Ozone (O) pollution can alter carbon allocation and reduce tree growth - both above and below ground, but the extent of these effects depends on the variation in soil water and nutrient availability. Here we present the accumulation and allocation of biomass in poplar clone 546 (Populus deltoides cv. '55/56' × P. deltoides cv. 'Imperial') for one growing season at two O concentrations (charcoal-filtered air [CF] and non-filtered air + 40 ppb of O [E-O]), two watering regimes (well-watered [WW] and reduced watering at 40% of WW irrigation [RW]) and two soil nitrogen addition treatments (no addition [N0] and the addition of 50 kg N ha year [N50]). We found that the deleterious effects of E-O depended on the supply of water and nitrogen. Specifically, when the supplies of water and/or N (WW and/or N50) were abundant, E-O significantly reduced whole plant biomass by >15% but had no significant effect on biomass when these supplies were limited (RW and N0). A significant reduction of biomass by E-O occurred earlier in fine roots than in other plant organs, indicating greater sensitivity of fine root to E-O. These results suggest that rising O concentrations may not ubiquitously lead to a large reduction in plant biomass since plant growth is often jointly constrained by water and nutrients.