Introduction Of Evergreen Forests

Forest-savanna transitions occur in a variety of localities around the tropical world. Furley et al. (1992) provide an extensive review of the processes at the forest-savanna boundary, its dynamics, the role of physical factors, environmental change and vegetation history. However, apart from Nigeria, regional accounts concerned mainly Eastern and Southern Africa, Australia, and the Neotropics. We present here a review on the topic for West Africa.

Distribution of vegetation types in West Africa is tightly bound to the climatic gradient from the shores of the Gulf of Guinea north to the Sahara (Bongers et al. chapter 4). In the absence of major orographic features, this gradient is mainly oriented on a south-north direction with the associated vegetation types forming latitudinal belts: from the lowland evergreen rainforest to drier vegetation types like dry forests, the mosaic of woodlands and savanna, and further north to a Sahelian pseudosteppe and finally desert.

Phytogeographically, western Africa is the home of two main regional centers of endemism. The lowland rainforests in the south correspond to the western block of the Guineo-Congolian centre of endemism, whereas the savannas, woodlands and dry forests north of c. 9° latitude belong to the western part of the Sudanian centre of endemism (White 1983).

Physiognomically and floristically, most steps in this transition are gradual, like the one from evergreen to semievergreen lowland rainforest, or from Guinean to Sudanian savanna and woodlands. On the contrary, the transition from semi-evergreen lowland rainforest to Guinean savanna is very sharp and can be unambiguously mapped at the continental scale by a solid line separating two very distinct landscapes (Figure 2.2). The multistrate closed forest which dominates the whole toposequences on the southern side abuts on a very different and more open landscape to the north, dominated by annually burning vegetation types. These vegetation types share a dense herbaceous layer made mainly of hemicryptophytic gramineous plant species, but whose tree cover may vary widely, ranging from almost pure grasslands, through savannas, to woodlands. In this open landscape, forest elements are limited to a network of gallery forests on the bottom of the slopes, but sometimes also to plateau forests

Figure 3.1 An example of a forest-savanna mosaic: Reserve de Lamto, south of the V-Baoulé, central Côte d'Ivoire.

on hilltops (Figure 3.1). Their structure and floristic composition is very similar to those of the adjacent drier types of the lowland rainforest.

In this chapter we describe the forest-savanna boundary at different spatial scales, and how its location has shifted in the past. We discuss the main environmental factors that explain the present distribution of the two vegetation types in West Africa, such as climate, substrate, fire, and men. Finally, we describe the present trend in vegetation dynamics under natural conditions and under the growing influence of man.

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