Are species shade and drought tolerance reflected in leaf-level structural and functional differentiation in Northern Hemisphere temperate woody flora?

Leaf-level determinants of species environmental stress tolerance are still poorly understood. Here, we explored dependencies of species shade (T-shade) and drought (T-drought) tolerance scores on key leaf structural and functional traits in 339 Northern Hemisphere temperate woody species. In general, T-shade was positively associated with leaf life-span (L-L), and negatively with leaf dry mass (M-A), nitrogen content (N-A), and photosynthetic capacity (A(A)) per area, while opposite relationships were observed with drought tolerance. Different trait combinations responsible for T-shade and T-drought were observed among the key plant functional types: deciduous and evergreen broadleaves and evergreen conifers. According to principal component analysis, resource-conserving species with low N content and photosynthetic capacity, and high L-L and M-A, had higher T-drought, consistent with the general stress tolerance strategy, whereas variation in T-shade did not concur with the postulated stress tolerance strategy. As drought and shade often interact in natural communities, reverse effects of foliar traits on these key environmental stress tolerances demonstrate that species niche differentiation is inherently constrained in temperate woody species. Different combinations of traits among key plant functional types further explain the contrasting bivariate correlations often observed in studies seeking functional explanation of variation in species environmental tolerances.