Cool Tree Architecture and Measurement Protocols

Trees and green infrastructures are recognized as essential to help adapt cities to deal with urban heat extremes. But while it is common knowledge that trees contribute to cooling through shade and evapotranspiration, it remains unclear what the relationship is between these mechanisms and the various thermal metrics used in the built environment. A comprehensive and reliable approach to equipment and protocols to measure the various tree cooling performances is by extension, also lacking. Accurate data on the performances of different tree species in relation to different thermal metrics is an expected outcome of such an approach. This paper presents the results of a study addressing these questions. A protocol was developed outlining the relevant metrics to measure, the selection and calibration of relevant instruments and equipment, the locations in and around trees for the placement of instruments, the minimum climatic conditions needed for measurement campaigns, and procedures for the setup, duration and validation of measurements. Relevant metrics to measure include Mean Radiant Temperature (MRT), Physiological Equivalent Temperature (PET) and Air Temperature. Five tree architecture traits impacting MRT were identified, expanding on current assumptions such as crown density, gap fraction and leaf area (index). Cooling performance values were generated for 69 tree species in all three metrics. The median difference in MRT between a reference point in the sun and beneath the trees ranged from -32.2°C to -22.5°C, illustrating the high impact on MRT but also the variability in cooling performance across different species and conditions. PET values under the analyzed trees ranged between 31°C and 45°C, demonstrating substantial differences in thermal comfort according to the tree species. The air-cooling performance of a tree was found to vary relative to the weather conditions, with cooling performance decreasing with the increase of atmospheric temperature.