干旱区生态环境知识资源中心

Following 25 years of below average annual rainfall in the Sahel between 1970 and 1995, the return to more humid conditions has led to rapid postdrought recovery of the woody cover. However, the increase in the woody cover is not spatially homogeneous raising questions about the resilience of some woody vegetation types. Based on the analysis of field and remote sensing data collected on the tiger bush systems in the northern Sahel in Mali, this study noted the current and persistent degradation of these systems in the Sahel since the 1970s despite the recent improvement in rainfall since the mid-1990s and the general Sahel re-greening. Profound changes in the woody population pattern, tree density and cover, and floristic composition took place regardless of the site location along the south-north rainfall gradient. Associated with definite structural changes of the woody population, surface hydrology shifted from a sheet to concentrated run-off accelerating the collapse of the patterned woody population. Currently, there is no evidence in favour of reversing the current degradation process, at least at a decadal scale, although very sparse recolonization by pioneer woody vegetation has been observed in the driest sites along recently formed gullies. These observations support the hypothesis of an ecosystem shift, with long-term implications for the structure and functioning of the patterned vegetation, as well as the whole watershed landscape through increased run-off leading to stronger water flows in enlarged wadis, increased soil erosion upstream and sediment deposition downstream, enhanced water storage in ponds, and greater recharge of aquifers, which is an illustration of the "Sahelian paradox".