Understanding urban tree shade: characterizing direct shortwave transmissivity through urban tree canopies using Terrestrial LiDAR Scanning

Tree shading is one of the most effective mechanisms to improve daytime human thermal comfort, specifically in urban contexts where exposure to direct shortwave radiation dominates heat stress. The quality and quantity of tree shading are heavily controlled by canopy closure and crown architecture. However, with limited data on the link between tree structure in different tree species and their shade quality, these relationships are frequently overlooked both in practice and in urban microclimate modelling.

In this study, we present a new framework to quantify tree shading potential using Terrestrial Laser Scanning (TLS). The TLS scan of a tree is used to derive its canopy gap fraction, which represents a proxy for direct shortwave radiation transmissivity. We evaluate different processing methods (laser pulse-based and point-based) and perform a digital validation of the different approaches. After validating and selecting the most appropriate method, the transmissivity values are linked back to the tree’s structural characteristics which can be derived from the TLS point cloud information. By including indices such as tree height, crown volume, and leaf area density, we investigate the link between tree structure and shading behavior.

The proposed framework is applied to a database of over 50 individually scanned urban trees, all measured in summer across multiple cities in Belgium. This allows for a comparison of shading capabilities between different tree species and morphologies. In addition to the expected differences in overall transmissivity between trees, preliminary results reveal strong variations in gap fraction across different zenith angles, with transmissivity values ranging from 3 to 28% at low and high zenith angles, respectively. This indicates not only a variation in shading intensity between individual trees, but also potential differences throughout the day.

These results are informative for comparing urban tree species and their management strategies when the goal is to optimize pedestrian shading and thermal comfort. Additionally, they provide empirically derived parameters to improve the representation of tree structure and shading effects in urban microclimate models.