Context-dependence of fungal community responses to dominant tree mycorrhizal types in Northern hardwood forests

Dominant tree community mycorrhizal associations can influence soil biogeochemistry and nutrient cycling, suggesting a prominent role of mycorrhizas in shaping belowground microbial community composition and function. The degree to which the mycorrhizal type of dominant trees interacts with natural environmental gradients to influence belowground microbial communities is, however, unclear. Likewise, it is unknown if community-level mycorrhizal associations can influence the local microbial community encountered by an individual tree through spillover effects. To address these questions, we studied fungal communities from soil, roots, and leaf litter surrounding individual arbuscular (AM) and ectomycorrhizal (ECM) trees embedded in gradients of tree mycorrhizal dominance from three climatically distinct locations in the Adirondack Mountains, NY, USA. We found that dominant tree mycorrhizal types interact with site location to explain more variation in fungal community composition, richness, and function than specific soil properties, such as pH. This finding was consistent for all three sample types, but soil-associated fungi demonstrated the largest amount of explainable variation compared to root- and leaf litter-associated fungi. The relative abundance of plant pathogens was especially responsive to tree mycorrhizal dominance, increasing with AM dominance around individual AM trees but not around ECM trees in the same forests. These "mycorrhizal-spillover" effects on AM trees were also strongest in our warmest, driest site and weakest in our coolest, wettest site, indicating that the strength of mycorrhizal spillover is context-dependent in mixed-mycorrhizal forests.