Wrong trees in the wrong place can make cities hotter at night, study reveals

While trees can cool some cities significantly during the day, new research shows that tree canopies can also trap heat and raise temperatures at night. The study aims to help urban planners choose the best combinations of trees and planting locations to combat urban heat stress.

Temperatures in cities are rising across the globe and urban heat stress is already a major problem causing illness, death, a surge in energy use to cool buildings down, heat-related social inequality issues and problems with urban infrastructure.

Some cities have already started implementing mitigation strategies, with tree planting prominent among them. But a University of Cambridge-led study now warns that planting the wrong species or the wrong combination of trees in suboptimal locations or arrangements can limit their benefits.

The study, published today in Communications Earth & Environment found that urban trees can lower pedestrian-level air temperature by up to 12°C. Its authors found that the introduction of trees reduced peak monthly temperatures to below 26°C in 83% of the cities studied, meeting the ‘thermal comfort threshold’. However, they also found that this cooling ability varies significantly around the world and is influenced by tree species traits, urban layout and climate conditions.

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