The value of local allometries from airborne laser scanning for tropical forest biomass estimates

This study addresses one of the major limitations in the accurate assessment of forest carbon essential for robust climate projections: the limited availability of ground-based data, especially in carbon-rich tropical forests. Although pantropical allometries have reduced regional-level bias in tree biomass estimation, local-level inconsistencies persist due to the variable quality of reference data. Classical methods for measuring tree height, such as clinometers and telemeters, show significant limitations in dense, closed canopy forests and are applicable only at small scales. To overcome these limitations, this study investigates the effectiveness of airborne laser scanning (ALS) in determining site-specific allometric relationships between tree diameter and height. This aims to improve the accuracy of aboveground biomass (AGB) estimates at plot level. The study is based on field inventory data from nearly 120 hectares in an extensive network of permanent plots in a tropical rainforest in French Guiana. These plots include both undisturbed forest along hydrographic gradients and areas treated with selective cutting. Ground-based data were combined with high-quality ALS data to establish allometric height-diameter (H-DBH) models, using a multilevel Bayesian modelling approach and an individual forest model based on canopy construction (Canopy Constructor approach, Fischer et al., 2020). The study reveals that the error in the prediction of mean height was almost halved by replacing the best universal pantropical allometry with a locally derived allometric H-DBH relationship. The inclusion of species identity contributed more than 50% to the overall reduction in prediction error in the Bayesian models, and this effect could be reliably inferred by the Canopy Constructor, even without direct canopy measurements. Both modelling approaches provided similar AGB predictions, which were 11 to 13% higher (40 to 54 t ha^-1 higher, depending on forest type) than those obtained by pantropical allometry. This finding underlines the importance of incorporating site-specific data and species identity in improving aboveground biomass estimates in tropical forests.