Infrared radiative performance of urban trees: spatial distribution and interspecific comparison among ten species in the UK by in-situ spectroscopy

Understanding the ways in which tree species interact with solar radiation has previously focused on transmission and reflection of sunlight, typically by examining individual leaves. Here we used a tree crown spectroscopy measurement method to conduct in-situ tests on the radiative performance of ten commonly planted tree species in the UK. Tree crown transflectance (comprehensive effect of transmission and reflection) was examined to determine i), how radiative performance of individual trees varies spatially within a species, and ii), how infrared radiative performance differs between tree species. Our results show that tree crown transflectance depends on the combination of tree crown morphology, local foliage distribution (leaf density, gaps in crown foliage contour, concave or convex crown shapes), solar altitude and leaf size. Spatially, the strongest tree crown transflection was found primarily towards sky on the sunlit side of trees rather than towards the zenith, meaning that infrared transflection towards surrounding buildings and pedestrians is substantial. For all ten species, the tree crown transflectance in the frontal sunlit area was linearly correlated with solar altitude on sunny days. Hence, a solar altitude of 45 degrees was chosen as the benchmark condition for comparing interspecific differences. Interspecific comparison indicated that interspecific differences in the infrared radiative performance levels were strongly dependent on leaf size when no obvious gaps or concave shapes were present within the tree crowns. Our findings provide insights for understanding radiative interactions between urban trees and surrounding built environment, as well as for tree species selection in urban heat stress mitigation.