Interactive effects of ozone and elevated carbon dioxide on the growth and physiology of black cherry, green ash, and yellow-poplar seedlings

Potted seedlings of black cherry (Prunus serotina Ehrh.) (BC), green ash (Fraxinus pennsylvanica Marsh.) (GA), and yellow-poplar (Liriodendron tulipifera L.) (YP) were exposed to one of the four treatments: (1) charcoal-filtered air (CF) at ambient CO2 (control); (2) twice ambient O3 (2×O3); (3) twice ambient CO2 (650 μl l−1) plus CF air (2×CO2); or (4) twice ambient CO2 (650 μl l−1) plus twice ambient O3 (2×CO2+2×O3). The treatments were duplicated in eight continuously stirred tank reactors for 10 weeks. Gas exchange was measured during the last 3 weeks of treatment and all seedlings were destructively harvested after 10 weeks. Significant interactive effects of O3 and CO2 on the gas exchange of all three species were limited. The effects of elevated CO2 and O3, singly and combined, on light-saturated net photosynthesis (Amax) and stomatal conductance (gs) were inconsistent across species. In all three species, elevated O3 had no effect on gs. Elevated CO2 significantly increased Amax in GA and YP foliage, and decreased gs in YP foliage. Maximum carbon exchange rates and quantum efficiencies derived from light-response curves increased, while compensation irradiance and dark respiration decreased in all three species when exposed to 2×CO2. Elevated O3 affected few of these parameters but any change that was observed was opposite to that from exposure to 2×CO2–air. Interactive effects of CO2 and O3 on light-response parameters were limited. Carboxylation efficiencies, derived from CO2-response curves (A/Ci curves) decreased only in YP foliage exposed to 2×CO2–air. In general, growth was significantly stimulated by 2×CO2 in all three species; though there were few significant growth responses following exposure to 2×O3 or the combination of 2×CO2 plus 2×O3. Results indicate that responses to interacting stressors such as O3 and CO2 are species specific.