Un cadre d'évaluation de la vulnérabilité écologique des forêts dans les fronts de déforestation tropicaux à partir de l'évaluation de la dégradation des forêts dans une approche paysagère : études de cas au Brésil et Vietnam

Conservation of the world's tropical forest cover is essential to ensure sustainable ecosystem services. However, population growth, increasing demand for agricultural products and land use changes are affecting their sustainability. This thesis aimed to develop a multi-dimensional approach to assess forest degradation and how it is related to land use dynamics, in order to estimate forest vulnerability. The studies for this thesis were conducted in Paragominas (Brazil) and Di Linh (Vietnam). In Paragominas, forest degradation is directly linked to selective logging and forest fires, which alter the structure of forests. We used remote sensing tools and data on carbon stocks to map aerial forest biomass, and investigated the consequences of degradation on forest structure. We observed that 58% of forest degradation was accounted for by changes in land use. The distribution of degraded forests is driven by their fragmentation due to the expansion of soya cultivation, but is also associated with recent deforestation and slash-and-burn practices. In Di Linh, forest degradation is due to encroachment by coffee plantations. By combining an inventory of different types of land use with Sentinel-2 images, we were able to map current land cover. We then built up the trajectories of landscape dynamics to characterise the expansion of the agricultural frontier. We identified the degradation and fragmentation trajectories that produce effects of greater or lesser intensity on the forest cover. Taken together, these indicators bring out vulnerability hotspots. Thanks to the remote sensing approaches and indicators developed for forests on the landscape scale, we were able to provide a holistic diagnosis of forests within landscapes altered by humans. This thesis aims to open up avenues for appropriate and priority management of degraded forests on the scale of the landscape.