Deforestationtrees, often as a result of human activities. Deforestation has been practiced by humans for thousands of years chiefly as a result of clearing land for commercial and industrial development, intensive collection of firewood, clearing of land for growing crops and to develop pasture for grazing animals. The rate of clearance increased during the second half of the 19th century due to agricultural expansion in Europe. There have been massive increases since then. Currently major worries concern the loss of tropical rainforest, one fifth of which was destroyed between 1960 and 1990. Estimates of deforestation of tropical forest for the 1990s range from ca. 55,630 km² to ca. 120,000 km² each year.
Deforestation is often cited as one of the major causes of the enhanced greenhouse effect. Trees remove carbon (in the form of carbon dioxide) from the atmosphere during the process of photosynthesis. Both the rotting and burning of wood releases this stored carbon carbon dioxide back in to the atmosphere.
Orbital photograph of human deforestation in progress in the Tierras Bajas project in eastern Bolivia. Photograph courtesy NASA
Pressure has been exerted on forests by the worldwide demand for wood and by local people who clear forests in their quests to establish an agrarian land base. Clearing of forests for the development of pasture for cattle has also resulted in deforestation as has the encroachment upon forests due to increasing human populations.
Deforestation promotes erosion of soil. Under normal circumstances trees and bushes and the forest floor act as a 'sponge' for rainfall, slowing its' overland and underground flow and releasing it back into the atmosphere through transpiration. Without the buffering effect of forest cover, rain impacting bare soil runs off, often causing flooding. In this environment, nutrients in the soil are leached off and the microorganisms which can replenish these nutrients are disturbed.
Some forests are rich in biological diversity. Deforestation can cause the destruction of the habitats that support biological diversity.
Some societies are making efforts to stop or slow deforestation. In China, where large scale destruction of forests has occurred, each citizen must plant at least 11 trees every year. In western countries, increasing consumer demand for wood products that have been produced and harvested in a sustainable manner are causing forest landowners and forest industries to become increasingly accountable for their forest management and timber harvesting practices.
While the earliest example of deforestation is unclear, (there is some controversy over the origin of the Sahara Desert for example) see history of Easter Island for clear evidence of the ecological impact of human activity.