A new study suggests that the cooling potential of green spaces decreases in sweltering heat.

Many cities have launched initiatives to plant more trees to counteract climate-induced urban overheating, but these bold planting efforts may be less effective in tackling the problem than we once thought.

A study by UNSW Sydney researchers Kai Gao, Dr Jei Feng and Scientia Professor Mattheos (Mat) Santamouris found that the heat protection effect of trees is significantly impaired in extreme heat. The results, published in Sustainable cities and societyshow that conventional climate models overestimated the cooling ability of trees during heat waves by 60 percent.

“Increasing green infrastructure, particularly tree planting, is a commonly used measure to mitigate urban heat,” says Prof Santamouris, co-author of the study and Anita Lawrence Chair in High Performance Architecture at UNSW Arts, Design & Architecture. “However, as our research shows, trees have reached their limits in mitigating urban heat during heatwaves.

“This is not to say that tree planting initiatives to mitigate heat are not important, but large-scale planting efforts need to be carefully considered.”

Reduced transpiration rates for cooling

Typically, the temperature reduction provided by green infrastructure under normal summer conditions is about 1–2 °C during the day. Much of this cooling effect is due to transpiration, where trees release water vapor through their leaves, absorbing heat and lowering the ambient air temperature.

However, when trees are under heat stress, they take precautions to conserve moisture and avoid drought by retaining their sap. This reduces transpiration flow and thus their ability to cool themselves – something that is overlooked in current climate projections.

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

The researchers found that two-thirds of the trees in the sample had overestimated their ability to cool under extreme heat by 60 percent.

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

According to Prof. Santamouris, decision-makers need to be more selective in choosing the tree species to use and their different temperature thresholds. During extreme temperatures, trees can also emit high concentrations of biogenic volatile organic compounds (BVOCs), such as isoprene, which can interact with other air pollutants and affect air quality.

“Without the right tree selection, heat mitigation during heatwaves is not only minimal, but there is also a risk of adverse health phenomena, including increased pollution,” says Prof. Santamouris. “This shows that there is a real need to refine the mitigation strategy to take into account heatwave conditions.”

Need for adequate irrigation

The availability of water is also crucial if trees are to be used to mitigate heat in cities. According to Prof. Santamouris, not watering trees limits their cooling potential during times of high temperatures.

“Tree irrigation is a serious consideration because cities cannot provide large amounts of freshwater to enable trees to maintain their high cooling potential,” says Prof. Santamouris. “Therefore, water management plans must be an integral part of any tree planting strategy.”

Sydney’s urban areas and suburbs are covered with large areas of evergreen broadleaf forests, mainly eucalyptus trees, and the city has recently discussed planting millions more trees to combat rising temperatures.

“For these trees to provide the cooling capacity required in Sydney, we would need to increase water use by about 20%,” says Prof. Santamouris. “To achieve the full cooling effect, we would need to consider ways to use other types of water, such as wastewater or greywater, that would otherwise be washed out to sea.”

Although plant physiologists are working on breeding genetically modified trees with higher temperature thresholds, it will be at least a decade before these represent a viable alternative.

“Instead of a threshold of around 34°C, these genetically modified trees could have a threshold of up to 45°C, making them better able to withstand extreme heat,” says Prof. Santamouris. “However, most of these efforts are still in the early stages and will not be widely available for about 10 years.”

Prof Santamouris says it is vital that efforts to curb urban heat are based on the latest science, particularly as global temperatures continue to rise. By the end of the century, average urban heatwaves are expected to be up to 6°C higher.

“This study should serve as a warning to those who think that the problem of urban heat can be solved by simply planting trees,” says Prof. Santamouris. “It’s not that simple.”

“Any urban cooling strategy based on tree planting must take scientific evidence into account. Otherwise, there is a risk that the huge investments will not produce the desired cooling results and, in the worst case, will have adverse health effects.”