Trees may not cool cities during heat waves as much as we thought

A new study suggests that the cooling potential of greenery is reduced in sweltering conditions.

Many cities have undertaken initiatives to plant more trees to help combat climate-driven urban overheating. However, those bold planting efforts may not be as effective in tackling the issue as we once thought.

A study by UNSW Sydney researchers Kai Gao, Dr Jei Feng and Scientia Professor Mattheos (Mat) Santamouris found that the heat mitigation benefit of trees is significantly hampered in extremely hot conditions. 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.

“Adding green infrastructure, especially planting trees, is a commonly employed measure to mitigate against urban heat,” says Prof. Santamouris, co-author of the study and the Anita Lawrence Chair in High-Performance Architecture at UNSW Arts, Design & Architecture. “But as our research shows, trees have some limits when mitigating urban heat during heat waves.

“It’s not to say that tree planting initiatives aren’t important for heat mitigation, but that large-scale planting policies require careful consideration.”
Reduced transpiration rates for cooling

Typically, the temperature reduction caused by green infrastructure in normal summer conditions is around 1 – 2°C during the day. Much of this cooling effect results from transpiration, where trees release water vapour through their leaves, absorbing heat and reducing the surrounding air temperature.

However, trees take precautions under heat stress to maintain moisture and avoid dryness by holding onto their sap. This reduces transpiration flow and thus their ability to cool – something overlooked in current climate forecasts.

For the study, the researchers used data from a global tree sap flow database to model changes in the transpiration behaviour of more than 700 tree types during heatwaves. They also conducted a field experiment to measure the transpiration changes, specifically in eucalyptus trees in Sydney, to validate the phenomenon.

The researchers found that two-thirds of the trees in the sample were overestimated in their ability to provide cooling by 60 per cent under extreme heat conditions.

“Once trees reach a threshold temperature of around 34°C, they try to protect themselves by reducing their circulation of sap,” Prof. Santamouris says. “This decreases their transpiration rate, which significantly reduces their ability to cool the ambient temperature, and in rare cases, may even result in heating.”

Continue reading: https://www.unsw.edu.au/