Slash-and-burn is an agricultural system used in tropical countries, in which a forest is cut, the debris is burned, and the land is then used to grow crops. Slash-and-burn conversions are relatively stable and long-term in nature. However, they are the leading cause of tropical deforestation.
Usually, some type of slash-and-burn system is used when extensive areas of tropical forest are converted into large scale, industrial agriculture, usually intended to supply commodities for an export market, rather than for local use. The slash-and-burn system is also widely used by individual, poor farmers when they develop agricultural land for subsistence farming and to supply cash goods to a local market. The poor farmers operate on a smaller scale, but there are many such people, so that huge areas are ultimately affected.
Slash-and-burn agriculture often follows soon after the natural tropical forest has been commercially logged, mostly because the network of logging roads that is constructed allows access to the otherwise almost impenetrable forest interior. Slash-and-burn agriculture may also be facilitated by government agencies, through the construction of roads that are specifically intended to help poor, landless people convert the forest into agricultural land. In other cases, slash-and-burn occurs in the absence of logging and planned roads, as a rapidly creeping deforestation that advances as poor people migrate to the forest frontier in search of land on which to grow food.
The slash-and-burn method differs from a much more ancient system known as shifting cultivation. Shifting cultivation has long been used by humans for subsistence agriculture in tropical forests worldwide. Variants of this system are known as swidden in Africa, as caingin in the Philippines, as milpa in Central America, and by other local names elsewhere. The major difference between the slash-and-burn system and shifting cultivation is in the length of time for which the land is used for agriculture. In the slash-and-burn system, the conversion is long-term, often permanent. Shifting cultivation is a more ephemeral use of the land for cultivation.
Shifting cultivation begins when a small area of tropical forest, typically less than one to several acres, is cleared of trees and shrubs by an individual farmer. The biomass is burned, and the site is then used to grow a mixture of agricultural crops for a few years. After this time, vigorous developments of weeds and declining fertility due to nutrient losses require that the land be abandoned for afallow period of 15 to 30 years or more. Meanwhile new tracts of forest are successively cleared and cultivated for several years. Clearly, the shifting cultivation system is only sustainable if the population density is small, and if the major goal of agriculture is subsistence, rather than market farming.
Because the slash-and-burn system is a longer-term, often permanent conversion of the tropical forest into agriculture, without an extended fallow period, its associated environmental problems tend to be more severe than those that are normally caused by the smaller scale, shifting cultivation systems. However, severe environmental problems can also be caused if too many people practice shifting cultivation in a small area of forest.
In spite of the fact that many mature tropical forests sustain an enormous biomass of many species of trees, the soil of many forested sites is actually quite infertile. The intrinsically poor fertility of many tropical soils is due to: (1) their great age, (2) the often large rates of precipitation, which encourage nutrient losses through leaching, and (3) the moist, warm climate, which encourages microbial decomposition and causes tropical forest soils to contain relatively little organic matter, so there is little ability to retain organic forms of nutrients in soil. The natural tropical-forest ecosystem and its species are well adapted to this soil infertility, being efficient at absorbing nutrients occurring in small concentrations in soil, and at recycling nutrients from dead biomass. As a result, much of the total nutrient capital of tropical forests is typically present in the living vegetation,
particularly in trees. When these trees are felled and burned, there is a pulse of increased nutrient availability associated with ash. However, this is a short-term phenomenon and much of the nutrients are rapidly leached or washed away under the influence of the wet climate. The overall effect of slash-and-burn forest conversions, and to a lesser degree shifting cultivation, is a rapid decline in fertility of the land.
In addition, some tropical soils are subject to a degrading process known as laterization, in which mineral silicates are dissolved by rainwater and carried downward, leaving behind insoluble oxides of iron and aluminum. Lateritic soils are very infertile, and in extreme cases can become rock like in consistency. Once this stage of degradation is reached, it can be impossible to cultivate the land because it is too hard to plow, and plant roots cannot penetrate into the substrate. The rate of laterization is greatly increased by clearing the tropical forest, and in cases of extreme damage by this process, the productive capability of the land can remain degraded for centuries.
Tropical deforestation also carries other important environmental risks. Tropical forests store huge quantities of carbon in their living biomass, especially in trees. When tropical forests are converted into agriculture, much less carbon is stored on the land, and the difference is made up by a large emission of carbon dioxide to the atmosphere. During the past several decades, tropical deforestation and the use of fossil fuels have been the major causes of the increasing atmospheric concentrations of carbon dioxide, which may have important implications for global climatic warming. In addition, old-growth tropical forests are the most highly developed and biodiverse ecosystems on the Earth. Tropical deforestation, mostly caused by slash-and-burn agriculture, is the major cause of the great wave of extinction that is presently afflicting Earth’s biodiversity.
Late in the twentieth century and continuing into the twenty-first century, slash-and-burn agriculture was used on an expanding basis in South America, especially in Columbia, in order to grow illegal drugs such as marijuana and opium poppy. It is estimated that over one hundred thousand acres (about 400 square kilometers) are lost annually to slash-and-burn techniques in Columbia alone. Some governments, aided by ecological groups, are attempting to introduce legal but profitable crops to grow instead of these illegal ones.
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