Leaf nutrient concentrations associated with phylogeny, leaf habit and soil chemistry in tropical karst seasonal rainforest tree species

Abstract

Leaf nutrient concentrations are predictors of plant growth variation and crucial for biogeochemical cycling. We aimed to explore the effects of phylogeny, leaf habit and soil chemistry on leaf nutrient concentrations in tropical karst environments. We sampled top-soils and leaves of co-existing evergreen and deciduous tree species along the continuum of mountain valley, slope and peak in a tropical karst seasonal rainforest. We used phylogenetic comparative methods to determine how leaf nutrient concentrations varied in response to phylogeny, leaf habit and soil chemistry and interacted with each other. Tree species had large inter- and intra-nutrient variability and were characterized by the combination of P limitation and Ca hyperaccumulation in leaves. The phylogenetic signals in leaf nutrient concentrations were not significant but increased with decreasing evolutionary rates as a result of the best fitted evolutionary process, i.e., stabilizing selection towards an optimum value. Compared with deciduous species, evergreen species had lower nutrient concentration requirements to fulfill specific biochemical functions in leaves. Along the valley-slope-peak continuum, the correlations between leaf and soil nutrient concentrations were positive for Ca, Mg, P, Cu and Zn and negative for N, S, K and Fe. The strength of interactions differed among leaf nutrients and this largely depended on the divergent biochemical functions among leaf nutrients. Our results suggest that stabilizing selection combined with the biochemical constraints could select the locally adapted evergreen and deciduous species with sufficient phylogenetic variations to produce leaf nutrient concentrations and certain nutrient combinations that should be well-fitted in tropical karst environments.