lower water loss via changes in leaf gas exchange. However, uncertainties remain if gas‐exchange regulation strategies are homeostatic or dynamical in response to increasing ca, as well as evolving climate and pollution inputs.Using a suite of tree ring‐based δ13C‐derived physiological parameters (Δ13C, ci, iWUE) and tree growth from a mesic, low elevation stand of canopy‐dominant Tsuga canadensis in north‐eastern USA, we investigated the influence of rising ca, climate and pollution on, and characterised the dynamical regulation strategy of, leaf gas exchange at multidecadal scales.Isotopic and growth time series revealed an evolving physiological response in which the species shifted its leaf gas‐exchange strategy dynamically (constant ci; constant ci/ca; constant ca − ci) in response to rising ca, moisture availability and site conditions over 111 yr. Tree iWUE plateaued after 1975, driven by greater moisture availability and a changing soil biogeochemistry that may have impaired a stomatal response.Results suggested that trees may exhibit more complex physiological responses to the changing environmental conditions over multidecadal periods, and complicating the parameterisation of Earth system models and the estimation of future carbon sink capacity and water balance in midlatitude forests and elsewhere. [ABSTRACT FROM AUTHOR]

Summary: C2 photosynthesis is a carbon concentrating mechanism that can increase net CO2 assimilation by capturing, concentrating and re‐assimilating CO2 released by photorespiration. Empirical and modelling studies indicate that C2 plants assimilate more carbon than C3 plants under high temperature, bright light, and low CO2 conditions. I argue that engineering C2 photosynthesis into C3 crops is a promising approach to improve photosynthetic performance under these – and temporally heterogeneous – environments, and review the modifications that may re‐create a C2 phenotype in C3 plants. Although a C2 engineering program would encounter many of the same challenges faced by C4 engineering programmes, the simpler leaf anatomical requirements make C2 engineering a feasible approach to improve crops in the medium term. [ABSTRACT FROM AUTHOR]

The author discusses the role of coccolithophore Emiliania huxleyi (E. huxleyi) in ocean acidification. He says that E. huxleyi accounts on global ocean calcium carbonate production. Also investigated is the study conducted by L. T. Bach and colleagues which shows that E. huxleyi provides evidence on carbon concentrating mechanism (CCM) that is not driven by calcification and suggests that E. huxleyi growth in marine system is not stimulated by carbon dioxide as the climate change progresses.

The article comments on a study of lateral diffusion of carbon dioxide conducted by R. Pieruschka et al and published in a 2006 issue of "New Phytologist." It outlines the major arguments of the study and presents a discussion on leaf anatomy and patchy stomata behavior of leaves. It considers the importance of lateral carbon dioxide diffusion.

Summary: Plants grow and transpire during the night. The aim of the present work was to assess the relative flows of carbon, water and solutes, and the energy involved, in sustaining night‐time transpiration and leaf expansive growth under control and salt‐stress conditions.Published and unpublished data were used, for barley plants grown in presence of 0.5–1 mM NaCl (control) and 100 mM NaCl.Night‐time leaf growth presents a more efficient use of taken‐up water compared with day‐time growth. This efficiency increases several‐fold with salt stress. Night‐time transpiration cannot be supported entirely through osmotically driven uptake of water through roots under salt stress. Using a simple three‐ (root medium/cytosol/vacuole) compartment approach, the energy required to support cell expansion during the night is in the lower percentage region (0.03–5.5%) of the energy available through respiration, under both, control and salt‐stress conditions. Use of organic (e.g. hexose equivalents) rather than inorganic (e.g. Na+, Cl−, K+) solutes for generation of osmotic pressure in growing cells, increases the energy demand by orders of magnitude, yet requires only a small portion of carbon assimilated during the day.Night‐time transpiration and leaf expansive growth should be considered as a potential acclimation mechanism to salinity. See also the Commentary on this article by Sanders, 225: 1047–1048. [ABSTRACT FROM AUTHOR]

Summary: Photosynthesis in bryophytes and lycophytes has received less attention than terrestrial plant groups. In particular, few studies have addressed the nonstomatal diffusion conductance to CO2gnsd of these plant groups.Their lower photosynthetic rate per leaf mass area at any given nitrogen concentration compared with vascular plants suggested a stronger limitation by CO2 diffusion. We hypothesized that bryophyte and lycophyte photosynthesis is largely limited by low gnsd. Here, we studied CO2 diffusion inside the photosynthetic tissues and its relationships with photosynthesis and anatomical parameters in bryophyte and lycophyte species in Antarctica, Australia, Estonia, Hawaii and Spain.On average, lycophytes and, specially, bryophytes had the lowest photosynthetic rates and nonstomatal diffusion conductance reported for terrestrial plants. These low values are related to their very thick cell walls and their low exposure of chloroplasts to cell perimeter.We conclude that the reason why bryophytes lie at the lower end of the leaf economics spectrum is their strong nonstomatal diffusion conductance limitation to photosynthesis, which is driven by their specific anatomical characteristics. [ABSTRACT FROM AUTHOR]

Summary: Phosphenolpyruvate carboxylase (PEPC)‐catalysed fixation of bicarbonate to C4 acids is commonly believed to represent a rather small flux in illuminated leaves. In addition, its potential variation with O2 and CO2 is not documented and thus is usually neglected in gas‐exchange studies.Here, we used quantitative NMR analysis of sunflower leaves labelled with 13CO2 (99% 13C) under controlled conditions and measured the amount of 13C found in the four C‐atom positions in malate, the major product of PEPC activity.We found that amongst malate 13C‐isotopomers present after labelling, most molecules were labelled at both C‐1 and C‐4, showing the incorporation of 13C at C‐4 by PEPC fixation and subsequent redistribution to C‐1 by fumarase (malate–fumarate equilibrium). In addition, absolute quantification of 13C content showed that PEPC fixation increased at low CO2 or high O2, and represented up to 1.8 μmol m−2 s−1, that is, 40% of net assimilation measured by gas exchange under high O2/CO2 conditions.Our results show that PEPC fixation represents a quantitatively important CO2‐fixing activity that varies with O2 and/or CO2 mole fraction and this challenges the common interpretation of net assimilation in C3 plants, where PEPC activity is often disregarded or considered to be constant at a very low rate. [ABSTRACT FROM AUTHOR]

ContentsSummary652I.Introduction652II.Discrepancy in predicting the effects of rising [CO2] on the terrestrial C sink655III.Carbon and nutrient storage in plants and its modelling656IV.Modelling the source and the sink: a plant perspective657V.Plant‐scale water and Carbon flux models660VI.Challenges for the future662Acknowledgements663Authors contributions663References663 Summary: The increase in atmospheric CO2 in the future is one of the most certain projections in environmental sciences. Understanding whether vegetation carbon assimilation, growth, and changes in vegetation carbon stocks are affected by higher atmospheric CO2 and translating this understanding in mechanistic vegetation models is of utmost importance. This is highlighted by inconsistencies between global‐scale studies that attribute terrestrial carbon sinks to CO2 stimulation of gross and net primary production on the one hand, and forest inventories, tree‐scale studies, and plant physiological evidence showing a much less pronounced CO2 fertilization effect on the other hand. Here, we review how plant carbon sources and sinks are currently described in terrestrial biosphere models. We highlight an uneven representation of complexity between the modelling of photosynthesis and other processes, such as plant respiration, direct carbon sinks, and carbon allocation, largely driven by available observations. Despite a general lack of data on carbon sink dynamics to drive model improvements, ways forward toward a mechanistic representation of plant carbon sinks are discussed, leveraging on results obtained from plant‐scale models and on observations geared toward model developments. [ABSTRACT FROM AUTHOR]

• Reassimilation of internal CO2 via corticular photosynthesis (PScort) has an important effect on the carbon economy of trees. However, little is known about its role as a source of O2 supply to the stem parenchyma and its implications in consumption and movement of O2 within trees. • PScort of young Populus nigra (black poplar) trees was investigated by combining optical micro‐optode measurements with monitoring of stem chlorophyll fluorescence. • During times of zero sap flow in spring, stem oxygen concentrations (cO2) exhibited large temporal changes. In the sapwood, over 80% of diurnal changes in cO2 could be explained by respiration rates (Rd(mod)). In the cortex, photosynthetic oxygen release during the day altered this relationship. With daytime illumination, oxygen levels in the cortex steadily increased from subambient and even exhibited a diel period of superoxia of up to 110% (% air sat.). By contrast, in the sapwood, cO2 never reached ambient levels; the diurnal oxygen deficit was up to 25% of air saturation. • Our results confirm that PScort is not only a CO2‐recycling mechanism, it is also a mechanism to actively raise the cortical O2 concentration and counteract temporal/spatial hypoxia inside plant stems. [ABSTRACT FROM AUTHOR]