Nature 438, 846-849 (8 December 2005) | doi:10.1038/nature04070; Received 26 April 2005; Accepted 22 July 2005

Determinants of woody cover in African savannas

Mahesh Sankaran1, Niall P. Hanan1, Robert J. Scholes2, Jayashree Ratnam1, David J. Augustine3, Brian S. Cade4, Jacques Gignoux5, Steven I. Higgins6, Xavier Le Roux7, Fulco Ludwig8, Jonas Ardo9, Feetham Banyikwa10, Andries Bronn11, Gabriela Bucini1, Kelly K. Caylor12, Michael B. Coughenour1, Alioune Diouf13, Wellington Ekaya14, Christie J. Feral15, Edmund C. February16, Peter G. H. Frost17, Pierre Hiernaux18, Halszka Hrabar19, Kristine L. Metzger20, Herbert H. T. Prins21, Susan Ringrose22, William Sea1, Jörg Tews23, Jeff Worden1 & Nick Zambatis24

  1. Natural Resource Ecology Laboratory, Colorado State University, Fort Collins, Colorado 80523, USA
  2. Division of Forest Science and Technology, CSIR, PO Box 395, Pretoria 001, South Africa
  3. USDA Forest Service, Commanche National Grassland, PO Box 12, Springfield, Colorado 81073, USA
  4. US Geological Survey, Fort Collins Science Center, 2150 Centre Avenue, Building C, Fort Collins, Colorado 80526-8818, USA
  5. Ecole Normale Superieure, Laboratoire d'ecologie, UMR 7625 CNRS, Université de Paris 6 ENS, 46 Rue d'Ulm, 75230 Paris Cedex 05, France
  6. Umweltforschungszentrum Leipzig Halle, Sekt Okosyst Anal, Permoserstrasse 15, D-04318 Leipzig, Germany
  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
  8. CSIRO Centre for Environment and Life Sciences, CSIRO Plant Industry, Private Bag 5, Wembley, Western Australia 6913, Australia
  9. Department of Physical Geography and Ecosystems Analysis, Lund University, Sölvegatan 12, 223 62 Lund, Sweden
  10. Department of Botany, University of Dar es Salaam, PO Box 35060, Dar es Salaam, Tanzania
  11. Department of Agriculture and Game Management, Private Bag X6011, Port Elizabeth Technikon, Port Elizabeth 6000, South Africa
  12. Department of Civil & Environmental Engineering, Princeton University, Princeton, New Jersey 08544, USA
  13. Centre de Suivi Ecologique, BP 15532, Dakar, Senegal
  14. University of Nairobi, Department of Range Management, PO Box 29053, Nairobi, Kenya
  15. Environmental Sciences Department, University of Virginia, PO Box 400123, 291 McCormick Road, Charlottesville, Virginia 22904-4123, USA
  16. Department of Botany, University of Cape Town, University Private Bag, Rondebosch 7700, South Africa
  17. Institute of Environmental Studies, University of Zimbabwe, PO Box MP 167, Mount Pleasant, Harare, Zimbabwe
  18. CESBIO, 18 Avenue E. Belin, 31401 Toulouse Cedex 9, France
  19. Mammal Research Institute, University of Pretoria, Pretoria 002, South Africa
  20. Department of Zoology, University of Wisconsin, 430 Lincoln Drive, Madison, Wisconsin 53706, USA
  21. Resource Ecology Group, Wageningen University, Bornsesteeg 69, 6708 PD Wageningen, The Netherlands
  22. Harry Oppenheimer Okavango Research Centre, University of Botswana, Private Bag 285, Maun, Botswana
  23. University of Potsdam, Institute of Biochemistry & Biology, Plant Ecology & Nature Conservation, Maulbeerallee 2, D-14469 Potsdam, Germany
  24. Scientific Services, Kruger National Park, Private Bag X402, Skukuza 1350, South Africa

Correspondence to: Mahesh Sankaran1 Correspondence and requests for materials should be addressed to M.S. (Email: mahesh@nrel.colostate.edu).

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 approx650 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 approx650 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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