Tree species diversity of tropical forest vegetation in Xishuangbanna, SW China

Tree species diversity of four tropical forest vegetation types was investigated in Xishuangbanna, southwestern China. These are: tropical seasonal rain forest, tropical montane rain forest, evergreen broad-leaved forest and monsoon forest over limestone. A total of 17 samples were taken and four species diversity indices were calculated: Shannon-Wiener's H', the complement of Simpson's index, d', Fisher's alpha and evenness index E. The results reveal the long-tailed rank/abundance diagrams of these forests. However, this feature is greatly reduced in the samples of monsoon forest over limestone. Tropical seasonal rain forest shows the highest tree species diversity of all four vegetation types. Owing to the variation of microenvironment, diversity values within the same vegetation type vary between the samples from different patches. The tree species diversity of single-dominant rain forest is not significantly lower than that of mixed rain forest, because the dominant species of some single-dominant rain forests are principally in the emergent layer. This is composed of sparse and huge trees of one species and, consequently, creates a unique canopy architecture and more heterogeneous microenvironments for the more diversified species composition under the emergent layer. The occurrence of tree species with small population sizes, particularly of species represented by only one individual, is highly correlated with the tree species diversity of the local forest vegetation. They are crucial elements in the richness of local biodiversity.