The effect of elevated CO(2) and O(3) on apparent quantum yield (varphi), maximum photosynthesis (P(max)), carboxylation efficiency (V(cmax)) and electron transport capacity (J(max)) at different canopy locations was studied in two aspen (Populus tremuloides) clones of contrasting O(3) tolerance. Local light climate at every leaf was characterized as fraction of above-canopy photosynthetic photon flux density (%PPFD). Elevated CO(2) alone did not affect varphi or P(max), and increased J(max) in the O(3)-sensitive, but not in the O(3)-tolerant clone. Elevated O(3) decreased leaf chlorophyll content and all photosynthetic parameters, particularly in the lower canopy, and the negative impact of O(3) increased through time. Significant interaction effect, whereby the negative impact of elevated O(3) was exaggerated by elevated CO(2) was seen in Chl, N and J(max), and occurred in both O(3)-tolerant and O(3)-sensitive clones. The clonal differences in the level of CO(2)xO(3) interaction suggest a relationship between photosynthetic acclimation and background O(3) concentration. 2009 Elsevier Ltd. All rights reserved.