Definition

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

	Definitions Savanna Formations	Acknowledgements References
	DEFINITION
	The classification of "savanna" is inexact and has been based on different, often subjective, criteria. Savannas are most commonly defined as tropical seasonal ecosystems with a more-or-less continuous herbaceous cover and a discontinuous cover of trees or shrubs in varying proportions (Frost et al, 1986). It is this coexistence and close interaction of herbaceous and woody species that make savannas unique and complex. The proportions of woody to grass biomass are highly variable and subject to environmental and management changes within broad climatic and edaphic constraints (Scholes & Archer, 1997). The one constant and defining characteristic of savannas is the seasonality of rainfall: "Tropical savannas, woodlands and grasslands can be defined as those formations constrained by water rather than temperature with an annual dry season of sufficient duration and intensity to cause woody plants to shed their leaves and grasses to dry out, thus providing dry fuel for periodic fires." (Huntley & Walker, 1982). Authors often distinguish between "humid" and "arid" savannas based on a rainfall amount - typically of 600 mm (Walker, 1985), or length of dry season whereby 2.5 - 5 months is classed as "humid" and 5 - 7.5 months is "arid" (Walter, 1964), in some areas the dry season may even last 9 months (Solbrig et al, 1996a). Seasonality may be bimodal with long and short rainy seasons. Fire and grazing are also integral elements in natural and managed savannas. Savannas form a continuum between tropical forests and grasslands and have often been classified as either one or the other in the past. The distinction between what is forest, grassland and different structural savanna types can only be set with arbitrary limits and descriptions such as those defined by Scholes and Hall (1996): Forests: Complete tree canopy cover and three or more overlapping vegetation strata Woodlands: 50-100% tree canopy cover, and a graminaceous layer Savannas: 10-50% cover by woody plants and well-developed grass Grasslands: less than 10% tree cover Different authors and classification systems vary as to whether "savanna" includes dense woodlands or treeless tropical grasslands. Some authors use the term "tropical grasslands" to include "the mixed grass and tree communities of the savanna and savanna forest" (Long et al, 1992), and others use "rangelands" in the same context. In this chapter we use the term "savanna" to include the whole range from treeless grasslands to closed-canopy woodlands (with a graminaceous layer) unless otherwise stated.
	Savanna Formations
	A number of distinctive savanna formations exist, and some common definitions (scientific and vernacular) are listed below (Menaut, 1983; Sarmiento, 1984). Grass savannas or grasslands: without woody species taller than the herbaceous stratum Bushlands/shrublands: Low-statured trees (<3 m), <2% = shrub, 2-15% = bush savanna Dwarf shrubland: Woody plants <1m or <3m tall (depending on author) Wooded grasslands, savanna grasslands, savanna woodland: 2-15% tree cover Savanna parkland: Mosaic patches of trees Thicket/scrub forest: Low-statured, multi-stemmed woodlands or forests, often impenetrable and clumped, with absent or discontinuous grass layer Tiger savanna: Tree patches in linear bands parallel to slope contours Bushveld, lowveld, thornveld: Different savanna formations in Southern Africa Steppes: Savanna grasslands sometimes with woody component
	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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