Ectomycorrhizal tree islands in arbuscular mycorrhizal forests: Hotspots of fungal inoculum important for seedling establishment of historically dominant trees

Abstract Deciduous forests in northeastern North America are comprised of a mixture of ectomycorrhizal (EM) and arbuscular mycorrhizal (AM) trees. However, land use legacies and other factors have decreased EM and concomitantly increased AM tree dominance, potentially limiting forest ecosystem functioning. We investigated how scattered EM tree patches and soil inoculum from local EM tree‐dominated forests influence EM fungal colonisation, survival and growth of EM Tsuga canadensis , Pinus strobus and Quercus spp. tree seedlings in AM‐dominated secondary forests. Seedlings were planted next to dominant AM Acer and scattered EM Betula trees across three New York secondary forests. A subset of seedlings also received soil inoculum from local EM‐dominated forests. We monitored the survival and height growth of seedlings for 2 years and then measured seedling shoot biomass, assessed EM colonisation and identified EM fungi from seedling roots. All genera of seedlings planted near EM Betula had higher EM colonisation and fungal richness than those planted near AM Acer trees. Inoculation with EM forest soil increased EM colonisation and fungal richness of seedlings only near AM Acer , showing great effectiveness in areas lacking native EM fungi. Total EM fungal diversity on seedling roots was highest near EM Betula , which included numerous taxa usually associated with mature trees. In contrast, EM fungal communities near AM Acer were sparse and mostly dominated by relatively few taxa of spore‐bank fungi. Although seedling survival did not differ between treatments, soil inoculation and proximity to EM Betula increased second‐year height growth of Pinus and Tsuga while soil inoculation alone significantly enhanced shoot mass of Quercus seedlings. Synthesis . Agricultural land use legacies have resulted in expansive secondary forests dominated by AM trees. In these forests, establishment of EM tree seedlings outside existing EM tree patches may be hindered by a lack of EM fungi, but local soil inoculum from EM tree‐dominated forests can reintroduce native EM fungi into secondary forests lacking established EM trees.