Identifying the coolest trees on the block: Study on drone-based thermal image analysis of species-specific tree crown temperatures

Mitigating the impacts of climate change in cities—where rising temperatures and the intensifying urban heat-island effect (UHI) increasingly threaten public health—has become a critical priority. Varied strategies are needed to alleviate complications due to UHI and climate change, and trees as nature-based solutions are one of the most popular and easiest to implement. Trees help alleviate heat-related problems in urban environments through reducing the physiological equivalent temperature (PET) by 10–13% in large cities (Zölch et al. 2016), and land surface temperature by 8–12°C on average in European cities (Schwaab et al. 2021). However, the identity, diversity and canopy density of trees can influence their cooling effects. It is therefore imperative to choose the correct species to plant in the right geographic locations to best improve the local cooling effects. This study investigated the effectiveness of an unmanned aerial vehicle (UAV or drone) equipped with a thermal camera to measure the surface temperature of tree crowns in a similar environment. We aimed to identify tree species with consistent temperature differences compared to ambient air temperature (always cooler or always warmer than other species) in different weather conditions. The thermal images were mosaicked, and areas for all surfaces were identified for analysis. We extracted the emissivity-corrected mean temperatures for all trees and subtracted air temperature at the time of the flights from the mean surface temperatures to compare across dates and different weather conditions. Tree species differed with some species having consistently mean crown temperatures warmer than air temperature across all dates and tree species with mean temperature always cooler than air temperature. Surface temperatures of trees were significantly correlated with specific leaf area (SLA), suggesting leaf morphology has an influence over the cooling of the tree crown and a diversity of tree species might lead to enhanced ecosystem services in urban areas.