Skip to main content
SpringerLink
Log in

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

  • Published:
Journal of Arid Land Aims and scope Submit manuscript

Abstract

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.

This is a preview of subscription content, log in via an institution to check access.

Access this article

Log in via an institution

Price excludes VAT (USA)
Tax calculation will be finalised during checkout.

Instant access to the full article PDF.

Institutional subscriptions

Similar content being viewed by others

References

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Google Scholar 

  • Adler P B, Raff D A, Lauenroth W. 2001. The effect of grazing on the spatial heterogeneity of vegetation. Oecologia, 128(4): 465–479.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • Archer S, Schimel D S, Holland E A. 1995. Mechanisms of shrubland expansion: land use, climate or CO2?. Climatic Change, 29(1): 91–99.

    Article  Google Scholar 

  • 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.

    Chapter  Google Scholar 

  • Ayres D, Melville G, Bean J, et al. 2001. Woody Weeds, Biodiversity and Landscape Function in Western New South Wales. Dubbo: WEST 2000, 221.

    Google Scholar 

  • Bahre C J. 1991. A Legacy of Change: Historic Human Impact on Vegetation in the Arizona Borderlands. Tucson: University of Arizona Press, 231.

    Google Scholar 

  • 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.

    Google Scholar 

  • Belay T A, Totland Ø, Moe S R. 2013. Woody vegetation dynamics in the rangelands of lower Omo region, southwestern Ethiopia. Journal of Arid Environments, 89: 94–102.

    Article  Google Scholar 

  • Bester F, Reed A. 1997. Bush encroachment: a thorny problem. Spotlight on Agriculture, 1: 175–177.

    Google Scholar 

  • Bews J W. 1917. The plant succession in the Thorn Veld. South African Journal of Science, 14: 163–172.

    Google Scholar 

  • 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.

    Article  Google Scholar 

  • Bothma J P. 2002. Game Ranch Management (4th ed.). Pretoria, South Africa: Van Schaik Publishers.

    Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Google Scholar 

  • Dregne H E. 2002. Land degradation in the drylands. Arid Land Research and Management, 16(2): 99–132.

    Article  Google Scholar 

  • 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.

    Chapter  Google Scholar 

  • 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.

    Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Google Scholar 

  • 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.

    Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • Idso S B. 1992. Shrubland expansion in the American Southwest. Climatic Change, 22(1): 85–86.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • Köchy M, Wilson S D. 2001. Nitrogen deposition and forest expansion in the northern Great Plains. Journal of Ecology, 89(5): 807–817.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • Morgan J A, Milchunas D G, Le Cain 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.

    Article  Google Scholar 

  • 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.

    Google Scholar 

  • 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.

    Google Scholar 

  • 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.

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • O’Connor T G. 1991. Local extinction in perennial grasslands: a life-history approach. The American Naturalist, 137(6): 753–773.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • Ratajczak Z, Nippert J B, Collins S L. 2012. Woody encroachment decreases diversity across North American grasslands and savannas. Ecology, 93(4): 697–703.

    Article  Google Scholar 

  • Reich P, Eswaran H, Kapur S, et al. 2000. Land degradation and desertification in desert margins, International Symposiumon Desertification/2000-Konya. [2004-10-29]. http://www.toprak.org.tr/isd/isd.

    Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • Sankaran M, Hanan N P, Scholes R J, et al. 2005. Determinants of woody cover in African savannas. Nature, 438(7069): 846–849.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

    Google Scholar 

  • Scholes R J, Archer S R. 1997. Tree-grass interactions in savannas. Annual Review of Ecology and Systematics, 28: 517–544.

    Article  Google Scholar 

  • Skarpe C. 1990. Structure of the woody vegetation in disturbed and undisturbed arid savanna, Botswana. Vegetation, 87(1): 11–18.

    Article  Google Scholar 

  • 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.

    Google Scholar 

  • Smith D L, Johnson L. 2004. Vegetation-mediated changes in microclimate reduce soil respiration as woodlands expand into grasslands. Ecology, 85(12): 3348–3361.

    Article  Google Scholar 

  • 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.

    Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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.

  • 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.

    Article  Google Scholar 

  • Van Auken O W. 2000. Shrub invasions of North American grasslands. Annual Review of Ecology and Systematics, 31: 192–215.

    Google Scholar 

  • 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.

    Article  Google Scholar 

  • Vetter S. 2005. Rangelands at equilibrium and non-equilibrium: recent developments in the debate. Journal of Arid Environments, 62(2): 321–341.

    Article  Google Scholar 

  • Ward D. 2005. Do we understand the causes of bush encroachment in African savannas? African Journal of Range and Forage Science, 22: 101–105.

    Article  Google Scholar 

  • 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.

    Article  Google Scholar 

  • 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)

    Google Scholar 

  • Walker H. 1971. Ecology of Tropical and Subtropical Vegetation. Edinburgh, UK: Oliver and Boyd.

    Google Scholar 

  • Westoby M, Walker B, Noy-Meir I. 1989. Opportunistic management for rangelands not at equilibrium. Journal of Range Management, 42(4): 266–274.

    Article  Google Scholar 

  • 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.

    Google Scholar 

  • 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.

    Article  Google Scholar 

Download references

Acknowledgments

The authors would like to acknowledge the information used in this review paper.

Author information

Authors and Affiliations

  1. Department of Biology, College of Natural and Computational Sciences, Haramaya University, Haramaya, 282, Ethiopia

    Anteneh Belayneh

  2. Rangeland Ecology and Biodiversity Program, School of Animal and Range Sciences, College of Agriculture and Environmental Sciences, Haramaya University, Dire Dawa, 138, Ethiopia

    Zewdu K. Tessema

Authors
  1. Anteneh Belayneh
    View author publications

    You can also search for this author in PubMed Google Scholar

  2. Zewdu K. Tessema
    View author publications

    You can also search for this author in PubMed Google Scholar

Corresponding author

Correspondence to Zewdu K. Tessema.

Rights and permissions

Reprints and permissions

About this article

Check for updates. Verify currency and authenticity via CrossMark

Cite this article

Belayneh, A., Tessema, Z.K. Mechanisms of bush encroachment and its inter-connection with rangeland degradation in semi-arid African ecosystems: a review. J. Arid Land 9, 299–312 (2017). https://doi.org/10.1007/s40333-016-0023-x

Download citation

  • Received:

  • Revised:

  • Accepted:

  • Published:

  • Issue Date:

  • DOI: https://doi.org/10.1007/s40333-016-0023-x

Keywords

Search

Navigation