Allometric equations underestimate woody volumes of large solitary trees outside forests

Abstract

Large solitary trees (LSTs) represent important wood volumes and carbon stocks outside forests. However, quantification remains difficult as most allometric relationships have been developed using trees in forests. Here, we explore the volumetric assessment of aboveground woody biomass of LSTs outside forests and compare them to forest trees. Using terrestrial laser scanning, we captured detailed 3D point clouds of 215 solitary trees for three widely distributed species, i.e. Quercus robur, Tilia sp. & Fraxinus excelsior, across nine cities in temperate Europe. Leaf-off lidar point clouds were processed using a quantitative structure model (QSM) to estimate the aboveground volumes and develop new species-specific allometric volume equations for solitary trees. Our findings show that the woody volumes of LSTs estimated by QSMs are, on average, 83 % higher than those predicted by species-specific allometric equations tailored to forest trees of similar height and diameter at breast height. To validate this discrepancy, we applied the same laser scanning and modeling methods to both LSTs and forest trees. Regression analysis confirmed that LSTs have significantly greater woody volumes than forest trees. However, this difference diminishes with increasing height, resulting in converging volumes in the tallest height class. Our results highlight the substantial carbon storage and wood volume potential of LSTs in both urban and rural areas, and underpin the urgent need for dedicated allometric volume equations tailored specifically to large solitary trees.