This paper provides an overview of the biology and species diversity of xylotrophic basidiomycetes, as well as the biotopic relationships of xylotroph mycobiota with phytocenoses. Ecological groups of these organisms are considered in connection with substrate specialization and their species similarity in birch and aspen small-leaved forests of different pharmacy. A brief assessment of the influence of the anthropogenic factor on the biological diversity of the represented group of organisms is given. For the first time, the Kostroma region is divided into southern, central, and northern parts to assess species diversity depending on weather, climate, and phytocenotic factors. The data on the study of changes in the degree of integration of mycocenoses of birch and aspen forests within the studied areas of the region are presented. In total, over 60 species of xylotrophic basidiomycetes were found in the small-leaved forests in the region, among which 38 species primarily affected birch forests and 40 species of pathogenic xylotrophic basidiomycetes have tropism for P. tremula. It was established that species belonging to the Coriolaseae family predominate in the species composition of wood-destroying mycobiota; they comprise about 27% of the total species composition in birch forests and about 24% of the species composition in aspen forests.