The forestsavanna boundary at different spatial scales

At the sub-continental scale, the sharp transition between the two landscapes occurs at a latitude of c. 8°N, with notable exceptions in central Côte d'Ivoire, where savanna penetrates as far south as 6°N in a region called "V-Baoulé", and in Togo and Benin, where a savanna dominated-landscape reaches the coast. Even now that deforestation has cleared most of the lowland rainforest,

Figure 3.2 Forest-savanna mosaic in Lamto, Cote d'lvoire. A) the forest-riparian type, B) the savanna-riparian type.

characteristic of this landscape: two distinct vegetation types intertwine, as well as two distinct floras, while few species occur on both side of the forest edge (Aubréville 1966).

This interpenetration of forest and savanna is evident when looking at aerial photographs or maps at the scale of 1:50,000 (Blanc-Pamard 1979). Swaine et al. (1976) make the distinction between the "forest riparian type" and "savanna riparian type". In the former strips of middle-slope savanna penetrate forest which persists on hilltops and along streams (Figure 3.2A) whereas in the latter the savanna dominates the whole of the topography except for fringing forest by streams (Figure 3.2B). It should be added that included savannas can also be found deep inside the continuous forest zone.

At higher resolution, in the field, the transition between forest and savanna is very clear-cut (Blanc-Pamard 1979) (Figure 3.3A). It is only at this last level of analysis that the term "forest edge" makes sense. Most authors stress the abruptness of the change at the field level (Morgan & Moss 1965, Swaine et al. 1976) and define the forest edge as a narrow line where the domination of savanna grasses stops. Spichiger (1975) points out, however, that the forest edge should be interpreted as a transitional belt of varying width where species of the two floras coexist and compete (Figure 3.3B).

Detailed field studies of the forest-savanna transition in Côte d'Ivoire by using transects perpendicular to the forest edge have allowed Spichiger (1975) to go beyond a simple forest/savanna species dichotomy. The Guineo-Congolian forest species have been classified in four groups of increasing tolerance to the savanna environment based on the occurrence of the species in the transects relative to the position of the boundary and its sharpness. Accordingly, the Sudano-Zambesian species have been classified in four groups of increasing tolerance to the forest environment. The fourth Sudanian group, which consist of species that are commonly found inside the forest, surprisingly include species of dry forest that are otherwise found only much further north in the Sudanian zone. They could thus be considered as relicts of ancient dry forest nowadays absent from the transition zone due to the impact of more frequent fires.

Figure 3.2 Forest-savanna mosaic in Lamto, Cote d'lvoire. A) the forest-riparian type, B) the savanna-riparian type.

the transition is still obvious when looking at classifications of low-resolution satellite images (Figure 2.2) and these images still reflect the shape of the limit of the previously continuous rainforest zone as mapped for example by White (1983). In a latitudinal belt from Senegal and Sierra Leone eastwards to Uganda, there is a zone where savanna dominates with dense humid forest elements scattered in the landscape. It crosses all West Africa as a strip of 100 to 500 km wide and has been referred to as the "forest-savanna mosaic", "secteur preforestier"(Monnier 1981) or the "Guineo-Congolian/Sudanian transition zone" (White 1983). This terminology underlies a very important

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