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Fig. 9.8A-D Aspects of the cerrado (A,B) and the cerradao (C,D) near Brasilia (Fazenda Agua Limpa), Brazil

and size of woody plants, i.e. shrubs and trees is an important feature of this system. In Africa one also encounters the distinction between "wooded savanna", where the trees stand more or less isolated, and "woodland savanna," where the canopies of individual trees touch each other (Fig. 9.9).

Fig. 9.9A,B Wooded savanna (A) and woodland savanna (B), Great Rift Valley, Ethiopia

9.2 Seasonality

Water and nutrient availability distinguish different types of savannas (Baruch 2005). In his climatic-hydrological classification Sarmiento (1984) separates mainly four types of savannas based on water status and seasonality (Fig. 9.10):

• a semi-seasonal savanna with a long rainy period but without excess of water (i.e. flooding) and a short period with a water deficit,

• a seasonal savanna with changes between periods with sufficient water and periods of drought,

• a hyperseasonal savanna, where periods of excess of water and of drought provide strong seasonal contrast (see quotations of A. von Humboldt in Sect. 9.1),

• a marsh savanna, where long periods of water excess are interrupted by drier periods, when water, however, still is in sufficient supply.

The latter merges into wetland ecosystems of which there is a large variety of types which are difficult to separate and define (Esteves 1998). Many of them are river floodplains in wet tropical forests (Sect. 3.2.3) and also associated with ponds and lakes.

Fig. 9.10 Scheme of the water budgets of savannas in the climatic-hydrological classification of Sarmiento (1984). Following the annual cycle described by the circumference of each circle shows the extensions of annual cycles dominated by water excess (hatched area), normal water availability (white area) and water deficit (dotted area) (Reprinted by permission of Harvard University Press)

Fig. 9.10 Scheme of the water budgets of savannas in the climatic-hydrological classification of Sarmiento (1984). Following the annual cycle described by the circumference of each circle shows the extensions of annual cycles dominated by water excess (hatched area), normal water availability (white area) and water deficit (dotted area) (Reprinted by permission of Harvard University Press)

9.3 The Savanna Problem: Why Do Savannas Exist?

A more fundamental problem is why there are savannas at all. Why is closed forest not growing all over these sites in the tropics? Are savannas natural plant communities or only products of human activities? There is no generally accepted hypothesis, and a number of possibilities are listed by Huber (1982) as follows:

• The climatic hypothesis. This must be rejected for several reasons, but most simply because of the co-occurrence of forest with closed canopies of trees and open savanna under the same climatic conditions.

• The edaphic hypothesis, especially including the importance of the nutrient limitation.

• The fluvial hypothesis, i.e. colonization of ancient riverbeds by savanna.

• The hydrological hypothesis, i.e. the important influence of the water regime including limitations due to insufficient or excessive drainage.

• The relict or refuge hypothesis, where savannas are considered as relicts of a formerly more widespread dry vegetation type.

• The anthropogenic hypothesis, implying the role of man in establishing, maintaining and extending savannas especially by forest clearing and burning.

A steady state model of grass/tree coexistence based on separated rooting niches, where trees have sole access to water in deeper soil horizons and grasses have preferential access to and are superior competitors for water in the surface soil (Sect. 10.1), is criticized by Higgins et al. (2000). Instead they propose a non-equilibrium nonlinear model which is a very detailed model essentially based on a "storage" function considering tree seedling establishment and recruitment. Variations in rainfall (Sect. 10.1.2), where establishment of tree seedlings requires several humid years in sequence (Baruch et al. 1996) and fire (Sect. 10.3) on a background of low levels of adult tree mortality allow the storage effect. The grass/tree coexistence is then supported by the limited opportunities for tree seedlings to escape both drought and the flame zone during fires into the adult stage. This prevents forest formation but stores those individuals that have escaped.

In any event, savannas are not only man made. The cerrado of central Brazil also is a natural, original vegetation and not derived from tropical mesophytic forest by man's destruction. Cerrados and natural savannas in the tropics are highly valuable biotopes both floristically and ecologically. It is mostly overlooked, that they are under enormous economic pressure and just as much threatened by the current destruction as tropical forests (Skole et al. 1994; Ribeiro et al. 2005). The cerrados of Brazil belong to the 25 biodiversity hotspots of the world's vegetation (Myers et al. 2000; Oliveira and Marquis 2002; Gottsberger and Silberbauer-Gottsberger 2006a). They originally covered 1,783,200 km2 and are now already reduced to 20% of their original area (356,630 km2, 22,000 km2 or 6.2% of which are protected) and they have 10, 000 plant species, 4400 of which are endemic (Myers et al. 2000). In the 20 years from 1975 to 1996 the cerrado area covered by the crops soybean, maize, rice and beans increased from 4.20 x 106 (15%) to 9.17 x 106 (28%) ha, where the numbers in brackets give the cerrado area in per cent of the total area which is covered in Brazil by these crops, and it is seen that the whole increment between 1975 and 1996 was due to cerrado cultivation (Resck et al. 2000; Gottsberger and Silberbauer-Gottsberger 2006a). There may even arise some kind of unsavoury contest in that destruction will increasingly turn towards savannas and cerrados as forests are protected.

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