In-situ spectroscopy and shortwave radiometry reveals spatial and temporal variation in the crown-level radiative performance of urban trees

In conventional microclimate environment modelling, and the development of tree planning strategies for urban heat mitigation, tree crown surface albedo for any given species is assumed to be a constant. However, our recent research into urban tree radiative performance at the crown level implied that tree crown surface albedo changes over time. Based on the in-situ spectroscopy protocols established previously to measure tree crown transflectance, variation in the characteristics of tree crown surface albedo was explored combining spectroscopy and solar shortwave radiometry. Three commonly planted native UK tree species, Carpinus betulus, Acer campestre, and Taxus baccata, were sampled. Spatial distribution profiles of tree crown transflectance measured at fixed solar altitudes were normalised by in-situ spectroradiometry. Tree crown transflectance in the near infrared (NIR) region was found to be proportionally linked to tree crown surface albedo. Within each species, mean tree crown transflectance in the NIR region of 800-900 nm was approximately 2.5 times tree crown surface albedo. It was further found that infrared radiation (700-2500 nm) accounted for more than 90% of the total transflected shortwave radiation from tree crowns. The results demonstrate that tree crown surface albedo linearly increases with momentary solar altitude and the maximum tree crown surface albedo corresponds to maximum solar altitude at solar noon on sunny days in summer. Tree crown surface albedo across species tends to be strongly dependent on leaf size if considering visibly dense crown foliage. Our findings provide important insights into tree radiative shading effects resulting from temporal variation in tree crown surface albedo, with consequences for urban microclimate modelling and the development of urban heat mitigation strategies.