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Determinants of woody cover in African savannas

Nature volume 438, pages 846–849 (2005)Cite this article

Abstract

Savannas are globally important ecosystems of great significance to human economies. In these biomes, which are characterized by the co-dominance of trees and grasses, woody cover is a chief determinant of ecosystem properties1,2,3. The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover1,2,4,5, but perceptions differ on which of these are the primary drivers of savanna structure. Here we show, using data from 854 sites across Africa, that maximum woody cover in savannas receiving a mean annual precipitation (MAP) of less than ∼650 mm is constrained by, and increases linearly with, MAP. These arid and semi-arid savannas may be considered ‘stable’ systems in which water constrains woody cover and permits grasses to coexist, while fire, herbivory and soil properties interact to reduce woody cover below the MAP-controlled upper bound. Above a MAP of ∼650 mm, savannas are ‘unstable’ systems in which MAP is sufficient for woody canopy closure, and disturbances (fire, herbivory) are required for the coexistence of trees and grass. These results provide insights into the nature of African savannas and suggest that future changes in precipitation6 may considerably affect their distribution and dynamics.

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Figure 1: Change in woody cover of African savannas as a function of MAP.
Figure 2: Woody cover as a function of MAP, soil properties and disturbance regimes in arid and semi-arid savannas.
Figure 3: Regression tree showing generalized relationships between woody cover and MAP, fire-return interval and percentage of sand.
Figure 4: The distributions of MAP-determined (‘stable’) and disturbance-determined (‘unstable’) savannas in Africa.

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Acknowledgements

This paper arose from a workshop on savanna complexity funded by the NSF. We thank R. Boone, I. McHugh, R. Grant, H. Biggs, W. T. Starmer, P. M. Barbosa, D. Ruess, J. Rettenmayer, C. Williams, J. Klein, M. T. Anderson, W. J. Parton, J. C. Neff, N. Govender and the Kruger Park Scientific Services for comments, help with data collection and analysis, and for providing access to otherwise unpublished data. Author Contributions All authors contributed data or intellectual input to the project.

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Authors and Affiliations

  1. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, 80523, Colorado, USA

    Mahesh Sankaran, Niall P. Hanan, Jayashree Ratnam, Gabriela Bucini, Michael B. Coughenour, William Sea & Jeff Worden

  2. Division of Forest Science and Technology, CSIR, PO Box 395, 001, Pretoria, South Africa

    Robert J. Scholes

  3. USDA Forest Service, Commanche National Grassland, PO Box 12, Springfield, 81073, Colorado, USA

    David J. Augustine

  4. US Geological Survey, Fort Collins Science Center, 2150 Centre Avenue, Building C, Fort Collins, 80526-8818, Colorado, USA

    Brian S. Cade

  5. Ecole Normale Superieure, Laboratoire d'ecologie, UMR 7625 CNRS, Université de Paris 6 ENS, 46 Rue d'Ulm, 75230 Cedex 05, Paris, France

    Jacques Gignoux

  6. Umweltforschungszentrum Leipzig Halle, Sekt Okosyst Anal, Permoserstrasse 15, D-04318, Leipzig, Germany

    Steven I. Higgins

  7. Microbial Ecology Laboratory, UMR 5557 CNRS, Université Lyon 1 USC INRA 1193, Bâtiment G. Mendel, 43 Boulevard du 11 Novembre 1918, 69622, Villeurbanne, France

    Xavier Le Roux

  8. CSIRO Centre for Environment and Life Sciences, CSIRO Plant Industry, Private Bag 5, Western Australia, 6913, Wembley, Australia

    Fulco Ludwig

  9. Department of Physical Geography and Ecosystems Analysis, Lund University, Sölvegatan 12, 223 62, Lund, Sweden

    Jonas Ardo

  10. Department of Botany, University of Dar es Salaam, PO Box 35060, Dar es Salaam, Tanzania

    Feetham Banyikwa

  11. Department of Agriculture and Game Management, Port Elizabeth Technikon, Private Bag X6011, 6000, Port Elizabeth, South Africa

    Andries Bronn

  12. Department of Civil & Environmental Engineering, Princeton University, New Jersey, 08544, Princeton, USA

    Kelly K. Caylor

  13. Environmental Engineering, Princeton University,

    Kelly K. Caylor

  14. Centre de Suivi Ecologique, BP 15532, Dakar, Senegal

    Alioune Diouf

  15. Department of Range Management, University of Nairobi, PO Box 29053, Nairobi, Kenya

    Wellington Ekaya

  16. Environmental Sciences Department, University of Virginia, PO Box 400123, 291 McCormick Road, Charlottesville, 22904-4123, Virginia, USA

    Christie J. Feral

  17. Department of Botany, University of Cape Town, University Private Bag, Rondebosch, 7700, South Africa

    Edmund C. February

  18. Institute of Environmental Studies, University of Zimbabwe, PO Box MP 167, Mount Pleasant, Harare, Zimbabwe

    Peter G. H. Frost

  19. CESBIO, 18 Avenue E. Belin, 31401, Cedex, 9, Toulouse, France

    Pierre Hiernaux

  20. Mammal Research Institute, University of Pretoria, 002, Pretoria, South Africa

    Halszka Hrabar

  21. Department of Zoology, University of Wisconsin, 430 Lincoln Drive, Madison, 53706, Wisconsin, USA

    Kristine L. Metzger

  22. Resource Ecology Group, Wageningen University, Bornsesteeg 69, 6708 PD, Wageningen, The Netherlands

    Herbert H. T. Prins

  23. Harry Oppenheimer Okavango Research Centre, University of Botswana, Private Bag 285, Maun, Botswana

    Susan Ringrose

  24. University of Potsdam, Institute of Biochemistry & Biology, Plant Ecology & Nature Conservation, Maulbeerallee 2, D-14469, Potsdam, Germany

    Jörg Tews

  25. Biology, Plant Ecology & Nature Conservation, Maulbeerallee 2, D-14469,

    Jörg Tews

  26. Scientific Services, Kruger National Park, Private Bag X402, Skukuza, 1350, South Africa

    Nick Zambatis

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  1. Mahesh Sankaran
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Corresponding author

Correspondence to Mahesh Sankaran.

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Supplementary information

Supplementary Methods

This file provides additional background, methods and results of statistical analyses, and references for data sources. (DOC 40 kb)

Supplementary Tables

This file contains Supplementary Tables 1 and 2 (correlations between soil parameters; summary of results for regression-tree analysis). (DOC 36 kb)

Supplementary Figures

This file contains Supplementary Figures 1–4 (location of sample sites in Africa; relationship between woody cover, MAP and fire-return interval; frequency distributions for soil parameters, herbivore biomass and fire-return intervals; and comparison of woody cover estimates from densiometer and summed canopy area methods). (DOC 135 kb)

Supplementary Data

This file contains the data used for the analysis. (XLS 253 kb)

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Sankaran, M., Hanan, N., Scholes, R. et al. Determinants of woody cover in African savannas. Nature 438, 846–849 (2005). https://doi.org/10.1038/nature04070

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