Weak vertical canopy gradients of photosynthetic capacities and stomatal responses in a fertile Norway spruce stand.

The sensitivity of carbon (C) assimilation to within-canopy nitrogen (N) allocation and of stomatal conductance (g s) to environmental variables were investigated along a vertical canopy gradient in a fertile Norway spruce [Picea abies (L.) Karst.] stand. Maximum rates of ribulose bisphosphate-saturated carboxylation (V (cmax)) and electron transport (J (max)) exhibited weak relationships with needle N content. Using these relationships together with a combined stomatal-photosynthesis model, it was found that the sensitivity of C assimilation of 12 1-year old shoots to within-canopy N allocation pattern was very weak. Modelled C assimilation based on optimal compared to observed N allocation pattern increased by only 1-2 %, and altering total needle N content by ± 30 % resulted in a 2-4 % change in modelled C assimilation. C assimilation was more sensitive to water use and changed by 8-12 % in response to ± 30 % altered stomatal conductance. No indications of significant limitations of photosynthesis by other nutrients or non-optimal within-canopy allocation of water were detected. The sensitivity of g s to photosynthetic photon flux density (PPFD) was found to be stronger in the lower canopy, while no significant within-canopy variation was observed in light-saturated g( s) or stomatal sensitivity to vapour pressure deficit (VPD). The results of this study show that, at this N rich site, photosynthesis integrated for shoots at different canopy positions is only marginally affected by N allocation pattern and that increased stand-scale N availability would only be truly beneficial to canopy photosynthesis if it resulted in increased leaf area.