The theory of island biogeography applies to ectomycorrhizal fungi in subalpine tree “islands” at a fine scale

Abstract The theory of island biogeography, which predicts that species richness is a function of island size and distance from the mainland, is well tested with macro‐fauna and flora. Yet, in many ways, microbes are more appropriate for testing this and other ecological theories due to their small size and short generation times that translate to an ease of replication. We used a natural experimental system of isolated “host islands” to test the generality of the theory of island biogeography. Specifically, we tested whether ectomycorrhizal fungal ( EMF ) richness increased with tree size and decreased with distance from forest in a subalpine basin in Yosemite National Park for two congeneric pine species, Pinus albicaulis and Pinus contorta . We determined EMF richness with next‐generation sequencing, measured the size and age of each tree island ( n = 40), and calculated geographic distances from each tree to the nearest forest edge. We found that EMF richness increased with island size (as measured by tree volume) and tree age for both pine species and decreased with distance from forest edge for P. albicaulis . Thus, we show the applicability of the theory to microbial symbionts in harsh, dry, and likely non‐equilibrium systems. In addition, we found that despite the fact that our tree islands had a mean age of 65 yr, a pioneer community of EMF dominated. We interpret this as evidence that water stress interacts with succession to create a sustained period of early‐stage fungi even in mature trees.