Tree-grass ratios in savannas – challenging paradigms

Abstract

news and update ISSN 1948-6596 commentary Tree–grass ratios in savannas: challenging paradigms One of the greatest challenges for plant ecologists is to ascertain which factors allow trees and grasses to co-exist in savannas (Moustakas et al. 2010; Figure 1). Two major hypotheses that ad- dress this issue have been proposed (Figure 2). The resource-based hypothesis states that grasses and trees have different root depths, and thus occupy different competitive niches (Scholes and Archer 1997). More recently, various versions of the disturbance hypothesis have been formulated, which postulate that disturbances such as fires and herbivory are major factors restricting the growth of trees (Higgins et al. 2000, Sankaran et al. 2005). In both hypotheses, rainfall is thought to benefit trees. Higher rainfall should cause deeper water infiltration, thereby benefitting the deeper- lying roots of trees and allowing them to grow faster and outcompete grasses by shading. Distur- bances that reduce woody cover, such as fires, would explain lower woody cover in relatively high-rainfall areas. Recently, these ideas have been the focus of a number of macroecological studies, which have found some support for the positive influ- ence of rainfall, and the negative influence of dis- turbances, on woody cover. In their seminal pa- per, Sankaran et al. (2005) showed that, for 854 sites across Africa, maximum woody cover in- creased with precipitation at sites with lower lev- els of precipitation (< 650 mm/year), but showed no relationship with precipitation at higher rainfall levels. They thus suggested that arid and semi-arid savannas are ‘stable’: here, even in the absence of disturbances, trees and grasses would continue to coexist. In contrast, in higher rainfall (mesic) sa- vannas, tree–grass coexistence would be facili- tated by disturbances such as fire and herbivory; in the absence of disturbances, high-rainfall sa- vannas would be expected to become forests. Other continental studies have corroborated these findings (Bucini and Hanan 2007, Sankaran et al. 2008, Staver et al. 2011). These studies have contributed to the idea that, at continental scale, rainfall, with the mediating effect of disturbances, plays a pivotal role in the tree–grass ratio of sa- vannas. However, two studies that were recently published have challenged various aspects of these two hypotheses. February et al. (2013) set up experimental plots to assess the effect of rain- fall manipulation on tree growth in a nutrient- poor mesic and a nutrient-rich semi-arid savanna Figure 1. What de- termines tree–grass ratios in tropical savannas? (Photo- graph taken at Weenen Game Re- serve, South Africa). frontiers of biogeography 5.3, 2013 — © 2013 the authors; journal compilation © 2013 The International Biogeography Society