The forest belt in Upper Guinea stretches over 2000 km from Senegal in the west to Togo in the east. Variation in floristic composition of these forests is mostly gradual although most vegetation maps are based for practical reasons on vegetation types and focus on classes rather than gradients (e.g. Taylor 1960, Mooney 1959, Guillaumet 1967, Guillaumet & Adjanohoun 1971, White 1983). Generally the vegetation of the region has been described in terms of the most conspicuous (mostly in terms of abundance) species that compose a vegetation type (e.g. Chevalier 1909, Schnell 1952, Taylor 1960, Mangenot 1955, Aubréville 1959, Voorhoeve 1965, Guillaumet & Adjanohoun 1971). Later studies have used mathematical techniques to classify the vegetation types (Hall & Swaine 1976, 1981 for Ghana, de Rouw 1991 for the Taï region, van Rompaey 1993 for southeast Côte d'Ivoire - Fig 4.1 - and east Liberia). In general, information on the forests of the Upper Guinea is scattered and available at a local level only, and international level vegetation descriptions and vegetation maps are scarce (except for Africa as a whole, e.g. White 1983).
In this chapter we present a vegetation map for the whole Upper Guinea region. A common classification allows for better comparisons between forests in different countries and a better exchange of information. For such a map we cannot simply combine the existing maps because of differences in criteria used, resulting in different vegetation types. For the present study we used data from forest inventories. Several types of data are available, from large-scale forest inventory data, collected by national forest services, to smaller scale data collected by individual researchers, or research groups. The large-scale data are mainly collected for large trees or smaller trees of large-tree species, especially commercially interesting ones. Other inventories include large trees as well as other life forms, like lianas, shrubs, understorey tree species, herbs and grasses (de Rouw 1991, Hall & Swaine 1976, 1981, Kouamé 1998b).
For the purpose of this book we have chosen to use the large-scale data on large trees collected mainly by the national forest services. The main reason is the large coverage: many forest areas in all countries concerned have been inventoried in this way. A drawback is that in most inventories only a restricted number of large tree species are included. However, for a general classification of forests
this seems to be fine: large trees are better known than small-stature species. Another drawback is that most if not all of the selected species are timber species (that was the main reason for the inventories, after all), and thus the abundance of larger individuals of these species may be reduced due to past logging practices.
We examine the major environmental factors that could determine the large-scale gradient. The best known factor is the rainfall gradient from c. 4 m per year along the coast to less than 1 m more inland. This rainfall gradient is generally linked to a gradual change from wet evergreen forest along the coast to semi-deciduous forest further inland, to savanna vegetation when the rainfall becomes too low. Small pockets of dry evergreen forest also occur at the lower end of the forest rainfall gradient. Other important environmental factors are soil fertility (Hall & Swaine 1981, Swaine 1996, White 1983) and altitude (Voorhoeve 1965, Hall & Swaine 1981). Most of the area is in the lowlands but in some places (e.g. Mount Nimba) altitude rises above 1000 m with an associated change in climate leading to a different vegetation.
In this chapter we describe the main vegetation gradient in the forests of our area, and determine which are the major environmental factors that give rise to these gradients. We focus on large tree species and highlight the larger scale vegetation patterns.
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