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Determinants of woody cover in African savannas

Mahesh Sankaran 1 Niall P. Hanan 1 Robert J. Scholes 2 Jayashree Ratnam 1 David J. Augustine 3 Brian S. Cade 4 Jacques Gignoux 5 Steven I. Higgins 6 Xavier Le Roux 7 Fulco Ludwig 8 Jonas Ardo 9 Feetham Banyikwa 10 Andries Bronn 11 Gabriela Bucini 1 Kelly K. Caylor 12 Michael B. Coughenour 1 Alioune Diouf 13 Wellington Ekaya 14 Christie J. Feral 15 Edmund C. February 16 Peter G. H. Frost 17 Pierre Hiernaux 18 Halszka Hrabar 19 Kristine L. Metzger 20 Herbert H. T. Prins 21 Susan Ringrose 22 William Sea 1 Jörg Tews 23 Jeff Worden 1 Nick Zambatis 24
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 properties1–3. 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 ,650mm 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 ,650mm, 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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Mahesh Sankaran, Niall P. Hanan, Robert J. Scholes, Jayashree Ratnam, David J. Augustine, et al.. Determinants of woody cover in African savannas. Nature, Nature Publishing Group, 2005, 438 (8), pp.846-849. ⟨10.1038/nature04070⟩. ⟨halsde-00304266⟩

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