Leaf photosynthetic properties in a willow (Salix viminalis and Salix dasyclados) plantation in response to fertilization

Specific leaf area (SLA), nitrogen and chlorophyll concentrations and photosynthetic characteristics were studied in upper and lower canopy leaves of Salix viminalis and S. dasyclados grown at two nutrition levels. Fertilization increased SLA and leaf mass-based nitrogen concentration in most cases. Positive effects of fertilization on leaf light-saturated photosynthetic rate (A(max)(A)) and maximum carboxylation rate (V-cmax) were not detected. Significant differences between the leaves from upper and lower canopy layers in area-based nitrogen, A(max)(A), SLA, mass-based chlorophyll, V-cmax and stomatal conductance were found for most plots. We attempted to estimate the fraction of non-photosynthetic nitrogen and found that it tended to be higher due to fertilization. Thus, the insensitivity of leaf photosynthesis to fertilization could be caused by higher proportion of non-photosynthetic nitrogen in the leaves of fertilized plots. Though leaf-level photosynthesis was not increased by fertilization, considerably higher leaf area index of fertilized plots still resulted in increased canopy carbon gain.