Relationships between Tree Species Diversity and Aboveground Biomass Are Mediated by Site-Dependent Factors in Northeastern China Natural Reserves on a Small Spatial Scale

Biodiversity loss has accelerated over decades and probably exerted adverse impacts on ecosystems. As a key forest ecosystem function, tree productivity and its relationship with the change in species diversity are a hotspot in ecology research. However, the changing patterns of the relationships between these two variables across ecosystems with different environmental conditions and the underlying mechanisms are not well understood but key to make environmental context-specific strategies for forest ecosystem conservation and management. Here, we examined changes in relationship (RDI-AGB) between the tree species diversity index (DI) and aboveground biomass (AGB) across temperate forest ecosystems with different environmental contexts on a small spatial (plot) scale as well as clarified the driving mechanisms using ground-based data from 3 natural reserves in northeastern China, surveying 22,139 trees across 77 species and 185 plots. We found substantial changes in RDI-AGB across natural reserves with different environmental conditions on a small spatial scale. These changes were probably modulated by site-specific factors. A positive RDI-AGB was attributed to positive impacts of local climate conditions (i.e., mean annual temperature and potential evapotranspiration) and stand age on both DI and AGB, which was observed in the Changbai Mountains Natural Reserve. In contrast, a negative RDI-AGB was likely due to opposite directions in the effects of the site-dependent factors (elevation, mean annual precipitation, vapor pressure deficit, and seasonality of temperature) on DI and AGB, appearing in the Honghuaerji Natural Reserve. In addition, complex impacts of site factors on DI and AGB leading to no obvious RDI-AGB was observed in the Shengshan Natural Reserve. Our results highlight the importance of site-dependent factors in modulating species diversity–productivity relationships and the need to use site-specific strategies in management and conservation for forest-dominated natural reserves and other forest ecosystems with heterogeneous environmental conditions.