Diagnostic gas exchange measurements and foliar chemical assays were conducted on hybrid poplar (Populus tristis Fisch. ×P. balsamifera L. cv. Tristis) and sugar maple (Acer saccharum Marsh.) seedlings grown under contrasting light and ozone treatments. Seedlings were grown in low irradiance (c. 2.5 mol m−2 d−1) and six-fold greater irradiance (c. 16.6 mol m−2 d−1) in combination with low (< 10 nl I −l) and elevated (99–115 nl 1−1) ozone. Analysis of light response curves showed ozone-induced reductions in photosynthetic capacity and quantum yield for unshaded poplar and shaded sugar maple, but not the contrasting light treatments. Photosynthesis at saturating CO2 concentrations was decreased in the elevated ozone treatment in both the unshaded and shaded poplar and in shaded sugar maple. Poplar had significant reductions in chlorophyll concentration due to ozone exposure in both unshaded and shaded treatments. Older leaves of unshaded poplar plants had significantly greater reductions in chlorophyll levels due to ozone than older leaves of shaded plants. In maple, only shade-grown leaves had significant decreases in chlorophyll concentration due to ozone exposure. The diagnostic gas exchange measurements in conjunction with chlorophyll measurements indicate that in hybrid poplar, unshaded leaves may be more sensitive to ozone than shade leaves, while in sugar maple, shade leaves are more sensitive to ozone. For hybrid poplar a decrease in photosynthetic capacity, quantum yield and chlorophyll concentration in the unshaded, moderately high light environment due to elevated ozone is consistent with prior studies. The results indicating that sugar maple seedlings may be more detrimentally affected by elevated ozone in the lower light environment may have serious implications for this and other shade-adapted species with respect to their performance in an understorey environment.