Non-lichenized fungi on attached parts of woody plants form a group which is taxonomically different from that on herbaceous plants. Outside the tropics, tree-inhabiting fungi are all rather similar, so groups of dominant fungi associated with woody plants can be distinguished. Predicted proportions of various taxa are as follows: anamorphic ascomycetes, 60-70% (coelomycetes, 90%, Sphaeropsidales, 50%, Sphaeropsidaceae, 40%); teleomorphic ascomycetes, 20-30% (pyrenomycetes, 50%, Sphaeriales, 25%, Cucurbitariaceae, 15%); basidiomycetes, 10-20 (hymenomycetes, 70%, Aphyllophorales, 50%, Polyporaceae, 30%). Predicted proportions of fungi associated with various organs are: shoots, 60%; leaves, 20%; trunks, 18%; fruits, 1%; flowers, 0.5%; roots, 0.5%. Fungal diversity is provided by their host specialization. If the total number of woody plant species worldwide is 30000, the same number of associated fungal species can be found on these substrata. However, the niches fungi occupy are not random. Up to 200 distinct fungal niches can be recognized. At least 10 niches on main plant organs can be distinguished by any researcher. A minimum of four host-specific species are predicted in every niche. This number depending on location of the plant in zone of range includes vicariant species as well. Taking into account spatial ecological niches and geographic distribution of woody plants from optimum to extreme habitats 40 species of fungi seem to be found on each tree, making a total of 1200000 species on all woody plants. Every niche is characterized by fungal succession. For example, three to four species are observed on shoots during three years. If three species is regarded as a minimum for temporal change in every niche the total number could be 120 fungal species on one plant species and 3600000 species on all woody plants.