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Desertification means land degradation in arid, semi arid and dry sub-humid areas resulting from various factors, including climatic variations and human activities. Modern desertification arises from the demands of increased populations that settle on the land in order to grow crops and graze animals.

Desertification is widespread in many areas of China. The populations of rural areas have increased since 1949 for political reasons as more people have settled there. While there has been an increase in livestock, the land available for grazing has decreased. Also the importing of European cattle such as Fresian and Simmental, which have higher food intakes, has made things worse.

Some arid and semi-arid lands can just support crops, but additional pressure from greater populations or decreases in rainfall can lead to the few plants present disappearing. The soil becomes exposed to wind, causing soil particles to be deposited elsewhere. The top layer becomes eroded. With the removal of shade, rates of evaporation increase and salts become drawn up to the surface. This is salinisation, and inhibits plant growth. The loss of plants causes less moisture to be retained in the area, which may change the climate pattern leading to lower rainfall.

A number of schemes have been tried to reduce the rate of desertification and regain lost land. Leguminous plants, which use nitrogen they extract from the air, can be planted. Stones placed around the base of trees increase the shade available for plants and insects. Artificial grooves in the ground can be dug to retain moisture and trap wind-pollinated seeds. In Iran oil is being sprayed over semi-arid land with crops. This coats seedlings to prevent moisture loss and stop them being blown away. Windbreaks made from trees and bushes to reduce soil erosion and evapotranspiration was widely encouraged by development agencies from the middle of the 1980s in the Sahel area of Africa.

Desertification is a historic fenomenon. Through dated fossil pollen, it has been found that todays Sahara desert has been changing between desert and fertile savanna. Studies also show that this desert advance and retreat depending on rainfall, every year.

See also: deforestation.

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