Background/Question/Methods Fungal endophytes are ubiquitous associates of terrestrial plants, occurring within overtly healthy photosynthetic tissues of living plants from the arctic to the lowland tropics. Although the ecological roles of these highly diverse fungi are largely unknown, increasing evidence suggests that endophytes have important, if cryptic, effects on their hosts and the organisms with which their hosts interact. To date, the factors shaping the species diversity, phylogenetic diversity, host specificity, and structure of endophyte communities have remained poorly known, reflecting a lack of large-scale studies that surmount problems with identifying operational taxonomic units among these often sterile symbionts. Our study examines the structure of endophyte communities at spatial scales ranging from individual leaves to landscapes.

Results/Conclusions Using molecular sequence data to explicitly compare communities of cultivable fungi, we highlight broad trends in the species-level diversity, phylogenetic diversity, and host specificity of endophytes for related plants at sites ranging from the lowland tropics to southern boreal forest. Then, using datasets generated by Native American students through an on-Nation research-training program for Diné (Navajo) undergraduates, we examine endophyte communities for multiple sympatric host species over gradients of elevation, rainfall, and air pollution associated with oil prospecting in the biotically rich mountains of eastern Arizona. Together, our data provide a first estimation of the distinctive symbiont communities encountered by plants in different locations, show that sympatric tree species harbor distinctive endophyte communities relative to one another, demonstrate the tremendous turnover in fungal communities as a factor of large- and small-scale differences among sites, and underscore the remarkable diversity of fungi capable of forming endophytic associations with ecologically and economically important plants.