Can urban trees sometimes warm the air instead of cooling it?

Details

Increasing urban tree cover is often proposed to mitigate urban heat because trees are expected to cool cities through evapotranspiration and shade. But trees also alter wind flow and urban aerodynamic roughness, potentially limiting heat dissipation. Previous studies have found that tree cooling varies across climates and times of day; these differences have not been systematically explained because separating individual tree effects is difficult and impossible through observations alone.

Researchers used a mechanistic urban ecohydrological model for numerical experiments that removed and added the effects of trees on radiation, evapotranspiration and aerodynamic roughness. They simulated four cities in different climates—Phoenix, Singapore, Melbourne and Zurich—considering seasonal and daily cycles of air and surface temperatures.

In summer across the four climates, evapotranspiration from well-watered trees alone lowered local air temperature at 2 meters by a maximum of 3.1–5.8 °C. At peak temperatures, high vapor pressure deficits limited transpiration through stomatal closure, preventing further cooling.

Although shade lowered surface temperatures, radiation interacting with a non-transpiring tree could raise local air temperature at 2 meters by up to 1.6–2.1 °C at certain hours, partially counteracting evapotranspirative cooling. In the scenarios analyzed, which did not account for trees blocking wind, trees reduced urban roughness, inhibiting turbulent energy exchange and raising daytime air temperature. Individual tree effects varied at night, likely because of differences in atmospheric stability within the urban canyon.

The researchers say the results explain reported daily, seasonal and climatic differences in tree cooling and can guide future field campaigns, planning strategies and species selection to improve local microclimates with urban greenery.