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Tree-grass coexistence in a flood-disturbed, semi-arid savanna system

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An Erratum to this article was published on 06 February 2010

Abstract

The coexistence of trees and grasses in savanna ecosystems is a contentious phenomenon. Fire and herbivory disturbances are often cited as major structuring forces that create a sustainable tree–grass relationship. However, periodic flooding of savanna patches may also enable coexistence. The aim of this study was to investigate the effects of flood-disturbance on the recruitment patterns of Acacia karroo trees in a semi-arid savanna system in South Africa. We analysed the spatial coincidence of A. karroo seedlings with tussocks of the tall spiny grass Stipagrostis namaquensis in the riverbed and related herbivory intensity to spatial position. The data showed that A. karroo seedlings were significantly positively associated with S. namaquensis (Chi-square test, \( \chi_{1}^{2} = 4 5. 20 \), n = 118, P < 0.001); A. karroo seedlings growing inside of tussocks experienced less browsing pressure than those growing in the floodplain (Kruskal–Wallis test, H = 11.90, n = 118, P < 0.01); and recruitment success of A. karroo trees was spatially discrete (K–S test, D = 0.78, n = 196, P < 0.01). We suggest that floods create an enemy-free zone, which S. namaquensis colonises and then facilitates successful A. karroo establishment. High levels of A. karroo recruitment in the riverbed may replenish the woodlands fringing the river, which appear to be sink areas for A. karroo seedlings. Thus, the interaction between disturbances at different spatial and temporal scales (flooding versus herbivory) seems to maintain the inherently unstable coexistence of tree and grass species in this ecosystem. These findings also suggested that flood disturbances alter the tree–grass relationship.

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Acknowledgements

We thank the Percy FitzPatrick Institute of African Ornithology for providing funding for the project. The study was stimulated by discussions with Dr. Shayne Jacobs, Conservation Ecology and Entomology, University of Stellenbosch. We also thank three anonymous reviewers for greatly improving this manuscript.

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Authors and Affiliations

  1. DST/NRF Centre of Excellence at the Percy FitzPatrick Institute of African Ornithology, University of Cape Town, Rondebosch, 7701, South Africa

    Matthew F. Child, Sue J. Milton, Richard W. J. Dean, Marisa K. Lipsey, James Puttick, Tessa N. Hempson, Gareth K. Mann, Hassan Babiker, Jamshed Chaudrey, Glynis Humphrey, Grant Joseph, Nicola C. Okes, Reda Potts & Thuli Wistebaar

  2. Conservation science group, Department of Zoology, University of Cambridge, Downing Street, Cambridge, CB2 3EJ, UK

    Matthew F. Child

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Correspondence to Matthew F. Child.

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An erratum to this article can be found at http://dx.doi.org/10.1007/s10980-010-9455-4

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Child, M.F., Milton, S.J., Dean, R.W.J. et al. Tree-grass coexistence in a flood-disturbed, semi-arid savanna system. Landscape Ecol 25, 315–326 (2010). https://doi.org/10.1007/s10980-009-9409-x

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