From north to south — from the Sahara to the humid southern coast — West Africa can be subdivided into five broad east-west belts that characterize the climate and the vegetation. These are the bioclimatic zones known as the Saharan, Sahelian, Sudanian, Guinean, and Guineo-Congolian Regions, shown in the map above. The lines between these regions represent more of a transition along a continuous ecological gradient than sharp boundaries. There is considerable variation among different authors in the definition and geographic delineation of these regions, though most use long-term rainfall averages to define the boundaries. Since long-term rainfall levels have generally decreased since the 1960s (but increased somewhat in the past two decades), some authors consider these bioclimatic regions to have shifted somewhat southward (Gonzalez, 1997). Since these regions are often referenced in this atlas, it is useful to present their general characteristics. They are presented from driest to wettest climatic regimes.
The Sahara, or Saharan Region, stretches across the whole northern extent of West Africa, formed by the Sahara Desert. It consists of a variety of arid landscapes varying from sandy sheets and dune fields to gravel plains, low plateaus, and rugged mountains. Vegetation cover is sparse to absent, except in depressions, wadis, and oases, where water is present at or just below the surface. Average annual rainfall ranges from 0 to 150 mm per year.
The Sahel, or Sahelian Region, is a broad semiarid belt, extending from the Atlantic Ocean to Sudan (and to the Red Sea), averaging about 350 km wide. Climatically, it is characterized by average annual rainfall between 150 and 600 mm, with great variability in amount and timing in a given year. It has an ecologically dry season of 8 to 9 months. Vegetation in the Sahel is generally characterized by open herbaceous types (steppe and short grass savanna) often mixed with woody plants. It is known for its thorny trees, particularly from the genus Acacia, and mostly annual grasses from the genera Aristida and Cenchrus. The number of woody plant species is relatively low. The present physiognomy of Sahelian vegetation results from long-term human and animal presence. Annual grass fires often sweep across its landscapes where there is ample grass cover. The Sahel is also home to countless small wetlands, like in eastern Mauritania, as well as some major ones including the Senegal Delta, the Inland Niger Delta, and the Lake Chad area.
The Sudan, or Sudanian Region, consists of a very large belt immediately south of the Sahel, with average annual rainfall between 600 and 1,200 mm and an ecologically dry season of 5 to 7 months. It is the domain of the savanna — ranging from open tree savannas to wooded savannas to open woodlands. As in the Sahel, rainfall is spread over the months when the sun is high (typically May to October). The short, annual grasses of the Sahel are replaced in the Sudan Region by tall, perennial grasses, mainly of the genus Andropogon.
The savannas almost always have a woody component, with trees growing among the tall grasses. There are at least 80 species of trees specific to this bioclimatic region (Aubréville, 1938). In the northern part of the Sudanian Region, tree savannas tend to dominate, whereas the southern reaches of this region typically transition into denser wooded savannas and open woodlands. Fire has been part of the region’s ecology for millennia. Both natural and human-induced bush fires sweep though the savanna areas, burning up to 80 percent of their area each year. Gallery forests, with tall tree species more common in the Guinean Region to the south, follow watercourses, penetrating deep into the Sudanian Region. They are generally not affected by bush fires and often act as natural fire breaks.
The Guinean Region lies immediately south of the Sudanian Region, generally defined by average annual rainfall between 1,200 and 2,200 mm. This is the domain of the seasonally wet-and-dry deciduous or semi- deciduous forest. Despite the relatively high rainfall, this region has a distinct dry season of 7 to 8 months, which distinguishes it from the Guineo-Congolian Region. The forest canopy is generally dense and closed, forming over a heterogeneous woody understory. Tree height is high, averaging 18 to 20 m. Guinean forests in their natural setting are generally not affected by bush fires. Present day landscapes of the Guinean Region are mostly altered by human activity, particularly slash-and- burn agriculture, so that the actual extent of Guinean forest is rather limited. Most of what remains has been modified by humans. The tree and wooded savannas are also extensive. Some authors consider that the forests have been replaced by “derived savanna,” a mosaic of cropland, bush fallow, and secondary forest resulting from centuries of human influence (Keay, 1959). Gallery forests of varying width follow watercourses.
The Guineo-Congolian Region is the wettest in West Africa, with average annual rainfall between 2,200 and 5,000 mm. The rainfall can be distributed across most of the year, or in two rainy seasons with short drier periods between the rains. This region is split geographically into western and eastern blocks, separated by the Dahomey Gap where savanna reaches the coast. These blocks are often referred to as the Upper Guinean and Lower Guinean Forests, respectively (Church, 1966). This region is thought to have been mostly forested in the past, but today only a fraction of the land is forested. Nevertheless, the forest flora is the richest in West Africa. The forests are dense, with trees reaching over 60 m. The upper tier usually has a discontinuous canopy, towering over a lower, dense canopy. In the undergrowth, woody climbers and epiphytes are characteristic. Herbaceous ground cover may be found but can also be absent.