From Trunk Diameter to Crown: Point Cloud-Based Allometric Equations for Urban Trees and a Scalable Approach for City-Wide Biomass Estimation

Background Urban trees provide essential ecosystem services, including carbon storage. However, quantifying the wood volume and biomass of urban trees remains challenging, primarily due to a lack of species-specific allometric equations tailored to urban growth conditions. In addition, most existing equations rely solely on diameter at breast height (DBH), which is hard to obtain from remote-sensing data, thus limiting scaling to city level. Methods This study addresses these gaps by developing both DBH-based and crown parameter-based allometric equations for urban tree species and cultivars in Braunschweig, Germany, using terrestrial laser scanning (TLS). Quantitative structure models (QSMs) were generated from TLS data, corrected for twig diameter overestimation in point clouds. Two reference trees were manually disassembled and measured for validation. Results Species/cultivar specific allometric equations mostly align with established literature equations. The irregularly shaped cultivar ‘Fastigiata’ shows a different allometry compared to standard form Quercus robur . DBH-based generalized equations perform better than crown parameter-based generalized equations. Comparing biomass estimates based on DBH values from a municipal tree cadaster and airborne LiDAR-derived tree-crown parameters showed that the latter performed better because of outdated entries in the municipal data. Conclusions Species-specific equations may not be precise enough for irregularly shaped cultivars. While equations based on crown parameters are less accurate than equations based on DBH, they have greater potential for city-wide application, because in contrast to DBH, crown parameters are increasingly available for whole cities.