Abstract: In a study on the vegetation in Jiuding Mountain, the source of Tuojiang River, we analysed the law between species diversity and the altitude and identified the factors influencing the vertical distribution pattern of vegetation and biodiversity. In the east of Jiuding mountain, arbors dominated the upper layer of the community, and their shade has a significant effect on shrubs and grasses underneath from the valley to the forest borderline. In communities with a higher coverage of arbors, shrubs and grasses are sparsely distributed; in secondary forests with lower coverage and areas of higher altitude above the forest borderline, species and quantities of shrubs and grasses increase to a large extent. Similar change was found in Hill index, N1 and N2 indices: below the forest borderline, the value of diversity index decreases in the order of arbor>grass>shrub. This becomes much more significant at the altitude of more than 3 500 m, where the diversity index of grasses is the highest, second to it is shrub, while arbors no longer exist.

Key words: Jiuding mountain, biodiversity, altitudinal gradient, vertical pattern