Determinants of woody cover in African savannas

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Park Scientific Services for comments, help with data collection and analysis, and for providing access to otherwise unpublished data.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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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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