Abstract
The increase in biomass and abundance of woody plant species, often thorny or unpalatable, coupled with the suppression of herbaceous plant cover, is a widely recognized form of rangeland degradation. Bush encroachment therefore has the potential to compromise rural livelihoods in Africa, as many depend on the natural resource base. The causes of bush encroachment are not without debate, but fire, herbivory, nutrient availability and rainfall patterns have been shown to be the key determinants of savanna vegetation structure and composition. In this paper, these determinants are discussed, with particular reference to arid and semi-arid environments of Africa. To improve our current understanding of causes of bush encroachment, an integrated approach, involving ecological and indigenous knowledge systems, is proposed. Only through our knowledge of causes of bush encroachment, both direct and indirect, can better livelihood adjustments be made, or control measures and restoration of savanna ecosystem functioning be realized. Resumen
Highlights
Savanna ecosystems are characterized by a continuous layer of herbaceous plants, e.g. grasses, and sparsely populated patches of trees and shrubs
The proliferation of woody plants in savanna ecosystems is known as bush encroachment and an increase of 10%
Several factors, such as overgrazing, fire frequency, soil moisture, nutrients and global warming, have been associated with bush encroachment but it is still controversial how each factor contributes to increased woody plant cover
Summary
Savanna ecosystems are characterized by a continuous layer of herbaceous plants, e.g. grasses, and sparsely populated patches of trees and shrubs. The proliferation of woody plants in savanna ecosystems is known as bush encroachment (van Auken 2009) and an increase of 10%. Pastoralists in Africa have indicated that woody plants contribute significantly towards livestock feed, especially during drought periods (Moleele 1998; Kgosikoma et al 2012a), thereby reducing the cost of supplementary feed. Despite bush encroachment being observed in many grasslands and savannas in Africa and elsewhere, the mechanisms that promote it are not clearly understood (Ward 2005). Several factors, such as overgrazing, fire frequency, soil moisture, nutrients and global warming, have been associated with bush encroachment (van Auken 2009) but it is still controversial how each factor contributes to increased woody plant cover. Commonly cited causes of bush encroachment are briefly reviewed and an integrated approach proposed for understanding causes of bush encroachment and sustainable management of savanna ecosystems
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