Measuring tree diameter at breast height in urban green spaces using ForestScanner: Implications for citizen science

A free mobile application called ForestScanner (FS) was recently developed to enable LiDAR-based forest inventories using LiDAR-equipped iPhones or iPads. Although FS has been shown to reduce the cost, labor, and time required to measure the diameter at breast height ( D ) of trees within forests, its applicability to trees in urban green spaces (UGS) has not been evaluated. Here, we conducted two experiments to evaluate the performance of FS in the UGS. The first experiment involved the measurement of 70 trees at various distances and directions. The results revealed that FS tended to underestimate D compared to the traditional diameter tape (DT), with greater bias and variability at longer distances. These results suggest that measurements should be taken as close to the tree as possible to improve the accuracy and precision. In the second experiment, 39 trees were measured repeatedly by multiple observers. The results demonstrated that FS produced slightly greater variability between and within observers than did DT, although the overall agreement remained high. The underestimation was more pronounced for larger trees across both experiments, and for broad-leaved trees than for conifers in the second experiment. Overall, these findings highlight the potential of FS for citizen science-based assessments of carbon stocks in UGS, while also emphasizing the need for careful application when measuring larger and broad-leaved trees.