Loss of moist broadleaf forest in Africa has turned a carbon sink into source

Summary

Assessing forest aboveground biomass dynamics over long time periods and at continental to global scale requires reference observations such as forest inventory field plots in combination with satellite Earth observation. To overcome the low availability and quality of reference data, we used the spaceborne Geoscience Laser Altimeter System (GLAS) onboard the Ice, Cloud, and land Elevation Satellite (ICESat)48, which operated from 2003 to 2010 and acquired millions of Light Detection and Ranging (LiDAR) footprints, providing measurements of canopy height and other biophysical metrics relating to canopy structure that are highly correlated with aboveground biomass49. The Global Ecosystem Dynamics Investigation (GEDI) LiDAR instrument22 onboard the International Space Station that commenced operation in 2019 uses similar technology to the GLAS/ICESat instrument but with smaller footprints and much denser coverage (narrower spacing between footprint locations along the orbit), providing a dense network of training and validation data on forest canopy height.Based on a machine learning algorithm, L-band Synthetic Aperture Radar (SAR) backscatter image mosaics acquired by ALOS PALSAR-1/PALSAR-248 and optical multispectral Landsat-derived percent tree cover maps29 were used as joint predictors to extend the canopy height obtained from GEDI to canopy height maps across the whole of Africa. Regional maps of aboveground biomass density derived from airborne LiDAR over a range of biomes in Africa were then used to derive an empirical model that estimates aboveground biomass density as a function of canopy height. The LiDAR-based aboveground biomass footprint estimates from LiDAR were used to train the machine learning model. The model was then used to produce annual Africa-wide maps of aboveground biomass density and its standard deviation (SD) at 100 m pixel spacing for the period 2007 to 2017. The maps were validated using a large independent dataset of field plot measureme...