Slash and burn
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Slash and burn (a specific practice that may be part of shifting cultivation or swidden-fallow agriculture) is an agricultural procedure widely used in forested areas. Although it was practiced historically in temperate regions, where it was termed assarting, it is most widely associated with tropical agriculture today. Slash and burn is a specific functional element of certain farming practices, often shifting cultivation systems. In some cases such as parts of Madagascar, slash and burn may have no cyclical aspects (e.g some slash and burn activities can render soils incapable of further yields for generations), or may be practiced on its own as a single cycle farming activity with no follow on cropping cycle. Shifting cultivation normally implies the existence of a cropping cycle component, whereas slash-and-burn actions may or may not be followed by cropping. In medieval England, this practice was the worst offense that could be committed in a forest.
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[edit] Slash-and-burn defined
An area of primary or secondary forest is selected, and the vegetation is cut and allowed to dry. Large trees are often girdled and allowed to die standing. Portions of the cut timber or saplings are often gathered to use for firewood or to make charcoal. After some period of time (a week to a few months) the residual dry vegetation is burned. Plots are cultivated for a few seasons (usually one to five years) and then abandoned as fertility declines and weeds invade.
Burning removes the vegetation and may release a pulse of nutrients to fertilize the soil. Ash also increases the pH of the soil, a process which makes certain nutrients (especially phosphorus) more available in the short term, but almost always to the detriment of long term.
Slash and burn requires a very low human population density to have any chance of success on its own as a single element farming cycle, since the recovery of forests may require many decades or even human generations. One of the side effects is erosion; for example, most of the Madagascar central highlands plateau is permanently rendered infertile and unproductive, due to large scale erosion that resulted from the adverse surface runoff deriving from the practice of slash-and-burn. A form of slash-and-burn has been used in different regions from the temperate coniferous forests of Northern Europe (e.g., Svedjebruk in Russia, Finland, Sweden and Norway) to the tropical moist broadleaf forests of Indochina and the Amazon Rainforest; however, much of the temperate forest cutting was followed by sustainable grazing or true crop rotation practices, whereas most of the current tropical practices of slash-and-burn are not associated with sustainable agriculture. The Tupi-Guarani of South America used slash-and-burn methods of farming.
[edit] Assessments of slash-and-burn
Slash-and-burn agriculture is usually labeled as ecologically destructive, but it may be workable when practiced by small populations in large forests, where fields have sufficient time to recover before again being slashed, burned, and cultivated. Given the present worldwide high population densities, it is not common to find such conditions. It is also more effective when used in mixed plots, with more than one crop (usually two or more plant species that complement each other's growth) being planted at or around the same time. Even when relatively small populations practice slash-and-burn agriculture, as in the eastern Madagascar rainforests, the cumulative effect has been destructive of the forest integrity, because of the slow regeneration times and the large number of vulnerable and endangered species. Problems with ecological unsustainability can arise with significant increases of population, leading to increased pressure on the land and failure to let fields lie fallow for enough time, as has been seen in the late 20th century in parts of the rainforests of Mexico and Brazil.
Slash and burn has been replaced by other methods in most temperate zones. It is still practised in some parts of Mexico, South America, Indonesia, India and Indochina. It is common in Madagascar, where it is known as tavy. A number of countries have established Biodiversity Action Plans that address the effect of human activities on the environment, and biodiversity in particular. Some, such as that of Australia, proscribe slash and burn practices.
Since the 1990s, a rise in the use of slash and burn agriculture to plant coca, marijuana and opium poppy as part of the illegal drugs trade has contributed to a yearly deforestation of more than 100,000 acres (400 km2) in Colombia.
[edit] Ecological implications
Although a dilemma for overpopulated tropical countries where subsistence farming may be the only viable method of sustaining many families, the consequences of slash-and-burn techniques to ecosystems are almost always deleterious when practiced on a large scale. The principal vulnerability is the nutrient-poor soil, pervasive in most tropical forests. When biomass is extracted even for one harvest of wood or charcoal, the residual soil value is heavily diminished for further growth of any type of vegetation. Sometimes there are several cycles of slash-and-burn within a few years time span; for example in eastern Madagascar the following scenario occurs commonly. The first wave might be cutting of all trees for wood use. A few years later, saplings are harvested to make charcoal, and within the next year the plot is burned to create a quick flush of nutrients for grass to feed the family zebu. If adjacent plots are treated in a similar fashion, large scale erosion will usually ensue, since there are no roots or temporary water storage in nearby canopies to arrest the surface runoff. Thus, any small remaining amounts of nutrients are washed away. The area may now be an example of desertification, and no further growth of any type may arise for generations.
The ecological ramifications of the above scenario are further magnified, because tropical forests are habitats for extremely biologically diverse ecosystems, typically containing large numbers of endemic and endangered species. Therefore, the role of slash-and-burn is significant in the current Holocene extinction event occurring on the planet Earth.
[edit] Trivia
Eero Järnefelt has painted the famous painting The Wage Slaves (Raatajat rahanalaiset or Kaski, 1893, External link) about slash-and-burn agriculture.
