Sub-Hectare Resolution Mapping of Forest Biomass with Global Dem Data and a Coarse Digital Terrain Model

Abstract

Accurate, sub-hectare resolution mapping of above-ground biomass density (AGBD) of the global forests is needed for many applications, including carbon accounting, forest resource management, and biodiversity research. Publicly available interferometric radar-based digital elevation models (DEMs) from Copernicus and SRTM can be used for global-scale AGBD mapping if a coarse digital terrain model (DTM) becomes available, e.g., from the upcoming ESA BIOMASS mission. For eleven test sites from three climate zones (boreal, temperate, tropical), we observe a consistent, near-linear relationship between phase height (PH), defined as the DEM-DTM difference, and AGBD maps derived from airborne laser scanning and in situ data. For PH derived from the Copernicus DEM and a 100 m lidar DTM, the estimated PH-to-AGBD scaling constant varies between 10.8 t/(ha·m) and 15.0 t/(ha·m) across sites, the relative root-mean-square error (RMSE) at 50 m resolution between 9% and 43%, and the coefficient of determination (R2) between 0.32 and 0.88. The best results are obtained in the tropical forests. Using a global scaling constant of 13.3 t/(ha·m) and 2200 0.25 ha pixels systematically subsampled from all eleven test sites, the RMSE is 52 t/ha (21%; R2=0.86). A comparison at 200 m resolution with AGBD estimates based on GEDI and ICESat-2 spaceborne lidar data and C- and L-band SAR backscatter data (ESA CCI Biomass project) demonstrates higher accuracy and precision of the PH-based approach. In conclusion, (i) PH is more closely related to AGBD than to top-of-canopy forest height because it is sensitive to both forest height and canopy density, (ii) the scaling model proposed by Solberg et al. [Rem. Sens. Env., 139(12), 2013] has good merit for near-global AGBD estimation from existing DEMs and the DTM expected from the ESA BIOMASS mission, (iii) there is an urgent need for follow-up studies to best utilise the co-existence of TanDEM-X and BIOMASS systems in the latter half of the 2020s, (iv) there is a need for continued operation of single-pass interferometric radar missions like TanDEM-X to support fine-scale forest AGBD (and carbon) monitoring.

Keywords: Forests, above-ground biomass density (AGBD), mapping, estimation, synthetic aperture radar (SAR), digital elevation model (DEM), interferometric SAR (InSAR)