Introduction to Savannas

By
J.I. House & D.O. Hall
Division of Life Sciences, King's College London


	INTRODUCTION
	The savanna biome is diverse including formations ranging from almost treeless grasslands to more-or-less closed canopy woodlands with considerable variation in plant composition, biomass and net primary productivity (NPP). Savannas cover an extensive area in the tropics, inhabited by a fifth of the human population and supporting the majority of the world’s livestock and large mammals. In most of these areas population pressure and land-use changes are high and are likely to increase in the foreseeable future. Savanna areas are both increasing due to deforestation and abandoned agriculture, and decreasing due to cultivation and degradation, the balance is probably a decrease. These changes often occur in remote areas and are poorly documented. Pollen records show that savanna systems are very old, natural systems, characterised by huge heterogeneity, and experiencing rapid changes in plant and animal composition (Menaut et al, 1985). Within this highly dynamic biome, the proportion of trees and grasses can vary enormously, sometimes leading to degradation. Savanna ecosystems and dynamics are currently poorly understood as little attention has been paid to these areas in the past compared to tropical forests or temperate grasslands. Yet it is emerging that savannas have a higher biodiversity (Solbrig et al, 1996), greater productivity (Long et al, 1989, 1992; Scholes & Hall, 1996) and larger impact on global carbon cycles (Amthor et al, 1998; Hall et al, 1995; Ojima et al, 1993; Scholes & Hall, 1996) than previously realised. Most research carried out in savanna ecosystems to date has been in the form of short-term site studies of one or two ecological aspects. Some sites have been studied in more detail or over long periods of time, with different emphases, e.g. Nylsvley, South Africa (Scholes & Walker, 1993 - structure, determinants); Lamto, Côte d’Ivoire (Menaut et al, 1999 - nutrient cycles, fire, biophysical properties); Orinocos Llanos, Venezuela (San Jose & Montes, 1997 - determinants, fire, biophysical properties); HAPEX-Sahel, Niger (Goutorbe et al, 1997 – hydrology, energy balances), etc. Due to the great heterogeneity of the savanna biome from site to site, and even from year to year, individual studies cannot provide a representative picture of the whole biome. More recently there has been a shift from individual studies to syntheses under specifically designed collaborative research programmes such as the "Responses of Savannas to Stress and Disturbance" (RSSD) 10-year programme set up in 1983 (Frost et al, 1986; Walker, 1987; Walker & Menaut, 1988; Solbrig, 1990); "The Primary Productivity and Photosynthesis of Semi-Natural Ecosystems of the Tropics and Sub-Tropics" programme, UNEP 1984 (Long et al, 1989, 1992); the International Geosphere-Biosphere Programme (IGBP) terrestrial transects which include the Kalahari Transect, the North Australian Terrestrial Transect (NATT) and SALT (Savanna in the Long Term), Ivory Coast to Mali (Koch et al, 1995); The Miombo Network (http://miombo.gecp.virginia.edu); SAFARI 2000 (Southern African Fire-Atmosphere Research Initiative http://safari.gecp.virginia.edu); and the SCOPE tree-grass modelling group (House et al, 2000, http://www.nceas.ucsb.edu/ search under projects, "tree-grass"). Over the years, several good synthesis reviews have been published on savannas: Bourlière & Hadley, 1970; Huntley & Walker, 1982; Bourlière, 1983; Sarmiento, 1984; Tothill & Mott, 1985; Cole, 1986; Werner, 1991; Young & Solbrig, 1993; Scholes & Hall, 1996; Solbrig et al, 1996; and Scholes & Archer, 1997.
	ACKNOWLEDGEMENTS
	Many thanks to Bob Scholes (CSIR, South Africa), Xavier Le Roux (INRA, Clermont-Ferrand, France), Jonathan Scurlock (ORNL, USA) and Joe Scanlan (Department of Natural Resources, Queensland, Australia) for providing information and making corrections to the manuscript. Dale Kaiser & Sonja Jones (ORNL, USA) for calculating tropical % of the Olson et al (1983) "grasslands" category. Sadly, David Hall passed away in August 1999 before this chapter was published. His knowledge and love of savannas was only surpassed by his eagerness to learn and teach.

REFERENCES

	Table 1: Previous estimates of area, biomass and NPP of savannas and grasslands
	Table 2: Broad plant functional types found in African savannas (from Scholes et.al., 1997)

Table 3: Biomass reported for tropical grasslands and savannas

	Table 4: Primary production reported for tropical grasslands and savannas
	Table 5: Biophysical properties, fluxes and efficiencies
	Figure 2: The relationship between total NPP and aboveground NPP

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