Urban heat island mitigation by tree transpiration and shading

Urban areas are heating up much faster than their surrounding environments, creating a heat island that critically affects the health of their inhabitants. As a consequence, more and more people are being exposed to extreme urban heat, which has in the recent past resulted in recurrent high mortality among the urban population, especially during heat waves.

Infrared image of a field site in Lancy near Geneva. Surface temperatures in the shade of the trees are significantly lower than on the sun-exposed pavement.

Transpiration and shading from vegetation, particularly from large trees, helps to mitigate urban heat islands. The individual effects of shading and transpiration on the urban microclimate are, however, not yet well quantified, because direct measurements of transpiration are still rare. As the complex interactions of urban vegetation with the urban microclimate, including air temperature and humidity, are far from being understood, it is difficult for city planners to make the best use of the beneficial effect of vegetation.

Currently, we aim at quantifying the effect of urban tree transpiration on heat island mitigation in a research project at EPFL. For this project, we are measuring tree sap flow and canopy microclimate on urban trees in the city of Lancy (near Geneva), and starting in 2024 also on the campus of EPFL. Using the cities of Geneva and the EPFL campus as a test platform, our goals are to establish the link between climate variability, tree transpiration and urban cooling potential for a broad range of tree species in diverse urban settings (ecology/ecophysiology).


Installation of sap flow sensors to measure tree transpiration on a Platanus tree in Lancy near Geneva.
Installation of canopy temperature and humidity sensors to measure tree canopy cooling on a Platanus tree in Lancy near Geneva.

For more information contact Christoph Bachofen.