Proximity to an old-growth forest edge and ectomycorrhizal tree islands enhance ectomycorrhizal fungal colonization of Betula lenta L. (black birch) seedlings in secondary forest soils

Aims The few remaining old-growth forests in the northeastern United States are often comprised of ectomycorrhizal (EM) tree-dominated patches surrounded by arbuscular mycorrhizal (AM) treedominated secondary forests. We examined how (1) distance from old growth and tree neighborhood composition influenced EM colonization, fungal richness, and fungal community composition of Betula lenta L. (black birch) seedlings, a common EM tree that colonizes abandoned agricultural fields, and (2) potential effects of EM fungal genera on seedling physiological performance. Methods We sampled soils and tree composition from the edge of an EM-dominated old-growth forest into an adjacent AM-dominated secondary forest. We used soils to grow black birch seedlings in a growth chamber bioassay. We measured seedling EM colonization and investigated effects of EM fungi and soil characteristics on seedling physiological performance. Results We identified 20 EM fungal species and found decreases in EM colonization and fungal richness with distance from old growth, with many taxa present only near the edge. Neighborhood EM tree abundance best explained EM colonization while distance interacted with EM tree basal area to best explain EM fungal richness of seedlings. Soils from neighborhoods lacking EM trees resulted in sparse EM colonization of seedlings. We found no clear effects of EM fungal genera on seedling performance, but we detected a slight decrease in seedling photosynthetic rate with distance from old growth. Conclusions Old-growth forests can be reservoirs of EM fungi, and EM tree patches can function as localized inoculum sources in AM-dominated secondary forests, potentially facilitating EM tree establishment.