Slash-and-Burn Agriculture

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 nutrient is 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 rocklike 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 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.