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Leaf photosynthesis ( Ps ), nitrogen (N) and light environment were measured on Populus tremuloides trees in a developing canopy under free-air CO 2 enrichment in Wisconsin, USA. After 2 years of growth, the trees averaged 1·5 and 1·6 m tall under ambient and elevated CO 2 , respectively, at the beginning of the study period in 1999. They grew to 2·6 and 2·9 m, respectively, by the end of the 1999 growing season. Daily integrated photon flux from cloudfree days (PPFD day,sat ) around the lowermost branches was 16·8 ± ± ± ± 0·8 and 8·7 ± ± ± ± 0·2% of values at the top for the ambient and elevated CO 2 canopies, respectively. Elevated CO 2 significantly decreased leaf N on a mass, but not on an area, basis. N per unit leaf area was related linearly to PPFD day,sat throughout the canopies, and elevated CO 2 did not affect that relationship. Leaf Ps light-response curves responded differently to elevated CO 2 , depending upon canopy position. Elevated CO 2 increased Ps sat only in the upper (unshaded) canopy, whereas characteristics that would favour photosynthesis in shade were unaffected by elevated CO 2 . Consequently, estimated daily integrated Ps on cloudfree days ( Ps day,sat ) was stimulated by elevated CO 2 only in the upper canopy. Ps day,sat of the lowermost branches was actually lower with elevated CO 2 because of the darker light environment. The lack of CO 2 stimulation at the midand lower canopy was probably related to significant down-regulation of photosynthetic capacity; there was no down-regulation of Ps in the upper canopy. The relationship between Ps day,sat and leaf N indicated that N was not optimally allocated within the canopy in a manner that would maximize whole-canopy Ps or photosynthetic N use efficiency. Elevated CO 2 had no effect on the optimization of canopy N allocation.
Leaf photosynthesis ( Ps ), nitrogen (N) and light environment were measured on Populus tremuloides trees in a developing canopy under free-air CO 2 enrichment in Wisconsin, USA. After 2 years of growth, the trees averaged 1·5 and 1·6 m tall under ambient and elevated CO 2 , respectively, at the beginning of the study period in 1999. They grew to 2·6 and 2·9 m, respectively, by the end of the 1999 growing season. Daily integrated photon flux from cloudfree days (PPFD day,sat ) around the lowermost branches was 16·8 ± ± ± ± 0·8 and 8·7 ± ± ± ± 0·2% of values at the top for the ambient and elevated CO 2 canopies, respectively. Elevated CO 2 significantly decreased leaf N on a mass, but not on an area, basis. N per unit leaf area was related linearly to PPFD day,sat throughout the canopies, and elevated CO 2 did not affect that relationship. Leaf Ps light-response curves responded differently to elevated CO 2 , depending upon canopy position. Elevated CO 2 increased Ps sat only in the upper (unshaded) canopy, whereas characteristics that would favour photosynthesis in shade were unaffected by elevated CO 2 . Consequently, estimated daily integrated Ps on cloudfree days ( Ps day,sat ) was stimulated by elevated CO 2 only in the upper canopy. Ps day,sat of the lowermost branches was actually lower with elevated CO 2 because of the darker light environment. The lack of CO 2 stimulation at the midand lower canopy was probably related to significant down-regulation of photosynthetic capacity; there was no down-regulation of Ps in the upper canopy. The relationship between Ps day,sat and leaf N indicated that N was not optimally allocated within the canopy in a manner that would maximize whole-canopy Ps or photosynthetic N use efficiency. Elevated CO 2 had no effect on the optimization of canopy N allocation.
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 CO 2 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 [CO 2 ], and there is little evidence of the long-term loss of sensitivity to CO 2 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 CO 2 enrichment. The relative effect of CO 2 on above-ground dry mass was highly variable and greater than that indicated by most syntheses of seedling studies. Effects of CO 2 concentration on static measures of response are confounded with the acceleration of ontogeny observed in elevated CO 2 . The trees in these open-top chamber experiments were in an exponential growth phase, and the large growth responses to elevated CO 2 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 CO 2 . There is no support for the conclusion from many studies of seedlings that root-to-shoot ratio is increased by elevated CO 2 ; 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 CO 2 -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 CO 2 ; 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 CO 2 under field conditions, confirmed some of the conclusions from previous seedling studies, ...
We tested the main and interactive effects of elevated carbon dioxide concentration ([CO 2 ]), nitrogen (N), and light availability on leaf photosynthesis, and plant growth and survival in understory seedlings grown in an N-limited northern hardwood forest. For two growing seasons, we exposed six species of tree seedlings (Betula papyrifera, Populus tremuloides, Acer saccharum, Fagus grandifolia, Pinus strobus, and Prunus serotina) to a factorial combination of atmospheric CO 2 (ambient, and elevated CO 2 at 658 lmol CO 2 mol À1 ) and N deposition (ambient and ambient 130 kg N ha À1 yr À1 ) in open-top chambers placed in an understory light gradient. Elevated CO 2 exposure significantly increased apparent quantum efficiency of electron transport by 41% (Po0.0001), light-limited photosynthesis by 47% (Po0.0001), and light-saturated photosynthesis by 60% (Po0.003) compared with seedlings grown in ambient [CO 2 ]. Experimental N deposition significantly increased light-limited photosynthesis as light availability increased (Po0.037). Species differed in the magnitude of light-saturated photosynthetic response to elevated N and light treatments (Po0.016). Elevated CO 2 exposure and high N availability did not affect seedling growth; however, growth increased slightly with light availability (R 2 5 0.26, Po0.0001). Experimental N deposition significantly increased average survival of all species by 48% (Po0.012). However, seedling survival was greatest (85%) under conditions of both high [CO 2 ] and N deposition (Po0.009). Path analysis determined that the greatest predictor for seedling survival in the understory was total biomass (R 2 5 0.39, Po0.001), and that carboxylation capacity (V cmax ) was a better predictor for seedling growth and survival than maximum photosynthetic rate (A max ). Our results suggest that increasing [CO 2 ] and N deposition from fossil fuel combustion could alter understory tree species recruitment dynamics through changes in seedling survival, and this has the potential to alter future forest species composition.
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