Ponds as Ecological Refuges: Dynamics of Woody Vegetation in Ferlo (1972–2016)

This study was conducted over a 34 km<sup>2</sup> area in the Ferlo region (northern Senegal). It aims to analyze the spatio-temporal dynamics of tree vegetation and the distribution of temporary ponds. The ponds were identified using satellite images acquired in September 2015 (rainy season), while woody vegetation was mapped for two periods: April 1972 and May 2016 (dry season), enabling a diachronic analysis over 44 years. The data were extracted from very high spatial resolution satellite images (<3 m): SPOT 6-7 for 2015-2016 and CORONA for 1972. Image processing (ortho-rectification, enhancement, automatic detection, mapping) enabled the creation of accurate maps, which were then analyzed according to the principles of landscape ecology using the open-source software R, GRASS 7.6, and QGIS 3.4. A total of 244 ponds were automatically detected, representing 1.29% of the study area (43.4 ha), organized in a NE-SW alignment. Diachronic analysis shows a significant collapse in tree cover between 1972 (12.3%) and 2016 (2%). However, this decline is not observed in ponds, where woody cover has remained stable. Between 1972 and 2016, the Ferlo ponds showed a significant improvement in vegetation cover: while half of the ponds had less than 55% cover in 1972, this value exceeded 60% in 2016. These results shed new light on the distribution of vegetation in Ferlo and highlight the central ecological role of ponds in plant resilience in the Sahel.