Study: Urban Trees Lose Cooling Benefits in Extreme Heat

A new study from the University of New South Wales reveals that the ability of trees to combat overheating is reduced in extremely hot temperatures. “The findings, published in Sustainable Cities and Society, opens in a new window, show that conventional climate models overestimated the ability of trees to cool during heat waves by 60 per cent.”

The study poses a serious problem for cities relying on planting trees as a key part of their heat mitigation strategy. “It’s not to say that tree planting initiatives aren’t important for heat mitigation, but that large-scale planting policies require careful consideration,” said one of the study’s co-authors, Professor Mattheos (Mat) Santamouris.

As Ben Knight explains in the article, trees typically reduce surrounding air temperatures by 1 to 2 degrees Celsius in the daytime by releasing vapor through their leaves and absorbing heat. In high heat conditions, trees limit their transpiration, a factor not accounted for in current climate forecasts.

In the study’s sample of eucalyptus trees in Sydney, two-thirds of the trees were overestimated in their cooling ability during extreme heat by 60 percent. “Prof. Santamouris says decision-makers need to be more selective about the types of trees used and their various temperature thresholds. Under extreme temperatures, trees can also emit high concentrations of Biogenic Volatile Organic Compounds (BVOCs) like Isoprene, which can harm air quality when they interact with other atmospheric pollutants.”

Santamouris also warns that urban trees need adequate irrigation to provide optimal cooling benefits.