Most forest trees form symbiotic relationships with mycorrhizal fungi, where some species, such as beech and oak, are colonized by ectomycorrhizal (ECM) fungi and others, such as maple, are colonized by arbuscular mycorrhizal (AM) fungi. There is evidence to suggest that some ECM fungal-associated tree species form associations with fungal endophytes when they are young and, as they mature, the endophyte symbioses are lost in favor of ECM fungal symbioses. In forests of northeastern Ohio, surface root mats from mature beech and oak trees are not visually colonized by ECM fungi and, based on preliminary staining and DNA sequencing data, we hypothesize that these root mats may be colonized by ericoid mycorrhizal (ERM) fungi. To investigate this hypothesis, surface root mats were sampled from 62 mature trees of three species: Acer saccharum, Fagus grandifolia, and Quercus rubra. Trees were located in 12 experimental plots that were manipulated to elevate soil pH and phosphorus availability with the addition of limestone. Mycorrhizal fungi were identified from cleaned root mats with cloning and sequencing of rDNA and compared between ECM fungal-associated tree species (F. grandifolia and Q. rubra) and an AM-associated tree species (A. saccharum), as well as between lime-treated and untreated plots.
DNA sequencing of the fungal ITS2 region revealed matches to putative ericoid mycorrhizal fungi, including the genera Rhizoscyphus and Meliniomyces, the order Heliotiales, and the class Leotiomycetes. However, the prevalence of putative ERM fungal sequences varied between AM fungal-associated root mats (5%) and ECM fungal-associated root mats (38%). This suggests that ERM fungi do colonize the surface root mats of our plots, but are more likely to colonize root mats of F. grandifolia or Q. rubra. The majority of sequence matches from the A. saccharum root mats were to saprotrophic fungi (38%), including the genera Agrocybe and Mycena and to fungi in the family Thelephoraceae (33%), including the genus Tomentella, which are corticoid fungi known to possess both saprotrophic and mycorrhizal capabilities. A number of matches to corticoid fungi (17%), including the genera Tomentella and Trechispora were also found in the ECM fungal-associated root mats, likely because these groups decay plant material on the soil surface. Sequence matches to dark septate fungi of the genus Phialocephala and root colonizers of the genus Cadophora were found only in A. saccharum root mats, while matches to ECM fungi, including Russula and Tricholoma were, unsurprisingly, found only in F. grandifolia/Q. rubra root mats.