Introduction

In Africa the lowland rain forest occurs under significantly drier conditions than in other continents, within an average precipitation of 1,600 to 2,000 mm/yr, although higher rainfall is observed around the Atlantic coast of Cameroon, Gabon, and in the Central Zaire Basin. Seasonal distribution of precipitation is far from being uniform (White, 1983). Variations in the duration of the dry season follow distance from the equator in both hemispheres and also along a west-to-east gradient. The Biafran Gulf is the only region where the minimum monthly precipitation value always exceeds the 50-mm threshold for the driest month, therefore experiencing no dry season. However, great annual rainfall variability is registered at most of the meteorological stations. Mean monthly temperatures remain constant. Inside the area occupied by the African rainforest, topography is not uniform. Low elevations are found in the coastal Atlantic plain, and in the Zaire Basin, that lie below 400 m. The undulating plateaus of Gabon and Cameroon, generally located between 600 and 800 m, can reach up to 1,500 m, whereas the eastern part of the Zaire Basin joins the slopes of high mountains above 2,000 m bordering the Rift in the Kivu region. Mount Cameroon exceeds 4,000 m in elevation.

The geographical distribution of plant species is complex (Richards, 1981). Relationships between geographical plant distribution and ecological variables—such as rainfall, available moisture, and seasonality—within the Guineo-Congolian domain are far from being well-established, although there are significant variations of these factors inside the areas occupied by rainforest. The only comprehensive review relies on the vegetation-mapping done for Africa. However, the classification of the different types (or variants after White, 1983) was difficult. This is partly because variation in floristic composition, physiognomy, and phenology is largely gradual and continuous (Aubreville, 1951). This chapter concerns the oriental part of the Guineo-Congolian domain, where the degree of endemism is high, representing 80% of the total 8,000

Guineo Congolian Rainforest

Figure 5.1. Distribution of different vegetation units within the Guineo-Congolian rainforest (6°S and 6°N) (after White 1983; Letouzey, 1965, 1985) with location of modern (O) and fossil pollen (*) and wood sites (+). Bm: Barombi Mbo; Mb: Mboandong, O: Ossa; M: Mengang; K: Kandara; L: Lobeke; B: Belinga; Mk: Makokou; G: Guibourtia; Ka: Kamalete; Bi: Bilanko; N: Ngamakala; Si: Sinnda; Ki: Kitina; C: Coraf; So: Songolo.

Figure 5.1. Distribution of different vegetation units within the Guineo-Congolian rainforest (6°S and 6°N) (after White 1983; Letouzey, 1965, 1985) with location of modern (O) and fossil pollen (*) and wood sites (+). Bm: Barombi Mbo; Mb: Mboandong, O: Ossa; M: Mengang; K: Kandara; L: Lobeke; B: Belinga; Mk: Makokou; G: Guibourtia; Ka: Kamalete; Bi: Bilanko; N: Ngamakala; Si: Sinnda; Ki: Kitina; C: Coraf; So: Songolo.

species and 25% of the genera, with the greatest number of endemic genera being found among the Leguminosae-Caesalpiniaceae. We present modern and fossil pollen data relating pollen and plant distribution within the present day and for the latest Quaternary (Figure 5.1).

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