The vegetation zones in West Africa follow the rainfall gradient that is running north-south, with the arid Sahara desert and Sahel biomes in the north and the moist forests in the south. In between these extremes lies the open to wooded savanna zone. The ecosystems along this gradient are a clear expression of rainfall and represent different climate or eco-climatic zones.
Only a small part of the Upper Niger Basin, in the very south of Guinea, belongs to the forest transition zone or Upper Guinea forests. This zone has a high rainfall (>1,800 mm) and sustains dense rainforests, though most of the contiguous forest has disappeared or is degraded. Patches of humid forest are also found in the transition zone between forests and moist woodland savannas, on steep mountain slopes and as gallery forest bordering rivers.
Most of the Guinean part of the UNB falls within the moist savanna zone or Guinea zone with 900-1,800 mm rainfall and a wet season of 6 months or more. This zone is generally heavily wooded, but due to frequent burning and lower rainfall compared to the forest zone, grasslands are maintained and form a landscape consisting of alternating grasslands, woodlands and gallery forest.
North of the moist savanna zone lies the Sudan zone or dry savanna, which receives less rainfall (500-900 mm) and has a longer dry season. The southern part consists of wooded savanna and forms a transition zone into the moist Guinea savanna. The northern zone is more open and harbours characteristic tree species such as Faidherbia albida and Parkia biglobosa. In the Sudan zone grazing is very important, with increasing livestock densities to the north. Open grassland savannas are dominating. Within the Sudan zone maize and cotton are dominant crops in agriculture as well as peanuts in the northern parts.
Further north, towards the Sahel with 150-500 mm rainfall, land use shifts towards Millet and Sorghum and in the east also to cotton. Along the Niger and Bani flood recession culture and irrigated agriculture are practiced in perimeters or large irrigation zones along the rivers. The rice granary in West Africa in this respect is the Zone d’irrigation d’Office du Niger where currently >50.000 ha is under irrigation for rice, sugar cane and other cultures. Also near the Sélingué lake, downstream the dam, an irrigation complex is situated.
IIn the Sahel the strong seasonal and annual variation in rainfall cause marked patterns in temporal and spatial availability in resources (grazing grounds). These variable resources in space and time are since time immemorial effectively exploited by pastoralists moving their herds from one place to the other (transhumance). In the dry season, the floodplains of the Inner Niger Delta and surrounding areas are extremely important as grazing grounds for livestock and the production of fish.
Key habitats in the Upper Niger Basin
In general terms, vulnerable habitats at the level of the Upper Niger Basin can be defined as habitats in a more or less natural state that are under pressure. In the northern part of the basin these habitats are prone to droughts in combination with (over)grazing and wood cutting. In the middle and southern part of the basin agriculture, mining, (illegal) logging and water extraction are important drivers of habitat degradation. A highly informative overview of land use changes in West Africa is given by the recently published book and website Landscapes of West Africa – A Window on a Changing World (USGS 2016).
Various forest types can be found in the basin, ranging from patches of humid closed-canopy forest in the south, to gallery forest along watercourses in the savanna zone, and to flooded Acacia nilotica forests in the northern parts. The forested areas, interspersed with water courses, can be considered as key habitats in the headwaters of the basin. The protected Haut Niger N.P. including the buffer zones are essential in the protection of these ecosystems. In particular the forests in the northern parts of the basin are directly affected by the water regime in the Niger River and its tributaries, since the southern forests are more governed by rainfall. Changes in water management, for example due to hydropower developments, could seriously affect the flood forests due to changes in flood patterns.
A typical feature of the savanna landscape in (mostly) Guinea are the bowé (singular: bowal). These are almost impermeable ferruginous plateaus which hardly bear any woody vegetation because of the near-absence of a soil layer, but a herbaceous vegetation is usually present.
Obviously, the distribution and health of wetlands across the basin depend on water from the Niger River. These wetlands often have high biodiversity values and deliver ecosystem services that are important to the livelihoods of local people (drinking water, small-scale irrigation, fishing etc). In most of the UNB, wetlands are part of the Niger river system, concerning smaller or larger floodplains and/or exposed (sand)banks along the river, (storage) lakes and riverine forests. In particular the riverine floodplains and exposed banks cover extensive areas. Important wetlands in the UNB include:
- Riverine forests interspersed with water courses or along the river, which are temporarily flooded during the wet season. These habitats are essential for a wide range of biodiversity.
- Exposed banks and floodplains in the riverbed, which are temporarily flooded during the wet season. These habitats can be important for mammals, and as foraging areas or breeding areas for threatened bird species (for instance African Skimmer, Kittlitz’s Plover). To a small extent still flood forests (with Acacia nilotica, Ziziphus spec.) can be found on the banks.
- Lac Sélinkegny, a large storage lake, created in 1987 by the Sélingué dam, without aquatic vegetation.
- Temporal lakes and depressions. Between the Malian border and the Inner Niger Delta, a number of smaller or large temporal lakes and depressions can be found, which are flooded during the wet season, depending on rainfall and connection to the river system. Some of these depressions are more or less isolated and are rainfed. The vegetation depends on flood duration, depth and land use. Some lakes hold aquatic and helophytic vegetations like Echinochloa-vegetations or resembling habitats (with Vossia cuspidata). Often these depression are used for cultivation, when the water recedes. These types of wetlands or very important for water birds and a range of other species (amphibians, small mammals, fish species).
- Falas Delta Mort – Office du Niger. In the depression of the Delta mort, where the irrigation zone of the Office du Niger has been developed, the old branches of the Niger river are called Falas. These natural wetlands are now used for the irrigation system, but still are important wetland areas with marshy habitats.
- Irrigated ricefield complexes. Large-scale irrigated rice cultivation in the Upper Niger Basin is mainly found in the Malian part of the basin. Also in the Guinean part of the basin cultures are found, which are mainly flood recession cultures. These may however host very important wetlands values. Large irrigated rice complexes (perimeters) are found just downstream the Sélingue dam (3,000 ha), before the confluence of the Sankarani with the Niger proper, between Bamako and Ségou, east of Ségou, and in the Office du Niger. The latter is one of the largest irrigation zones of West Africa, nevertheless hosting important biodiversity values.
Key habitats in the Inner Niger Delta
The Inner Niger Delta forms a huge floodplain ecosystem of 20.000 km2 with a diverse mix of vegetation types. Although it is a wetland of global importance the area at the same time is used by almost 2 million people for rice cultivation, grazing and fisheries. It is therefore no natural and pristine ecosystem anymore, and hasn’t been for centuries. The Inner Niger Delta is vegetated with plant and tree species adapted to steep fluctuations in water level, seasonal flooding and long dry periods. The presence of the main vegetation types is determined by the depth of water. These are, with increasing depth, the tall Black vetever grass Vetiveria nigritana, cultivated (floating) rice Oryza glaberrima, Nymphaea lotus, Wild rice Oryza longistaminata / barthii, Dideré Vossia cuspidata, and wild and planted Bourgou Echinochloa stagnina. During flooding, wild rice, bourgou, and also Vossia cuspidata (known in other parts of Africa as Hippo Grass, but as didere in the Delta), form huge floating meadows.
The permanent water bodies consist of water bodies which are also holding water during the dry season, hence at the lowest flood level (étiage) when the floodplain is exposed and dry. In practice this refers to the Niger river proper and the deepest parts of the lakes.
Bourgoutieres are floating grass fields of Echinocloa stagnina (water depth 4-6 m) or Vossia cuspidata (water depth < 4m) and are an essential habitat in the IND floodplain ecosystem. Their structure (stems up to 6 m in length) constitutes an ideal habitat for the reproduction and growth of first-year fish populations. Bourgou constitutes a key habitat for populations of several bird species, including those of resident and migratory herons and egrets. Bourgou fields are essential as grazing ground for livestock through the dry season. Being essential as feed for the large herds, bourgou is often planted by the local populations.
Wild rice vegetations
Wild rice Oryza barthii and O. longistaminata produce long stems and occupy the zone where the water column reaches up to 2 metres.
Controlled submerged rice
These rice fields are surrounded by small dikes and get flooded during high water levels. The dikes also keep the water in during the retreat of the flood.
People are cultivating an increasing proportion of the floodplains to grow rice. Cultivated rice Oryza glaberrima requires the same water depth zone as wild rice and flood forests, and so extension of cultivated rice fields occurs at the expense of natural habitats.
Mares (ponds) and water lilies
Ponds are temporary habitats for fish, being used by local inhabitants for communal fishing when the flood retreats. In addition, the vegetation is exploited by waterbird populations, especially by Garganey feeding on water lily seeds.
The flooded forests form a unique habitat in the IND. Few tree species support prolonged flooding alternating with several months of drought. In the IND flood forest are dominated by Acacia kirkii. The flood forests are a key habitat for the large breeding colonies of waterbirds, as spawning area and nursery for fish and habitat for high densities of migratory land birds. Since time immemorial, the local populations have managed these forests according to a system of shared responsibilities.
Strictly speaking the non-flooded forests around the delta are not dry since they are located on more or less moist soils in depressions and may be temporarily flooded; tree species include Acacia seyal but other species are also present. Wood production in these forests is important for local communities, for example for drying fish. The forests are a hotspot for migratory land birds.