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Despite colonizing nearly every plant on Earth, foliar fungal symbionts have received little attention in studies on the biogeography of host-associated microbes. Evidence from regional scale studies suggests that foliar fungal symbiont distributions are influenced both by plant hosts and environmental variation in climate and soil resources. However, previous surveys have focused on either one plant host across an environmental gradient or one gradient and multiple plant hosts, making it difficult to disentangle the influence of host identity from the influence of the environment on foliar endophyte communities. We used a culture-based approach to survey fungal symbiont composition in the leaves of nine C 3 grass species along replicated elevation gradients in grasslands of the Colorado Rocky Mountains. In these ecosystems, the taxonomic richness and composition of foliar fungal symbionts were mostly structured by the taxonomic identity of the plant host rather than by variation in climate. Plant traits related to size (height and leaf length) were the best predictors of foliar fungal symbiont composition and diversity, and composition did not vary predictably with plant evolutionary history. The largest plants had the most diverse and distinctive fungal communities. These results suggest that across the ~ 300 m elevation range that we sampled, foliar fungal symbionts may indirectly experience climate change by tracking the shifting distributions of plant hosts rather than tracking climate directly.

Climate change is shifting altitudinal species ranges, with potential to disrupt species interactions. Altitudinal gradient studies and warming experiments can both increase understanding of climate effects on species interactions, but few studies have used both together to improve predictions. We examined whether plant-fungal symbioses responded similarly to altitude and 23 yr of experimental warming. Root- and leaf-associated fungi, which can mediate plants' climate sensitivity, responded divergently to elevation vs. warming. Fungal colonization, diversity, and composition varied with altitude, but climate variables were generally weak predictors; other factors such as host plant identity, plant community composition, or edaphic variables likely drive fungal altitudinal distributions. Manipulated warming altered fungal colonization, but not composition or diversity. Leaf symbionts were more sensitive to climate and experimental warming than root symbionts. Altitudinal patterns and responses to warming differed among host plant species and fungal groups, indicating that predicting climate effects on symbioses will require tracking both host and symbiont identities. Combining experimental and observational methods can yield valuable insight into how climate change may alter plant-symbiont interactions, but our results indicate that altitude does not always serve as an adequate proxy for warming effects on fungal symbionts of plants., (© 2019 by the Ecological Society of America.)