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

  • Community ecology - Methods Paper
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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.

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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).

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  1. Kari E. Veblen

    Present address: Department of Wildland Resources, Utah State University, Logan, UT, 84322, USA

Authors and Affiliations

  1. Department of Plant Sciences and Graduate Group in Ecology, University of California, Davis, CA, 95616, USA

    Lauren M. Porensky & Kari E. Veblen

  2. Mpala Research Centre, P.O. Box 555, Nanyuki, 10400, Kenya

    Lauren M. Porensky & Kari E. Veblen

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  1. Lauren M. Porensky
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  2. Kari E. Veblen
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Correspondence to Lauren M. Porensky.

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Communicated by Scott Collins.

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Porensky, L.M., Veblen, K.E. Grasses and browsers reinforce landscape heterogeneity by excluding trees from ecosystem hotspots. Oecologia 168, 749–759 (2012). https://doi.org/10.1007/s00442-011-2123-9

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  • DOI: https://doi.org/10.1007/s00442-011-2123-9

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