perspective: Learning new tricks from old trees: revisiting the savanna question

Abstract

Savannas are ecosystems comprising of a mixture of woody species (trees and bushes), grasses and forbs. They cover about a fifth of the global land surface and about half of the area of Africa, Aus-tralia and South America (Scholes and Archer 1997, Sankaran et al. 2004). Savannas are charac-terized by a continuous grass understorey and a discontinuous tree layer. A savanna, where trees and grasses co-dominate (often referred to as co-existence), may be viewed as an intermediate eco-system between grassland (grass dominance) and forest (tree dominance). Tree-grass ratios vary widely in savannas, with higher precipitation usu-ally leading to a more continuous tree layer (Sankaran et al. 2005). However, tree canopies in mesic savannas are still discontinuous enough, with significant understorey grass biomass for the system to be characterised as a savanna and not forest. There are several different savanna eco-regions worldwide, each containing different sub-sets of species and displaying substantial variation in physical and structural attributes (Scholes and Archer 1997, House et al. 2003). In this paper, we focus primarily on tropical and subtropical savan-nas, although some of the conclusions might be valid for a broader extent of savannas.