Grasses and browsers reinforce landscape heterogeneity by excluding trees from ecosystem hotspots
- 432 Downloads
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.
KeywordsBoma Grazing lawn Tree–grass interactions Spatial heterogeneity Edge interactions
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).
- 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
- DigitalGlobe (2003) QuickBird scene 000000058088_01_P001, Level Standard 2A. DigitalGlobe, Longmont, Colorado, 6/20/2003Google Scholar
- 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
- 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
- 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
- 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
- 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
- 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
- Veblen KE (2011) Savanna glade hotspots: plant community development and synergy with large herbivores. J Arid Environ (in revision)Google Scholar
- Walter H (1971) Ecology of tropical and subtropical vegetation. Oliver and Boyd, EdinburghGoogle Scholar
- 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