On the Classification of Forest Vegetation in Xishuangbanna, Southern Yunnnan

Xishuangbanna of southern Yunnan is a region of extremely interest to biologists and also a hotspot for biodiversity conservation.It is located in a transitional zone from tropical Southeast Asia to temperate East Asia biogeographically.The present paper reviewed vegetation types of Xishuangbanna and suggested a revised classification system based on the updated study results over the last two decades.By combining physiognomic and floristic characteristics with ecological performances and habitats,the primary forest vegetation in Xishuangbanna can be organized into four main vegetation types:tropical rain forest,tropical seasonal moist forest,tropical montane evergreen broad-leaved forest and tropical monsoon forest.The tropical rain forest can be classified into two subtypes,i.e.tropical seasonal rain forest in the lowlands and tropical montane rain forest on higher elevations.The tropical seasonal rain forest in this region shows similar forest profile and physiognomic characteristics to those of equatorial lowland rain forests and is a type of world tropical rain forest.Because of conspicuous similarity on floristic composition,the tropical seasonal rain forest in Xishuangbanna is a type of tropical Asian rain forest.However,since the tropical seasonal rain forest occurs at the northern edge of tropical SE Asia,it differs from typical lowland rain forests in equatorial areas in maintaining some deciduous trees in the canopy layer,fewer megaphanerophytes and epiphytes but more abundant lianas and more plants with microphyll.It is a type of semi-evergreen rain forest at the northern edge of the tropical zone.The tropical montane rain forest occurs in wet montane habitats and is similar to the lower montane rain forests in equatorial Asia in floristic composition and physiognomy.It is a variety of lower montane rain forests at the northern tropical edges of tropical rain forests.The tropical seasonal moist forest occurs on middle and upper limestone mountains and is similar to the tropical montane evergreen broad-leaved forest of the region in physiognomy,but it differs from the latter in floristic composition.The monsoon forest in Xishuangbanna is a tropical deciduous forest under the influence of a strong monsoon climate and is considered to be a transitional vegetation type between tropical rain forest and savanna in physiognomy and distribution.The tropical montane evergreen broad-leaved forest is the main vegetation type in mountain areas.It is dominated by the tree species of Fagaceae,Euphorbiaceae,Theaceae and Lauraceae in majority.It differs from the tropical montane rain forests in lack of epiphytes and having more abundant lianas and plants with compound leaves.It is considered to be a distinct vegetation type in the northern margin of mainland southeastern Asia controlling by a strong monsoon climate,based on its floristic and physiognomic characteristics.