Canopy-scale relationships between foliar nitrogen and albedo are not observed in leaf reflectance and transmittance within temperate deciduous tree species.

摘要

Strong positive correlations between the maximum rate of canopy photosynthesis, canopy-averaged foliar nitrogen concentration, and canopy albedo have been shown in previous studies. While leaf-level relationships between photosynthetic capacity and foliar nitrogen are well documented, it is not clear whether leaf-level relationships between solar-weighted reflectance and nitrogen underlie the canopy-scale patterns. Using an integrating sphere, we measured the reflectance and transmittance (350-2500 nm) of both individual leaves and multileaf stacks. Samples were collected from 12 broadleaf species at the Harvard Forest in central Massachusetts, USA. Across all species, foliar nitrogen (both mass-based nitrogen concentration and area-based nitrogen content) and leaf mass / area ratio were negatively, rather than positively, correlated with solar-weighted reflectance and transmittance in ultraviolet-visible and near-infrared wavelength bands (350-700 nm and 700-2500 nm, respectively). Leaf-level anatomy and biochemistry, therefore, do not appear to drive the canopy-level association between increasing foliar nitrogen content and increasing canopy albedo. This suggests that interactions between leaf optical properties and structural canopy-scale traits that correlate with nitrogen content (perhaps some combination of crown shape, leaf area index, leaf angular distribution, or other structural characteristics of the canopy), may instead underlie the previously observed relationship between nitrogen and canopy-level shortwave albedo.