Location and General Description
The Yunnan Plateau is an expansive upland region comprised of low hills and broad intermontane basins. It is situated between the limestone hills of Southeast China and the Tibetan Plateau. It also separates the Sichuan Basin and Changjiang (Yangtze) River gorges from the hill country of Northern Indochina. Lacking the winter fog characteristic of basins to the north and east, and high enough to avoid intense pre-monsoon heat, the Yunnan Plateau has a mild climate that has supported humans and their ancestors for more than a million years. A number of large, freshwater lakes are situated on the plateau. Dian Chi, located near the city of Kunming, is large but shallow and ecologically distressed due to eutrophication and pollution from nearby urban areas. Er Hai, farther west near the city of Dali, is a deep lake with a high diversity of freshwater fish.
Precipitation patterns are monsoonal. Most of the Yunnan Plateau receives about 1,200 mm of rain per year, 80 percent of which falls during the summer. Winter storms bring cold weather, and light snow can fall in the mountains, although snow is very rare in the basin areas.
Characteristic vegetation over much of the Yunnan Plateau consists of seasonally humid, evergreen-broadleaved forest. It is adapted to wet summers and an extended cool, dry season that lasts from November until April. Although elevations across the plateau are moderately high (1,700 to 2,300 m), these massive elevated areas exert a sufficient warming effect that the forests here have a distinct subtropical affinity. Here, temperate cloud-forest communities may occur on the crests of the taller hills, but low hills and intermontane basins support seasonally dry, subtropical forest. Local residents exploit this climatic feature, practicing the world’s highest-elevation rice propagation at nearly 3,000 m and growing citrus at nearly 2,500 m in the northwestern intermontane valleys.
Yunnan Plateau vegetation is floristically similar to the lower temperate forests of the Eastern Himalaya, although species richness here tends to be greater than in the eastern Himalaya. Dominant taxa belong to the oak family, Fagaceae (Castanopsis spp., Lithocarpus spp., and Cyclobalanopsis spp.), and the laurel family, Lauraceae (Lindera spp., Persea spp.). Leaves may be sclerophyllous or coated on the undersurface with appressed hairs to conserve water, and they remain evergreen throughout an extended dry season.
The plateau historically supported many mammal species that have been extirpated as a result of millennia of human activity. Examples include tigers (Panthera tigris) and other large cats as well as their prey base of deer and other mammals.
Although most of the plateau consists of low mountain ranges and basins below 2,300 m, the western third includes outlier ranges of the Hengduan mountain system that exceed 2,500 m. These ridges have a cooler, more humid climate with persistent cloud cover during the summer growing season, and cool winters with some snow accumulation at the higher elevations. These ridgetop habitats support a temperate cloud forest community distinct from the subtropical forests of the Yunnan Plateau. Trees are cloaked with mosses and laden with epiphytic ferns, orchids, and other flowering plants. The understory consists of small-statured bamboo (Sinarundinaria spp.), ferns (Dryopteris spp.), and shade-tolerant plants like Arisaema spp., Impatiens spp. and the curious root parasite, Balanophora spp..
Mammal species that occupy these ridge crest mountain refuges include black gibbons (Hylobates concolor), bamboo rats, Asiatic black bears (Selenarctos thibeticus), and some large cats. Severely threatened populations of black gibbons occur in the Ailao Mountain Nature Reserve (504 km2). This species could act as an effective "umbrella species" under which to protect the habitat of this elongate, ridgetop protected area.
The Yunnan Mountains are an Endemic Bird Area (Stattersfield et. al. 1998) included within the Yunnan Plateau Ecoregion. Three restricted-range bird species occur here, and their habitats tend to be fragmented because the mountain habitat is generally restricted to ridgetop locations. Yunnan nuthatches (Sitta yunnanensis) breed in pine forests above 2,440 m and are widely distributed within the endemic bird area, from western Guizho to southeastern Tibet. Brown-winged parrotbills (Paradoxornis brunneus) have a large elevation range and occur here in a variety of habitats, extending west as far as northeast Myanmar. White-speckled laughingthrushes (Garrulax bieti) occur in bamboo at elevations above 3,000 m in the Yunnan-Sichuan border regions. Other threatened bird species with a broad habitat range include giant nuthatches (Sitta magna) and white-eared pheasant (Crossoptilon crossoptilon). Birds include several babbler species (Timiliidae) and green peafowl (Pavo muticus). Pheasant also occur in the area, although populations have been greatly reduced by hunting.
Today, the evergreen broadleaf forests have been cleared throughout the Yunnan Plateau. In many places, especially in the western half of the plateau, they have been replaced by thin forests of Yunnan pine (Pinus yunnanensis) which may co-occur with Michelia yunnanensis (Magnoliaceae) and various species of Rhododendron spp. Today, these forests of P. yunnanensis is the most widespread vegetation-type on the plateau. Because Yunnan pine is dominant over such large areas, it is not clear that this plant community is the result of human disturbance. Indeed, photographs taken here during the 1920s show a condition similar to today. Relict stands of subtropical evergreen broadleaf forest still occur in the West Hills near Kunming, on the slopes of Jizu Mountain near Dali, and in small temple forests throughout the region.
Types and Severity of Threats
Intermontane basins of the Yunnan Plateau were converted to agriculture centuries (possibly millennia) ago. The mountains have been deforested in recent centuries, and today this process continues on the more remote mountain ridges. Nature reserves tend to be places at the crests of the mountains because elevations above 2,400 m are foggy during the growing season and less attractive for settlement. Populations of animals restricted to the mountaintop locations are probably not in their optimum habitat. Future conservation efforts should endeavor to protect pockets of existing habitat on the lower mountain slopes, and undertake restoration activities wherever possible.
Justification of Ecoregion Delineation
The Yunnan pine community is the dominant vegetation cover that now replaces much of the original evergreen broadleaf forests. Dominant classes according to the CVMCC (1979) Vegetation Map of China include: Yunnan pine (9); warm-temperate evergreen or deciduous shrubs (32b and 33 ); evergreen and deciduous mixed forests (19 and 21b ); cultivated areas or plantation (73c, 72b, and 74a). According to Mackinnon et al. (1996), the ecoregion is comparable to the Yunnan Plateau biogeographic subunit (39a) in the Sichuan-Yunnan Highlands.
Chinese Vegetation Map Compilation Committee. 1979. Vegetation map of China. Map (1:10,000,000). Science Press, Beijing, China.
Laidler L. and K. Laidler. 1996. China’s Threatened Wildlife. Blandford, London.
MacKinnon, J. 1996. Wild China. The MIT Press, Cambridge MA.
Mackinnon, J., M. Sha, C. Cheung, G. Carey, Z. Xiang, and D. Melville. 1996. A biodiversity review of China. World Wide Fund for Nature, Hong Kong.
Stattersfield, A. J., M.J. Crosby, A.J. Long and Devid C. Wege 1998 Endemic Bird Areas of the World: Priorities for Biodiverstiy Conservation. Birldlife International, Cambridge, UK
Young, Stephan S., Chris Carpenter, and Wang Zhijun. 1992. A study of the structure and composition of an old growth and secondary broad-leaved forest in the Ailao Mountains of Yunnan, China. Mountain Research and Development. 12(3): 269-284.
Prepared by: Chris Carpenter
Reviewed by: In process