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Assessing woody vegetation trends in Sahelian drylands using MODIS based seasonal metrics

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Woody plants play a major role for the resilience of drylands and in peoples' livelihoods. However, due to their scattered distribution, quantifying and monitoring woody cover over space and time is challenging. We develop a phenology driven model and train/validate MODIS (MCD43A4, 500 m) derived metrics with 178 ground observations from Niger, Senegal and Mali to estimate woody cover trends from 2000 to 2014 over the entire Sahel. The annual woody cover estimation at 500 m scale is fairly accurate with an RMSE of 4.3 (woody cover %) and r2 = 0.74. Over the 15 year period we observed an average increase of 1.7 (5.0) woody cover (%) with large spatial differences: No clear change can be observed in densely populated areas (0.2 4.2), whereas a positive change is seen in sparsely populated areas (2.1 5.2). Woody cover is generally stable in cropland areas (0.9 4.6), reflecting the protective management of parkland trees by the farmers. Positive changes are observed in savannas (2.5 5.4) and woodland areas (3.9 7.3). The major pattern of woody cover change reveals strong increases in the sparsely populated Sahel zones of eastern Senegal, western Mali and central Chad, but a decreasing trend is observed in the densely populated western parts of Senegal, northern Nigeria, Sudan and southwestern Niger. This decrease is often local and limited to woodlands, being an indication of ongoing expansion of cultivated areas and selective logging. We show that an overall positive trend is found in areas of low anthropogenic pressure demonstrating the potential of these ecosystems to provide services such as carbon storage, if not over-utilized. Taken together, our results provide an unprecedented synthesis of woody cover dynamics in the Sahel, and point to land use and human population density as important drivers, however only partially and locally offsetting a general post-drought increase.

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