50 years of woody vegetation changes in the Ferlo (Senegal) assessed by high-resolution imagery and field surveys

Woody vegetation dynamics in the Sahel have been debated since the great droughts of the 1970s-1980s. Here, we combined high-resolution satellite and aerial imagery, field inventory, and historical botanical records to study woody vegetation trends over the years 1965, 1980, 2008, and 2018 in the Ferlo, the Sahelian sylvo-pastoral zone of Senegal. While tree density has decreased from 1965 (14.8 trees ha(-1)) to 1980 (13.4 trees ha(-1)) and 2008 (11.9 trees ha(-1)), tree density has stabilized in 2018 (12.2 trees ha(-1)). The relatively moderate decrease in tree density over 50 years characterized by extensive human pressure and droughts, as well as the rather stable woody cover following the drought years after the 1980s, do not support narratives of widespread desertification in this region. However, we observed a shift in the composition of species. While the drought-resistant tree Balanites aegyptiaca showed a stable abundance, Acacia tortilis showed strong increases and other species like Sclerocarya birrea and Combretum glutinosum decreased. In addition, recent field surveys show that the ratio between shrubs and trees has increased towards more shrubs. The observed loss of species diversity combined with the increase of drought-resistant species is in line with current observations for savanna ecosystems in the context of an increased aridity.