The importance of the various abiotic and biotic factors that structure biodiversity patterns is well-studied among animals and plants, but less so for lichens in many regions globally. Temperature and productivity are two factors often strongly linked to species richness latitudinally and elevationally. Lichens are obligate symbioses between fungi and photosynthesizing partners that offer a tractable system to study the influence of biotic interactions contrasted with an array of abiotic factors on patterns of species richness. We quantified micro- and macrolichen (i.e., total) taxonomic diversity in the southern Appalachian Mountains, eastern U.S.A., an area more than 33,000 km2 in size and larger than the country of Belgium. This quantification involved intensive sampling of 208 one-hectare plots spanning gradients of elevation, latitude, anthropogenic disturbance and habitat type. Using 16,309 vouchered species occurrences, extensive field-collected ecological variables, and GIS-derived variables, we examined elevational species richness trends. We tested the importance of 12 variables in structuring these trends, including multiple factors of climate, habitat, biotic interactions, anthropogenic disturbance, and pollution, using multivariate, random forest models, a machine-learning approach that excels in detecting non-linear relationships. We detected a strong mid-elevational peak in species richness across the region. Lichen richness also strongly declined with increasing anthropogenic disturbance. For all lichens, analyses based on just the low disturbance sites implicated greater tree richness as the most critical factor for higher lichen richness. Sites with very low temperatures, very low precipitation, and very high levels of sulfur dioxide pollution were also associated with lower lichen richness. The importance of lichen photobionts, as delineated by the three major groups of photosynthetic partners, was reflected in divergent elevational distributions and differing primary drivers of richness. In contrast, abundance of primary substrates (tree density, log density, rock coverage) and variable habitats (habitat richness-heterogeneity) were not critical contributors to lichen richness. This study emphasizes the interplay of both biotic factors and abiotic environments as key to understanding the keystone obligate symbioses in a global biodiversity hotspot.