Broad belts of tropical grassland flanking each side of the equatorial forest of Africa and South America. Furley (2006) PPG 30, 1 reviews the determinants of savanna: climate and water, fire, soil–vegetation relationships, grazing. See also Boone et al. (2002) Afr. J. Ecol. 40. The savanna belts are associated with the sinking of high-level equatorial air on its return to the inter-tropical convergence zone. Rainfall is therefore slight, and trees are modified to minimize water loss, with small leaves, and often thorny (Chidumayo (2001) J. Veg. Sci. 12).

The boundaries of the savanna are far from clear; there is a gradual change from tall grasses, 1–3 m high with scattered trees, to grassy woodland, and finally to the rain forest. However, where savanna vegetation has been repeatedly burnt (Balfour (2002) Afr. J. Range & Forest Sci. 19, 1) there can be a sharp division between this and the equatorial forest, which is less easily fired. Laris (2011) AAAG 101, 5, 1067, explains that anthropogenic burning differs from the one based on ecological theory in that its spatiotemporal pattern is relatively consistent from year to year.