Abstract: Nestedness theory reflects the patterns of species dispersal and migration within the metacommunities. Mycorrhizal fungi are vital symbionts of vascular plants and show significant distance-decay patterns in community composition and fungal spores, i.e., dispersion-limited. A comprehensive understanding of the migration and dispersion characteristics of mycorrhizal fungi will help to protect and utilize microbial resources in fragmented habitats, which, in turn, serve as a foundation for the development of biodiversity conservation policies. Using the altitude gradient of the oak forest in Dongling Mountains that is comprised of ten semi-sunny slopes with different altitudes, we investigated the ectomycorrhizal fungal communities in the roots of Quercus wutaishanica tree islands and plant properties on these semi-sunny slopes. We used plant properties on the slope to characterize the properties of the tree islands and used fungal phylum to characterize the diffusion level of ectomycorrhizal fungi. We employed the nestedness metric based on the overlapping and decreasing fill index (NODF) and null model tests to determine the nestedness values and the significance of mycorrhizal fungal assemblages compared with random conditions. The objective of this study was to determine whether ectomycorrhizal fungal assemblages associated with Q. wutaishanica tree islands exhibit a nested pattern and to identify the underlying driving factors. The results showed that: (1) The NODF of the fungal assemblages (57.35) of the Quercus wutaishanica tree islands was higher than that of equiprobable columns-equiprobable rows (EE) nulls and fixed columns-equiprobable rows (FE) nulls (fungal population fixed) (P<0.05). (2) The Leotiomycetes, Agaricomycetes, and Dothideomycetes were the dominant classes of mycorrhizal fungi, accounting for 41.46%, 29.31% and 16.37% of the community abundance, respectively. The Leotiomycetes, which has the highest relative abundance, occupied significantly higher niche width than other classes (P<0.05), i.e., a higher tree island range. (3) When the sequence of tree island attributes was changed, the NODF_site of the fungal assemblages that were ranked by diameter of breast height (DBH) was significantly higher than that of the FE and EE nulls (P<0.05), indicating that DBH significantly affected the migration processes and diffusion path of mycorrhizal fungi. The NODF_site of the fungal assemblages ranked by soil temperature and habitat heterogeneity was significantly higher than those of the EE nulls (P<0.05), indicating that soil temperature and habitat heterogeneity have an impact on fungal assemblages by affecting species population size. This study found that the migration and diffusion process of the mycorrhizal fungal assemblages was primarily influenced by DBH. The large DBH tree island could recruit more mycorrhizal fungi through energy supply to become migration and diffusion sources in the aggregation community, shaping the nested pattern and increasing community stability. Therefore, it is necessary to protect large-diameter trees during forest management.