Nitrogen availability does not affect ozone flux-effect relationships for biomass in birch (Betula pendula) saplings

To investigate whether nitrogen (N) load affects the ozone (O₃) stomatal flux-effect relationship for birch biomass, three-year old birch saplings were exposed to seven different O₃ profiles (24 h mean of 35-66 ppb) and four different N loads (10, 30, 50 and 70 kg ha^-1 yr^-1) in precision-controlled hemispherical glasshouses (solardomes) in 2012 and 2013. Stomatal conductance (g_s) under optimal growth conditions was stimulated by enhanced N supply but was not significantly affected by enhanced O₃ exposure. Birch root, woody (stem + branches) and total biomass (root + woody) were not affected by the Phytotoxic Ozone Dose (POD₁SPEC) after two seasons of O₃ exposure, and enhanced N supply stimulated biomass production independent of POD₁SPEC (i.e. there were no POD₁SPEC × N interactions). There was a strong linear relationship between the stem cross-sectional area and tree biomass at the end of the experiment, which was not affected by O₃ exposure or N load. Enhanced N supply stimulated the stem cross-sectional area at the end of season 2, but not at the end of season 1, which suggests a time lag before tree biomass responded to enhanced N supply. There was no significant effect of POD₁SPEC on stem cross-sectional area after either the first or second growing season of the experiment. Contrasting results reported in the literature on the interactive impacts of O₃ and N load on tree physiology and growth are likely due to species-specific responses, different duration of the experiments and/or a limitation of the number of O₃ and N levels tested.