Endophyte diversity is influenced by host species and environment but community composition is structured by spatial distance

Foliar fungal endophytes are microbial symbionts that inhabit the leaves of nearly all plant species and can significantly influence host performance, but the relative influence of dispersal limitation, host filtering, and environmental variation on endophyte communities remains poorly understood. We co-sampled fungal endophyte communities from three herbaceous plant species across a complex landscape of the Sierra Nevada, California spanning 100 km where environmental and spatial distance are largely uncorrelated. Our results reveal that across all three host taxa, spatial distance was the primary driver of endophyte community composition, with similarity declining with distance regardless of environmental variation. By contrast, host identity and environmental factors were the predominant predictors of endophyte diversity, with hosts showing differing responses to environmental gradients of temperature, precipitation, solar radiation, and vegetation cover. While most fungal taxa were unique to a single host species at the site level, host filtering was inconsistent at larger spatial scales. Overall, our study reveals that spatial, environmental, and host factors drive different aspects of foliar fungal endophyte community structure, with interactive effects highlighting the context-dependent nature of microbial community assembly.