Vegetation optical depth at L-band and above ground biomass in the tropical range: Evaluating their relationships at continental and regional scales

•L-VOD retrieved by SMOS shows an increasing relation with forest AGB, reaching saturation at about 350 Mg ha−1 in Africa and South America Forests.•The saturation level of 350 Mg ha−1 found in this study includes most of the forested areas in the tropical range.•L-band VOD added to climate variables is useful to retrieve forest AGB at continental and regional scales using machine learning methods.•VOD - AGB scatterplots for ecoregions display some clear data clusters for those units encompassing large amounts of biomass in both continents.