Tree–Grass Interactions in Savannas: Paradigms, Contradictions, and Conceptual Models
The savannas (mixed woody-herbaceous systems in drought-seasonal regions) are one of the most fascinating and enigmatic of the world’s biomes. The relatively recent appearance and rapid global expansion of the savannas occurred only 6 to 8 million years ago (6-8 M BP; Beerling and Osborne, 2006), after the earlier evolution of C4 grasses (15-35 M BP; Sage, 2004), to now cover more than 20% of the Earth’s land surface (Hill et al., Chapter 1, this volume; Scholes and Archer, 1997). The C4 expansion was associated with changing paleo-climate, increasing evidence of fire in the Earth system, and the evolution and numerical expansion of large herbivore guilds specialized in grazing (Pagani et al., 1999; Keeley and Rundel, 2005; Beerling and Osborne, 2006). In many semiarid regions, the increase in C4 dominance displaced woody angiosperms that were drought tolerant but sensitive to the frequent fires which the C4 grasses brought with them. Climate, atmospheric CO2 concentrations, fire, and herbivory have all been associated with, and may partially explain, the historical savanna expansion; and these factors remain critical in determining their modern-day geographic extent (Beerling and Osborne, 2006; Osborne, 2008).