Seedlings of Quercus petraea (oak), Fraxinus excelsior (ash) and Pinus sylvestris (Scots pine) were grown at two CO2 concentrations with two O3 and two water supply treatments for 3 yr in a factorial experiment. Oak was the most responsive species to all three treatments; elevated CO2 and irrigation increased biomass by an average of 79% and 41%, respectively, whereas the ozone treatment resulted in a 30% reduction in growth. Significant treatment interactions in this species demonstrated that CO2 ameliorated and irrigation exacerbated the effects of ozone. For Scots pine and ash, irrigation and elevated CO2 increased growth by approx. 60% and 20%, respectively, whereas ozone had no detectable effect on ash and resulted in a 15% reduction in growth of Scots pine. Carbon partitioning to the shoot was enhanced by both the CO2 and H2O treatments in oak, while branching was also increased in this species in response to elevated O3, resulting in changes to the allometric relationships. CO2 enhanced leaf production in oak and Scots pine, and together with the promotion of shoot allocation, this indicates an increased susceptibility to windthrow. Biomass accumulation expressed as relative growth rate, suggested three different time-dependent growth responses to elevated CO2; the CO2 fertilization effect was maintained through the third year of growth in oak, had disappeared in Scots pine and a negative effect was evident in ash. Foliar nitrogen and chlorophyll concentrations indicated a CO2-induced nitrogen deficiency in oak and ash, but not in Scots pine. Chlorophyll degradation in response to ozone was observed in oak, an effect that was enhanced by irrigation and reduced by CO2, presumably through stomatal mediated changes in effective ozone dose. These results therefore suggest that elevated atmospheric CO2 concentrations will enhance growth of some UK forest tree species, although this might only be apparent during the juvenile phase. However, nitrogen deficiencies might limit this enhancement on some sites while changes in allocation and leaf area might promote susceptibility to windthrow. Elevated CO2 also provides some protection against ozone pollution, especially in combination with limited soil moisture availability. These interactions between CO2, ozone and water supply should be taken into account when predicting the effects of environmental change on tree growth and forest productivity.