Functional diversity enhances, but exploitative traits reduce tree mixture effects on microbial biomass

Soil micro-organisms play key roles in terrestrial biodiversity and ecosystem functions. Despite recent progress in elucidating the association between plant diversity and soil micro-organisms, it remains unclear whether the functional properties of plant mixtures might alter this association. We examined whether the effects of tree species mixtures on soil microbial biomass were impacted by the functional diversity (FD) and community-weighted mean (CWM) of tree mixtures, by conducting a global meta-analysis involving 123 paired observations of tree mixtures and the corresponding monocultures from 38 studies in forests. We found that the tree mixture effect on microbial biomass increased with the FD of specific leaf area (SLA) and leaf N and P contents, as well as the FD based on all of these traits plus leaf dry matter content. Meanwhile, the responses of microbial biomass to tree mixtures decreased with the CWM of SLA and leaf N and P contents. The effects of FD and CWM remained consistent, despite variable tree species richness, stand age and climatic factors. Our results provide a new insight that the functional properties of plants may alter the magnitude of the association between plant diversity and soil micro-organisms. A free Plain Language Summary can be found within the Supporting Information of this article.