Slash and burn consists of cutting and burning of forests or woodlands to create fields for agriculture or pasture for livestock, or for a variety of other purposes. It is sometimes part of shifting cultivation agriculture, and of transhumance livestock herding.
Historically, the practice of slash and burn has been widely practiced throughout most of the world, in grasslands as well as woodlands, and known by many names. In temperate regions, such as Europe and North America, the practice has been mostly abandoned over the past few centuries. Today the term is mainly associated with tropical rain forests. Slash and burn techniques are used by between 200 and 500 million people worldwide.
Older English terms for slash and burn include assarting, swidden, and fire-fallow cultivation.
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.
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. Some trees are often left standing, especially those viewed as useful, such as food producing trees like chestnuts or economically valuable trees like teak. 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.
Such abandoned plots often become used as pasture for livestock. If the forest is allowed to recover, pasture becomes rough pasture for a while. Recovering woodlands are sometimes treated as "fallow" land, which means it is to be subjected to another round of slash and burn in the future.
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. Burning also temporarily drives off soil microorganisms, pests, and established plants long enough for crops to be planted in their ashes. Before artificial fertilizers were available, fire was one of the most widespread methods of fertilization.
Slash and burn requires a relatively low human population density or a continuing supply of new "frontier" lands, since the recovery of forests may require many decades or even human generations.
Various forms of slash-and-burn have been used in nearly every forested environment, 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. Much of the temperate forest cutting was followed by sustainable grazing or crop rotation practices. An almost total conversion of forests to farmland and pasture has occurred in many temperate regions, such as England. In many tropical forests, sustainable forms of slash and burn have been practiced for millennia, but population growth and large-scale industrial logging, among other factors, have made traditional slash and burn practices less sustainable and more likely to result in catastrophic wildfires.
During the Neolithic Revolution, or "new stone age revolution" which included agricultural advancements, groups of prehistoric humans started domesticating various plants and animals, shifting from a hunter-gatherer lifestyle toward agriculture and pastoralism. The origins of domestication are not known. One theory is that it was mainly due to the end of the Ice Age (ie. about 9-11,000 years ago), resulting in the extinction of many of prehistoric man's game, such as the wooly mammoth. Due to this decrease in food from hunting, some groups started to turn to agriculture. Some groups could easily plant their seeds in open fields, but others had forests blocking their farming land. Since Neolithic times, slash and burn techniques have been widely used for converting forests into crop fields and pasture. Fire was used before the Neolithic as well, and by hunter-gatherers up to present times. Clearings created by fire were made for many reasons, such as to draw game animals and to promote certain kinds of edible plants such as berries and mushrooms.
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 practiced in some parts of Mexico, South America, Indonesia, India and Indochina. It is common in parts of Africa such as Zambia and southern DR Congo where it is known as chitemene, and 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. The chief cause of desertification in the Sahel is slash-and-burn farming practised by an expanding human population. The Sahara is expanding south at an average rate of 30 miles per year.
Although a dilemma for overpopulated tropical countries where subsistence farming may be the easiest 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 is 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.
Eero Järnefelt has painted the famous painting The Wage Slaves (Raatajat rahanalaiset or Kaski), 1893) about slash-and-burn agriculture.