Desertification is land degradation in arid, semi-arid and dry sub-humid areas arising mainly from human activity. 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.
There is some evidence that desertification is reversible. The southern boundary of the Sahara has been gradually retreating, enabling vegetation to return to previously uninhabitable areas like northern Burkina Faso. The reasons for this reversal remain a mystery and it is possible that it may only be temporary, but it is already causing people to return to areas they had previously given up.
- See also: deforestation.