A dynamic leaf gas-exchange strategy is conserved in woody plants under changing ambient CO₂: Evidence from carbon isotope discrimination in paleo and CO₂ enrichment studies

Rising atmospheric [CO₂], c_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 [CO₂], c_i, a constant drawdown in CO₂ (c_a - c_i), and a constant c_i/c_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_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_a. To assess leaf gas-exchange regulation strategies, we analyzed patterns in c_i inferred from studies reporting C stable isotope ratios (δ¹³C) or photosynthetic discrimination in woody angiosperms and gymnosperms that grew across a range of c_a spanning at least 100 ppm. Our results suggest that much of the c_a-induced changes in c_i/c_a occurred across c_a spanning 200 to 400 ppm. These patterns imply that c_a - c_i will eventually approach a constant level at high c_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_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_a, when additional water loss is small for each unit of C gain, and increasingly water-conservative at high c_a, when photosystems are saturated and water loss is large for each unit C gain.