Drivers and implications of distance decay differ for ectomycorrhizal and foliar endophytic fungi across an anciently fragmented landscape.

Fungal communities associated with plants often decrease in similarity as the distance between sampling sites increases (i.e., they demonstrate distance decay). In the southwestern USA, forests occur in highlands separated from one another by warmer, drier biomes with plant and fungal communities that differ from those at higher elevations. These disjunct forests are broadly similar in climate to one another, offering an opportunity to examine drivers of distance decay in plant-associated fungi across multiple ecologically similar yet geographically disparate landscapes. We examined ectomycorrhizal and foliar endophytic fungi associated with a dominant forest tree (Pinus ponderosa) in forests across ca. 550 km of geographic distance from northwestern to southeastern Arizona (USA). Both guilds of fungi showed distance decay, but drivers differed for each: ectomycorrhizal fungi are constrained primarily by dispersal limitation, whereas foliar endophytes are constrained by specific environmental conditions. Most ectomycorrhizal fungi were found in only a single forested area, as were many endophytic fungi. Such regional-scale perspectives are needed for baseline estimates of fungal diversity associated with forest trees at a landscape scale, with attention to the sensitivity of different guilds of fungal symbionts to decreasing areas of suitable habitat, increasing disturbance, and related impacts of climate change.