The importance of plant traits in diversity-productivity relationships
Our results confirm that functional traits of tree communities have an important effect on fungal community composition (Yang et al.2020; Prada-Salcedo et al. 2021). The tree functional identity axis represents a gradient in tree communities with more acquisitive traits, higher Nmass and Amax (positive FI) to more conservative traits, higher LL and LMA (negative FI). Therefore, the direct negative effect of functional identity on productivity suggests that communities with more conservative traits have higher community productivity. Fungal community composition mediated a positive, indirect effect of tree functional identity on productivity, in contrast to the observed negative, direct effect of tree functional identity on productivity. Therefore, tree communities with more conservative traits alter fungal communities in a way which lowers productivity and fungal communities counteract the direct effect of tree functional traits on productivity. Pathogen richness was also negatively associated with functional identity, indicating that acquisitive plant traits, such as higher Nmass and Amax, are associated with lower pathogen richness, contrary to the results of previous studies (Blumenthal et al. 2009; Parker & Gilbert 2018). Pathogen and ECM richness interactively mediated an indirect effect of functional identity on productivity. Functional identity was also an important driver of NE and CE, but only through positive, indirect effects of fungal community composition. Therefore, variation in plant traits among tree communities interact with fungal communities to promote productivity of mixed tree communities.