Unique host effect of Tilia japonica on ectomycorrhizal fungal communities independent of the tree’s dominance: A rare example of a generalist host?

Trees of the genus Tilia (Malvaceae) are known to establish symbiosis with ectomycorrhizal (EM) fungi, however, most of what is known about their EM relations is based on few European Tilia species, despite the highest diversity of the genus found in the Asian subcontinent. To the authors’ knowledge, this is the first report on EM fungal communities hosted by Tilia in Japan. Three study sites, on Hokkaido (H) where Tilia was a dominant forest component, Honshu (C) and Kyushu (K) islands where Tilia was extremely rare, were established. From each site 30 root samples of T. japonica were collected after tracing from the trunk, and additional 30 random soil core samples near the traced trees. EM root tips in the samples were subjected to molecular identification of EM fungal species and hosts. The study identified 30 EM fungal lineages (246 OTUs), with 28 lineages (164 OTUs) recorded from T. japonica EM communities. In sites H, C, and K, Tilia communities comprised of 19, 20, and 15 lineages, respectively. 127 EM fungal OTUs were never reported from Japan, and 69 OTUs had no previous records in the UNITE database. Contrarily, about 25% of the recorded OTUs have been exclusively reported from Japan. The observed EM richness hosted by T. japonica exceeded those of cooccurring EM trees as identified from the random soil samples and of other previously studied Tilia species. In each study site, T. japonica was found to exert a consistent “generalist” host effect on its EM fungal communities. About 36% of the EM fungal taxa found in random soil samples were also confirmed in T. japonica root samples, indicating a high connectance of Tilia in the local EM networks. The studied EM communities showed nested structure, with T. japonica as the main contributor to the local community nestedness.