Annuals naturally at home in the Sahel
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CTA. 1994. Annuals naturally at home in the Sahel. Spore 50. CTA, Wageningen, The Netherlands.
Permanent link to this item: http://hdl.handle.net/10568/49363
The flat, sandy landscape of the Sahel is the last place one would expect to find a green revolution taking place. 'Yet, a revolution of a sort has quietly taken place in the Sahelian flora that contradicts observations in other arid and semi-arid...
The flat, sandy landscape of the Sahel is the last place one would expect to find a green revolution taking place. 'Yet, a revolution of a sort has quietly taken place in the Sahelian flora that contradicts observations in other arid and semi-arid areas,' reports Dr Pierre Hiernaux, an ecologist working with the International Livestock Centre for Africa. The vegetation of the Sahel is mostly herbaceous and is dominated by annual grasses. Unlike other dry environments, such as the Kalahari Desert in Southern Africa, perennial grasses and herbs are rare in the Sahel and succulents are generally absent. Some environmentalists believe that the predominance of annuals in the Sahel is a sign of degradation. But the presence of perennials in southern Sahara contradicts the hypothesis that annuals assert themselves with increased aridity. Observations have shown that the increase in annual plants in the Sahel is neither due to increased aridity nor the result of heavy grazing. The fact that annuals dominate the natural vegetation of the Sahel may to be due to their ability to adapt to the two main characteristics of the Sahelian environment, namely extreme aridity toward the end of the dry season and a relatively regular seasonal distribution of the rains. Perennials have to expend huge amounts of energy just to stay alive between two rainy seasons and, as a result, they are unable to take full advantage of the available moisture when the rains start. Annuals on the other hand, invest a large part of their energy in seed production and protection. Seed dormancy during the dry season reduces the risk of seed wastage, while diverse germination strategies enable annuals to compete effectively with perennials for soil moisture and nutrients. Comparisons with Australia's Northern Territory have shown that the big-climate gradients in this region is the exact opposite of that in the Sudano-Sahelian zone of West Africa. Both regions have poor soils but they differ in terms of their rainfall distribution. In the Northern Territory of Australia, the seasonal mon-soonal rains become less regular with increasing aridity and may occur at any time during the year in the region's very arid southern parts. In the steppes of the southern Sahara, where rains are so scarce that they can hardly be described as seasonal, as well as in the Australian deserts, perennials have a comparative advantage over annuals because of the Irregular distribution of rainfall. The subhumid savannas of northern Australia have evolved under low grazing pressure, from large ruminants or other large grazers. Thus if annual plants dominate there, this cannot be due to overgrazing. The northern parts of the Sahel are also subjected to relatively low grazing pressure. Since the distribution of seasonal rains in the Sahel is as regular as in Australia's northern savannas, the predominance of annuals in the Sahelian vegetation would appear to be the result of adaptation to climate rather than a sign of degradation. Inca Alipui ILCA, BP 5689 Addis Ababa ETHIOPIA