Species-specific urban heat mitigation through shade and transpiration

Urban trees mitigate heat stress through transpiration (latent heat flux, LE) and shading, thereby altering the surface energy balance and cooling their surroundings. The magnitude of this cooling varies among species and may be linked to functional traits such as wood anatomy (diffuse- vs. ring-porous xylem) and canopy structure (leaf area density, LAD). However, species-specific physiological responses to extreme urban climates, including heatwaves, remain insufficiently understood, limiting our knowledge of their role in heat stress alleviation.

We continuously monitored sap flow to assess LE cooling of Quercus robur, Quercus petraea, and Tilia × europaea in Lausanne during the 2025 growing season, which included two major heatwaves. We further assessed canopy and stomatal conductance, canopy surface temperatures (T_can), and canopy morphology (LAD). Cooling effects were assessed using below- and outside-canopy measurements of air temperature (T_air), relative humidity, and black globe temperature (T_BG).

Our results show that the diffuse-porous T. europaea achieved up to three times higher LE cooling compared to the two Quercus species. Combined with its higher LAD, this resulted in significantly lower T_air and T_BGbeneath its canopy, particularly under high irradiance and temperature conditions. This translated into substantially reduced heat stress for people beneath T. europaea canopies. T_can was similar across species and did not approach critical thresholds for photosystem damage.

Despite repeated heatwaves, all species maintained high transpiration rates and effective shading. Our findings suggest that species with diffuse-porous xylem and dense canopies are particularly effective for urban cooling, provided their physiological tolerance to heat is not exceeded. In cities experiencing intermittent heatwaves, urban vegetation can therefore continue to provide reliable microclimatic cooling through transpiration and shade.