Nitrogen addition and warming modulate the pathogen impact on plant biomass by shifting intraspecific functional traits and reducing species richness

Foliar fungal pathogens can substantially reduce plant biomass. This effect can be modulated by environment conditions, such as soil nitrogen availability and air temperature. The ongoing global changes are altering these variables and thus interact with pathogens to influence plant biomass, but experimental test of their interactions is scarce. We conducted a 4-year field experiment in a Tibetan alpine meadow to examine the interactive effects of nitrogen addition, warming and foliar pathogens (via fungicide application) on plant biomass. We also measured plant functional traits, species richness and abundance to test the possible mechanisms underlying these interactions. Our results showed that foliar fungal pathogens reduced plant community biomass under nitrogen addition, which in turn weakened the positive nitrogen effect on community biomass. Mechanistically, nitrogen addition shifted the plant communities towards fast-growing traits; this happened predominantly because of changes in within-species trait values, including an increase in specific leaf area and height. These trait changes resulted in greater suppression of plant biomass by pathogens, likely because of the trade-offs associated with the allocation of resources to plant growth and defense. Moreover, the reduction in species richness amplified the pathogen effect under nitrogen addition due to the increased density and susceptibility of the most dominant species (i.e. Kobresia capillifolia). Furthermore, warming did not interact with pathogens and nitrogen addition to influence plant community biomass, but their three-way interaction modified the biomass of K. capillifolia. Specifically, warming enhanced the positive effect of nitrogen addition on the biomass of K. capillifolia in the fungicide, low infection plots, while it weakened the nitrogen effect in the no fungicide, high infection plots. Synthesis. Our results demonstrate how pathogens interact with nitrogen addition and warming to influence the biomass of dominant species and the whole plant community. Our study highlights the importance of considering foliar fungal pathogens when assessing ecosystem responses to multiple global change factors.