perspectives ISSN 1948-6596 Learning new tricks from old trees: revisiting the savanna question Aristides Moustakas 1,* , Kerstin Wiegand 2 , Katrin M. Meyer 2 , David Ward 3 & Mahesh Sankaran 1,4 Institute of Integrative & Comparative Biology, Faculty of Biological Sci- ences, University of Leeds, Leeds LS2 9JT, UK Ecosystem Modelling, University of Gottingen, Gottingen, Germany School of Biological and Conservation Sciences, University of KwaZulu- Natal, Scottsville 3209, South Africa National Centre for Biological Sciences, Tata Institute of Fundamental Research, GKVK, Bellary Road, Bangalore 560065, India *Corresponding author. e-mail: arismoustakas@gmail.com; http://www.fbs.leeds.ac.uk/staff/ profile.php?tag=Moustakas_A Introduction 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. Until the early 1990s, it was generally be- lieved that trees and grasses coexist because of a separation of rooting niches (Scholes and Archer 1997). This idea was based on Walter’s (1939) two-layer hypothesis, a version of niche separa- tion theory (Walker et al. 1981). According to this theory, water is the limiting factor for woody spe- cies as well as grasses. It was assumed that grasses were the better competitors for topsoil moisture (usually 30 cm). Topsoil is usually defined by its fast reaction to climatic re- gimes, getting wet during light rain, drying out with a few days of sunshine (Knoop and Walker 1985). The two-layer theory has been verified by field experiments which reported that in the Nylsvlei savanna (South Africa) grasses reduced the supply of water to the trees (Knoop and Walker 1985). Furthermore, subsoil competition between the trees, probably for water but possi- bly also for nutrients, was reported (Smith and Goodman 1986). Several studies have concluded that savanna stability is based on the two-layer theory (e.g. Knoop and Walker 1985, Skarpe There is a growing body of literature that questions the validity of the two-layer theory. A number of field studies have reported facilitative effects of trees on grass biomass, where grass bio- mass is greater below tree canopies rather than away from trees (e.g. Belsky et al. 1989). Among others, Ludwig et al. (2004) reported a field ex- periment where the two-layer theory is inappro- priate. More specifically, according to Ludwig et al. (2004) “prevention of tree–grass interactions frontiers of biogeography 2.2, 2010 — © 2010 the authors; journal compilation © 2010 The International Biogeography Society