The appearance of desertlike conditions that were nonexistent previously in an area is termed desertification. More specifically, desertification may be defined as land degradation in arid, semiarid, and subhumid areas resulting from climatic variation and human activities (Hare, 1993). Desertification is a widespread and discrete process of land degradation. With desertification, the fraction of bare soil increases, and vegetation is reduced to small patches. With more bare soil, fine mineral and organic material is rapidly removed by wind. Gully and sheet erosion by occasional heavy rainfall tends to accumulate the eroded material on the low-lying areas or the valley floors.
More than 250 million people in 110 countries are directly affected by desertification, while more than 1 million people are threatened by it. Six million hectares each year are affected worldwide by desertification, causing famine, death of livestock, and the loss of cultivated land (di Castri, 1990; Horstmann, 2001). According to another estimate, approximately 70 percent of the susceptible drylands are undergoing various forms of land degradation (Ayoub, 1998). The West African Sahelian region is one example of increasing desertification and its impact. In this region, original field data show that forest species richness and tree density have declined in the last half of the twentieth century. Average forest species richness of areas of
4 km in northwest Senegal fell from 64 ± 2 species in 1945 to 43 ± 2 species in 1993. Densities of trees of height greater than or equal to 3 m declined from 10 ± 0.3 trees/ha in 1954 to 7.8 ± 0.3 trees/ha in 1989. Arid Sahel species have expanded in the north. The changes also decreased human-carrying capacity to below actual population densities. For example, the carrying capacity for firewood from shrubs in 1993 was of 13 people/km2 compared to the rural population density of 45 people/km2 (Gonzalez, 2001).
Significant climatic anomalies, both in time and space, may result in lengthening dry periods, higher temperatures, and strong winds resulting in the permanent loss of vegetation from an area (Abdel-Samie, Gad, and Abdel-Rahman, 2000; Oba, Post, and Stenseth, 2001). Likewise, increasing human pressure results in extension of cultivated areas and overuse of dryland natural resources. Details of the desertification processes vary from region to region. However, the common processes are water and wind erosion; overgrazing by livestock; deforestation for more firewood and building; bush and forest fires; alkalization; and waterlogging (Kerley and Whitford, 2000). Desertification is further accelerated by disadvantageous social factors (Zhang and Bian, 2000). None of these processes is capable of affecting the natural ecosystems seriously. The entire structure of these systems is adapted to seasonal distribution of rainfall. Desertification increases very rapidly when human misuse of land combines with the occurrence of drought. The documented impacts of desertification foreshadow possible future effects of climate change. Under that situation, the natural mechanism of repair and renewal cannot cope with the added stresses.
Livestock overgrazing causes the semiarid grasslands to shift in community structure toward the shrublands, with associated changes in the structure and functioning of faunal communities (Kerley and Whitford, 2000). Vegetation cover, plant height, herbage yield, and root weight decrease, and the composition of the grasses changes. The surface is exposed to wind and soil-holding ability is decreased, soil erosion increases, and sands begin to accumulate on the leeward slope. This process is sped up by the heavy tread of animals.
There is a sequential degradation of soil properties in the process of desertification. The concentration of fine soil particles (clay and silt) decreases, but the concentration of sand particles increases with the deterioration of the ecosystem. Bulk density is at its maximum in a degraded land ecosystem, and consequently, pore space is at its minimum. Water holding and field capacity of soil decreases with the degradation of the ecosystem and exhibits a positive correlation with the clay content of soil. Eventually, desertification causes progressively drier soil conditions. The concentration of organic carbon, total nitrogen, and available phosphorus significantly decreases, whereas concentration of calcium, magnesium, potassium, and sodium significantly increases with the deterioration of ecosystem. The reduction of concentration of organic carbon, total nitrogen, and available phosphorus and increase of concentration of sodium, potassium, and calcium are related to the reduction of the amount of soil macroaggregates. Electrical conductivity and pH of soil increases, suggesting that soil salinity develops with desertification (Pandey, Parmar, and Tanna, 1999).
Irrigation expansion results in ever-increasing water withdrawal from rivers. The main desertification processes associated with irrigation are a decline in the groundwater level, increased mineralization and chemical pollution of watercourses, and soil salinization. Many of the salt-sensitive plant species stop growing in such soils. (Saiko and Zonn, 2000).
Regular monitoring of climate and associated hydrological and ecological processes is the first and most essential part of any program for checking desertification. Because desertification is a discrete process, its tackling requires continued monitoring and research. We need to know how the climate systems in tropical and subtropical areas work. In many arid zones of the world, the network of weather stations is still very poor and climate records are not complete (Sivakumar, Stigter, and Rijks, 2000; Stigter, Siv-kumar, and Rijks, 2000). More weather stations and more weather records are needed in fragile areas. Using satellite remote sensing facilities, vegetation changes should be observed regularly (Wang, 1990). Another important action for combating desertification is the control on land use (Olulumazo, 2000). A land-use control is also a microclimate control. Good land-use practices lead to enhanced biological productivity and maintain healthy ecosystems. Apart from control measures, a political and managerial will is needed to control the processes of desertification.
Realizing the consequences of the impacts of desertification, the United Nations held Conventions to Combat Desertification in mid-1990s. The UN conventions recommended that efforts should move from global assessment to national and local level assessments, and focus should shift from physical parameters and be directed more to people issues (Ayoub, 1998). Acting on these recommendations, a majority of countries have taken initiatives for the halting of desertification (Wang, Zheng, and Yang, 2000; Hoven, 2001).
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