Tree functional diversity was positively related to tree productivity via fungal richness
The SEM including fungal richness (Fig. 1A) explained 34% of the variability in tree productivity and provided a better fit than the null SEM (Fig. 1D; Fisher’s C = 14.125, P = 0.001 on 2 df; Table S2). Tree functional diversity had a negative effect on fungal richness (std. coef. = -0.13, P = 0.043), and fungal richness was negatively related to tree productivity (std. coef. = -0.23, P < 0.001), therefore, fungal richness mediated a positive, indirect effect of functional diversity on productivity. The direct, negative effect of fungal richness on productivity was similar in magnitude to the direct effects of tree richness (std. coef. = 0.26, P = 0.003), functional diversity (std. coef. = 0.25, P = 0.006), and functional identity (std. coef. = -0.21, P = 0.001) on productivity.