Fungi mediated the relationship between water availability and
productivity
In the null SEM, water availability was not significantly related to
productivity (P = 0.221; Table S5; Fig. 1D). However, in the SEM with
fungal richness there was a direct positive association between water
availability and productivity (std. coef. = 0.16, P = 0.047, Fig. 1A)
and a similar trend for the SEM with fungal community composition (std.
coef. = 0.24, P = 0.069; Fig. 1 C). Water availability marginally
increased fungal richness (std. coef. = 0.32, P = 0.094) which was
negatively associated with productivity (std. coef. = -0.23, P = 0.001).
Furthermore, water availability had a negative effect on fungal NMDS1
(std. coef. = -0.57, P = 0.030) which was positively associated with
productivity (std. coef. = 0.24, P = 0.005). In both cases the result
was a negative, indirect effect of water availability on productivity
via fungi (Fig. 1A, B). These negative indirect effects counteracted the
positive direct effects of water availability on productivity.