Terrestrial laser scanning improves LiDAR and radar biomass calibration in tallest mangrove forest on Earth

High-resolution global-scale estimates of aboveground biomass density will soon be available from a suite of space-borne LiDAR and radar missions. Calibration and validation of biomass products for each instrument relies on ground-based inventory plots, where diameter measurements are converted to biomass estimates with allometric relationships. Allometry in undersampled systems may be biased, especially if predictions are made outside the measured diameter range. Given the critical link between allometric equations and global estimates of biomass and carbon density, improvements to these equations could reveal substantial biases in current estimates of terrestrial carbon storage. Terrestrial laser scanning (TLS) non-destructively quantifies forest structure and biomass with unprecedented precision and accuracy. The greatest improvements in biomass estimates from TLS will be in forests with massive, rare, and/or understudied trees. Here, we deploy TLS in the tallest known mangrove forest on Earth - Pongara National Park, Gabon. For the first time, we provide precise estimates of biomass from mangrove trees that reach nearly 63 m and leverage these measurements to create non-destructive allometry. We evaluate the implications for future satellite missions, by comparing our allometry to commonly used mangrove allometry across 18 biomass calibration plots, all of which overlap with acquisitions from LVIS, UAVSAR, and Tandem-X as part of the AfriSAR airborne campaign. Further, we evaluate the impact of allometry on single-sensor and fused biomass products, including a globally calibrated Tandem-X biomass product. Our findings are directly relevant for space-borne missions estimating terrestrial carbon storage - GEDI, ICESat-2, NISAR, BIOMASS, Tandem-X, and Tandem-L - highlighting the essential role of TLS in global biomass product calibration and validation.