S07 - Session O3 - Climatic benefits of street trees on the microclimate and thermal comfort: an in-situ experiment in Paris

Authors: Julien Thierry *, Sophie Herpin, Fabrice Rodriguez, Mathilde Renard, Marie Gantois, Pierre-Emmanuel Bournet

Global warming is accompanied by a multiplication of extreme events such as heatwaves. In cities, this phenomenon is amplified by the urban configuration and the nature of the materials whose optical and thermal properties enhance overheating. Vegetation appears to be a solution to partially overcome this issue. Trees in particular offer two main benefits: cast shadows and evapotranspiration, which explains why several municipalities launched important tree planting programmes (e.g. 170,000 new trees in Paris by 2026). But the climatic benefits induced by trees in their immediate proximity still remain poorly quantified. In this context, the green spaces department of the city of Paris started in 2018 an in-situ study ("Trees and Climate") aiming at measuring the microclimate inside the crown of various tree species. 36 trees representing 9 different species and located in 5 different sites in Paris, consisting mostly in canyon streets configuration, were equipped with meteorological sensors (air and globe temperature, air relative humidity and wind speed). Next to these trees, the same sensors were installed on lamp posts. Field surveys were conducted from May 2019 to April 2022. The ability of trees to mitigate the microclimate was monitored. A particular attention was paid to periods including heatwaves. Measurements made it possible to quantify the overheating generated by the street and the air temperature reduction provided by the trees, together with the human thermal comfort improvement (through the use of the Universal Thermal Climate Index). A statistical analysis was also carried out to investigate the influence of various parameters on these benefits, including tree species, crown volume and crown density. From this study, it should be possible in the near future to provide stakeholders with tools and data to help plan the greening of urban areas, and give guidance on the most adapted species to climate change.