An open-top chamber experiment was carried out in Curno (Northern Italy) in 2004 and 2005 on seedlings of Fagus sylvatica (FS), Quercus robur (QR), and an ozone-sensitive Populus (POP) clone, to investigate the role of two stress factors: tropospheric ozone and water shortage. Treatments were filtered air to achieve a 50% reduction in the environmental ozone concentrations (charcoal filtered, CF); and non-filtered air, with a 5% reduction in the environmental ozone concentrations (non-filtered, NF). Overall ozone exposure (AOT40) in open air (April-September) was 26,995 ppb h in 2004 and 25,166 ppb h in 2005. The plants were either watered (W) or not watered (dry, D). We investigated the above-ground biomass, tree-ring growth, stable carbon isotopes ratio, i.e. δ¹³C of tree rings, and the photosynthetic parameter Driving forces (DF_ABS), derived from chlorophyll a fluorescence analysis. Ozone-induced growth reduction (in terms of biomass) in POP, and that reduction was more pronounced in D plots. A synergistic effect of ozone and drought stress was evidenced by DF_ABS in POP and QR, but not in FS. The water availability was revealed as the main factor influencing the isotopic ratio δ¹³C. In drought-stressed seedlings, the increase in δ¹³C value was accompanied by the reduction in stomatal conductance and increased DF_ABS. Fast-growing plant species with high water requirements are more susceptible to ozone and drought stress.