بخشی از مقاله انگلیسی:
Environmental degradation is a major contemporary globally issue that is bedeviling most parts of the arid environment. This is due to the several centuries of poor management of the earth’s natural resources, and the increasing pressure associated with rapid population growth. Degradation of natural resources especially vegetation is most felt in the fragile environment of arid regions of the world which is home to several millions of people (United Nations Environment Programm (UNEP), 2008; Federal Ministry of Environment (FME), 2008). While drought and desertification are largely natural occurrence (Oladipo, 1993) these phenomena are significantly exacerbated by anthropogenic factors particularly farming, grazing and tree felling. Desertification, as defined in Chapter 12 of “Agenda 21”, Rio declaration on environment and development “is the degradation of the land in arid, semi-arid and sub-humid dry areas resulting from various factors, including climatic variations and human activities” (UNICED, 1992). It reduces the ability of land to support many life forms, limiting biodiversity and constraining socio-economic activities and development. It also stimulates accelerated soil erosion and deposition particularly by wind. Open land with little or no vegetal cover is highly vulnerable to both wind and water erosion (Cao, et al. 2008). In the absence of vegetation, rain water is mainly disposed as runoff due to minimal infiltration rate. This further constrains the growth and productivity of plants. Even long-lived plants: trees, shrubs and other perennials that would normally survive droughts eventually find survival an insurmountable challenge. A reduction in plant cover also results in lowering the quantity of humus in the soil, and plant productivity drops further (Adesina, 2008). As protective plant cover disappears, the probability of flash floods becomes higher and further degradation occurs. Thus, in general terms, desertification is self-reinforcing, that is, once the process starts it creates conditions for self sustenance. Desertification and drought have severe impacts on food security, livelihood, socio-economic and cultural activities of the region where they occur. It is accompanied by a reduction in the natural potential of the land and depletion in surface and ground-water resources. Desertification is not only a feature of natural deserts, but also takes place on land which is exposed to persistent drought and human pressure. In West Africa and in Nigeria in particular, food insecurity associated with desertification is mounting (UNEP, 2008). This is causing significant socio-economic challenges including mass movement of people from arid environment and reduced supply of food as well as disruption in the economic and social activities of people living in semi-arid environment (National Action Plan (NAP), 2007). Shelterbelt has been adjudged as one of the effective ecological means of contending the effects of desertification in an arid environment. Shelterbelts are rows of trees planted perpendicular to the direction of the prevailing winds to reduce their velocities. Igugu and Osemeobo (1991) reported that between 1963 and 1989 over 236,500 hectares of shelterbelts were established in the States threatened by desertification in Nigeria. This coverage of shelterbelts apart from being inadequate has sadly been allowed to deteriorate by the activities of farmers, loggers and fuelwood extractors. The Nigerian Forest Policy specifies that at least 20% of the country’s total land area should be reserved as forest estates to effectively fight environmental degradation (Odigie and Obiaga, 1991; FAO, 2005). Considering the magnitude of shelterbelt development needed to fight the menace of desertification in Nigeria, both government and people living in the arid and semi-arid region need to actively rise up to the challenges. In semi-arid regions, the short rainy season provides limited opportunities for plants to grow. This can however be exploited to grow and nurture adaptable varieties of plants in afforestation projects. Afforestation is the deliberate planting of trees where it has not existed before or where original tree cover had been removed. Without afforestation and support for the trees, it is almost impracticable to have re-establishment of tree cover in most tropical arid lands. Growing seasons are short and wild grazing is uncontrolled. In the event that natural regrowths happen, removal by grazing animals makes the plants short-lived. Afforestation can be achieved by establishing shelterbelts, increasing densities of farm-trees, planting live fences and by establishing woodlot (Igboanugo, 1991). Among these afforestation strategies, shelterbelts are the most effective and environmentally friendly approach to achieving success in combating desertification in the arid and semi-arid environments. Shelterbelts are established both to ensure their survival and enhance their capacity to yield optimum environmental benefits. Apart from the ecological and socio-economic benefits that they provide, shelterbelts reduce wind velocities as well as modify micro-climates (Ojo, et al., 1987; Igugu and Osemeobo, 1991; Wang and Takle, 1996; Mohammed, et al., 1996; Cornelis and Gabriels, 2005; Torita and Satou, 2007). In particular, the planting of shelterbelts in rows perpendicular to the direction of the prevailing winds, makes it possible for the trees to act as windbreak and so protect vegetation, and soils on the lee side against wind damages. Shelterbelt also improves the microclimatic conditions by the cooling effect of transpiration of the trees and conservation of available water resources. In this way, shelterbelts provide ‘safe sites’ for other plants to thrive through the process of establishment and succession (Pascal, 2003). They can thus become “growth” areas for the expansion of vegetation cover in the area. Among the specific objectives of establishing shelterbelts in Nigeria as highlighted by Igugu and Osemeobo, (1991) are to: i. Create windbreaks against high winds, check moving sand dunes and create conducive microclimate for sustainable agricultural production and recreation. ii. Provide habitat for small animals including migratory birds; iii. Improve soil productivity and carrying capacities of biotic and abiotic resources. Shelterbelts are established on communal lands and in forest reserves. The traditional shelterbelts in Nigeria are composed of pure stands of Azadirachta indica (neem tree) or Eucalyptus camaldulensis with ten rows of trees in an escapement of 2.5 m x 2.5 m to give a size of 200 m x 30 m per belt (Okefiena, 1988). Shelterbelts are being used to curb the expansion of desert condition and reduce the subsisting impacts of aridity in northern region. It is crucial that these shelterbelts are successfully established otherwise the objectives behind them may never be realized. This study assesses the influence of some selected shelterbelts in arid environment of Yobe State, Nigeria on vegetation characteristics. This is with intention of identifying its significance in fighting desert encroachment.