Stomatal uptake of O-3 in aspen and aspen-birch forests under free-air CO2 and O-3 enrichment

Rising atmospheric carbon dioxide (CO2) may alleviate the toxicological impacts of concurrently rising tropospheric ozone (O-3) during the present century if higher CO2 is accompanied by lower stomatal conductance (g,), as assumed by many models. We investigated how elevated concentrations of CO2 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 CO2-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 CO2. Instead, there was a significant CO2 x 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 CO2-induced reductions in g, alleviating the adverse effect of rising tropospheric O-3 may not be reasonable for northern hardwood forests. (C) 2009 Elsevier Ltd. All rights reserved.