Location and General Description
Consisting of low hills and alluvial plains, the Sichuan Basin is located where several rivers converge to flow into the middle reaches of the Changjiang (Yangtze) River. Although it is ringed by rugged mountains, the Sichuan Basin itself is an extremely fertile and productive agricultural area. The climate is subtropical with cool, cloudy winters and very warm, hazy summers.
The basin includes extensive areas of reddish sandstone and purple shales which give rise to fertile agricultural soils. Plains are broken by low ranges of limestone hills that reach 700 m elevation, and the western part of the Sichuan Basin holds alluvium deposited by the many rivers than drain into the basin from mountain ranges that form the eastern rim of the Tibetan Plateau. When the rivers reach the basin, they slow and deposit their sediment loads onto the plain.
Temperatures in the Sichuan Basin are mild with very warm summers (26 to 29°C) and cool winters (5 to 8°C). Mountains to the north of the basin moderate winter cold by blocking the southward movement of cold air from Central Asia. Because the climate is humid and surrounding mountains impede air circulation, the Sichuan Basin is prone to temperature inversions. Summers are often overcast or very hazy and fog is present many days during the winter. A local saying holds that in Sichuan, dogs bark when the sun appears. In fact, Chengdu in the western part of the Sichuan Basin averages more than 300 days a year of foggy weather.
Because of its long history of intensive anthropogenic use, the lowlands of the Sichuan Basin today support very little original habitat. Some temple forests and holy mountains like Leshan or Dazushan support remnant patches of climax forest vegetation, and braided, depositional floodplains include some semiwild wetland areas that support a few migratory bird species. Black kites Milvus migrans forage along the river courses and bats are active at dusk. Apart from these small, semiwild enclaves, the most extensive natural forest occurs on the lower slopes of the mountains that fringe the Sichuan Basin, including Emei Mountain which, because it is extremely sacred, has been spared deforestation over the centuries.
Original vegetation in the Sichuan Basin probably consisted of a mixture of subtropical oak (Quercus, Castanopsis), laurels (Lauraceae), and Schima (Theaceae). Diverse laurel species are native to the basin; Machilus, Lindera, Litsea, and Cinnamomum are some of the more important genera. It is rather difficult to know the exact character of these forests because they have been extirpated for many years. It is thought that, like the agriculture, climax forests here tend to resemble formations known in other parts of China from more southerly latitudes.
Uncultivated areas in the Sichuan Basin include deforested slopes that now support a scrub of Rhododendron, Vaccinium bracteatum and Myrica nana. Places disturbed less recently may support thin stands of the pine Pinus massoniana or Japanese cypress Cryptomeria japonica, many of which have been planted. Limestone areas support scrub and forest vegetation that is florally distinctive, consisting of dyetrees Platycarya sp., "Sichuan pepper" Zanthoxylum planispinum, and rose Rosa spp.
Rare plants that probably once occurred in the Sichuan Basin and still occur at subtropical elevations in the surrounding foothills include the tree fern Cyathea spinulosa, dove tree Davidia involucrata and the conifer Cathaya argyrophylla. D. involucrata is renown for the large bracts, up to 30 cm long, that subtend the base of the inflorescence, giving the tree in flower a very striking appearance. Taxus chinensis is another rare conifer that occurs at low elevations in the hills adjoining the Sichuan Basin.
The sacred mountain of Emei Shan is perhaps one of the best places to see the remnants of the lush Sichuan Basin subtropical forests and associated animal species. In the words of Roy Lancaster, "If there is one mountain in China calculated to set a plantsman’s pulse racing, it is Emei Shan." Emei Shan also supports a number of plants and animals that are endemic or of restricted range. These include plants such as Nothophoebe omeiensis and Rosa omeiensis, and Tibetan stump-tailed macaque Macaca tibetana, a species of monkey with shorter-tail and longer hair than its more widely distributed relative, the rhesus macaque.
The Emei Shan Liocichla Liocichla omeiensis is a restricted range bird that occurs in Emei Shan and nearby forested hills.
The Sichuan Basin has a history of human settlement that extends more than 5,000 years. Massive irrigation projects like Dujiang were in place more than 2,000 years ago, and today the Sichuan Basin supports more than 100 million people on 260,000 km2 of land, including two municipalities, Chengdu and Chongqing that each contain more than 8 million residents. The population density here, more than 380 persons per square kilometer, makes the Sichuan Basin one of the most densely populated agricultural areas in the world.
Types and Severity of Threats
This is a place that has been severely modified by millennia of agriculture, centuries of commerce, and decades of heavy industry. Today urban landscapes are encroaching on agricultural land, and natural landscapes are restricted to steep hillsides and a few temple forests. Sichuan would be a good place to try to raise people’s awareness of the importance of conserving nature because, as the region continues to develop, increasing numbers of people will begin to visit the outlying areas for recreation and escape from the city. Farmers here have a visceral understanding of biological diversity because they raise an astonishing number of different kinds of plants and animals. A more enlightened attitude toward nature on the part of urban residents would also enable more species to live in and near the cities and towns of the Sichuan Basin.
Improving the air and water quality in the Sichuan Basin would also help make this inhabited landscape more congenial to all species, including humans.
Justification of Ecoregion Delineation
The ecoregion boundary was formed according to the CVMCC (1979) Vegetation Map of China warm-temperate conifers (8b and 11). Agricultural conversion has eradicated most of the original vegetation, so CMMCC’s agriculture class (73a,c) was included also. The region is comparable to the Sichuan basin biogeographic subunit (01b) in the Chinese subtropical forests according to Mackinnon et al. (1996).
Chinese Vegetation Map Compilation Committee. 1979. Vegetation map of China. Map (1:10,000,000). Science Press, Beijing, China.
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MacKinnon, John and Karen Phillipps. 2000. A Field Guide to the Birds of China. Oxford University Press, New York.
Field Guide to the Birds of China
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Lancaster, Roy. 1989. Travels in China, a Plantsman’s Paradise. Antique Collector’s Club, Woodbridge, Suffolk.
Zhao Ji editor. Zheng Guangmei, Wang Huadong, Xu Jialin. 1990. The Natural History of China. McGraw Hill Publishing Company, New York.