By
J.I. House & D.O. Hall
Division of Life Sciences, King's College
London
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EXTENT | |
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Savannas occupy a wide range of environments. Rainfall can vary from 200 mm to 2000 mm, and tends to be more erratic at the arid end of the scale, often occurring as short storm events (Walker 1985; Solbrig et al 1996a). Temperatures vary from hot tropical temperatures with little seasonality to sub-tropical temperatures that can approach frost in the winter. Soils vary from fertile to infertile. Typically, savannas in high rainfall areas have poor soils since wet areas with good soils tend to tropical rainforests. Similarly arid savanna areas tend to have nutrient-rich soils, since low rainfall plus poor soils leads to desert vegetation. However, there are exceptions to these rules. The extent of savannas is uncertain due to the variation in classification of this biome, and the sparsity of data in these areas. Table 1 shows a range of estimates. Whittaker & Likens (1973, 1975) estimated savanna cover in 1950 to be 15 M km2 using a modified UNESCO scheme. Atjay et al (1979) used the same classification as Whittaker & Likens for their estimate of 22.5 M km2 which incorporates updated reports and vegetation maps as well as a consideration of human interference such as the cutting of forest areas and subsequent formation of secondary or derived savannas. The Olson et al (1983) map reflects better knowledge of classification, mapping and ecosystem change, and the tropical and temperate tree-grass classes included in Table 1 represent a total land area of 33.7 M km2, a quarter of the global land surface. Scholes & Hall (1996) used the digitised version of this map published by the United States Environmental Protection Agency (U.S. EPA) for their estimate of 16.1 M km2 using the four "savanna" classes they considered tropical. However, 74.5% of the "warm or hot shrub and grasslands" class is within the tropics (Dale Kaiser, ORNL, personal communication), giving a total tropical savanna area of 27.6 M km2 (almost a fifth of the global land surface). Temperate tree-grass mixes occur in the warmer areas of North America (5 M km2) (McPherson, 1997), Mediterranean Europe and parts of Russia and Asia.
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Table 1: Previous estimates of area, biomass and NPP of savannas and grasslands | |
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Africa | |
| Savannas form a semicircle around the western central rainforest areas, bordered by the desert zones to the north and south, across a variety of soil conditions with rainfall ranging from 200-1800 mm. Broad-leaved savannas are found in the sub-humid interior plateau on old, highly weathered, infertile soils. Fine-leaved savannas are typical of the low-lying, semi-arid regions. The northern Sudan-type savannas are open xerophytic grasslands with scattered deciduous trees forming a transition with the Saharan desert vegetation. The Guinea-type savanna woodlands form the transition with the evergreen moist forests. The eastern African savannas, with bimodal rainfall typically totalling less than 700mm, are dominated by herbaceous vegetation with some shrubs or scattered trees. The southern savanna area is known as Miombo woodland due to its distinctive tree species. (Menaut et al, 1985; Backéus, 1992; Scholes & Walker, 1993; Solbrig, 1996). | ||
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Australia | |
| Australian savannas occur in two distinctive climatological areas - the cooler, wetter east coast and the warmer, drier north coast. Rainfall is mostly below 1000 mm, soils tend to be poor, fires frequent, and diversity high. Australian savannas include monsoon, tropical and subtropical tallgrass communities along coastal areas with Eucalyptus woodlands; the midgrass savannas on clay soils with Acacia harpophylla (Brigalow) and associated Eucalyptus woodlands; tussock and hummock grasslands (Mitchell grass); and Acacia shrublands (mulga and gidgee pastures) (Mott et al, 1985; Braithwaite, 1990; Burrows et al, 1990, McKeon et al, 1990; Solbrig, 1996). | ||
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South America | |
| The South American savannas tend to be wetter, with less contrasting seasonal variation, and very nutrient poor soils often high in aluminium. The largest savanna-type, the cerrado, covers 1.8 M km2 within Brazil and includes the pure grassland campo limpo, through the low open woodland of the true cerrado, to the closed dry-forest formations of the cerradao. In Colombia/Venezuela the llanos del Orinoco area is grassland with scattered trees (San Jose & Montes, 1989). Flooded savannas occur in Brazil and Bolivia. The caatinga region of Brazil and the chaco region of Paraguay/Bolivia/Argentina (which can suffer frost) are often not considered savannas but do fall into the descriptions used by African ecologists. Other smaller areas of distinct savanna types are scattered through South America, Central America and Cuba (Sarmiento, 1983; Medina & Silva 1990; Solbrig, 1996). | ||
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Asia | |
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Savannas are mostly
"secondary" or "derived" and are formed by
deforestation, abandoned cultivation and burning, and maintained by
repeated grazing, harvesting and burning. Savannas are fairly extensive in
India and Sri Lanka, with continued forest clearing increasing their
extent, although many areas are under threat from agriculture (Misra 1983;
Yadava 1990; Backéus, 1992; Pandey & Singh, 1992). Savannas are not
so common in South East Asia. Thailand, Laos, Cambodia and Vietnam have an
open deciduous dipterocarp forest with the ground covered by grasses,
similar to some vegetation types in Africa, which form a transition
between dense forest and shrub savanna. Treeless savannas occur in many
places but occupy only a small area (Blasco, 1983; Stott, 1990).
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ACKNOWLEDGEMENTS | |
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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.
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REFERENCES |
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Table 1: Previous estimates of area, biomass and NPP of savannas and grasslands |
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Table 2: Broad plant functional types found in African savannas (from Scholes et.al., 1997) |
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Table 3: Biomass reported for tropical grasslands and savannas | |
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Table 4: Primary production reported for tropical grasslands and savannas |
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Table 5: Biophysical properties, fluxes and efficiencies |
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Figure 2: The relationship between total NPP and aboveground NPP |
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