The nature of African savannas: future changes in precipitation may considerably affect their distribution and dynamics



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 Bucini
, 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 Ringrose
22, William Sea1, Jörg Tews23, Jeff Worden1 & Nick Zambatis2
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 properties.
The availability of resources (water, nutrients) and disturbance regimes (fire, herbivory) are thought to be important in regulating woody cover, 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 precipitation may considerably affect their distribution and dynamics.

Determinants of woody cover in African savannas. Available from: [accessed Jan 21, 2017].

Author: Willem Van Cotthem

Honorary Professor of Botany, University of Ghent (Belgium). Scientific Consultant for Desertification and Sustainable Development.

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