Nitrogen supply as a limiting factor determining the sensitivity of Populus tremuloides Michx. to ozone stress

Seedlings of trembling aspen (Populus tremuloides Michx.) were grown at six rates of supplemental nitrogen (N). Five of the N rates were sub‐optimal to optimal for growth, whilst the sixth provided an excess of N with no additional growth benefit. The biomass of plants stressed by ozone (O₃) in open‐top chambers was significantly reduced at optimal rates of N. Plants which received sub‐optimal or excess N showed no significant effects of O₃ on biomass accumulation. Ozone induced two compensatory responses; increased shoot/root ratio and accelerated senescence, at all N rates. Growth rate was strongly influenced by the amount of N supplied, and O₃‐induced reductions in total biomass were highly correlated with growth rate. We concluded that when growth rate was small as a result of N limitation, compensatory responses to O₃ stress were sufficient to prevent detectable losses in biomass.