Street tree cooling is modulated by the built environment: fine-scale evidence from mobile measurements

Increasing urban tree cover, a widely adopted approach for mitigating urban heat, is often challenged by the difficulty of designing cities that are both compact and green. Street trees provide a unique opportunity to integrate trees directly into the existing fabric of cities, only requiring a small permeable spot for planting. However, many cities are highly heterogeneous in their form, such as with variable building heights and surface characteristics. This heterogeneity can affect local microclimate conditions via changes to both amount of incoming radiation and local heat transfers. As the processes underlying the cooling properties of trees are directly affected by the local microclimate, the cooling potential of urban street trees may vary depending on the surrounding urban form. We use a high-frequency bike-based mobile measurement system to collect fine-scale air and ground surface temperature measurements within the densely built and mostly impervious area of downtown Montréal, Canada. We measure variation in daytime and nighttime temperatures across a range of urban microclimate conditions using a stratified transect survey analysis of temperatures at pedestrian height. Our analysis demonstrates that street trees cool daytime air temperature about three times more in areas with high building obstruction compared to open areas, while the cooling effect on daytime ground surface temperature remains consistent regardless of the surrounding built environment. At night, however, higher canopy cover led to slightly warmer air temperatures in open areas. Overall, these results support street trees as valuable nature-based solutions to mitigate extreme heat, even in highly dense urban environments.