Proximate Causes of Deforestation 126.96.36.199
Myers (1984) cited "shifting cultivation" as the most important cause of deforestation. There are various types of shifting cultivation. A destructive type of shifting cultivation is practiced by non-indigenous colonizers who often know little about farming, other than chopping down the forest, burning it, and planting corn or rice in the ashes. After 2 or 3 years, production gives out, and they are forced to move further into the wilderness. They may just abandon the land, or they may sell their land to a consolidator, perhaps a rancher, who is buying up land in the region, either for pasture or for speculation. In Brazil, some of these shifting cultivators are from the drought-ridden north-east and have migrated into the Amazon rain forest and cleared small patches for agriculture. Although they may follow roads built by the government or by loggers, they act on their own. Other colonizers may be participants in a government resettlement program, in which people from other regions of the country, including cities, are transported to wilderness areas and given some land and subsidized housing. Myers (1992) referred to this type of shifting cultivation as "shifted cultivation", i.e. practiced by people who would not be farmers, given the choice, in contrast to "shifting cultivation", practiced in a more "traditional" fashion. In the 1980s and 1990s, "shifted" cultivation apparently accounted for 35, 70, and 50% of deforestation of closed forests in America, Africa, and Asia, respectively.
Cultivation of illegal narcotic plants such as coca (the source of cocaine) and opium (the source of heroin) in rain forest clearings in Southeast Asia and South America is also a destructive type of shifting cultivation. When the plots are discovered by drug enforcement agents, the farmers move elsewhere.
Another type of shifting cultivation has been practiced by indigenous peoples for subsistence and by people whose ancestors moved into the forest and learned traditional techniques. A small area, approximately 1 ha, is cleared and then burned. Both annual crops such as manioc and perennial crops such as fruit and nut trees are planted. The annual crops produce well for 1 or 2 years, but declining nutrient availability and weed pressure rapidly diminish the production by annuals. However, by the third year, perennial crops such as plantain and cashew have become established and begin to yield. Successional species that are valuable for wood, medicines or other uses are favored, and other species may be weeded out.
In some areas, people who have lived in the forest for generations follow this same system. This system can be sustainable, as long as population density is low, because the land can be left fallow long enough to recover soil fertility. Organic debris from the surrounding forest quickly covers the soil, either directly through leaf and litter fall or through dispersion by animals. It is the organic matter that keeps the nutrients available in most tropical soils. When the plot is left fallow, the soil organic matter gradually builds up again, and the nutrients, especially phosphorus, become available (Jordan 1995 a). Some tribes, like the Kayapo in Brazil, plant perennial crops and fruit trees in the fallow, managing the plot as a long-term rotation (Posey 1982).
The length of time required for the site to regenerate sufficient nutrients to permit further cultivation depends on the soil quality and the intensity of cultivation, and can vary from a few years to almost a century (see also Chaps. 2 and 5). Younger, volcanically derived soils, for example, regenerate more quickly than highly weathered Oxisols of the lowland tropics. As forest areas become more populated, the fallow period becomes shortened. Because of the short fallow, nutrient stocks in the soil do not fully recover, and the period of cultivation must be shortened (Nye and Greenland 1960).
Permanent cultivation has become a more important cause of deforestation than shifting cultivation (Geist and Lambin 2002). In South and Central America, large areas of forest have been converted to pasture. Mega-farms that produce soybeans are encroaching on the southern fringes of the Amazon rain for est (Nepstad et al. 2002). Tree-crop plantations for rubber, oil palm, cocoa, coffee, and coconut have been an important cause of deforestation in Africa and Southeast Asia (Grainger 1993). In Central America, plantation crops such as coffee, cacao, palm oil, bananas, pineapple, and others have also been a major force driving deforestation. Timber is another key plantation crop. In the Jari project in the state of Para in Brazil in the eastern Amazon, tens of thousands of hectares of primary forest were cleared in order to plant the fast-growing species Gmelina arborea for pulp wood (Jordan 1995b). Commodity crops such as those mentioned above are subject to global economic cycles of boom and bust. When demand peaks, deforestation occurs as land is cleared for new plantings. When oversupply occurs, cleared land is often abandoned.
Tropical agriculture is not necessarily ecologically or economically unsustainable. The colony at Tomé Açu in the state of Para, Brazil, is an interesting example of colonizers who learned to farm sustainably on the poor soils of the Amazon. Japanese immigrants in the 1920s settled the area and experimentally devised a rotational scheme that included mixtures of perennials and annuals, and various animal stocks to supply manure as well as meat and milk (Subler and Uhl 1990). Because of the diversity of their agricultural practices and products, their soils retain fertility and their economic income is less influenced by global cycles that affect individual commodities.
Conversion of tropical forest to pasture has been a particularly important activity in Brazil. In the late 1970s, the Brazilian government instituted a program to encourage ranching in the Amazon region. Large cattle enterprises were promoted as the prototype for development. There was a perception that ranching actually improved the quality of the soil by increasing soil nutrients (Barbosa 2000). However, the soil tests that led to this conclusion presumably were taken immediately after the cutting and burning of the forest, before the ashes containing the nutrients had leached away (see Figs. 2.10-2.12 for nutrient dynamics following burning of forest, and see Fig. 2.13 for changes in the availability of soil nutrients following conversion of forest to pasture in the Amazon region of Brazil). Roughly 10 million ha of land was converted from forest to pasture by the early 1980s (Hecht 1985). Since then, government incentives to clear forest have been reduced, but conversion of forest to pasture still remains important in the Amazon region (Castellanet and Jordan 2002).
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