Shared Ecto-Mycorrhizal Associations among Tall Trees in a Mixed Forest

Individual species of ectomycorrhizal fungi (EMF) are known to form mutualistic symbiosis with different host species in temperate forests, and EMF form extensive mycelial networks in the soil connecting many mycorrhizal root tips to the same mycelium. As a consequence, unrelated neighbor trees may be connected to the same fungal network, potentially explaining how substantial amounts of carbon can be transferred among adult trees of different co-occurring species in a forest. Recently we reported such belowground carbon transfer based on stable carbon isotope signals for Picea, Pinus, Larix and Fagus trees. Here we document the actual EMF community for exactly these tree individuals using molecular tools. Among the nearly 1500 EMF root tips examined, the ancient Basidiomycota genus Sebacina (that commonly produces no prominent sporocarps) accounted for more than 60% of EMF associations common to >2 tree species and 40% of the associations common to the four tree species. Sebacina dimitica, the most abundant EMF species, was identified on all 4 tree species, and the additionally investigated Abies alba. Our data lend support to the existence of a common EMF network that facilitates carbon transfer as evidenced in our earlier study. Our results also underline the importance of Sebacina and a number of other EMF taxa (from the genera Russula, Amphinema, Clavulina, Tomentella, Peziza, Trichphaea, Inocybe, Lactarius and Tuber) as potential agents of transfer. Of the 39 EMF species of the 10 major genera found, 75% were not host-specific, indicating that a diverse, highly complex network of fungal hyphae is connecting those adult forest trees.