High Variability in Urban Tree Growth: Necessity of Size-Dependent Estimation with Emphasis on Large-Diameter Trees

The national level carbon sequestration estimation for urban trees generally relies on diameter at breast height (DBH) increments. However, limited research on urban tree growth has led to uncertain estimates. In this study, we determined the DBH increment of six major urban tree species in South Korea using three analytical methods (mean, linear regression, and generalized additive model) and two data classifications (total and segmented). Additionally, wedeterminded the aboveground woody biomass (AGWB) increment, which was calculated using the estimated DBH increment, and the accuracy of the estimates using different methods. We found wide variations in DBH increment (0.616 to 1.002 cm yr−1) among the assessed tree species. Metasequoia glyptostroboides exhibited the highest mean growth, while Pinus densiflora showed the lowest. The peak DBH increment occurred at a median of 25.4 cm for Quercus spp. and 40.0 cm for Prunus yedoensis. The total mean method led to overestimations of AGWB increment, especially for the largest trees (i.e., those in the top 5% of DBH). Our study suggests that relying on a simple total mean method can lead to significant uncertainties in estimating AGWB increment. Therefore, more accurate methods for estimating DBH increment, especially in urban environments, are needed.