Species richness and functional-trait effects on fine root biomass along a subtropical tree diversity gradient

Aims There is evidence that different facets of biodiversity such as species richness (SR), phylogenetic diversity (PD) and functional diversity (FD) can modulate ecosystem functioning via niche-complementarity or mass-ratio effects, but the support for these hypotheses on fine root biomass remains unclear. Methods In a tree diversity gradient in subtropical forests (ranging from 2 to 10 tree species), we identified the species identity of fine roots (<= 2 mm in diameter) using a molecular method. This allowed computation of PD as well as trait-based FD and community-weighted means (CWM) metrics, which were then tested as predictors of fine root biomass. Results Three metrics (SR, CWM of specific root length and PD) were positively related to root biomass. SR was the best predictor of fine root biomass and its predicting ability depended on stand density. The predicting power of CWM of specific root length varied with the soil nutrient status. Conclusions Overall, the results supported both tested hypotheses (mass-ratio and niche-complementarity) that seem to be non-mutually exclusive. Furthermore, our findings emphasize that stand density and soil nutrient status are the important variables driving fine root biomass and trait-values.