A Review of Vegetation Cover as a Natural Factor to Soil Erosion

Soil erosion is a major global environmental problem driven by a number of both natural and anthropogenic factors. The objective of this paper is to conduct a review of previous works on vegetation cover as a natural factor of soil erosion. The study made a review of academic/journal articles, internet materials, conference papers, books and publicly available materials on vegetation as a factor of soil erosion. From the review, previous authors had a unity of opinion that vegetation cover is an important index to evaluate the soils’ sensivity to erosion. Uncovering the soil exposes it to the vagaries of erosion ranging from rainsplash to gully erosion that may be irreversible in forms of badlands. Recommendations of the study include: (1) making legislations mandating landholders to cover their soils using mainly economic trees as a source of livelihoods as well as protecting the soil against the forces of raindrops and runoff; (2) agroforestry by farmers and reforestation programme by governments to increase vegetation cover of soils so as to reduce the impact of raindrops and runoff that detach and transport soil particles; and (3) creation of awareness among the generality of the populace on the importance of vegetation in protecting the soil against erosion and maintaining the


INTRODUCTION
Soil erosion is a major problem in the world (Lant, Kraft,Beaulieu, Bennette,Loftust and Nicklow, 2005).Godone and Stanche (2011) opined that soil erosion is one of the main soil threats comprising soil productive and protective function.Soil erosion is a serious environmental, economic and social problem; it does not only cause land degradation and soil productivity loss, but also threatens the stability and health of the society in general and sustainable development of rural areas in particular (Jing, Wang and Zheng, 2005).
Globally, it has been estimated that 1.1billion hectares of land are affected by soil erosion (Pathak,Wani and Sudi, 2005), with an annual global loss of agricultural land due to erosion estimated at 3million hectares (Woreka,2004).According to Morgan (2005), soil erosion is a hazard traditionally associated with agriculture in tropical and semi-arid areas, and is important for its long-term effects on soil productivity and sustainable agriculture.In Africa, 8.5 percent of the mean yield loss is associated with soil erosion (Eswaran, Lal and Reich, 2001).In Nigeria, Agulu-Nanka in Anambra State is an area badly affected by soil erosion; up to 250 tons per hectares have been lost (Kalu, 2001).Adinna (2001) defined erosion as the gradual or forceful removal of weathered rock from the point of weathering.Ahnert (2003) asserted that soil erosion is the displacement of soil particles by agents of tillage, wind, water and snow, and down slope movement to gravity.Igbokwe, Akinyede, Dang, Ono, Nnodu and Anike (2008) saw soil erosion as an accelerated process under which soil is bodily displaced and transported away faster than it can be formed.Francis (2012) said that soil erosion is the removal of top soil than the soil forming processes can replace it due to natural, animal and human activities such as overgrazing, over cultivation, deforestation and mechanical farming.Eyankware, Eyankware and Effam (2015) opined that soil erosion is denudation process that involves three stages of rock decay decomposition, transportation and deposition.The factors that affect soil erosion could be natural or anthropogenic; natural factors influencing soil erosion are soil texture, rainfall intensity, soil type, climate, erodibility, slope and vegetation cover (Kirchlof and Salako 2008).Soil erosion is triggered by anthropogenic factors such as deforestation, slash-and burn agriculture, intensive cultivation for agriculture and overgrazing (Blanco and Lal, 2008).Both natural and anthropogenic factors contribute in different degrees to rainsplash-sheet-rill-gully erosion processes by the detachment and transport of soil particles, thereby creating on-site and off-site environmental and socio-economic impacts.Vegetation cover is defined as the assemblage of plant species that provides cover to the ground, to control the harm and reduce the risk of soil erosion (Sharma, 2009).The natural vegetation protects the soil against impacts of rainfall.It improves infiltration and enhances the recharging of the ground water reservoirs.When vegetation cover is displaced, infiltration capacity is decreased (Zuazo and Plaguezuele, 2008;Vanoost, Govers and Desmet, 2008).Therefore, the issue of soil erosion deserves priority attention to mitigate its dire consequences on the land status and environmental quality.This paper focused on reviewing vegetation cover as a natural factor of soil erosion.

Statement of the Problem
Soil erosion is a natural geomorphological process resulting from water and land interactions but accelerated to become an environmental hazard by human activities such as clearing of forest for cultivation, poor farming practices and encroachment into marginal lands (Farayi, 2011).Accelerated soil erosion is a major ecological problem in the tropics (Hartemink, 2002).Human induced soil erosion and associate damage to all agricultural land over many years have resulted to the loss of valuable agricultural land.Due to abandonment and reduced productivity of the remaining land which is partly made up for, by addition of nitrogen and phosphate fertilizer (Pimentel, 2006).For instance, increased rates of soil erosion are directly associated with nutrient loss, which may reduce agricultural productivity, (Bakker, 2007) and cause water bodies' eutrophication (Istanovics, 2009).Soil erosion reduces the general productivity of terrestrial ecosystem (Pimentel, Petrotra, Riley, Jacquet, Honigman and Valero, 2006).It increases runoff thereby decreasing water infiltration and the water storage capacity of the soil (Troeh, Hobbs and Donahue, 2004).For example loss of soil organic matter increases water runoff which reduces the soil's water storage capacity that diminishes nutrient levels in soil and also reduces the natural biota, biomass and biodiversity of soil ecosystem (Brevik, 2013).Lack of vegetation cover exposes the soil to high intensity rainfall resulting to poor structure and increases the rate of runoff, which detaches soil particles and causes soil erosion (Egbai, Eric and Ogogo, 2012).When vegetation cover is displaced, infiltration capacity is decreased (Zuazo and Plaguezuele, 2008;Vanoost , Govers and Desmet, 2008).

Objective of the Study
The objective of this paper is to review vegetation cover as a natural factor of soil erosion.

II.
CONCEPTUAL FRAMEWORK: SUSTAINABLE DEVELOPMENT This research is based on the concept of sustainable development.In the opinion of World Conference on Environment and Development (WCED) (1987), sustainable development is development that meets the needs of the present without compromising the ability of future generation to meet their own needs.Morelli (2010) saw sustainable development as meeting the resources and service needs for current and future generations without compromising the health of the ecosystems that provide them and more specifically as a condition of balance, resilience and interconnection that allows human society to satisfy its needs while neither exceeding the capacity of its supporting ecosystems to continue to regenerate the services necessary to meet those needs nor by our actions diminishing biological diversity.Vegetation cover is a major factor of soil erosion which this research focuses on so as to build sustainability into soil erosion management.

III.
METHOD This research made use of academic articles, journals, conference paper, textbooks and internet materials on soil erosion and vegetation cover as a natural factor.The researchers had ( 43 ) materials for the research but summarized (10) that centered more on vegetation cover as a natural factor of soil erosion.This enabled the researchers to make a synthesis of various researchers' views on vegetation cover as a natural factor of soil erosion.

IV.
REVIEW OF RELATED LITERATURE Rey (2003) carried out a study on influence of vegetation distribution on sediment yield in forested Marly gullies in France and stated that plant cover protected soil against erosion by reducing water runoff.Gyssel, Poesen, Bochet and Li (2005) conducted a study on the impact of plant roots on the resistance of soil erosion by water and asserted that plant stabilized the soil with their roots.Kateb

RESULTS AND DISCUSSION
Vegetation cover provides a blanket cover to soil against raindrops and runoff that detach and transport soil particles.From Table 1, the researchers' topics are concerned with investigating vegetation cover as a very important factor of soil erosion.The methods used in collecting data are standard, ranging from measurement to laboratory analysis.From the review of previous works by researchers ( for example Eshghizedeh, Talebi, Dastorani and Azimzadeh, 2015; Rehiman, Rashid, Kausar, Alvi and Hussain, 2015; Puigefabregas 2004), plant cover reduces the impact of raindrops on soils and soil erosion.Similarly, plant roots increase the resistance of soil to erosion (Gyssel, Poesen, Bochet and Li, 2005; De-Baet, Poesen, Knappen, Babera and Navorro, 2007).All previous researchers whose works were reviewed agreed that increase in vegetation cover would lower water-induced soil erosion in many ways, including intercepting rainfall and stabilizing the soil through the plant root system.The implication is that plant cover protects the soil to a large extent against the influence of other factors of soil erosion as any area with thick vegetation cannot suffer serious accelerated soil erosion as it was before the anthropogenic activities of man began to uncover the soil.Generally, the studies made recommendation aimed at covering the soil to reduce soil erosion.

VI.
RECOMMENDATIONS The specific recommendations emanating from this paper are: 1. Creation of awareness among the generality of the populace on the importance of vegetation cover in protecting the soil against erosion and maintaining the world's climate.2. Agro forestry by farmers and reforestation programme by government to increase vegetation cover of soils so as to reduce the impact of raindrops and runoff that detach and transport soil particles.3. Researchers should review more works regularly on vegetation cover as a natural factor to soil erosion to enable the general public have a broad knowledge on the topic.4. Legislations should make landholders cover their soils using mainly economic trees that provide a source of livelihoods to them and protect the soil against the forces of raindrops and runoff.

VII. CONCLUSION
This paper reviewed works done by previous authors on vegetation cover as a natural factor of soil erosion.From the review of many studies on vegetation as a natural factor of soil erosion and based on the results, this study concludes that for the survival of the global environment which is the survival of man, maintenance of existing vegetation cover and planting of trees where the soil has been devegetated provide the needed panacea for soil erosion management.

International journal of Rural Development, Environment and Health Research(IJREH) [Vol-1, Issue-4, Nov-Dec, 2017] https://dx.doi.org/10.22161/ijreh.1.4.4 ISSN: 2456-8678 by
Beven (2002)ng and Mosandi (2013) carried out a study on soil erosion and surface runoff on different vegetation cover and slope gradients in Southern Shaanxi Province China and claimed that the amount of erosion was considerably influenced by changes in vegetation cover.A study conducted in China on effect of vegetation cover on grassland on runoff and sediment yield in loess hilly region by Zhao, Chen, Huang, Wu and Helmer (2013) revealed that runoff and sediment yields are meaningfully influenced vegetation.Bochet and Garcia-Fayos (2004), in their study on the factors controlling vegetation establishment and water on motor way slopes, in Valencia Spain noted that vegetation reduced water-caused erosion by intercepting rainfall, increasing water infiltration on associated soil-fertility islands, intercepting runoff at soil surface level and stabilizing the soil by root.In China, Zhou and Shangguan (2007) studied effects of dry grass roots and shoots on loess erosion under simulated rainfall and stated that increasing the amount of vegetation can lead to runoff generation and erosion control.In their research on soil erosion and runoff response to plant cover strips on semiarid slopes in Spain Martinez, Duran and Francia (2006) asserted that the role of vegetation is summarized by introducing water runoff and increasing soil infiltration.Moreira,Silva, Righetto, Medeinis (2008) conducted a research in Brazil on sediment and overflow of soil erosion, in undisturbed Brazilian North-Eastern semi-arid experimental plot, and reported that vegetation cover protects soil surface from splash, increase surface roughness and enhances soil structure and macro-porosity.In his research in England on runoff generation in semi-arid areasBeven (2002)noted that runoff generation and soil loss in semi-arid regions are affected by non-linear and complex interdependency among rainfall properties, soil moisture and vegetation characteristics.Furthermore, he observed that vegetation has an important effect on infiltration and soil water storage capacity.Eshghizadeh,Talebi, Dastorani and Azimzadeh ( 2015) investigated the effect of land cover on runoff and soil loss at hill-slope scale in Mashad Iran and reported that increase in land cover reduces soil loss and results in the increase of permeability in canopy and litter.Additionally, they opined that litter and plant cover caused overland flow, velocity reduction, and surface roughness increase, enhancing soil infiltration through greater macro-pore density and improving soil structure by contributing organic matter.