Between desertification and re-greening of the Sahel region : diagnosis of remote sensing data and field observations
The Sahel region is characterized by a strong bioclimatic gradient and is very sensitive to climatic fluctuations and especially to rainfall variability. Two successive and particularly severe droughts occurred in the 1970's and in the 1980's over the whole Sahel region, which had dramatic impact on the populations and their resources. The theory of a Sahel suffering from desertification was then refueled, as well as the view of a Sahara desert marching rapidly through the rest of the African continent. The analysis of the first remote sensing observations suitable for vegetation monitoring (available since the 1980's) proved to be at odds with these desertification assessments. Indeed, the satellite vegetation indices and particularly the NDVI evidenced strong positive trends, meaning that a significant increase in vegetation productivity occurred at the regional scale. This phenomenon was called "re-greening". However, no regional validation of such trends was possible due to the scarcity of field observations of vegetation. The objective of this work is to evaluate whether re-greening is actually happening over the Sahel or if some degradation trends of Sahelian ecosystems can be detected as well, using both field observations of vegetation and long-term NDVI data (GIMMS-3g NDVI). Within the framework of the AMMA-Catch observatory and of previous field programs, we now have almost 30 years of field observations in the Gourma region in Mali (1984-2011) and almost 20 years in the Fakara region in south-western Niger (1994-2011). The comparison of these two data sources (GIMMS-3g NDVI and vegetation productivity measured on the field) evidences strong re-greening patterns over the pastoral Gourma as well as an increase in vegetation productivity measured on the field. In contrast, the Fakara region in south-western Niger reveals decreasing trends for both variables. The consistency of satellite and field data confirms that NDVI can be used as a reliable tool for monitoring vegetation changes in semi-arid ecosystems and over long periods of time. At the Sahel scale, a strong re-greening pattern is found over the major part of the region over the 1981-2011 period. By analyzing the RUE index (the ratio of production to rainfall) we show that the re-greening observed over the Gourma is mainly explained by the rainfall recovery over the same period, thus showing the ecosystems' resilience to extreme climatic events. This resilience nature, however, is mainly found for the deep sandy soils of the area. Contrasted changes are observed over a small part of the landscape composed of shallow soils which experienced an increase in soil erosion and run-off coefficients, sometimes leading to vegetation decay. The re-greening observed at the Gourma scale should thus consider changes in the ecosystem functioning that may occur in a small portion of the landscape (the shallow soils). The decreasing trend observed over the Fakara region is not explained by rainfall. Changes in land use (strong increase in cultivated areas, shortening of the fallow duration) may lead to a decline in soil fertility which could explain the loss in vegetation productivity over the past 20 years. Thus, we do not observe any extensive desertification of the Sahel region over the past 30 years, but we do observe a global recovery of vegetation which mainly follows the precipitation recovery. It does not exclude though that some degradation may occur in a few regions or at the local scale, for instance over the surfaces under strong erosion or over some agricultural lands, which tends to reconcile both theories.