Exposure to moderate concentrations of tropospheric ozone impairs tree stomatal response to carbon dioxide

With rising concentrations of both atmospheric carbon dioxide (CO(2)) and tropospheric ozone (O(3)), it is important to better understand the interacting effects of these two trace gases on plant physiology affecting land-atmosphere gas exchange. We investigated the effect of growth under elevated CO(2) and O(3), singly and in combination, on the primary short-term stomatal response to CO(2) concentration in paper birch at the Aspen FACE experiment. Leaves from trees grown in elevated CO(2) and/or O(3) exhibited weaker short-term responses of stomatal conductance to both an increase and a decrease in CO(2) concentration from current ambient level. The impairement of the stomatal CO(2) response by O(3) most likely developed progressively over the growing season as assessed by sap flux measurements. Our results suggest that expectations of plant water-savings and reduced stomatal air pollution uptake under rising atmospheric CO(2) may not hold for northern hardwood forests under concurrently rising tropospheric O(3).