The rangelands of the Sahel

The Sahel is a biogeographic entity first defined by the arid to semiarid tropical climate controlled by the West African monsoon of the Gulf of Guinea and the Harmattan (Saharan trade wind). During the Quaternary the vegetation of the Sahel had to adjust to climate fluctuations between tropical humid and arid, and even hyper-arid. Precipitation distribution during the rainy season and water distribution from runoff and runon on the soil surface are leading factors of the diversity of the vegetation cover and production. The shades in soil water regime that result from interactions between rain water redistribution and soil textures explain the high differenciation of vegetation types, particularly for the perennial component, the trees and shrubs populations. Notwithstanding, the species composition of the herbaceous annuals largely vary from year to year depending on rain distribution in space and time (ca 1,500) and on the dynamics of the soil seed bank. The sahelian vegetation is species-poor, with only few endemics, it appears as a transition zone between two endemism hubs: Sudanian and Saharan. The Sahel vegetation differs from other arid zone vegetations in that it has very few perennial grasses herbaceous and almost no succulents. It is dominated instead by annual herbaceous, mostly C 4 grasses, associated with scattered woody plants. Highly patterned at the micro-scale, the herbaceous layer is also characterised by large interannual changes in species composition. The primary production is hampered by phosphorus and nitrogen deficiencies in soils that are also poor in organic matter. This constrains the crop and forestry potential, while the rhythm imposed to fodder resources by the monsoon both limits and protects pastoral production. If the history of management of natural resources by man is very long and shared all over the Sahel, its influence was extensive and diffuse until the 20th century. The demographic upsurge and urbanization experienced since the 1950s have deeply changed land use and resource management, and are challenging the resilience of the Sahelian ecosystem.