Slash and burn is a form of agriculture that involves the slashing and burning of forests to clear land for farming. Typically, after a farmer plants in the field and yields his crops, he leaves the field fallow. A fallow field is one that is left uncultivated so that the natural vegetation and soil nutrients may redevelop. People from many different regions of the world, from Brazil to Finland, have safely practiced slash and burn agriculture for centuries. Some scientists even suggest evidence of slash and burn agriculture that dates back to 4000 BP (Piperno, 2004).
According to the Intergovernmental Panel on Climate Change ([IPCC], 2007), forests cover 3952 million ha, about 30% of the Earth’s land surface. Deforestation is defined as the clearing of trees in a forest. Deforestation in the 2000’s is estimated to have removed 12.9-16.3 million ha of forest per year (IPCC, 2007; Loarie et. al., 2009). It is only in recent years with increasing population pressure that slash and burn agriculture has been identified as a problematic agricultural system and linked to deforestation. Population pressure has been shown to lead to decreases in fallow periods and the development of secondary forests* (Fearnside, 1995). When farmers do not allow for a sufficient fallow period in between crop cycles, they drain the nutrients of the soil. The forest is then unable to regenerate.
This graph illustrates the relationship between productivity (and therefore the ability of a forest to regenerate) and the length of fallow periods. Click on the image to enlarge it.
(Hauser & Norgrove, 2001)
Other sources of deforestation include clearing of forests for use as cattle pastures, road building and logging. Deforestation leads to decreases in biomass, which can have many different local effects (flooding, etc.). Because trees and soil hold large amounts of carbon, deforestation and burning of forests is also importantly tied to carbon dioxide emissions and climate change.
This image by the IPCC (2007) shows where deforestation is occuring on the earth. Click on the image to enlarge it.
These photos of the Brazilian Amazon show examples of slash and burn agriculture. The second photo is a zoom on a small area on the lower right of the first photo. The rectangular shapes in the lighter green areas are plots of land cleared for farming. The darker green represents the actual forest.
http://veimages.gsfc.nasa.gov/2997/Amazon.A2002182.1405.500m.jpg
*As opposed to primary forests which have more diversity and biomass. The secondary forest is the forest that regenerates early on after a disturbance to a primary forest.
