By 1993, the land area converted to agriculture amounted to 4,810,000,000 hectares (48,100,000 km2), or 36% of global terrestrial area, although recently the rate of land conversion has not kept pace with population growth due to technological improvements in agriculture (Goklany, 1998). (While these technological changes have reduced the rate of land conversion, they have caused significant environmental damage, and have permitted increased rates of human population growth, which in itself has necessitated increased land conversion.) Approximately 3.1 million hectares of tropical rainforest per year are cut to provide agricultural land (Achard, et al., 2002). Of the agricultural land in tropical countries, 38% in Africa, 38% in South America, and 67% in Asia has been converted from areas of tropical rain forest. The area of land converted from forest to agricultural purposes in the past 140 years is more than twice that converted between the origin of agriculture, about 10,000 years ago, until the mid nineteenth century. Almost 80% of tropical deforestation is the result of conversion of forest land for agricultural purposes, but surprisingly only about 5% of this loss between 1980 and 1990 was due to the establishment of large plantations and farms. Most was the result of population growth and the consequent search for small plots of arable land to feed the additional people. Tilman, et al. (2001b) estimate that by 2050, 18% more land will have been converted from natural ecosystems for agricultural purposes, or a net loss of 10 billion hectares, an area larger than the United States, and half of all potentially arable land remaining. As they stated, “Land use and habitat conversion are, in essence, a zero-sum game: land converted to agriculture to meet global food demand comes from forests, grasslands and other natural habitats.”
a. Conversion for large plantations and ranches: In the second half of the 20th century, the business of agriculture has shifted from small, independent farms to huge commercial operations, which require large tracts of land. This shift has been driven by the rise of the global agricultural economy and the consolidation of land into a few hands, and also by the growth in affluence of a number of countries. This has substantially increased the international demand for meat, coffee and other luxury goods, many of which are produced in tropical countries and exported. If everyone in the world were to consume meat and animal products at the level of North Americans (approximately 25% of calories), only about 2.8 billion people could be supported by our ecosystems. Presently, only about 20% of deforested land is used to increase local food production, and even much of that food is exported (beef, coffee, etc.). The impact of ranching and large-scale agriculture on forest systems are also exaggerated by government policies which encourage the clearance of land (see Section C7).
b. Swidden agriculture: Shifting cultivation, or swidden farming, is thought to be a major – if not the major – cause of deforestation in Southeast Asia. More than two million km2 of closed forest land may be included within these cultivation systems (Amelung, Torsten and Diehl, 1992). This is especially true in Africa. Up to 80% of deforestation in Southeast Asia has been attributed to this type of agriculture (Palm, et al., 1986). This is true for some types of traditional farming, and so-called “slash-and-burn” agricultural method produces huge amounts of greenhouse gases annually – as much as 1.6 metric tons (Kaiser, 1997). However, if properly practiced, it is the only type of agriculture which can be sustained in tropical forests. With this system, it is possible to farm in a sustainable manner, taking into account the ecology of the forest. In shifting cultivation (and there are many variants), vegetation is cut in small patches of forest (primary or secondary); generally, some trees are allowed to stand. This practice forms openings which allow the sunlight to penetrate, and the burning of the remaining vegetation enhances the fertility of the soil by providing organic material. The open area is planted with a variety of crop species. In Amazonia, for instance, the major crops are numerous varieties of cassava (as many as 48, according to Dufour, 1990), plantains, bananas and other fruits, medicinal plants, and sometimes poisonous plants. Valuable species of trees, such as timber and fruit trees, may also be planted in the swiddens. The crops are staggered, so that they provide harvests for several years. Trees are harvested after maturity and until forest regrowth reclaims the plot. Trees are harvested after maturity and until forest regrowth reclaims the plot. The diversity of species provides protection against plant pests, and also some insurance against the failure of any one crop. After a few years (two to three crops), soil fertility decreases, and the forest is allowed to regenerate for a number of years (the “fallow”). The crop and fallow ratios vary according to the soil and crop, but the fallow time must be lengthy. Fifteen to thirty years, in most places, is needed in fallow after one to three years of cultivation. During the fallow period, the farmers remove fruit, wood, and other items from the forest, and harvest staples from other swidden plots. Animals, too, share in the vegetables and fruits in the gardens. Since many trees are retained and others planted, natural regeneration is encouraged. However, the composition of the forest changes, since many crop plants, especially trees, remain in the regenerating forest, and it becomes a mosaic of swidden patches at various stages of forest regeneration or cultivation. This system retains the genetic pool of primary species, and is not very destructive if the area cut is small and allowed to reforest. In fact there is some evidence that trees in swidden plots may grow as rapidly as natural forest. However, biomass and species diversity remain lower than in primary forest, and it may take more than 100 years for a swidden area to revert to primary forest (Dufour, 1990). As population pressures mount near rainforests, less and less time is being allowed for fallow and forest (and, therefore, soil) regeneration. Now, too often, the soil becomes degraded because the fallow is too brief, leaving the seed bank depleted, shoots disrupted and allowing weeds to invade. Then the plot is abandoned, as it is no longer productive. Even more damaging, permanent agriculture is now replacing the traditional shifting agricultural systems.
c. Land degradation: The recent expansion of agricultural land and pastureland accounts for less than half of the deforestation currently occurring. The remainder is due to the degradation of abandoned farms and ranches, leading to demand for additional pristine forest land to be converted for agriculture (Hoffman and Carroll, 1995). Land conversion of most tropical rainforests is unsustainable; the land soon becomes infertile and less productive due to erosion, reduction in rainfall, salinization of soil and reduction in soil water retention, and consequent increased danger of flooding. Much agricultural land is soon abandoned because of soil impoverishment. Approximately one-third of the area of land deforested annually in the Brazilian Amazon, for instance, is abandoned every year (Houghton, et al., 2000). Not all of this land is capable of reforestation, however, and large areas of previously-forested land which had been used for agriculture have been replaced with grasslands rather than forest.
d. Coca cultivation: Some mention should be made of the contribution of the drug trade to deforestation. In South America, coca has been planted for centuries, but in small quantities and carefully. Peru had only 18,000 hectares of coca in 1972. Fifteen years later that figure had increased to 200,000 hectares, much of it in the forest interior and on slopes with bare soil, without regard for erosion. But the profit was more than US$1 billion (Amelung, Torsten and Diehl, 1992; Salati, et al., 1993). The same process is occurring elsewhere in many tropical forest regions of Southeast Asia (Laos, Myanmar) as well as in the Neotropics.