Degraded Land in South China Damaging agricultural and forestry practices over the past years have degraded half of the land in China bounded by the Tropic of Cancer and the South China Sea, dramatically reducing its biological productivity and environmental services. The damage to South China's hilly tropical/subtropical(亚热带的) forests probably began in the Sung Dynasty as population expanded to the south. The settlers most likely used cutting and bum techniques to clear land for fanning. Forest cover provided building materials and firewood to the settlers, but also served as a habitat(栖息地) for tigers, leopards, wild pigs, and deer. Population growth put severe pressure on the land. Eventually, erosion (腐蚀) washed away much of the top soil and much of the degraded land was abandoned. Today, only about one square kilometer of the original tropical evergreen broad-leaved forest remains. Impacts The most direct impact of this disastrous loss of productive soil is reduced agricultural production. Farmers trying to produce crops under these conditions face great difficulties even when using commercial fertilizers(化肥). The soil is poor at holding the plant nutrients(营养) contained in commercial fertilizers. The problem is deteriorated by the fact that fertilizer runs off the land quickly during heavy rains. China cannot afford to abandon potentially productive land, let alone lose more to erosion. It is already losing enormous amounts of land to new towns and other non-agricultural uses. Per capita amble land (人均可耕种土地) in China now is about 0.1 hectare, only 47 percent of the world average. But China estimates that it lost about 50,000 square kilometers of its amble land from 1986 to 1995, the equivalent of the total agricultural land area of Japan. Food production is not the only problem. The loss of much of China' s forest cover today has made wood products China' s leading import costing the country $ 2 to $ 5 billion annually. Economic losses also result from waterlogged (水涝的) fields and coastal sand storms that result directly from the soil erosion. Some 710 million tons of sediment(沉淀物)were carried through the Three Gorges(三峡) part of the Yangtze in 1998--a 33 percent increase over the loads measured before 1990. In parts of Guangdong Province, sediment-choked streams and rivers have caused the water table to rise and have waterlogged nearby farm fields. Since the waterlogged land is useless for conventional fanning, the Chinese introduced the 'dam-pond system'. Ponds are dug and used for raising fish and vegetables are grown on the tops of the dam surrounding the ponds. This system may have to be ridiculously stopped if erosion is controlled. Once controlled, the streams and rivers will cut deeper channels, thus lowering the water level in the fishponds. In the few areas where reforestation (重新造林) of eroded hills has reduced erosion and sediments in the rivers, the river has begun to cut through years of deposited sediment to return to its original bed. This lower water table(地下水位) reduces water logging of nearby land and the fishponds begin to dry up. In some places, pumps had to be installed to draw water from the streams and rivers to keep the water level in the fishponds stable. Sandstorms are another effect of the heavy sediment load created when granite (花岗岩)weathers (风化)in the wet tropics. Coarse quartz(粗的石英) sands move from the weathered hills to lower elevations during heavy rains, filling valleys and covering agricultural fields. Fine-grained quartz sands, however, are carried further downstream and deposited near river mouths. Strong sea winds and typhoons regularly blow the sand inland, burying agricultural fields and human settlements. To reduce the movement of sand by wind action, the Chinese built windbreaks(防风林)along much of the sandcovered coastal belt. Salt-tolerant (耐盐的)trees, satisf