Effects of CO2 enrichment on trees and forests: Lessons to be learned in view of future ecosystem studies

Because of their prominent role in global bioproductivity and because of th eir complex structure and function, forests and tree species deserve partic ular attention in studies on the likely impact of elevated atmospheric CO2 on terrestrial vegetation. Besides a synoptic review of some of the most pr ominent above-ground response processes, particular attention is given to b elow-ground responses of trees to elevated atmospheric CO2, while some feed back processes and interactions with Various biotic and abiotic factors are also briefly summarized. At the leaf level there is little evidence of the long-term loss of sensitivity to CO2 that was suggested by earlier experim ents with tree seedlings in pots. Future studies on photosynthesis measurem ents will probably not alter our conclusions about acclimation, but should focus more on respiration under elevated CO2, which is still poorly underst ood. At the tree level, the increase in growth observed in elevated CO2 res ults from an increase in both leaf area and leaf photosynthetic rate (per u nit leaf area). Tree growth enhancement is generally larger at high rates o f nutrient supply; when nutrient supply rates do not meet growth rates, tre e nutrient status declines and nutrients become limiting. In many studies a t the canopy level, a shift in whole-tree carbon allocation pattern towards below-ground parts has been associated with increased atmospheric CO2 conc entrations. At the ecosystem level, a larger amount of carbon being allocat ed below-ground could show up by either (1) more root growth and turnover, (2) enhanced activity of root-associated microorganisms, (3) larger microbi al biomass pools and enhanced microbial activity, or (4) increased losses o f soil carbon through soil respiration. Fine root production is generally e nhanced, but it is not clear whether this response would persist in a fores t. As elevated CO2 stimulates biomass production, litterfall and rhizodepos ition also increase. This increased delivery of labile organic matter to th e soil could influence soil microbial communities and subsequent decomposit ion rates, nutrient availability and carbon storage in soil. There are, how ever, contradictory hypothesis about the direction in which nutrient availa bility will be affected. Knowledge of the response of these and other ecoph ysiological processes to elevated CO2 is the key to understanding the funct ioning of the whole forest ecosystem. Our current knowledge is sufficiently large with regard to how the carbon uptake process and individual tree gro wth respond under atmospheric changes, but more emphasis should be put in f uture experiments on the interactions between various processes, such as th e carbon and nitrogen cycles, and on below-ground responses. (C) 1999 Annal s of Botany Company.