A dynamic leaf gas-exchange strategy is conserved in woody plants under changing ambient CO sub(2): evidence from carbon isotope discrimination in paleo and CO sub(2) enrichment studies

Rising atmospheric [CO sub(2)], c sub(a), is expected to affect stomatal regulation of leaf gas-exchange of woody plants, thus influencing energy fluxes as well as carbon (C), water, and nutrient cycling of forests. Researchers have proposed various strategies for stomatal regulation of leaf gas-exchange that include maintaining a constant leaf internal [CO sub(2)], c sub(i), a constant drawdown in CO sub(2) (c sub(a) - c sub(i)), and a constant c sub(i)/c sub(a). These strategies can result in drastically different consequences for leaf gas-exchange. The accuracy of Earth systems models depends in part on assumptions about generalizable patterns in leaf gas-exchange responses to varying c sub(a). The concept of optimal stomatal behavior, exemplified by woody plants shifting along a continuum of these strategies, provides a unifying framework for understanding leaf gas-exchange responses to c sub(a). To assess leaf gas-exchange regulation strategies, we analyzed patterns in c sub(i) inferred from studies reporting C stable isotope ratios ( delta super(13)C) or photosynthetic discrimination ( Delta ) in woody angiosperms and gymnosperms that grew across a range of c sub(a) spanning at least 100 ppm. Our results suggest that much of the c sub(a)-induced changes in c sub(i)/c sub(a) occurred across c sub(a) spanning 200 to 400 ppm. These patterns imply that c sub(a) - c sub(i) will eventually approach a constant level at high c sub(a) because assimilation rates will reach a maximum and stomatal conductance of each species should be constrained to some minimum level. These analyses are not consistent with canalization toward any single strategy, particularly maintaining a constant c sub(i). Rather, the results are consistent with the existence of a broadly conserved pattern of stomatal optimization in woody angiosperms and gymnosperms. This results in trees being profligate water users at low c sub(a), when additional water loss is small for each unit of C gain, and increasingly water-conservative at high c sub(a), when photosystems are saturated and water loss is large for each unit C gain.