Advertisement

Oecologia

, Volume 168, Issue 3, pp 749–759 | Cite as

Grasses and browsers reinforce landscape heterogeneity by excluding trees from ecosystem hotspots

  • Lauren M. PorenskyEmail author
  • Kari E. Veblen
Community ecology - Methods Paper

Abstract

Spatial heterogeneity in woody cover affects biodiversity and ecosystem function, and may be particularly influential in savanna ecosystems. Browsing and interactions with herbaceous plants can create and maintain heterogeneity in woody cover, but the relative importance of these drivers remains unclear, especially when considered across multiple edaphic contexts. In African savannas, abandoned temporary livestock corrals (bomas) develop into long-term, nutrient-rich ecosystem hotspots with unique vegetation. In central Kenya, abandoned corral sites persist for decades as treeless ‘glades’ in a wooded matrix. Though glades are treeless, areas between adjacent glades have higher tree densities than the background savanna or areas near isolated glades. The mechanisms maintaining these distinctive woody cover patterns remain unclear. We asked whether browsing or interactions with herbaceous plants help to maintain landscape heterogeneity by differentially impacting young trees in different locations. We planted the mono-dominant tree species (Acacia drepanolobium) in four locations: inside glades, far from glades, at edges of isolated glades and at edges between adjacent glades. Within each location, we assessed the separate and combined effects of herbivore exclusion (caging) and herbaceous plant removal (clearing) on tree survival and growth. Both caging and clearing improved tree survival and growth inside glades. When herbaceous plants were removed, trees inside glades grew more than trees in other locations, suggesting that glade soils were favorable for tree growth. Different types of glade edges (isolated vs. non-isolated) did not have significantly different impacts on tree performance. This represents one of the first field-based experiments testing the separate and interactive effects of browsing, grass competition and edaphic context on savanna tree performance. Our findings suggest that, by excluding trees from otherwise favorable sites, both herbaceous plants and herbivores help to maintain functionally important landscape heterogeneity in African savannas.

Keywords

Boma Grazing lawn Tree–grass interactions Spatial heterogeneity Edge interactions 

Notes

Acknowledgments

We thank Mpala Research Centre and Jessel Ranch for letting us perform this experiment on their properties. Fredrick Erii, John Lochukuya, Matthew Namoni, Jackson Ekadeli, Solveig Franziska Bucher and Michael Ellis provided invaluable field assistance. Truman P. Young, Neil Willits, Young Lab 2008–2011, Mary L. Cadenasso and Valerie Eviner provided helpful advice and comments. This research complies with Kenyan law and was approved by the Kenyan Ministry of Science and Technology. Financial support came from National Science Foundation (NSF) GRF and Jastro-Shields Fellowships (to L.M.P. and K.E.V.), NSF DDIG (to L.M.P), and NSF LTREB DEB-08-16453 (to T.P. Young).

References

  1. Augustine DJ (2003) Long-term, livestock-mediated redistribution of nitrogen and phosphorus in an East African savanna. J Appl Ecol 40:137–149CrossRefGoogle Scholar
  2. Augustine DJ (2004) Influence of cattle management on habitat selection by impala on central Kenyan rangeland. J Wildl Manag 68:916–923CrossRefGoogle Scholar
  3. Augustine DJ, McNaughton SJ (2004) Regulation of shrub dynamics by native browsing ungulates on East African rangeland. J Appl Ecol 41:45–58CrossRefGoogle Scholar
  4. Augustine DJ, Veblen KE, Goheen JR, Riginos C, Young TP (2011) Pathways for positive cattle–wildlife interactions in semi-arid rangelands. In: Georgiadis N (ed) Conserving wildlife in African Landscapes: Kenya’s Ewaso Ecosystem, vol Smithsonian Contributions to Zoology no. 632. Smithsonian Institution Scholarly Press, Washington, D.C., pp 55–71Google Scholar
  5. Blackmore AC, Mentis MT, Scholes RJ (1990) The origin and extent of nutrient-enriched patches within a nutrient-poor savanna in South-Africa. J Biogeogr 17:463–470CrossRefGoogle Scholar
  6. Bond WJ (2008) What limits trees in C-4 grasslands and savannas? Annu Rev Ecol Evol Syst 39:641–659CrossRefGoogle Scholar
  7. Cramer MD, van Cauter A, Bond WJ (2010) Growth of N-2-fixing African savanna Acacia species is constrained by below-ground competition with grass. J Ecol 98:156–167CrossRefGoogle Scholar
  8. Davies KW, Bates JD, Svejcar TJ, Boyd CS (2010) Effects of long-term livestock grazing on fuel characteristics in rangelands: an example from the sagebrush steppe. Rangel Ecol Manag 63:662–669CrossRefGoogle Scholar
  9. Deckers J, Sppargaren O, Nachtergaele F (2001) Vertisols: genesis, properties and soilscape management for sustainable development. In: Syers JK, Penning de Vries FWT, Nyamudeza P (eds) The sustainable management of vertisols. CABI, Wallington, pp 3–20CrossRefGoogle Scholar
  10. Dickie IA, Schnitzer SA, Reich PB, Hobbie SE (2007) Is oak establishment in old-fields and savanna openings context dependent? J Ecol 95:309–320CrossRefGoogle Scholar
  11. DigitalGlobe (2003) QuickBird scene 000000058088_01_P001, Level Standard 2A. DigitalGlobe, Longmont, Colorado, 6/20/2003Google Scholar
  12. Dublin HT, Sinclair ARE, McGlade J (1990) Elephants and fire as causes of multiple stable states in the Serengeti Mara woodlands. J Anim Ecol 59:1147–1164CrossRefGoogle Scholar
  13. Eckhardt HC, van Wilgen BW, Biggs HC (2000) Trends in woody vegetation cover in the Kruger National Park, South Africa, between 1940 and 1998. Afr J Ecol 38:108–115CrossRefGoogle Scholar
  14. Fox-Dobbs K, Doak DF, Brody AK, Palmer TM (2010) Termites create spatial structure and govern ecosystem function by affecting N-2 fixation in an East African savanna. Ecology 91:1296–1307PubMedCrossRefGoogle Scholar
  15. Goheen JR, Keesing F, Allan BF, Ogada DL, Ostfeld RS (2004) Net effects of large mammals on Acacia seedling survival in an African savanna. Ecology 85:1555–1561CrossRefGoogle Scholar
  16. Goheen JR, Young TP, Keesing F, Palmer TM (2007) Consequences of herbivory by native ungulates for the reproduction of a savanna tree. J Ecol 95:129–138CrossRefGoogle Scholar
  17. Goheen JR, Palmer TM, Keesing F, Riginos C, Young TP (2010) Large herbivores facilitate savanna tree establishment via diverse and indirect pathways. J Anim Ecol 79:372–382PubMedCrossRefGoogle Scholar
  18. Gregory NC, Sensenig RL, Wilcove DS (2010) Effects of controlled fire and livestock grazing on bird communities in east African savannas. Conserv Biol 24:1606–1616PubMedCrossRefGoogle Scholar
  19. Hodar JA, Zamora R, Castro J, Gomez JM, Garcia D (2008) Biomass allocation and growth responses of Scots pine saplings to simulated herbivory depend on plant age and light availability. Plant Ecol 197:229–238CrossRefGoogle Scholar
  20. Horvitz CC, Schemske DW (2002) Effects of plant size, leaf herbivory, local competition and fruit production on survival, growth and future reproduction of a neotropical herb. J Ecol 90:279–290CrossRefGoogle Scholar
  21. Jeltsch F, Weber GE, Grimm V (2000) Ecological buffering mechanisms in savannas: a unifying theory of long-term tree-grass coexistence. Plant Ecol 161:161–171CrossRefGoogle Scholar
  22. Knoop WT, Walker BH (1985) Interactions of woody and herbaceous vegetation in a southern African savanna. J Ecol 73:235–253CrossRefGoogle Scholar
  23. Lamprey R, Reid RS (2004) Expansion of human settlement in Kenya’s Maasai Mara: what future for pastoralism and wildlife? J Biogeogr 31:997–1032CrossRefGoogle Scholar
  24. Laporte I, Muhly TB, Pitt JA, Alexander M, Musiani M (2010) Effects of wolves on elk and cattle behaviors: implications for livestock production and wolf conservation. Plos One 5(8):e11954Google Scholar
  25. Levick SR, Asner GP, Kennedy-Bowdoin T, Knapp DE (2009) The relative influence of fire and herbivory on savanna three-dimensional vegetation structure. Biol Conserv 142:1693–1700CrossRefGoogle Scholar
  26. Levick SR, Asner GP, Kennedy-Bowdoin T, Knapp DE (2010) The spatial extent of termite influences on herbivore browsing in an African savanna. Biol Conserv 143:2462–2467CrossRefGoogle Scholar
  27. Lundholm JT (2009) Plant species diversity and environmental heterogeneity: spatial scale and competing hypotheses. J Veg Sci 20:377–391CrossRefGoogle Scholar
  28. Midgley JJ, Lawes MJ, Chamaille-Jammes S (2010) Savanna woody plant dynamics: the role of fire and herbivory, separately and synergistically. Aust J Bot 58:1–11CrossRefGoogle Scholar
  29. Muchiru AN, Western D, Reid RS (2009) The impact of abandoned pastoral settlements on plant and nutrient succession in an African savanna ecosystem. J Arid Environ 73:322–331CrossRefGoogle Scholar
  30. Odadi WO (2010) Competitive and facilitative interactions between cattle and wild ungulates in a semi-arid savanna rangeland in Laikipia, Kenya. PhD dissertation, Egerton University, Njoro, KenyaGoogle Scholar
  31. Okello BD, Young TP, Riginos C, Kelly D, O’Connor TG (2008) Short-term survival and long-term mortality of Acacia drepanolobium after a controlled burn. Afr J Ecol 46:395–401CrossRefGoogle Scholar
  32. Peles JD, Barrett GW (1996) Effects of vegetative cover on the population dynamics of meadow voles. J Mammal 77:857–869CrossRefGoogle Scholar
  33. Pellew RAP (1983) The impacts of elephant, giraffe and fire upon the Acacia tortilis woodlands of the Serengeti. Afr J Ecol 21:41–74CrossRefGoogle Scholar
  34. Porensky LM (2011) When edges meet: interacting edge effects in an African savanna. J Ecol 99:923–934CrossRefGoogle Scholar
  35. Reid RS, Ellis JE (1995) Impacts of pastoralists on woodlands in South Turkana, Kenya: livestock-mediated tree recruitment. Ecol Appl 5:978–992CrossRefGoogle Scholar
  36. Rietkerk M, Dekker SC, de Ruiter PC, van de Koppel J (2004) Self-organized patchiness and catastrophic shifts in ecosystems. Science 305:1926–1929PubMedCrossRefGoogle Scholar
  37. Riginos C (2009) Grass competition suppresses savanna tree growth across multiple demographic stages. Ecology 90:335–340PubMedCrossRefGoogle Scholar
  38. Riginos C, Grace JB (2008) Savanna tree density, herbivores, and the herbaceous community: bottom–up versus top–down effects. Ecology 89:2228–2238PubMedCrossRefGoogle Scholar
  39. Riginos C, Young TP (2007) Positive and negative effects of grass, cattle, and wild herbivores on Acacia saplings in an East African savanna. Oecologia 153:985–995PubMedCrossRefGoogle Scholar
  40. 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
  41. Sankaran M, Ratnam J, Hanan NP (2004) Tree-grass coexistence in savannas revisited—insights from an examination of assumptions and mechanisms invoked in existing models. Ecol Lett 7:480–490CrossRefGoogle Scholar
  42. Sankaran M, Hanan NP, Scholes RJ, Ratnam J, Augustine DJ, Cade BS, Gignoux J, Higgins SI, Le Roux X, Ludwig F, Ardo J, Banyikwa F, Bronn A, Bucini G, Caylor KK, Coughenour MB, Diouf A, Ekaya W, Feral CJ, February EC, Frost PGH, Hiernaux P, Hrabar H, Metzger KL, Prins HHT, Ringrose S, Sea W, Tews J, Worden J, Zambatis N (2005) Determinants of woody cover in African savannas. Nature 438:846–849PubMedCrossRefGoogle Scholar
  43. Scholes RJ, Archer SR (1997) Tree-grass interactions in savannas. Annu Rev Ecol Syst 28:517–544CrossRefGoogle Scholar
  44. Staver AC, Bond WJ, Stock WD, van Rensburg SJ, Waldram MS (2009) Browsing and fire interact to suppress tree density in an African savanna. Ecol Appl 19:1909–1919PubMedCrossRefGoogle Scholar
  45. Tamme R, Hiiesalu I, Laanisto L, Szava-Kovats R, Partel M (2010) Environmental heterogeneity, species diversity and co-existence at different spatial scales. J Veg Sci 21:796–801Google Scholar
  46. Tobler MW, Cochard R, Edwards PJ (2003) The impact of cattle ranching on large-scale vegetation patterns in a coastal savanna in Tanzania. J Appl Ecol 40:430–444CrossRefGoogle Scholar
  47. van der Waal C, de Kroon H, de Boer WF, Heitkonig IMA, Skidmore AK, de Knegt HJ, van Langevelde F, van Wieren SE, Grant RC, Page BR, Slotow R, Kohi EM, Mwakiwa E, Prins HHT (2009) Water and nutrients alter herbaceous competitive effects on tree seedlings in a semi-arid savanna. J Ecol 97:430–439CrossRefGoogle Scholar
  48. van der Waal C, Kool A, Meijer S, Kohi E, Heitkönig I, de Boer W, van Langevelde F, Grant R, Peel M, Slotow R, de Knegt H, Prins H, de Kroon H (2011) Large herbivores may alter vegetation structure of semi-arid savannas through soil nutrient mediation. Oecologia 165:1095–1107PubMedCrossRefGoogle Scholar
  49. van Langevelde F, van de Vijver C, Kumar L, van de Koppel J, de Ridder N, van Andel J, Skidmore AK, Hearne JW, Stroosnijder L, Bond WJ, Prins HHT, Rietkerk M (2003) Effects of fire and herbivory on the stability of savanna ecosystems. Ecology 84:337–350CrossRefGoogle Scholar
  50. Veblen KE (2008) Season- and herbivore-dependent competition and facilitation in a semiarid savanna. Ecology 89:1532–1540PubMedCrossRefGoogle Scholar
  51. Veblen KE (2011) Savanna glade hotspots: plant community development and synergy with large herbivores. J Arid Environ (in revision)Google Scholar
  52. Veblen KE, Young TP (2010) Contrasting effects of cattle and wildlife on the vegetation development of a savanna landscape mosaic. J Ecol 98:993–1001CrossRefGoogle Scholar
  53. Walker B, Ludwig D, Holling CS, Peterman RM (1981) Stability of semi-arid savanna grazing systems. J Ecol 69:473–498CrossRefGoogle Scholar
  54. Walter H (1971) Ecology of tropical and subtropical vegetation. Oliver and Boyd, EdinburghGoogle Scholar
  55. Walters M, Milton SJ, Somers MJ, Midgley JJ (2005) Post-dispersal fate of Acacia seeds in an African savanna. S Afr J Wildl Res 35:191–199Google Scholar
  56. Ward D, Esler KJ (2011) What are the effects of substrate and grass removal on recruitment of Acacia mellifera seedlings in a semi-arid environment? Plant Ecol 212:245–250CrossRefGoogle Scholar
  57. Western D, Dunne T (1979) Environmental aspects of settlement site decisions among pastoral Maasai. Hum Ecol 7:75–98CrossRefGoogle Scholar
  58. Western D, Maitumo D (2004) Woodland loss and restoration in a savanna park: a 20-year experiment. Afr J Ecol 42:111–121CrossRefGoogle Scholar
  59. Young TP, Patridge N, Macrae A (1995) Long-term glades in Acacia bushland and their edge effects in Laikipia, Kenya. Ecol Appl 5:97–108CrossRefGoogle Scholar
  60. Young TP, Okello BD, Kinyua D, Palmer TM (1998) KLEE: a longterm multi-species herbivore exclusion experiment in Laikipia, Kenya. Afr J Range Forage Sci 14:94–102CrossRefGoogle Scholar
  61. Young TP, Stanton ML, Christian CE (2003) Effects of natural and simulated herbivory on spine lengths of Acacia drepanolobium in Kenya. Oikos 101:171–179CrossRefGoogle Scholar
  62. Young TP, Palmer TA, Gadd ME (2005) Competition and compensation among cattle, zebras, and elephants in a semi-arid savanna in Laikipia, Kenya. Biol Conserv 122:351–359CrossRefGoogle Scholar

Copyright information

© Springer-Verlag 2011

Authors and Affiliations

  1. 1.Department of Plant Sciences and Graduate Group in EcologyUniversity of CaliforniaDavisUSA
  2. 2.Mpala Research CentreNanyukiKenya
  3. 3.Department of Wildland ResourcesUtah State UniversityLoganUSA

Personalised recommendations