Stomatal uptake of O 3 in aspen and aspen-birch forests under free-air CO 2 and O 3 enrichment

Abstract

Rising atmospheric carbon dioxide (CO 2) may alleviate the toxicological impacts of concurrently rising tropospheric ozone (O 3) during the present century if higher CO 2 is accompanied by lower stomatal conductance ( g s), as assumed by many models. We investigated how elevated concentrations of CO 2 and O 3, alone and in combination, affected the accumulated stomatal flux of O 3 (AFst) by canopies and sun leaves in closed aspen and aspen-birch forests in the free-air CO 2–O 3 enrichment experiment near Rhinelander, Wisconsin. Stomatal conductance for O 3 was derived from sap flux data and AFst was estimated either neglecting or accounting for the potential influence of non-stomatal leaf surface O 3 deposition. Leaf-level AFst (AFst l) was not reduced by elevated CO 2. Instead, there was a significant CO 2 × O 3 interaction on AFst l, as a consequence of lower values of g s in control plots and the combination treatment than in the two single-gas treatments. In addition, aspen leaves had higher AFst l than birch leaves, and estimates of AFst l were not very sensitive to non-stomatal leaf surface O 3 deposition. Our results suggest that model projections of large CO 2-induced reductions in g s alleviating the adverse effect of rising tropospheric O 3 may not be reasonable for northern hardwood forests.