Multiscale Structuring of Mycorrhizal Fungal Communities of Tropical Epiphytic Orchids.

Symbiotic interactions with root-associated fungi are essential for orchid germination, nutrient acquisition and survival, yet the factors structuring these symbioses in tropical epiphytic orchids remain poorly understood. Tropical montane forests provide an ideal system for examining these interactions because steep environmental gradients occur over short geographic distances. We investigated how elevation, variation among populations within elevational belts and host identity shape root-associated fungal communities in the southern Ecuadorian Andes, a region of exceptional epiphytic orchid diversity. Root samples from 699 individuals representing 11 orchid species were analyzed using ITS2 amplicon sequencing. A total of 4,697 operational taxonomic units (OTUs) were recovered, including 271 putative orchid mycorrhizal fungi. Fungal richness peaked at mid elevation and declined at higher elevations. Community composition varied significantly among elevational belts, sites and host species, with differences among elevations driven primarily by species turnover rather than nestedness. Despite this turnover, mycorrhizal assemblages maintained a persistent core across elevations, while site-level heterogeneity contributed to fine-scale differentiation. These results indicate that fungal community assembly in epiphytic orchids is structured across multiple spatial scales by elevational gradients, local environmental conditions and host identity. The mid-elevation diversity peak and stronger filtering at higher elevations indicate that orchid-fungus symbioses are highly sensitive to environmental gradients, with potential consequences for their stability under ongoing environmental change in tropical montane forests.