The relationship between species richness and biomass changes from boreal to subtropical forests in China

Diversity-manipulation experiments suggest a positive effect of biodiversity on ecosystem properties (EPs), but variable relationships between species richness and EPs have been reported in natural communities. An explanation for this discrepancy is that observed richness–EPs relationships in natural communities change with environment and species functional identities. But how the relationships change along broad-scale climatic gradients has rarely been examined. In this paper, we sampled 848 plots of 20 × 30 m2 from boreal to tropical forests across China. We examined plot biomass with respect to environmental factors, tree species richness and functional group identity (FGI, i.e. evergreen vs deciduous, and coniferous vs broadleaf). Variation partitioning was used to evaluate the relative effects of the three classes of factors. We found that, most of the ‘effects’ (percentage of variation explained) of richness and FGI on forest biomass were shared with environmental factors, but species richness and FGI still revealed significant effects in addition to environment for plots across China. Richness and FGI explained biomass mainly through their shared effects instead of independent effects, suggesting that the positive biodiversity effect is closely associated with a sampling effect. The relative effects of richness, FGI and environment varied latitudinally: the independent effects of environment and richness decreased from boreal to subtropical forests, whereas the total effect of FGI increased. We also found that the slope of richness–biomass relationship decreased monotonically from boreal to subtropical forests, possibly because of decreasing complementarity and increasing competition with increasing productivity. Our results suggest that while species richness does have significant effects on forest biomass it is less important than environmental gradients and other biotic factors in shaping large-scale biomass patterns. We suggest that understanding how and why the diversity–EPs relationships change along climatic gradient would be helpful for a better understanding of real biodiversity effects in natural communities.