Plant Ecology

, Volume 214, Issue 10, pp 1211–1222 | Cite as

Ecosystem responses to woody plant encroachment in a semiarid savanna rangeland

Article

Abstract

Woody plant encroachment alters the structure and function of rangeland ecosystems. The objective of this study was to explore the association between woody plant encroachment and various ecosystem properties (i.e. vascular plant species diversity, richness, evenness, soil organic matter, herbaceous biomass, leaf litter and bare ground cover) in a semiarid savanna rangeland, and also to test whether the relationships were influenced by woody species composition, elevation and site. We carried out a vegetation survey in four rangeland sites in the lower Omo region of southwestern Ethiopia, and regressed each one of the ecosystem properties, separately, against woody plant density, elevation and site using multiple linear regressions. We found that vascular plant species diversity, richness and evenness increased with woody plant density, most likely due to increased spatial heterogeneity and soil microclimate improvement. Bare ground cover increased significantly, whereas herbaceous biomass and soil organic matter did not respond to woody encroachment. In a subsequent investigation, we used a redundancy analysis to assess whether ecosystem properties were influenced by the identity of encroaching woody plant species. Species diversity and richness responded positively to Lannea triphylla, whereas leaf litter responded positively to Grewia tenax and G. villosa. Our findings suggest that woody plant encroachment in a semiarid rangeland does alter ecosystem properties. However, its impact is highly variable, influenced by a set of factors including the level of encroachment and identity of encroaching woody species.

Keywords

Bush encroachment Ecosystem properties Community structure and function Animal production Biodiversity Lower Omo region 

References

  1. Alberti G, Leronni V, Piazzi M, Petrella F, Mairota P, Peressotti A, Piussi P, Valentini R, Gristina L, La Mantia T, Novara A, Ruhl J (2011) Impact of woody encroachment on soil organic carbon and nitrogen in abandoned agricultural lands along a rainfall gradient in Italy. Reg Environ Change 11:917–924CrossRefGoogle Scholar
  2. Anderson MJ, Gribble NA (1998) Partitioning the variation among spatial, temporal and environmental components in a multivariate data set. Aust J Ecol 23:158–167CrossRefGoogle Scholar
  3. Angassa A (2005) The ecological impact of bush encroachment on the yield of grasses in Borana rangeland ecosystem. Afr J Ecol 43:14–20CrossRefGoogle Scholar
  4. Archer SR (2010) Rangeland conservation and shrub encroachment: new perspectives on an old problem. In: du Toit J, Kock R, Deutsch J (eds) Wild rangelands. Wiley-Blackwell, Oxford, pp 53–97CrossRefGoogle Scholar
  5. Belay TA, Moe SR (2012) Woody dominance in a semi-arid savanna rangeland—evidence for competitive self-thinning. Acta Oecol 45:98–105CrossRefGoogle Scholar
  6. Belay TA, Totland Ø, Moe SR (2013) Woody vegetation dynamics in the rangelands of lower Omo region, southwestern Ethiopia. J Arid Environ 89:94–102CrossRefGoogle Scholar
  7. Belsky AJ, Amundson RG, Duxbury JM, Riha SJ, Ali AR, Mwonga SM (1989) The effect of trees on their physical, chemical, and biological environments in a semi-arid savanna in Kenya. J Appl Ecol 26:1005–1024CrossRefGoogle Scholar
  8. Bertness MD, Callaway RM (1994) Positive interaction in communities. Trends Ecol Evol 9:191–193PubMedCrossRefGoogle Scholar
  9. Borcard D, Legendre P, Drapeau P (1992) Partialling out the spatial component of ecological variation. Ecology 73:1045–1055CrossRefGoogle Scholar
  10. Brantley ST, Young DR (2010) Shrub expansion stimulates soil C and N storage along a coastal soil chronosequence. Glob Change Biol 16:2052–2061CrossRefGoogle Scholar
  11. Cabral AC, De Miguel JM, Rescia AJ, Schmitz MF, Pineda FD (2003) Shrub encroachment in Argentinean savannas. J Veg Sci 14:145–152CrossRefGoogle Scholar
  12. Callaway RM, Walker LR (1997) Competition and facilitation: a synthetic approach to interactions in plant communities. Ecology 78:1958–1965CrossRefGoogle Scholar
  13. Cornwell WK, Cornelissen JHC, Amatangelo K, Dorrepaal E, Eviner VT, Godoy O, Hobbie SE, Hoorens B, Kurokawa H, Perez-Harguindeguy N, Quested HM, Santiago LS, Wardle DA, Wright IJ, Aerts R, Allison SD, van Bodegom P, Brovkin V, Chatain A, Callaghan TV, Diaz S, Garnier E, Gurvich DE, Kazakou E, Klein JA, Read J, Reich PB, Soudzilovskaia NA, Vaieretti MV, Westoby M (2008) Plant species traits are the predominant control on litter decomposition rates within biomes worldwide. Ecol Lett 11:1065–1071PubMedCrossRefGoogle Scholar
  14. Cramer MJ, Willig MR (2005) Habitat heterogeneity, species diversity and null models. Oikos 108:209–218CrossRefGoogle Scholar
  15. Dalle G, Maass BL, Isselstein J (2006) Encroachment of woody plants and its impact on pastoral livestock production in the Borana lowlands, southern Oromia, Ethiopia. Afr J Ecol 44:237–246CrossRefGoogle Scholar
  16. Donaldson CH (1969) In: Department of Agricultural Technical Services (ed) Bush encroachment with special reference to the Blackthorn problem of the Molopo area. Department of Agricultural Technical Services, PretoriaGoogle Scholar
  17. Eldridge DJ, Bowker MA, Maestre FT, Roger E, Reynolds JF, Whitford WG (2011) Impacts of shrub encroachment on ecosystem structure and functioning: towards a global synthesis. Ecol Lett 14:709–722PubMedCrossRefGoogle Scholar
  18. Finzi AC, Schlesinger AH (2002) Species control variation in litter decomposition in a pine forest exposed to elevated CO2. Glob Change Biol 8:1217–1229CrossRefGoogle Scholar
  19. Gill RA, Burke IC (1999) Ecosystem consequences of plant life form changes at three sites in the semiarid United States. Oecologia 121:551–563CrossRefGoogle Scholar
  20. Gomez-Aparicio L, Gomez JM, Zamora R, Boettinger JL (2005) Canopy vs. soil effects of shrubs facilitating tree seedlings in Mediterranean montane ecosystems. J Veg Sci 16:191–198CrossRefGoogle Scholar
  21. Grime JP (1979) Plant strategies and vegetation processes. Wiley, New YorkGoogle Scholar
  22. Hibbard KA, Archer S, Schimel DS, Valentine DW (2001) Biogeochemical changes accompanying woody plant encroachment in a subtropical savanna. Ecology 82:1999–2011CrossRefGoogle Scholar
  23. Hudak AT (1999) Rangeland mismanagement in South Africa: failure to apply ecological knowledge. Hum Ecol 27:55–78CrossRefGoogle Scholar
  24. Hudak AT, Wessman CA, Seastedt TR (2003) Woody overstorey effects on soil carbon and nitrogen pools in South African savanna. Austral Ecol 28:173–181CrossRefGoogle Scholar
  25. Huxman TE, Wilcox BP, Breshears DD, Scott RL, Snyder KA, Small EE, Hultine K, Pockman WT, Jackson RB (2005) Ecohydrological implications of woody plant encroachment. Ecology 86:308–319CrossRefGoogle Scholar
  26. Knapp AK, Briggs JM, Collins SL, Archer SR, Bret-Harte MS, Ewers BE, Peters DP, Young DR, Shaver GR, Pendall E, Cleary MB (2008) Shrub encroachment in North American grasslands: shifts in growth form dominance rapidly alters control of ecosystem carbon inputs. Glob Change Biol 14:615–623CrossRefGoogle Scholar
  27. Lett MS, Knapp AK (2005) Woody plant encroachment and removal in mesic grassland: production and composition responses of herbaceous vegetation. Am Midl Nat 153:217–231CrossRefGoogle Scholar
  28. Liao JD, Boutton TW (2008) Soil microbial biomass response to woody plant invasion of grassland. Soil Biol Biochem 40:1207–1216CrossRefGoogle Scholar
  29. Lunt ID, Winsemius LM, McDonald SP, Morgan JW, Dehaan RL (2010) How widespread is woody plant encroachment in temperate Australia? Changes in woody vegetation cover in lowland woodland and coastal ecosystems in Victoria from 1989 to 2005. J Biogeogr 37:722–732CrossRefGoogle Scholar
  30. Macarthur R, Macarthur JW (1961) On bird species-diversity. Ecology 42:594–598CrossRefGoogle Scholar
  31. Maestre FT, Bowker MA, Puche MD, Hinojosa MB, Martinez I, Garcia-Palacios P, Castillo AP, Soliveres S, Luzuriaga AL, Sanchez AM, Carreira JA, Gallardo A, Escudero A (2009) Shrub encroachment can reverse desertification in semi-arid Mediterranean grasslands. Ecol Lett 12:930–941PubMedCrossRefGoogle Scholar
  32. Magurran AE (1988) Ecological diversity and its measurement. Princeton University Press, PrincetonCrossRefGoogle Scholar
  33. Martens SN, Breshears DD, Meyer CW (2000) Spatial distributions of understory light along the grassland/forest continuum: effects of cover, height, and spatial pattern of tree canopies. Ecol Model 126:79–93CrossRefGoogle Scholar
  34. McCulley RL, Jackson RB (2012) Conversion of tallgrass prairie to woodland: consequences for carbon and nitrogen cycling. Am Midl Nat 167:307–321CrossRefGoogle Scholar
  35. Moleele NM, Ringrose S, Matheson W, Vanderpost C (2002) More woody plants? The status of bush encroachment in Botswana’s grazing areas. J Environ Manag 64:3–11CrossRefGoogle Scholar
  36. Mooney HA (1972) The carbon balance of plants. Annu Rev Ecol Syst 3:315–346CrossRefGoogle Scholar
  37. Mueller-Dombois D, Ellenberg H (1974) Aims and methods of vegetation ecology. Wiley, New YorkGoogle Scholar
  38. Parizek B, Rostagno CM, Sottini R (2002) Soil erosion as affected by shrub encroachment in northeastern Patagonia. J Range Manag 55:43–48CrossRefGoogle Scholar
  39. Petersen SL, Stringham TK, Roundy BA (2009) A process-based application of state-and-transition models: a case study of western juniper (Juniperus occidentalis) encroachment. Rangel Ecol Manag 62:186–192CrossRefGoogle Scholar
  40. Pickett STA (1989) Space-for-time substitution as an alternative to long-term studies. In: Likens GE (ed) Long-term studies in ecology—approaches and alternatives. Springer, New York, pp 110–135CrossRefGoogle Scholar
  41. Price JN, Morgan JW (2008) Woody plant encroachment reduces species richness of herb-rich woodlands in southern Australia. Austral Ecol 33:278–289CrossRefGoogle Scholar
  42. Pugnaire FI, Luque MT (2001) Changes in plant interactions along a gradient of environmental stress. Oikos 93:42–49CrossRefGoogle Scholar
  43. Pugnaire FI, Armas C, Maestre FT (2011) Positive plant interactions in the Iberian Southeast: mechanisms, environmental gradients, and ecosystem function. J Arid Environ 75:1310–1320CrossRefGoogle Scholar
  44. Ratajczak Z, Nippert JB, Collins S (2012) Woody encroachment decreases diversity across North American grasslands and savannas. Ecology 93:697–703PubMedCrossRefGoogle Scholar
  45. Richter C, Snyman H, Smit G (2001) The influence of tree density on the grass layer of three semi-arid savanna types of southern Africa. Afr J Range Forage Sci 18:103–109CrossRefGoogle Scholar
  46. Roques KG, O’Connor TG, Watkinson AR (2001) Dynamics of shrub encroachment in an African savanna: relative influences of fire, herbivory, rainfall and density dependence. J Appl Ecol 38:268–280CrossRefGoogle Scholar
  47. Schleicher J, Meyer KM, Wiegand K, Schurr FM, Ward D (2011) Disentangling facilitation and seed dispersal from environmental heterogeneity as mechanisms generating associations between savanna plants. J Veg Sci 22:1038–1048CrossRefGoogle Scholar
  48. Schlesinger WH, Reynolds JF, Cunningham GL, Huenneke LF, Jarrell WM, Virginia RA, Whitford WG (1990) Biological feedbacks in global desertification. Science 247:1043–1048PubMedCrossRefGoogle Scholar
  49. Scholes RJ, Archer SR (1997) Tree–grass interactions in savannas. Annu Rev Ecol Syst 28:517–544CrossRefGoogle Scholar
  50. Schulte EE, Hopkins BG (1996) Estimation of organic matter by weight loss-on-ignition. In: Magdoff FR, Tabatabai MA, Hanlon EA (eds) Soil organic matter: analysis and interpretation. SSSA Spec. Publ, Madison, pp 21–31Google Scholar
  51. Smith DL, Johnson LC (2003) Expansion of Juniperus virginiana L. in the Great Plains: changes in soil organic carbon dynamics. Glob Biogeochem Cycle 17:1–12CrossRefGoogle Scholar
  52. Springsteen A, Loya W, Liebig M, Hendrickson J (2010) Soil carbon and nitrogen across a chronosequence of woody plant expansion in North Dakota. Plant Soil 328:369–379CrossRefGoogle Scholar
  53. R Development Core Team (2010) A language and environment for statistical computing. R Foundation for Statistical Computing, Vienna. http://www.R-project.org
  54. ter Braak CJF, Smilauer P (2002) CANOCO reference manual and canodraw for windows user’s guide: software for canonical community ordination (version 4.5). Microcomputer Power, IthacaGoogle Scholar
  55. Ward D (2005) Do we understand the causes of bush encroachment in African savannas? Afr J Range Forage Sci 22:101–105CrossRefGoogle Scholar
  56. Wessman C, Archer S, Johnson L, Asner G (2004) Woodland expansion in US grasslands: assessing land-cover change and biogeochemical impacts. In: Gutman G, Janetos AC, Justice CO et al (eds) Land change science: observing, monitoring and understanding trajectories of change on the earth’s surface. Kluwer Academic Publishers, Dordrecht, pp 185–208Google Scholar
  57. Wiegand K, Ward D, Saltz D (2005) Multi-scale patterns and bush encroachment in an and savanna with a shallow soil layer. J Veg Sci 16:311–320CrossRefGoogle Scholar
  58. Wiegand K, Saitz D, Ward D (2006) A patch-dynamics approach to savanna dynamics and woody plant encroachment—insights from an and savanna. Perspect Plant Ecol Evol Syst 7:229–242CrossRefGoogle Scholar

Copyright information

© Springer Science+Business Media Dordrecht 2013

Authors and Affiliations

  • Tamrat A. Belay
    • 1
    • 2
  • Ørjan Totland
    • 1
  • Stein R. Moe
    • 1
  1. 1.Department of Ecology and Natural Resource ManagementNorwegian University of Life SciencesÅsNorway
  2. 2.Hawassa UniversityHawassaEthiopia

Personalised recommendations