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Deforestation is the removal of trees, often as a result of human activities. Deforestation has been practiced by humans for thousands of years chiefly in order to collect firewood, clear land for growing crops and to provide land for animals. The rate of clearance increased during the second half of the ninenteenth century due to agricultural expansion in Europe and there has been massive increases since then. Currently major worries concern the loss of tropical forest, one fifth of which was destroyed between 1960 and 1990. 12 million hectares of tropical forest are lost each year, a land approximately the size of England.

This is often cited as one of the major causes of the enhanced greenhouse effect for two reasons: 1) the burning or decomposition of the wood releases carbon dioxide; and 2) trees that once removed carbon dioxide from the atmosphere in the process of photosynthesis are no longer present and contributing to carbon storage.

Large scale clearance has been made possible due to the mechanisation of logging. Pressure has been exerted by the worldwide demand for wood and also to a lesser extent by local people. The forests provide wood for fuel, they are mined for valuable minerals and harvested for food. Modernisation of developing countries has seen the building of roads and hydroelectric stations. Ranching of cattle has encroached on forest land and overpopulation has necessitated space for houses.

Deforestation promotes erosion of soil. Under normal circumstances trees and bushes act as a 'sponge' for rainfall, holding it and releasing it slowly through transpiration. Without this effect, rain hits the soil directly and runs off, often causing flooding. Nutrients in the soil tend to be leached off, and microorganisms which replenish these nutrients are disturbed. Tropical soil is typically not very high in nutrients, these being locked up in trees. A natural cycle of decomposition passes on nutrients to new generations. This can not occur after deforestation has taken place.

Forests are rich in biological diversity of both plants and animals, many of the latter including thousands of undiscovered species. Deforestation causes the destruction of habitats. Some animals adapt or move to new locations but thousands do not survive. The gene pool is important to western agriculture as it can extend the variety of domesticated animals. Many plants are also promising sources of new medicines.

Some efforts have been made to stop deforestation. In China, where large scale destruction of forests has occured, it is obligatory for all citizens over 11 to plant three trees a year. Roadside checks have been introduced to curb logging. The future looks sustainable management of forests and schemes such as coppicing.