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

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.”

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.

“Without proper tree selection, not only is there minimal heat mitigation during heatwaves, but also the potential for adverse health phenomena, including increasing pollution,” Prof. Santamouris says. “It shows there is a real need for mitigation strategy refinement to account for the conditions of heat waves.”

Need for adequate irrigation

Water availability is also crucial for using trees to mitigate urban heat. Prof. Santamouris says not irrigating trees limits their cooling potential during high-temperature periods.

“Tree irrigation is a serious consideration because cities cannot allocate large amounts of freshwater so trees can maintain their high cooling potential,” Prof. Santamouris says. “So, water management plans must be essential to any tree planting strategy.”

Sydney’s urban and suburban areas boast substantial coverage of evergreen broadleaf forests, particularly eucalyptus trees, and the city has recently debated planting millions more to combat rising temperatures.

“In Sydney, for these trees to have the proper cooling performance, we would need to increase water usage by about 20 per cent,” Prof. Santamouris says. “We would need to consider ways to use different types of water, such as wastewater or greywater, that would otherwise be washed out to the ocean, if we were to see the full cooling benefits.”

While plant physiologists are working to create genetically modified trees with higher temperature thresholds, they are at least a decade away from becoming a viable alternative.

“Instead of having a threshold of around 34 degrees, these genetically modified trees may have a threshold of up to 45 degrees, making them more suited to endure extreme heat,” Prof. Santamouris says. “However, most of these efforts are in the early stages and will not be widely available for about 10 years.”

Prof. Santamouris says it’s vital that urban heat mitigation efforts be based on the latest scientific evidence, especially as global temperatures continue to rise. By the end of the century, the average urban heatwaves are projected to be up to 6°C hotter.

“This study should caution those who consider that the problem of urban heat can be solved by simply planting trees,” Prof. Santamouris says. “It’s not as straightforward as that.

“Any cooling policy in cities based on the plantation of trees must take into account the science or risk the tremendous investment not having the desired cooling results and, at worst, having adverse health effects.”