Estimating aboveground volume of diverse urban tree species: Developing allometric equations for higher taxonomic levels

Allometric equations are essential for quantifying carbon storage in trees; however, owing to the high species diversity in urban green spaces, the development of allometric equations for all urban tree species is not feasible. In this study, we estimated the aboveground volume of urban trees and determined the fitness of several allometric equations. We compared the equations for different taxonomic levels, i.e., division, family, and species. A field survey was conducted across three types of urban green spaces (street trees, parks, and residential gardens) in three cities (Seoul, Goyang, and Wonju) in South Korea. We estimated the aboveground volumes of 1080 trees from their diameter measurements using a laser dendrometer, carried out at successive points from the ground to the top of the trees. Various tree size metrics, such as the diameter at breast height (DBH), height, and/or crown diameter, were applied as independent variable(s) in the equations. For gymnosperm trees, the allometric equations for higher taxonomic levels (RMSE: 0.011 m3) yielded goodness-of-fits similar to those at the species level (RMSE: 0.009 m3). For angiosperm trees, the family-level allometric equations satisfactorily covered the inter-species heterogeneity of tree shape, whereas the division-level allometric equations did not. The scaling exponent coefficients of allometric equations for each family (b = 1.65−2.69) linearly increased with an increasing mean height-to-DBH ratio. Although the allometric equations based on crown diameter were more unstable than those based on the DBH, integrating the height variable into the equations could ensure the feasibility of the allometric equations based on crown diameter. Our study facilitates the development of nationally specific emission factors for settlement areas and suggests the applicability of generalized or crown-based allometric equations for urban trees.