Afforestation is the planting of trees to create a forest on non-forest land. It is different from reforestation, which is replanting trees where a forest has been depleted. The main purposes for implementing afforestation are commercial forestry and environmental restoration or preservation.
When afforestation is used for commercial purposes, it usually involves planting and harvesting of trees as agricultural crops. Areas where this is done are known as plantation forests. Harvesting trees from plantation forests helps to reduce deforestation in natural forests. On the other hand, there may be environmental consequences if this tree-farming is done incorrectly. The fast-growing trees often chosen for plantation forests consume large amounts of water and may deplete the area's water resources. Some types of trees also change the physical or chemical properties of the soil, which can damage indigenous species. Thus, sustainable commercial afforestation must take environmental factors into account to avoid damaging the local ecosystem.
Sustainable afforestation is also vital when it is implemented for environmental purposes. The appropriate types and amounts of trees to be planted vary depending on the environment, the climate, and the purpose of creating forested areas. Major environmental purposes include soil conservation and water quality improvement. For example, trees can be used to prevent soil erosion and reduce polluted runoff into nearby bodies of water. Trees may also be planted to create windbreaks. During the Dust Bowl in the United States' Great Plains area, for instance, the planting of long rows of native trees was encouraged in order to protect crops from the wind and reduce the loss of topsoil.
Afforestation is sometimes considered as a method for stopping or slowing desertification. Desertification is the deterioration of land in arid climates due to loss of vegetation and soil moisture. If done correctly, creating forestland in areas in danger of desertification can slow erosion and reduce its spread. There are efforts in the Gobi Desert in China and the Sahara in Africa to use afforestation to prevent the desert from claiming more land area.
In China, at least 3,600 square kilometers of land is taken over by sands from the Gobi Desert every year. The Green Wall is a massive, approximately 2,800 mile long tree-planting effort to prevent this. A similar Great Green Wall stretching from Senegal to Djibouti is proposed to help stop the spread of the Sahara. Critics of both say, however, that central planning would not be as effective as supporting local sustainable farming methods, since such projects require taking widely varying local conditions into consideration.