50 years of woody vegetation dynamics in the Senegalese Sahel (Ferlo) : bases for a functional diversity approach

Dendoncker, Morgane [UCL]

Sahelian woody vegetation has been the topic of many researches since the great drought that struck the Sahel in the 1970s-1980s. However, there is currently no consensus about the long-term trends of these savannas as observations sometimes seem contradictory according to the indicators used. Moreover, the observed trends often lack an understanding of the consequences on ecosystem functioning and subsequently on the provision of ecosystem services (ES), even if a decrease in ES is often assumed. As the inhabitants of the sylvo-pastoral zone of Senegal rely on these ES in their daily life, it is crucial to be able to predict the woody vegetation response to the current pressures acting on these ecosystems. In this research, we investigate the long-term trends of woody vegetation in order to discuss its persistence and its effects on two ecosystem functions (primary productivity and biogeochemical cycles) when facing three types of disturbances (drought, fire and grazing). This vegetation is examined from 1965 to 2018 using different indicators of the vegetation state and persistence, computed at several spatial scales and derived from multiple datasets (field inventory, remote sensing and literature). We first studied the spatial differentiation of the current woody vegetation according to the topography. Second, we evaluated changes since 1965 onward in terms of specific composition, density and woody cover. Third, we transposed the different indicators from the first two parts into a functional level of the woody communities, focusing on the vegetation response to pressure and effects on ecosystem. Based on this functional approach, we suggest some avenues for a potential management of this zone, in order to maintain the sylvo-pastoralism as land use.