Introduction

Savannas are the most common vegetation type in the tropics and subtro-pics. Broadly defined as ecosystems formed by a continuous layer of grami-noids {grasses and sedges) and a discontinuous layer of trees and/or shrubs of variable extent, savannas are found over a wide range of rainfall, temperature and soil conditions. The one constant climatic characteristic of tropical savannas is rainfall seasonality. Yet the duration of the dry season can vary from three to nine months, with a mode of five to seven months.

Almost one-fifth of the world's population lives in areas that are at present, or were recently, covered with savanna vegetation (Young and Solbrig 1993; Solbrig 1994), many of them in rural societies that depend on herding or subsistence agriculture. Per capita food production in these communities is usually low, the result of a variety of environmental, social and economic constraints. Bccause rainfall in these regions is highly seasonal and variable, primary production is uneven and unpredictable in space and time, particularly in low-rainfall areas. Many savanna soils are nutrient-poor, particularly in high-rainfall areas, which is reflected in Jow crop yields and the poor nutritional quality of natural pastures. Compounding the effects of these factors is the dual nature of land use in many savanna areas: the most productive lands are set aside for cash crops, while the poorer lands are used for local food production or extensive cattle grazing (Kowa! and Kassam 1978; Klink et al. 1993; Lane and Scoones 1993). Combined with the rapid growth of human populations in the tropics, this pattern of use is bringing about a rapid transformation of savanna landscapes, resulting in many cases in degraded environments, reduced productivity, low carrying capacity and loss of species.

'An extensive report on this subject can be found in Solbrig, O.T., Medina, E. and Silva, J. (1996} Biodiversity and Savanna Ecosystem Processes: A Global Perspective. Springer, Berlin, in press.

Functional Roles of Biodiversity: A Global Perspective -m-.

Edited by H.A. Mooney, J.H. Cushman, E. Medina, O.E. Sala and E.-D. Schulze IM! © 1996 SCOPE Published in 1996 by John Wiley & Sons Ltd unflp

Tropical savannas have high species diversity, especially when compared with temperate grasslands and dry tropical woodlands. So, for example, 2366 species of phanerogams (Filgueiras and Pereira 1994) grow in the savannas of the small area of the Federal District of Brasilia. It has been estimated that the entire cerrado has over 10 000 vascular plant species (Ratter, personal communication 1995), while there are fewer than 6000 species in the pampas of Argentina, Uruguay and southern Brazil (Cabrera 1968; Rosengurtt et al. 1970), an area of similar size. However, there is still no complete inventory of the biota of any tropical savanna. Best known are vascular plants, birds and mammals; least known are invertebrates, especially non-arthropods, fungi and protists.

Savannas from different continents share very few Linnaean species, particularly among the woody elements. The invasion of American and Australian savannas by African grasses is a recent phenomenon of human origin. Within an area, however, different savanna types often share common species (Sarmiento 1984, 1994; Medina and Huber 1992). Savanna species are usually more closely related to species in other local vegetation types than to savanna species in other continents. So, for example, the phyloge-netic affinities of the woody flora of the Brazilian savannas, known as cerrado, are with the Amazonian flora rather than with the flora of west Africa; physiognomically, however, the cerrado is more similar to the savannas of West Africa than to the Amazonian forest. In turn, African savanna species, as is also true for non-savanna ecosystems, are phylogcncti-cally more related among themselves and with species from the wet forest than they are with savanna vegetation in other continents. African savannas (Menaut 1983) are almost as rich in species as the African rain forests, and Australian savannas have more species than neighboring wet forests.

The physiognomic similarity of tropical savannas in different regions of the world is supposedly the result of convergent evolution, predicated primarily on the basis of the vascular vegetation, especially the presence of a continuous layer of grasses. However, the Gramincae are a very stereotyped family of angiosperms, i.e. all grasses have the same basic architecture, and therefore the characteristics of savanna grass species cannot necessarily be attributed to convergence evolution. It is more likely that savannas result from parallel evolution from common ancestors under more or less similar circumstances, rather than from dissimilar ancestors. It is likely that the differences between savanna grasses are as great as the similarities (Eiten 1972), but no rigorous studies have been conducted.

Tropical savannas are very heterogeneous systems at all scales of analysis, from the individual patch to the regional level (Solbrig 1991a). This heterogeneity makes it difficult to define tropical savannas with precision and inclusiveness, and no general consensus has emerged among researchers regarding what is to be considered a tropical savanna. Broadly defined,

Table 8.1 Physiognomic types of savanna (according to Sarmiento 1984)

1. Savannas without woody species taller than the herbaceous stratum: Grass savannas or grasslands

2. Savannas with low (less than 8 m) woody species forming a more or less open stratum

(a) Shrubs and/or trees isolated or in groups; total cover of woody species less than 2%: Tree and shrub savanna

(b) Total tree/shrub cover between 2% and 15%: Savanna woodland, wooded grassland, or bush savanna

(c) Tree cover greater than 15%: Woodland

3. Savannas with trees over 8 m

(a) Isolated trees with less than 2% cover. Tail-tree savanna

(b) Tree cover 2-15%: Tall savanna woodland

(c) Tree cover 15-30%: Tall wooded grassland

(d) Tree cover above 30%: Tall woodland

4. Savannas with tall trees in small groups: Park savanna

5. Mosaic of savanna units and forests: Park savannas can be subdivided into a number of regional types (Table 8.1; Sarmiento 1984, 1992) based on rainfall seasonality characteristics and density of woody vegetation. A first distinction is between dry (roughly less than 700 mm) and moist (more than 700 mm rainfall) savannas. Within the moist savannas, we can distinguish seasonal savannas, where a period of positive moisture balance alternates with a negative one; semi-seasonal savannas, which suffer a long period of water surplus, and hyperseasonal savannas, where plants suffer a period of water deficit during the dry season and one of water surplus during the wet season. Seasonal wet savannas prevail in America; both wet and dry savannas are found in Africa, and in Australia dry savannas predominate. Natural and anthropogenically induced changes in climate, in nutrients, in fire regime and in herbivory can displace the borders of the areas occupied by different types of savanna vegetation, as well as the borders with other types of vegetation: humid forests and semi-deserts. A good example is provided by the border between the Brazilian cerrado and the tropical forest. It is well documented (Van dcr Hammen 1983; Furley et al. 1992) that during the Pleistocene dramatic expansions and shrinkages took place in the extent of the cerrado.

Savannas are also very heterogeneous at a more restricted scale. Small gallery forests with entirely different floristic elements along streams and moist areas grow within a sea of graminoid-dominated savanna vegetation. In turn, wooded savannas are dotted with small islands of woodlands from which grasses are essentially absent, and with strips of pure grassland

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