Productivity and carbon fluxes of tropical savannas


*John Grace, Institute of Atmospheric and Environmental Science, School of GeoSciences, The University of Edinburgh, Crew Building, The King's Buildings, Edinburgh EH9 3JN, UK.


Aim  (1) To estimate the local and global magnitude of carbon fluxes between savanna and the atmosphere, and to suggest the significance of savannas in the global carbon cycle. (2) To suggest the extent to which protection of savannas could contribute to a global carbon sequestration initiative.

Location  Tropical savanna ecosystems in Africa, Australia, India and South America.

Methods  A literature search was carried out using the ISI Web of Knowledge, and a compilation of extra data was obtained from other literature, including national reports accessed through the personal collections of the authors. Savanna is here defined as any tropical ecosystem containing grasses, including woodland and grassland types. From these data it was possible to estimate the fluxes of carbon dioxide between the entire savanna biome on a global scale.

Results  Tropical savannas can be remarkably productive, with a net primary productivity that ranges from 1 to 12 t C ha−1 year−1. The lower values are found in the arid and semi-arid savannas occurring in extensive regions of Africa, Australia and South America. The global average of the cases reviewed here was 7.2 t C ha−1 year−1. The carbon sequestration rate (net ecosystem productivity) may average 0.14 t C ha−1 year−1 or 0.39 Gt C year−1. If savannas were to be protected from fire and grazing, most of them would accumulate substantial carbon and the sink would be larger. Savannas are under anthropogenic pressure, but this has been much less publicized than deforestation in the rain forest biome. The rate of loss is not well established, but may exceed 1% per year, approximately twice as fast as that of rain forests. Globally, this is likely to constitute a flux to the atmosphere that is at least as large as that arising from deforestation of the rain forest.

Main conclusions  The current rate of loss impacts appreciably on the global carbon balance. There is considerable scope for using many of the savannas as sites for carbon sequestration, by simply protecting them from burning and grazing, and permitting them to increase in stature and carbon content over periods of several decades.