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Mechanisms of bush encroachment and its inter-connection with rangeland degradation in semi-arid African ecosystems:a review

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Journal of Arid Land 2017年第2期9卷 299-312页

作者： Anteneh BELAYNEH Zewdu K TESSEMA Department of Biology College of Natural and Computational Sciences Haramaya University Haramaya 282 Ethiopia Rangeland Ecology and Biodiversity Program School of Animal and Range Sciences College of Agriculture and Environmental Sciences Haramaya University Dire Dawa 138 Ethiopia

Many studies show that semi-arid rangelands throughout the world have been rapidly converted from a grassland state to a bush encroachment state during the past 50 years. Bush encroachment includes the spread of local woody species and/or incursion of woody species introduced from other ecosystems into semi-arid savannas and grassland ecosystems. Rangeland degradation due to bush encroachment causes several challenges, affecting the production of livestock and pastoral people livelihoods in most parts of Africa. Scientists have long been attempting to develop schematic and mathematical theories to explain the observed phenomenon of bush encroachment, and several theories were proposed and developed. The well-regarded theories include：（1） Waiter＇s two-layer model, （2） Moir＇s one-layer model, （3） state-and-transition theory, （4） equilibrium theory, （5） disequilibrium theory, and （6） non-equilibrium theory. Within those theories, the most frequently-indicated driving factors that explain bush encroachment include over-grazing, availability of soil nutrient and moisture, elevated CO2 levels, frequency and intensity of fire, spread of seeds of woody species by livestock and wild animals. It should be stressed that couplings and interactions among diverse driving factors are more often at work in determining the condition of bush encroachment. To summarize, the effort in managing semi-arid ecosystems needs critical knowledge to understand the cause-effect relationships of underlying factors through integrated approach. Therefore, future research on encroachment of woody plants should be multi-discipline oriented and multi-partnership involved.

关键词： climate change exotic woody species indigenous woody species Moir's one-layer model rangelandtheories Walter's two-layer model

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