Ectomycorrhizal infection intensity in the eight primary forests namely hollyleaf alpine oak (Quercus aquifolioides), China spruce(Abies faxoniana), chinapaper birch (Betula albo-sinensis), wild poplar (Populus davidiana), China hemlock (Tsuga chinensis), huashan pine (Pinus armandi), larch (Larix japonica), taxon fir (Abies faxoniana) in subalpine forest zone of western Sichuan China was investigated with sampling method in August 2003. The results showed that the investigated forests were all infected by ectomycorrhizae fungi. Intensity index of ectomycorrhizal infection in different forest types were different. In a homogeneous forest, the altitude, slope and forest age influenced the infection rates, infection intensity index of mycorrhizae and fine-root biomass. In the alpine oak (Quercus aquifolioides) forests, infection rate of mycorrhizae in upper soil layer（0—20cm）and infection intensity index of mycorrhizae in lower layer (20—40cm) increased but the concentration of available P decreased in both layers with the increase of attitude. Fine-roots biomass and infection rate of mycorrhizae in both upper and lower layers were higher, but the Mycorrhizal infection intensity index were lower in the lower-slope than the upper-slope of Chinese spruce (Picea asperata) forest. In the same site of China spruce forest, infection rate and infection intensity index of mycorrhizae were obviously higher at initial stage (young age) and aged stage (matured and over-matured) than at steady stage of the community. In fertile soil, Mycorrhizal infection rate and infection intensity index have no remarkable correlation with nutrient concentrations. In the wild poplar, larch, taxon fir and chinapaper birch forests infection rates of mycorrhizae in upper soil layer（0—20cm were over 65%. Due to the slope of Huashan pine forest was the highest (50°), its intensity index of ectomycorrhizal infection was also the highest (55.78%). The infection rate in China hemlock forest was the lowest, but biomass of fine roots was the highest.
In subalpine forests, fine-root biomasses in upper soil layers were higher than those in lower layers, indicating that plant nutrition was mainly conveyed by upper root system.