Can a 25-year trend in Soudano-Sahelian vegetation dynamics be interpreted in terms of land use change? A remote sensing approach

This study is based on the premise that, in the Sahel/Sudanian belt of Africa, the main determinants of interannual variation in vegetation dynamics are rainfall and land cover type. We analyzed the spatio-temporal sensitivity of the NOAA–AVHRR 8 km-resolution vegetation index (NDVI) to (i) annual rainfall variability (0.5° × 0.5° resolution) acquired over a 25-year period (1982–2006); and (ii) land use changes in the different eco-climatic regions of the Bani catchment in Mali (130 000 km2). During the period 1982–2006, there was no clear trend in rainfall over the catchment, whereas there was a strong positive trend in the NDVI, both when the NDVI values were corrected using annual rainfall variability and when they were not. We divided the catchment into three eco-climatic regions based on the relationship between the annual NDVI and rainfall. In each region, we analyzed the observed greening in relation to changes in land use after correcting for the effect of annual rainfall on the NDVI. Results show that there is a mixed level of agreement between the land cover changes at the grid cell scale and the spatial pattern of the NDVI trend. Increased cropping does not explain the increase in the annual NDVI, except in the Sahelian part of the catchment. We hypothesize that the natural vegetation dynamics related to the non-linear rainfall patterns during the 25-year study period were responsible for these results.

Research highlights
► A positive trend in Soudano-Sahelian vegetation dynamics (a greening) is observed by satellite.
► There is no clear trend in the recent 25-year rainfall in the study area of eastern Mali.
► Increased cropping could explain the greening trend in the Sahelian region.
► Human activities cannot explain the greening trend in the Soudanian region.
► Vegetation dynamics are complex due to non-linear temporal rainfall patterns.