Remote sensing based analysis of urban tree temperatures during extreme heat and drought

Trees have a pivotal role in mitigating heat island effects and several studies analyzed temperature differences of various species. However, the potential of trees to decrease temperatures during heat waves, drought and extreme events is inadequately studied, particularly at the city scale. Therefore, we present a remote sensing based approach that evaluates surface temperatures of trees in the urban environment of Forchheim (Germany) during the heatwave 2022. To provide an example of extreme conditions, we conducted measurements on July 20, 2022, the day with the current absolute daily heat record of the region since temperature measurements started in 1949. The three-month period before the survey flight (May-July) was the second warmest and third driest May-July period ever measured, leading to an ideal setting to assess the role of trees in urban regions during projected climate extremes. Analyzing such situations is highly relevant for city planning as existing research showed that sap flow is only reduced after several weeks of drought. We performed a low-altitude flight campaign (350 meters above ground) during the daily maximum temperature period (2-4 pm) with a thermal camera (Optris PI 450) for surface temperatures and a multispectral camera (Micasense RedEdge M) for vegetation parameters and land cover. We compared derived surface temperatures at field mapped locations of more than 3000 trees covering more than 30 species (n ≥ 20) to assess species patterns and the influence of urban parameters such as imperviousness. We show differences between species and interrelationship with vegetation parameters (e.g. NDVI) to provide insights into mitigation effects and patterns of urban trees during extreme events.