Conclusions
Our study underscores the importance of considering the effects of biotic interactions, especially between plants and foliar fungi, on ecosystem carbon fluxes in addition to carbon stocks (e.g., plant biomass). Together with previous studies, our results suggest that these interactions influence carbon fluxes by a combination of effects ranging from leaf to community scale (Mitchell 2003; Borer et al. 2015; Seabloom et al. 2017). Foliar fungal endophytes and pathogens colonize the leaves of all plants (Rodriguez et al. 2009; Dobson & Crawley 1994); examining whether the large effects of foliar fungi reported here are ubiquitous across biomes and ecosystems will shed light on the generality of our findings. The role of foliar fungi is likely to become even more important in the future with climatic changes (Harvell et al. 2002; Olofsson et al. 2011) and loss of plant diversity (Mitchell et al. 2002, Civitello et al. 2015), especially since we found that foliar fungi acted differently on carbon fluxes in low vs high plant diversity communities: they suppressed mass-specific flux rates in low diversity communities, but suppressed plant biomass in high diversity communities. These results further suggest that integrating plant disease ecology into global change and ecosystem ecology will be a fruitful avenue for additional research into human alterations of carbon cycling (Preston et al. 2016), as foliar fungi have large but largely unquantified effects on ecosystem flux rates.