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a

a Infiltration deeper than 2.3 m: > 600 mm p.a. (in savannas = 0)

a Infiltration deeper than 2.3 m: > 600 mm p.a. (in savannas = 0)

vannas are dominated by C4-grasses (Sect. 10.1.1.2). Thus, the soil-organic matter beneath, fed by the decomposing litter of savanna plants, should be much less negative than the soil under forest, where C3-trees dominate. On this basis horizontal and vertical S 13C-analyses in soils have allowed to unravel historical savanna-forest ecotone dynamics. Figure 9.12 shows that at the study site deeper soil layers at 80 cm and below had more negative S13C values indicating that the whole area once

Fig. 9.11 Separation of various types of savannas and tropical forests based on nutrient and water availability. (Medina 1987)
Fig. 9.12 Vertical and horizontal profiles of S13 C values in the soil organic matter along a savanna-forest topological gradient in Kattinkar, Western Ghats (13°57' N, 77°44' E), India. (After Mariotti and Peterschmitt 1994)

was dominated by forest. The soil directly under the present savanna shows the less negative 8 13C values expected from the predominant C4-photosynthesis by the vegetation and that under the present forest corroborates prevailing C3-photosynthesis by the forest trees. However, in the zone between forest and savanna, less negative 813C values extend deep under the present forest, indicating that the savanna must have had a larger extension in the past, and that the forest must be currently expanding.

9.4.2 Savanna-Desert Ecotone Dynamics: The Sahel Problem as a Case Story

In Sect. 9.4.1 we have considered the savanna ecotone to the wetter side, i.e. the forest. There is another important interface, to the drier side, namely that between savanna and desert. The large deserts of the world lie mainly outside the tropics, and thus, it is not within the scope of this book to treat the ecophysiology of desert plants. It is interesting, however, to consider briefly the ecotone between savanna and desert in addition to that between savanna and forest.

To do this, desertification in the Sahel region of Africa offers itself as an appropriate case story as it has caused much public concern due to the dramatic economic and social implications. The Arab word "sahel" in fact means coast or shore, referring to the southern delineation of the sand-ocean of the Sahara, a pertinent way to portray the ecotone. The region is characterized by summer rain with an 8-10 months long drought period. According to the annual precipitation a north-south zonation is given as follows:

• saharo-sahelian transition zone 100-200 mm,

• sudano-sahelian transition zone 400 - 600 mm

(Fig. 9.13; Walter and Breckle 1984). The latitudinal position of the saharo-sahelian transition zone, the savanna-desert ecotone, may vary due to climatic oscillations. The case of the sahel is particularly interesting because very strong climatic oscillations have been documented both for the extended period of the last 30,000 years and for much shorter intervals in the last century. Figure 9.14 illustrates the large changes of the area occupied by the Sahara over the ages. During the last ice age (18,000-12,000 years B.C.) the Sahara had enlarged considerably and then, in the post-glacial period (9,500-4,500 years B.C.) contracted again. At that time the river Niger near Timbuctu had a large inland delta with a flooding plain of 20,000 km2. At present the Sahara again occupies a large area similar to that in the last ice age (Petit-Maire 1984). The stochastic appearance of drought periods interchanging with wetter intervals or the movement of the saharo-sahelian transition zone more to the south and more to the north respectively, during the last century is shown in Fig. 9.15. Remote-sensing of the vegetation density (Sect. 2.3) resolves differences for individual years. The long wet period between 1942 and 1966 led to the

Fig. 9.13 Klimadiagramm graphs of four stations in the Sahel zone. (Walter and Breckle 1984, with kind permission of S.-W. Breckle and G. Fischer-Verlag)

extension of savannas and was followed by an increase of the human population and the herds of the nomads. However, this prepared the ground for the catastrophes of the years after 1966. During the increasingly frequent and extended drought periods the land could not sustain the population growth any longer. The example is both tragic and illustrative. It shows that it is impossible to make long term prognoses on the basis of few singular events during periods of short or medium duration.

Fig. 9.14 Oscillations of the area occupied by the Sahara desert during the past 18,000 years. (Petit-Maire 1984, with kind permission of La Recherche)
Fig. 9.15 Annual means of precipitation in the Sahel zone from 1922 to 1984 related to the conventional Saharo-Sahelian transition line given here at 150 mm (dashed horizontal line); wetter years hatched vertically, drier years dotted. (Petit-Maire 1984, with kind permission of La Recherche)

9.5 Productivity

Productivity of savannas is largely determined by nutrients. Generally, conditions required for intensive and productive agriculture in the tropics can be listed after Eiten (1972):

i) above rocks, which lead to soils rich in minerals, e.g. limestone and volcanic rocks, ii) in flood-plains with periodically rising water-tables, leading to repeated renewal of the mineral content of the soils, e.g. in the Amazonas, the Nile, the Ganges, iii) strong organic fertilization, iv) use of chemical fertilizers.

Limitations to the third and fourth possibilities are the economic costs, and with only small areas available for organic fertilization. Table 9.3 gives some data for productivity of seasonal and hyperseasonal savannas.

Table 9.3 Productivity of savannas, biomass in gm 2 (Sarmiento 1984)

Savannas

Above ground

Below ground

Total

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