Fungal pathogens and nitrogen addition alter community spatial variability via different mechanisms.

In plant communities, biomass varies considerably in both space and time. Both top down (e.g., pathogens) and bottom up (e.g., nutrients) can influence this variation, but their relative importance and the pathways in which they do so remain poorly understood. Here, we examined the separate and interactive influence of fungal pathogen exclusion and nitrogen addition on the spatial variability of plant community biomass and the underlying mechanisms in an alpine meadow. We found that fungal pathogen exclusion and nitrogen addition independently increased community spatial variability by increasing the variance of plant biomass more than the mean biomass, but there was no interaction between the treatments. Fungal pathogen exclusion increased spatial variation in community biomass by enhancing species covariation. In contrast, nitrogen addition increased community spatial variability mainly by enhancing species variability through increasing beta diversity among communities. Additionally, our observed increase in the spatial mean and variance of biomass in the pathogen exclusion treatment was mainly driven by dominant grasses, whereas all functional groups responded to nitrogen addition. Our results suggest that higher trophic groups and resources can regulate spatial variability of biomass distribution through distinct mechanisms. This enhances our knowledge regarding the roles of top-down and bottom-up forces in maintaining ecosystem functions across spatial scales.