Cooling potential of large solitary trees: insight for urban green management

Large solitary trees (LSTs) are keystone structures in urban and rural landscapes, serving as green islands that host a broad range of biodiversity and ecosystem functions. With their large and well-developed crowns, LSTs significantly influence local microclimatic conditions. However, LSTs are globally declining due to land-use changes, infrastructure development and age-related pests and pathogens. Their isolated nature makes them more susceptible to climatic stresses, such as droughts and heat waves, compared to forest trees. In urban areas, these effects are further exacerbated by the urban heat island effect, where buildings and concrete surfaces amplify warming. Climate change challenges the growth and vitality of LSTs, while their mitigating effects on the climate are needed now more than ever.

This study explores the cooling potential of LSTs during heat events with a particular focus on the relationship between cooling effects and the tree’s morphological characteristics, such as the starting height of the crown, crown volume, and branching angle. We measured temperatures at a height of three meters along the trunks of 216 LSTs from summer 2022 to spring 2024 and compared these to temperatures around the tree extracted from the R-package microclimf. We focused on three widely spread and iconic tree species: Quercus robur, Fraxinus excelsior, and Tilia cordata. These trees were selected across an urban-rural gradient in nine cities throughout temperate Europe. Using a terrestrial laser scanner, we obtained leaf-off point clouds for each tree, providing a unique dataset on their morphology. We hypothesized that species selection and specific morphological characteristics, controllable by management practices, will significantly affect the cooling ability of urban trees. Our findings aim to provide valuable insights for urban planners and managers, highlighting how urban green management can optimize the cooling benefits of trees in cities.