Is shade all there is to it? Quantifying the contribution of tree transpiration to cooling cities

Tree planting is a leading strategy for mitigating the Urban Heat Island (UHI) effect. However, current urban climate models often conflate the physiological transpiration cooling of trees with their physical shading effects. To isolate and quantify this specific "biological bonus," we introduced the “Living Control” framework.

In a controlled field experiment on mature trees in a Mediterranean climate, we applied chemical anti-transpirants to inhibit stomatal conductance (g_s) and transpiration (E), while maintaining identical canopy geometry and aerodynamic properties. This effectively decoupled latent heat flux from radiative shading.

Our results demonstrate the efficacy of the manipulation: the application of auxinic herbicides significantly reduced g_s by 32% and E by 23% compared to untreated controls. This suppression of physiological activity led to a statistically significant rise in leaf surface temperature. However, this distinct physiological warming revealed an intriguing contrast: it did not translate into significant differences in human thermal comfort metrics, specifically Mean Radiant Temperature (MRT) and the Universal Thermal Climate Index (UTCI).

These findings challenge the prevailing assumption that canopy cooling linearly improves pedestrian comfort. They suggest that in water-limited environments, the primary contribution of trees to thermal comfort is static shading rather than active transpiration. This study highlights the complexity of microclimatic interactions and provides a vital baseline for future experiments evaluating Nature-Based Solutions (NBS) and water-wise strategies for urban heat mitigation.