Monitoring vegetation dynamics with open earth observation tools: the case of fire-modulated savanna to forest transitions in Central Africa

Abstract

Woody encroachment and forest progression are widespread in forest-savanna transitional areas in Central Africa. Quantifying these dynamics and understanding their drivers at relevant spatial scales has long been a challenge. Recent progress in open access imagery sources with improved spatial, spectral and temporal resolution combined with cloud computing resources, and the advent of relatively cheap solutions to deploy laser sensors in the field, have transformed this domain of study. We present a study case in the Mpem & Djim National Park (MDNP), a 1,000 km2 protected area in the Centre region of Cameroon. Using open source algorithms in Google Earth Engine (GEE), we characterized vegetation dynamics and the fire regime based on Landsat multispectral imagery archive (1975–2020). Current species assemblages were estimated from Sentinel 2 imagery and the open source biodivMapR package, using spectral dissimilarity. Vegetation structure (aboveground biomass; AGB) was characterized using Unmanned Aerial vehicle (UAV) LiDAR scanning data sampled over the study area. Savanna vegetation, which was initially dominant in the MDNP, lost about 50% of its initial cover in <50 years in favor of forest at an average rate of ca. 0.63%.year−1 (6 km2.year−1). Species assemblage computed from spectral dissimilarity in forest vegetation followed a successional gradient consistent with forest age. AGB accumulation rate was 3.2 Mg.ha−1.year−1 after 42 years of forest encroachment. In savannas, two modes could be identified along the gradient of spectral species assemblage, corresponding to distinct AGB levels, where woody savannas with low fire frequency store 40% more AGB than open grassy savannas with high fire frequency. A fire occurrence every five year was found to be the fire regime threshold below which woody savannas start to dominate over grassy ones. A fire frequency below that threshold opens the way to young forest transitions. These results have implications for carbon sequestration and biodiversity conservation policies. Maintaining savanna ecosystems in the region would require active management actions to limit woody encroachment and forest progression, in contradiction with global reforestation goals.