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Journal of Arid Land  2017, Vol. 9 Issue (2): 299-312    DOI: 10.1007/s40333-016-0023-x
Review Article     
Mechanisms of bush encroachment and its inter-connection with rangeland degradation in semi-arid African ecosystems: a review
BELAYNEH Anteneh1, K TESSEMA Zewdu2,*()
1 Department of Biology, College of Natural and Computational Sciences, Haramaya University, Haramaya 282, Ethiopia
2 Rangeland Ecology and Biodiversity Program, School of Animal and Range Sciences, College of Agriculture and Environmental Sciences, Haramaya University, Dire Dawa 138, Ethiopia
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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) Walter’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.

Key wordsclimate change      exotic woody species      indigenous woody species      Moir’s one-layer model      rangeland theories      Walter’s two-layer model     
Received: 14 April 2016      Published: 20 April 2017
Corresponding Authors: K TESSEMA Zewdu     E-mail:
Cite this article:

BELAYNEH Anteneh, K TESSEMA Zewdu. Mechanisms of bush encroachment and its inter-connection with rangeland degradation in semi-arid African ecosystems: a review. Journal of Arid Land, 2017, 9(2): 299-312.

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1 Abule E, Smit G N, Snyman H A.2005. The influence of woody plants and livestock grazing on grass species composition, yield and soil nutrients in the Middle Awash Valley of Ethiopia. Journal of Arid Environments, 60(2): 343-358.
2 Abule E, Snyman H A, Smit G N.2007. Rangeland evaluation in the middle awash valley of Ethiopia: II. Woody vegetation. Journal of Arid Environments, 70(2): 272-292.
3 Adams M E.1966. A study of the ecology of Acacia mellifera, A. seyal and Balanites aegyptiaca in relation to land-clearing. Journal of Applied Ecology, 4(1): 221-237.
4 Adler P B, Raff D A, Lauenroth W.2001. The effect of grazing on the spatial heterogeneity of vegetation. Oecologia, 128(4): 465-479.
5 Allred B W, Fuhlendorf S D, Smeins F E, et al.2012. Herbivore species and grazing intensity regulate community composition and an encroaching woody plant in semi-arid rangeland. Basic and Applied Ecology, 13(2): 149-158.
6 Angassa A, Baars R M T.2000. Ecological condition of encroached and non-encroached rangelands in Borana, Ethiopia. African Journal of Ecology, 38(4): 321-328.
7 Angassa A, Oba G.2007. Relating long-term rainfall variability to cattle population dynamics in communal rangelands and a government ranch in southern Ethiopia. Agricultural Systems, 94(3): 715-725.
8 Archer S.1990. Development and stability of grass/woody mosaics in a subtropical savanna parkland, Texas. U.S.A. Journal of Biogeography, 17(4-5): 453-462.
9 Archer S, Schimel D S, Holland E A.1995. Mechanisms of shrubland expansion: land use, climate or CO2?. Climatic Change, 29(1): 91-99.
10 Archer S R.2010. Rangeland conservation and shrub encroachment: new perspectives on an old problem. In: du Toit J T, Kock R, Deutsch J C. Wild Rangelands: Conserving Wildlife While Maintaining Livestock in Semi-arid Ecosystems. Chichester: John Wiley and Sons Ltd., 53-97.
11 Ayres D, Melville G, Bean J, et al.2001. Woody Weeds, Biodiversity and Landscape Function in Western New South Wales. Dubbo: WEST 2000, 221.
12 Bahre C J.1991. A Legacy of Change: Historic Human Impact on Vegetation in the Arizona Borderlands. Tucson: University of Arizona Press, 231.
13 Behnke R H, Scoones I, Kerven C.1993. Range Ecology at Disequilibrium: New Models of Natural Variability and Pastoral Adaptation in African Savannas. London: Overseas Development Institute, 248.
14 Belay T A, Totland ?, Moe S R.2013. Woody vegetation dynamics in the rangelands of lower Omo region, southwestern Ethiopia. Journal of Arid Environments, 89: 94-102.
15 Bester F, Reed A.1997. Bush encroachment: a thorny problem. Spotlight on Agriculture, 1: 175-177.
16 Bews J W.1917. The plant succession in the Thorn Veld. South African Journal of Science, 14: 163-172.
17 Biederman L A, Boutton T W.2009. Biodiversity and trophic structure of soil nematode communities are altered following woody plant invasion of grassland. Soil Biology and Biochemistry, 41(9): 1943-1950.
18 Bothma J P.2002. Game Ranch Management (4th ed.). Pretoria, South Africa: Van Schaik Publishers.
19 Bowman D M J S, Balch J, Artaxo P, et al.2011. The human dimension of fire regimes on Earth. Journal of Biogeography, 38(12): 2223-2236.
20 Bradstock R A, Auld T D.1995. Soil temperatures during experimental bushfires in relation to fire intensity: consequences for legume germination and fire management in south-eastern Australia. Journal of Applied Ecology, 32(1): 76-84.
21 Briske D D, Fuhlendorf S D, Smeins F E.2003. Vegetation dynamics on rangelands: a critique of the current paradigms. Journal of Applied Ecology, 40(4): 601-614.
22 Brown J R, Archer S.1989. Woody plant invasion of grasslands: establishment of honey mesquite (Prosopis glandulosa var. glandulosa) on sites differing in herbaceous biomass and grazing history. Oecologia, 80(1): 19-26.
23 Buitenwerf R, Bond W J, Stevens N, et al.2012. Increased tree densities in South African savannas: >50 years of data suggest CO2 as a driver. Global Change Biology, 18: 675-684.
24 Clarke P J, Davison E A, Fulloon L.2000. Germination and dormancy of grassy woodland and forest species: effects of smoke, heat, darkness and cold. Australian Journal of Botany, 48(6): 687-699.
25 Dregne H E, Choun N T.1992. Global desertification dimensions and costs. In: Dregne H E. Degradation and Restoration of Arid Lands. Lubbock: Texas Technology University, 249-282.
26 Dregne H E.2002. Land degradation in the drylands. Arid Land Research and Management, 16(2): 99-132.
27 Ehleringer J R.2005. The influence of atmospheric CO2, temperature, and water on the abundance of C3/C4 taxa. In: Baldwin I T, Caldwell M M, Heldmaier G, et al. A History of Atmospheric CO2 and its Effects on Plants, Animals, and Ecosystems. New York: Springer, 214-231.
28 Eldridge D J, Wilson B R, Oliver I.2003. Regrowth and Soil Erosion in the Semi-arid Woodlands of New South Wales. Sydney: NSW Department of Land and Water Conservation, 42.
29 Eldridge D J, Bowker M A, Maestre F T, et al.2011. Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis. Ecology Letters, 14(7): 709-722.
30 Ernst W H O.1988. Seed and seedling ecology of Brachystegia spiciformis, a predominant tree component in Miombo woodlands in South Central Africa. Forest Ecology and Management, 25(3-4): 195-210.
31 Fatunbi A O, Dube S.2008. Land degradation in a game reserve in Eastern Cape of South Africa: soil properties and vegetation cover. Scientific Research and Essay, 3: 111-119.
32 Friis I, Vollesen K.2005. Flora of the Sudan-Uganda border area east of the Nile. II. Catalogue of vascular plants, 2nd part, vegetation and phytogeography. Biologiske Skrifter, 51(2): 390-855.
33 Gemedo-Dalle, Maass B L, Isselstein J.2006. Rangeland condition and trend in the semi-arid Borana lowlands, southern Oromia, Ethiopia. African Journal of Range and Forage Science, 23(1): 49-58.
34 Gibbens R P, McNeely R P, Havstad K M, et al.2005. Vegetation changes in the Jornada Basin from 1858 to 1998. Journal of Arid Environments, 61(4): 651-668.
35 Gottfried G J, Allen L S, Warren P L, et al.2009. Private-public collaboration to reintroduce fire into the changing ecosystems of the southwestern borderlands region. Fire Ecology, 5(1): 85-99.
36 Grover H D, Musick H B.1990. Shrubland encroachment in southern New Mexico, U.S.A.: an analysis of desertification processes in the American southwest. Climate Change, 17(2-3): 305-330.
37 Hottman M T, O’Connor T G.1999. Vegetation change over 40 years in the Weenen/Muden area, KwaZulu-Natal: evidence from photo-panoramas. African Journal of Range and Forage Science, 16(2-3): 71-88.
38 Idso S B.1992. Shrubland expansion in the American Southwest. Climatic Change, 22(1): 85-86.
39 Illius A W, O’Connor T G.1999. On the relevance of nonequilibrium concepts to arid and semiarid grazing systems. Ecological Applications, 9(3): 798-813.
40 Jeltsch F, Milton S J, Dean W R J, et al.1996. Tree spacing and coexistence in semiarid savannas. Journal of Ecology, 84(4): 583-595.
41 Kgosikoma O, Mojeremane W, Harvie B A.2012. Pastoralists’ perception and ecological knowledge on savanna ecosystem dynamics in semi-arid Botswana. Ecology and Society, 17(4): 27.
42 K?chy M, Wilson S D.2001. Nitrogen deposition and forest expansion in the northern Great Plains. Journal of Ecology, 89(5): 807-817.
43 Maestre F T, Bowker M A, Puche M D, et al.2009. Shrub encroachment can reverse desertification in semi-arid Mediterranean grasslands. Ecology Letters, 12(9): 930-941.
44 Moleele N M, Perkins J S.1998. Encroaching woody plant species and boreholes: is cattle density the main driving factor in the Olifants Drift communal grazing lands, south-eastern Botswana? Journal of Arid Environments, 40(3): 245-253.
45 Moleele N M, Ringrose S, Matheson W, et al.2002. More woody plants? The status of bush encroachment in Botswana’s grazing areas. Journal of Environmental Management, 64(1): 3-11.
46 Morgan J A, Milchunas D G, LeCain D R, et al.2007. Carbon dioxide enrichment alters plant community structure and accelerates shrub growth in the shortgrass steppe. Proceedings of the National Academy of Sciences of the United States of America, 104(37): 14724-14729.
47 Moshe D, Bailey C L, Scholes R J.2000. The effect of elevated atmospheric carbon dioxide on selected savanna plants. In: Seydack A H W, Vermeulen W J, Vermeulen C. Proceedings: Natural Forests and Savanna Woodlands Symposium II: Towards Sustainable Management Based on Scientific Understanding of Natural Forests and Woodlands. Knysna, South Africa: Department of Water Affairs and Forestry, 142-144.
48 Mphinyane W N, Moleele N M, Sebego R J.2011. Effect of burning interval on species composition, herbage yield and bush control in the savanna of eastern Botswana. Botswana Journal of Agriculture and Applied Science, 7(1): 96-106.
49 Noble J C.1997. The Delicate and Noxious Scrub: CSIRO Studies on Native Tree and Shrub Proliferation in the Semi-arid Woodlands of Eastern Australia. Canberra: CSIRO.
50 Nott K, Savage M J.1985. Observations on the utilization of a dune succulent by Namib faunae. South African Journal of Zoology, 20(4): 269-271.
51 Oba G, Post E, Syvertsen P O, et al.2000. Bush cover and range condition assessments in relation to landscape and grazing in southern Ethiopia. Landscape Ecology, 15(6): 535-546.
52 O’Connor T G.1991. Local extinction in perennial grasslands: a life-history approach. The American Naturalist, 137(6): 753-773.
53 O’Connor T G, Puttick J R, Hoffman M T.2014. Bush encroachment in southern Africa: changes and causes. African Journal of Range & Forage Science, 31(2): 67-88.
54 Polley H W, Johnson H B, Mayeux H S.1992. Carbon dioxide and water fluxes of C3 annuals and C3 and C4 perennials at subambient CO2 concentrations. Functional Ecology, 6(6): 693-703.
55 Price J N, Morgan J W.2008. Woody plant encroachment reduces species richness of herb-rich woodlands in southern Australia. Austral Ecology, 33(3): 278-289.
56 Prins H H T, Van der Jeugd H P.1992. Growth rates of shrubs on different soils in Tanzania. African Journal of Ecology, 30(4): 309-315.
57 Ratajczak Z, Nippert J B, Collins S L.2012. Woody encroachment decreases diversity across North American grasslands and savannas. Ecology, 93(4): 697-703.
58 Reich P, Eswaran H, Kapur S, et al.2000. Land degradation and desertification in desert margins, International Symposiumon Desertification/2000-Konya. [2004-10-29]. .
59 Reynolds J F, Smith D M S, Lambin E F, et al.2007. Global desertification: building a science for dryland development. Science, 316(5826): 847-851.
60 Richardson F D, Hahn B D, Hoffman M T.2005. On the dynamics of grazing systems in the semi-arid succulent Karoo: The relevance of equilibrium and non-equilibrium concepts to the sustainability of semi-arid pastoral systems. Ecological Modelling, 187(4): 491-512.
61 Richter C G F, Snyman H A, Smit G N.2001. The influence of tree density on the grass layer of three semi-arid savanna types of Southern Africa. African Journal of Range and Forage Science, 18(2-3): 103-109.
62 Roques K G, O’Connor T G, Watkinson A R.2001. Dynamics of shrub encroachment in an African savanna: relative influences of fire, herbivory, rainfall and density dependence. Journal of Applied Ecology, 38(2): 268-280.
63 Russell J, Ward D.2014. Vegetation change in northern KwaZulu-Natal since the Anglo-Zulu War of 1879: local or global drivers?. African Journal of Range and Forage Science, 31(2): 89-105.
64 Sankaran M, Ratnam J, Hanan N P.2004. Tree-grass coexistence in savannas revisited-insights from an examination of assumptions and mechanisms invoked in existing models. Ecology Letters, 7(6): 480-490.
65 Sankaran M, Hanan N P, Scholes R J, et al.2005. Determinants of woody cover in African savannas. Nature, 438(7069): 846-849.
66 Sankaran M, Ratnam J, Hanan N.2008. Woody cover in African savannas: the role of resources, fire and herbivory. Global Ecology and Biogeography, 17(2): 236-245.
67 Schmutz E M, Smith E L, Ogden P R.1991. Desert grassland. In: Coupland R T. Natural Grasslands: Introduction and Western Hemisphere. Amsterdam: Elsevier, 337-362.
68 Scholes R J, Archer S R.1997. Tree-grass interactions in savannas. Annual Review of Ecology and Systematics, 28: 517-544.
69 Skarpe C.1990. Structure of the woody vegetation in disturbed and undisturbed arid savanna, Botswana. Vegetation, 87(1): 11-18.
70 Smit G N.2003. The Importance of Ecosystem Dynamics in Managing the Bush Encroachment Problem in Southern Africa. Bloemfontein: University of the Free State. In: Proceedings of the Seventh International Rangelands Congress, 26 July-1 August 2003, Durban, South Africa, 14-22.
71 Smith D L, Johnson L.2004. Vegetation-mediated changes in microclimate reduce soil respiration as woodlands expand into grasslands. Ecology, 85(12): 3348-3361.
72 Tefera S, Snyman H A, Smit G N.2007. Rangeland dynamics in southern Ethiopia: (3). Assessment of rangeland condition in relation to land-use and distance from water in semi-arid Borana rangelands. Journal of Environmental Management, 85(2): 453-460.
73 Tessema Z K, de Boer W F, Baars R M T, et al.2012. Influence of grazing on soil seed banks determines the restoration potential of aboveground vegetation in a semi-arid savanna of Ethiopia. Biotropica, 44(2): 211-219.
74 UNCCD, UNDP, UNEP.2009. Climate Change in the African Drylands: Options and Opportunities for Adaptation and Mitigation. Bonn, New York and Nairobi: UNCCD, UNDP and UNEP.
75 Van Auken O W, Gese E M, Connors K.1985. Fertilization response of early and late successional species: Acacia smallii and Celtis laevigata. Botanical Gazette, 146(4): 564-569.
76 Van Auken O W.2000. Shrub invasions of North American grasslands. Annual Review of Ecology and Systematics, 31: 192-215.
77 Van Auken O W.2009. Causes and consequences of woody plant encroachment into western North American grasslands. Journal of Environmental Management, 90(10): 2931-2942.
78 Vetter S.2005. Rangelands at equilibrium and non-equilibrium: recent developments in the debate. Journal of Arid Environments, 62(2): 321-341.
79 Ward D.2005. Do we understand the causes of bush encroachment in African savannas? African Journal of Range and Forage Science, 22: 101-105.
80 Ward D, Hoffman M T, Collocott S J.2014. A century of woody plant encroachment in the dry Kimberley savanna of South Africa. African Journal of Range and Forage Science, 31(2): 107-121.
81 Walter H.1939. Grassland, savanna and bush in arid parts of Africa and their ecological dependence. Jahrbücher für Wissenschaftlike Botanik, 87: 750-860. (in German)
82 Walker H.1971. Ecology of Tropical and Subtropical Vegetation. Edinburgh, UK: Oliver and Boyd.
83 Westoby M, Walker B, Noy-Meir I.1989. Opportunistic management for rangelands not at equilibrium. Journal of Range Management, 42(4): 266-274.
84 White F.1983. The vegetation of Africa. A descriptive memoir to accompany the Unesso/AETFAT/UNSO vegetation map of Africa (3 plates), 1:5,000,000. Paris: UNESCO.
85 Zewdu T, Oustelet Y.2007. Vegetation composition, biomass production, carrying capacity and grassland types in Odolla Area of Shinile Zone, Eastern Ethiopia. East African Journal of Sciences, 1(2): 148-159.
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