South China Degraded Land Issues

A view of South China degraded-land issues

Widespread land degradation in South China started about 1,000 years ago as a major, north-south population migration began (Anderson, 1988; Grant, 1960). Although the hilly land’s forest cover was a source of building materials and firewood, it also was the habitat for tigers, leopards, wild pigs and deer. To protect themselves and their crops, the new migrants probably chose to burn large parts of the forest, thus, setting in motion the beginning of a severe soil erosion process (Grant, 1960). The region’s human population has continued to increase since that time. The land was subjected to damaging agricultural practices that led to increased soil erosion and landslides, sandstorms, water-logged agricultural lands, silt-choked rivers, and extinction of plant and animal species. Political strife and warfare, and extensive uncontrolled urbanization and industrialization contributed further to the land degradation. Today, large parts of South China’s hilly lands have been abandoned because they produce so little in their damaged condition. Consequently, the remaining fertile land here still is undergoing further pressure to satisfy the food, fuel, and fiber needs of the high, dense population.

Land degradation stemming from loss of a vegetative cover is common in many parts of the world’s tropical/subtropical regions. However, the prevalence of 30 to 60 meters of deeply weathered granitic bedrock (Luk and Yao, 1990) accentuates many of the problems in South China. The weathered granitic rocks have a narrow chemical composition consisting largely of silicon, aluminum, iron, and water (Table 1). Such deeply weathered areas make up 30 to 40 percent of Guangdong and Fujian Provinces, 10 to 20 percent of Hunan, Guangxi, and Jiangxi Provinces (Gong, 1986) and many near-shore islands as well. These fine-grained minerals are poor at holding plant nutrients. Thus, when commercial fertilizers are applied, the fertilizer runs off the land quickly contaminating surface water and ground water.

Controlling the processes that degrade South China’s tropical/subtropical lands, though difficult, has been accomplished on some small areas by Chinese researchers and farmers. These promising experiences offer hope that the extensive degraded lands here may be returned to an economic and ecological productive state.

Land Clearing’s Adverse Effects

Slash-and-burn agriculture and similar practices removed much of the primary monsoon evergreen broad-leaved forest in South China (Luo and He, 1986) (Figure 1). Today, only about 1 km² of this forest remains at the Dinghushan Biosphere Reserve, 86 km west of Guangzhou, Guangdong Province (Figure 2). Successive clearing and scarcity of seed-dispersing wildlife makes the process of natural revegetation of such forests difficult. Removal of vegetation promotes a variety of environmental changes that lead to degraded landscapes. These include increased soil temperatures, reduced soil organic matter, soil compaction, and soil erosion.

Soil-surface temperatures increase significantly following loss of vegetative cover. For example, maximum soil-surface temperatures in tropical China are reported to range from 50^o C to 80^o C under direct sunlight. Although such temperatures decrease rapidly with depth, these temperatures are sufficient to damage or kill seed sprouts. This is especially true for seeds that are accustomed to the cool, moist conditions of the forest floor that existed prior to forest clearing. In addition, soil-surface temperatures under slash-and-burn agriculture fires range from 200^o C to 300^o C (Phillips, 1965; Batchelder, 1967) further degrading the soil surface.

Soil organic matter decreases quickly as soil temperature and soil biotic decomposition rates increase. Further, removal of vegetation and litter inhibits buildup of new organic matter. Soil organic matter plays a large role in holding nutrients in a form available to plants and even small decreases in soil organic matter are significant (Montagnini and Sancho, 1990). For example, a decrease of soil organic matter from two percent to one percent is equivalent to a 1,125 kg loss of fixed nitrogen/ha (White and Collins, 1976).

Where the annual rainfall reaches 125 cm, two common weathering products of China are the clay minerals kaolinite and halloysite (Chang, 1963; Felix-Henningsen, Liu, and Zakosek, 1989; Gong, 1986; Li, Wang, Han, and Zhang, 1983; Lu, 1989; Parham, 1969a and 1969b; and Xu, Jiang, Yu, and Yang, 1986). Hydrated halloysite (4H₂O) can hold plant nutrients in an easily exchangeable form, however, exposure to prolonged, direct sunlight or the thermal effects of slash-and-burn agriculture over time can dehydrate the soil mineral irreversibly to halloysite 2H₂O. The dehydration reduces the cation exchange from 40 – 50 meq/100 gm to 5 -10 meq.100 gm (Grim, 1968). This ion exchange reduction coupled with the decrease in soil organic matter further damages the soil.

Erosion of soil clay minerals and destruction of soil organic matter results in up to a 27 percent decrease in important soil microelements including copper, zinc, manganese, cobalt, molybdenum, and boron, many of which are vital to the formation of vitamins, enzymes, and hormones that may be required by certain animals and plants. Quality and crop yields, and animal reproduction can decrease from microelement losses (Openlender, 1979).

With the loss of vegetative cover and soil organic matter, intense tropical/subtropical rains compact the soil surface. Clay-size particles plug small soil pores, inhibit water infiltration, and increase runoff and erosion. Water infiltration in such sealed or crusted soil in general is six to eight times less than under forest cover (Lal, 1986). Under these conditions, water runs off the land quickly causing pronounced soil drying on uplands. Sparse vegetation, characteristic of drier environments, commonly replaces original vegetation and can lead to a mistaken impression that a regional climate change has occurred (West, 1986).

The area of severe soil erosion in the nine provinces of South China increased from 60,000 km² in the 1950s to 170,000 km² in the 1980s largely as a result of damage or destruction of the land’s vegetative cover. Some 50 percent (about 100,000 km²) of China bounded by the Tropic of Cancer and the South China Sea is in a degraded condition. The land either supports a cover of mostly shrub or savanna vegetation (Figure 3) or is barren (Hou, 1979). Hainan Island’s forests covered 39.8 percent of the land in the early 1950s, 25.7 percent in 1956, and only 8.9 percent in the 1980s. However, damage has not been restricted to terrestrial vegetation. Hainan Island’s mangrove forest area, for example, has been reduced in size from 10,000 ha to 1,300 – 2,000 ha in just the past several decades.

Soil erosion rates from devegetated surfaces can be high. For example, a barren, hilly, deeply weathered granitic area of Deqing County, Guangdong Province now is seven to 13 m lower than 1,000 years ago, a result of sheet and rill erosion alone (Luk and Yao, 1990). Similarly, Hong Kong’s topography is estimated to be about 27 meters lower than it was 1,000 years ago from such erosion (Lam, 1977).

Incision of gullies to depths of 10 – 80 m can develop quickly in the weathered granite during exceptionally heavy rain storms that can carry away enormous volumes of sediment. Silts and sands, lying on barren Hong Kong hills for example, are eroded rapidly by typhoon associated torrential rains. Boulders of fresh granite (core stones), as large as automobiles, sometime slide and tumble down hillsides when the surrounding, soft, weathered material becomes saturated. In addition, eroded sediments damage aquatic productivity and bury what were once freshwater and near-shore marine aquatic breeding grounds.

Because runoff occurs so quickly where vegetation is absent, the water-carried sediment derived from weathered rock accumulates in the lowlands clogging streams and rivers causing flooding. Such sediments fill valley bottoms and marine and brackish-water inlets as well (Grant, 1960). Where sediments have blanketed valley floors and runoff is slow, drainage water sometimes carry enough iron in solution to make the waters toxic to plants and animals (Luk and Yao, 1990).

Waterlogging adversely affects the soils of many parts of South China and has damaged 130,000 ha of lowlands in Guangdong Province alone. The silting rate of the riverbeds of Guangdong Province generally is about 10 cm/yr (IDRC Final Report, 1990). Consequently, the water table rises and flooding and water logging of adjacent agricultural fields results. To offset the waterlogging, farmers have developed dike-pond systems comprised of fish ponds surrounded by constructed dikes on which they grow a variety of vegetable, tree, flower, and fiber crops. The dike-pond system has existed for about 600 years in the Zhujiang Delta and today covers 59,000 ha.

When granite undergoes chemical weathering, the quartz component remains largely unchanged and accumulates as a lag deposit on the land surface. Coarse quartz sands are swept from the weathered hills to lower elevations during heavy rains, filling valleys and covering agricultural fields. Fine-grained quartz sands on the other hand, are carried down stream and deposited near river mouths. Strong sea winds and typhoons periodically move the sand inland burying agricultural fields and human settlements. For example, wind-blown sand became a severe problem on Pingtan Island, a granite island lying off the southeast coast of Fujian Province (Zhou, 1990). The island had a covering of tropical, broad-leaf monsoon forest about 1,000 years ago (Sung Dynasty). Widespread forest and vegetation clearing led to soil erosion and the accumulation of quartz sands. During the fall of each year, strong winds caused sand storms so severe that sand covered the roads and disrupted communications. In some cases, the people had to abandon their villages. Similar conditions existed along much of South China’s mainland coast and on some near-shore islands.

To reduce the potential for sand movement inland, windbreaks have been established now along much of the sand-covered coastal belt. At first, salt-tolerant Casuarina trees were commonly planted for coastal windbreaks and for a fuelwood source for local farmers; today, the use of mixed tree species windbreaks is increasing. Tree cutting normally is not allowed but the local people are permitted to rake up litter beneath the trees for fuel. By doing so, few nutrients are returned to the soil, and when replanting is necessary, the newly planted trees have difficulty surviving.

The consequences of forest clearing on these South China hills obviously lead to a whole host of interrelated problems (e.g. soil erosion, water-logged fields, and coastal sand storms). Clearly, these problems are expressions of a land-use system gone wrong. Setting priorities to improve these degraded lands is easier where each problem is viewed as a part of the entire system rather than being viewed separately.

Sharing research information from all parts of the degraded system can reinforce a successful systems approach. For example, reforesting eroded hills will lead to reduced sediment loads in associated drainage ways. Reduced sediment loads in runoff will foster down cutting of silted river beds which in turn will reduce the amount of adjacent water-logged land thus lowering of the water level in the dike-pond systems. As the rivers continue scouring their channels, the wind-blown sand problem should diminish along the coast. However, with reduced amounts of sand being deposited at the river mouths and along the shore, unwanted coastal erosion ultimately may arise.

Controlling the processes that degrade South China’s tropical/subtropical lands, though difficult, has been accomplished on some small areas by Chinese researchers and innovative farmers. A variety of these techniques were collected from researchers across South China in a workshop held in Hong Kong in 1991. Support for the fieldwork and workshop was provided to the Bishop Museum in Honolulu by the Rockefeller Brothers Fund. The findings were subsequently published in 1993 by the Bishop Museum as Improving Degraded Lands: Promising Experiences from South China. These promising experiences offer hope that the extensive tropical/subtropical degraded lands here may be returned to an economic and ecological productive state with the extension of such promising techniques. Further work is underway to identify similar additional techniques. In addition, what is learned from the China experience may be useful to other countries where similar degraded land problems exist.

Walter E. Parham, PhD, August 2009

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