The Ivorian flora is estimated to number 3660 species (Davis et al. 1994) that are found in four main vegetation types (Figure 5.2). Ivorian closed forests, which correspond to the rainforests of White (1986) are divided in evergreen and semi-deciduous forests. In Côte d'Ivoire, the savanna penetrates the forest zone in a region called V-Baoulé (Guillaumet & Adjanohoun 1971, Gautier & Spichiger chapter 3). On each side of V-Baoulé, rainfall increases towards the southwest and southeast (Eldin 1971). Rainfall also increases from lower to higher altitudes. Along these rainfall gradients soil humidity increases, inducing a change in the vegetation from semi-deciduous to wet evergreen forests (Hall & Swaine 1981) and an increase in floristic diversity. The lowland evergreen forests in the southeast (Mabi, Songan, Tamin, Yaya) and southwest (Haute Dodo, Taï), and the mountain forests in the west (Mont Tonkoui) have the highest rainfall in the country (Eldin 1971). They have also been identified as the centres of greatest floristic diversity in Côte d'Ivoire (Aubréville 1949, Guillaumet 1967, Anonymous 1991, Beentje et al. 1994). Guillaumet and Adjanohoun (1971) have distinguished a floristic species group in southwest Côte d'Ivoire which they coined "Sassandrian". This group includes species that occur between the Cavally river and the Sassandra river, from Tabou to Taï. Another species group, called "Ghanaian", occurs in southwest Ghana (Hall & Swaine 1981), and appears to penetrate into the east of Côte d'Ivoire.
In this chapter, we investigate the environmental factors responsible for the floristic composition in the lowland closed forests of Côte d'Ivoire. The forest blocks of southeast and southwest Côte d'Ivoire are currently separated by the relatively dry region of the V-Baoulé. They have probably been separated by even drier savanna vegetation during the dry glacial periods (Gautier & Spichiger chapter 3). Nevertheless, we expect to find high floristic similarities between these forest blocks, because they experience similar high rainfall levels, and because they have been connected by wetter forest types during the interglacial periods.
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