Slash and burn is a forestry technique that involves cutting down trees and shrubs on a large swath of land, allowing them to dry, and then setting a fire. It is a rapid and effective method for clearing a chunk of land, typically with the intention of converting it to agricultural use. This land clearing method is also cited as a leading cause of deforestation worldwide, since it is extremely destructive when it is not practiced sustainably.
Although many people think of Amazonian forests when they visualize slash and burn, the practice is actually widespread around the world and quite old. Humans have been using fire to clear land for thousands of years, although early methods were practiced on a much smaller scale. Many European forests bear evidence of former slash and burn practices, and it is not uncommon in Africa and Australia as well.
When slash and burn is practiced on a small scale with the environment in mind, it is not terribly harmful. The cleared land experiences a brief burst in fertility, thanks to the large amount of biomass which was burned, and it is farmed for a few seasons before being allowed to return to the forest. If the technique is practiced on a very slow rotation, with several decades between each episode of burning, it can actually be healthy for the forest, allowing new growth to establish itself and replenish the soil.
However, more typically, slash and burn agriculture completely strips the earth, contributing to erosion and ultimate environmental collapse in the area that has been treated. Instead of taking a few years to recover, the land may take many decades to return to former levels of fertility, and nonnative species may establish themselves in the meantime, choking out more slow growing native trees and plants. It also strips animals of habitat, and it can contribute to poor air quality when practiced on a large scale, as it is in the Amazon.
Typically, before executing a burn, economically useful trees and plants will be removed. The remaining brush and junk trees are loosely piled to form large bonfires that may burn for weeks before the dried fuel is exhausted. Large scale slash and burnis actually visible from space, and many environmental activists show photographs of the technique being practiced in the Amazon when they are trying to convince people that the practice is dangerous and unsustainable.
People who are opposed to this form of agriculture have worked to promote alternatives to the practice, ranging from encouraging people to grow crops under the cover of the rainforest to promoting a vegetarian diet so that land in South America does not need to be cleared for cattle. There has also been a growing focus on the intrinsic economic value of forests as sources of new medicine, tourist revenue, and living space for animals and humans alike.