This paper reports the findings of an open-top chamber experiment carried out in northern Italy (Forest nursery at Curno), during the 2004 and 2005 growth seasons, on Fagus sylvatica and Quercus robur seedlings and on Populus nigra cuttings, in order to test their photosynthesis response to ambient ozone. The experimental protocols were non-filtered air (NF), charcoal-filtered air (CF) and open air (OA). Tests performed included morphological features of leaves; development of foliar symptoms; chlorophyll content, determined by non-destructive means; chlorophyll fluorescence (direct fluorescence and JIP test) and gas exchanges and net photosynthesis (PN). Main findings were as follows: (1) symptoms occurred early and were extensive in P. nigra, and they occurred later in F. sylvatica, whereas early degeneration of chlorophyll occurred in late summer in Q. robur; (2) in conditions of ozone exposure, the three species all presented a decline in photosynthesis efficiency and a decrease in PN, regardless of the symptomatology they displayed; (3) leaf traits are predictors of species-specific sensitivity to ozone--the high density of Q. robur foliar tissues prevents this species from developing visible symptoms and reduces the extent of physiological responses and (4) physiological responses varied from year to year in the same species--responses were lower in the second year of the experiment, when plants had become better acclimatized to plot conditions.