Mycorrhizal fungi and pathogens influenced the diversity-productivity relationship
The SEM that included richness of plant pathogens and ECM (Fig. 1C, Table S4) explained 35% of the variability in tree productivity and provided a better fit than the null model (Fisher’s C = 13.933, P = 0.008 on 4 df; Fig. 1D). Tree species richness (std. coef. = -0.18, P = 0.007) and functional identity (std. coef. = -0.32, P < 0.001) were negatively associated with pathogen richness but pathogen richness was not related to productivity (P = 0.767). Tree species richness had a positive effect on ECM richness (std. coef. = 0.25, P = 0.013), and ECM richness was negatively related to productivity (std. coef. = -0.19, P = 0.007). Pathogen and ECM richness had a negative, interactive effect on productivity (std. coef. = -0.14, P = 0.027). Therefore, pathogen and ECM richness mediated both positive and negative indirect effects of tree species richness on productivity.