ALBERT

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

Notes: Responses of photosynthesis and stomatal conductance were monitored throughout a 3-year field exposure of Liriodendron tulipifera (yellow-poplar) and Quercus alba (white oak) to elevated concentrations of atmospheric CO2. Exposure to atmospheres enriched with +150 and +300 umol mol-1 CO2 increased net photosynthesis by 12–144% over the course of the study. Net photosynthesis was consistently higher at +300 than at +150 umol mol-1 CO2. The effect of CO2 enrichment on stomatal conductance was limited, but instantaneous leaf-level water use efficiency increased significantly. No decrease in the responsiveness of photosynthesis to CO2 enrichment over time was detected, and the responses were consistent throughout the canopy and across successive growth flushes and seasons. The relationships between internal CO2 concentration and photosynthesis (e.g. photosynthetic capacity and carboxylation efficiency) were not altered by growth at elevated concentrations of CO2. No alteration in the timing of leaf senescence or abscission was detected, suggesting that the seasonal duration of effective gas-exchange was unaffected by CO2 treatment. These results are consistent with data previously reported for these species in controlled-environment studies, and suggest that leaf-level photosynthesis does not down-regulate in these species as a result of acclimation to CO2 enrichment in the field. This sustained enhancement of photosynthesis provides the opportunity for increased growth and carbon storage by trees as the atmospheric concentration of CO2 rises, but many additional factors interact in determining whole-plant and forest responses to global change.

Notes: The need to assess the role of forests in the global cycling of carbon and how that role will change as the atmospheric concentration of CO2 increases has spawned many experiments over a range of scales. Experiments using open-top chambers have been established at many sites to test whether the short-term responses of tree seedlings described in controlled environments would be sustained over several growing seasons under field conditions. Here we review the results of those experiments, using the framework of the interacting cycles of carbon, water and nutrients, because that is the framework of the ecosystem models that are being used to address the decades-long response of forests.Our analysis suggests that most of what was learned in seedling studies was qualitatively correct. The evidence from field-grown trees suggests a continued and consistent stimulation of photosynthesis of about 60% for a 300 p.p.m. increase in [CO2], and there is little evidence of the long-term loss of sensitivity to CO2 that was suggested by earlier experiments with tree seedlings in pots. Despite the importance of respiration to a tree's carbon budget, no strong scientific consensus has yet emerged concerning the potential direct or acclimation response of woody plant respiration to CO2 enrichment. The relative effect of CO2 on above-ground dry mass was highly variable and greater than that indicated by most syntheses of seedling studies. Effects of CO2 concentration on static measures of response are confounded with the acceleration of ontogeny observed in elevated CO2. The trees in these open-top chamber experiments were in an exponential growth phase, and the large growth responses to elevated CO2 resulted from the compound interest associated with an increasing leaf area. This effect cannot be expected to persist in a closed-canopy forest where growth potential is constrained by a steady-state leaf area index. A more robust and informative measure of tree growth in these experiments is the annual increment in wood mass per unit leaf area, which increased 27% in elevated CO2. There is no support for the conclusion from many studies of seedlings that root-to-shoot ratio is increased by elevated CO2; the production of fine roots may be enhanced, but it is not clear that this response would persist in a forest. Foliar nitrogen concentrations were lower in CO2-enriched trees, but to a lesser extent than was indicated in seedling studies and only when expressed on a leaf mass basis. The prediction that leaf litter C/N ratio would increase was not supported in field experiments. Also contrasting with seedling studies, there is little evidence from the field studies that stomatal conductance is consistently affected by CO2; however, this is a topic that demands more study.Experiments with trees in open-top chambers under field conditions have provided data on longer-term, larger-scale responses of trees to elevated CO2 under field conditions, confirmed some of the conclusions from previous seedling studies, and challenged other conclusions. There remain important obstacles to using these experimental results to predict forest responses to rising CO2, but the studies are valuable nonetheless for guiding ecosystem model development and revealing the critical questions that must be addressed in new, larger-scale CO2 experiments.

Notes: Light-saturated photosynthetic and stomatal responses to elevated CO2 were measured in upper and mid-canopy foliage of a sweetgum (Liquidambar styraciflua L) plantation exposed to free-air CO2 enrichment (FACE) for 3 years, to characterize environmental interactions with the sustained CO2 effects in an intact deciduous forest stand. Responses were evaluated in relation to one another, and to seasonal patterns and natural environmental stresses, including high  temperatures, vapour pressure deficits (VPD), and drought. Photosynthetic CO2 assimilation (A) averaged 46% higher in the +200 µmol mol−1 CO2 treatment, in mid- and upper canopy foliage. Stomatal conductance (gs) averaged 14% (mid-canopy) and 24% (upper canopy) lower under CO2 enrichment. Variations in the relative responses of A and gs were linked, such that greater relative stimulation of A was observed on dates when relative reductions in gs were slight. Dry soils and high VPD reduced gs and A in both treatments, and tended to diminish treatment differences. The absolute effects of CO2 on A and gs were minimized whenever gs was low (〈0·15 mol m−2 s−1), but relative effects, as the ratio of elevated to ambient rates, varied greatly under those conditions. Both stomatal and non-stomatal limitations of A were involved during late season droughts. Leaf temperature had a limited influence on A and gs, and there was no detectable relationship between prevailing temperature and CO2 effects on A or gs. The responsiveness of A and gs to elevated CO2, both absolute and relative, was maintained through time and within the canopy of this forest stand, subject to seasonal constraints and variability associated with limiting air and soil moisture.

Notes: Summary This study investigated ways in which genetically determined differences in SO2 susceptibility resulting from ecotypic differentiation inGeranium carolinianum were expressed physiologically. The SO2-resistant and SO2-sensitive ecotypes were exposed to a combination of short- and long-term SO2 exposures to evaluate the responses of photosynthesis, H2S efflux from foliage (sulfur detoxification), photoassimilate retention, leaf-diffusive resistance to CO2, and growth. When exposed to SO2, both ecotypes re-emit sulfur in a volatile, reduced form, presumably as H2S. Because H2S efflux rates at various SO2 concentrations were comparable between ecotypes, genetic differences inG. carolinianum could not be attributed to a re-emission of excess sulfur as H2S. Incipient SO2 effects on photosynthesis were observed as cumulative SO2 flux into the leaf interior excecded 0.40 nmol·m−2 in the resistant ecotype and 0.26 nmol·m−2 in the sensitive ecotype. Although initial SO2-induced changes in photosynthesis in both ecotypes were mediated through an increase in stomatal resistance to CO2, the ecotype-specific patterns as a function of pollutant concentration and exposure time were associated with marked increases in residual resistance to CO2. Patterns in photosynthesis, photoassimilate retention, and growth following long-term SO2 exposures were also ecotype-specific. Although physiological accommodation of SO2 stress was observed in both ecotypes, it was more pronounced in the resistant ecotype. The physiological mechanisms underlying genetic differences inG. carolinianum in response to SO2 stress were concluded to be (1) dissimilar threshold levels of response to SO2 and/or its toxic derivatives and (2) differences in homeostatic processes governing the rate of repair or compensation for physiological injury.