Australian cities are among many through the world to implement large scale tree planting in the belief it will combat climate-related urban heating.

But new research confirms that if not managed properly, trees aren’t as effective in combatting heat during heatwaves as may be thought, and could even be increasing temperature and adding to pollution.

A UNSW study published in Sustainable Cities and Society shows that conventional climate models overestimate the ability of trees to cool during heat waves by 60 per cent.

This is related to a process known as transpiration, where trees release water vapour through their leaves, absorbing heat and reducing the surrounding air temperature.

While this is fine in normal summer conditions, where trees can lower temperatures by up to 2C during the day, once the mercury hits 34C trees begin to protect themselves by reducing their sap circulation, which in turn reduces transpiration.

This not only significantly reduces their ability to cool the ambient temperature, but in rare cases can even result in heating, one of the study’s authors Professor Mattheos Santamouris, Anita Lawrence Chair in High-Performance Architecture at UNSW Arts, Design & Architecture, says.

“We know very well the cooling potential of urban greenery during normal climatic conditions, but our research shows trees have some limits when mitigating urban heat during heat waves,” Professor Santamouris told Government News.

“When temperatures are very high, trees stop circulating their sap. As a result, the cooling potential is decreased.  As the transpiration of plant decreases, the cooling potential decreases as well.”

Not the desired effect

Professor Santamouris says previous research looking at the south of LA has shown that planting about 1 million trees could increase the temperature by 1.2 degrees, while a study of Riyadh in Saudi Arab showed planting a million trees without additional mitigation measures could increase daytime temperatures by 0.4 degrees.

Other studies have show that during heatwaves, the temperature in urban parks can be higher than outside the park.

For the current study, the UNSW team considered 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 on eucalyptus trees in Western Sydney.

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

The research isn’t against greenery,  greenery is very positive. But we have to respect the physiology of trees.

Professor Mattheos Santamouris

Another problem with tree planting is that some trees, and particularly eucalypts, can respond to stress by producing chemicals known as BVOCs, or Biogenic Volatile Organic Compounds, which can be three times more toxic than car exhaust fumes.

The production of BVOCs is what gives the Blue Mountains in NSW their ‘blue’ haze.

“With proper irrigation this can actually  increase pollution in an area,” Professor Santamouris says.

Trees planting beneficial if managed properly

Professor Santamouris says the study should caution anyone who thinks the problem of urban heat can be solved by simply planting trees.

But that doesn’t mean we should stop urban greening initiatives.

Rather, urban planners, local authorities and governments need to think about complementing tree planting programs with other heat mitigation strategies, such as using cooling materials in the built environment and ensuring proper irrigation, which can boost transpiration in a heat wave.

“Tree irrigation is a serious consideration because cities cannot allocate large amounts of freshwater so trees can maintain their high cooling potential,” Professor Santamouris says.

“So, water management plans must be essential to any tree planting strategy.

“In Sydney, for these trees to have the proper cooling performance, we would need to increase water usage by about 20 per cent.

“We would need to consider ways to use different types of water, such as wastewater or greywater, if we want to see the full cooling benefits.”

Plant physiologists are working to create genetically modified trees with higher temperature thresholds, Professor Santamouris says, but this is at least ten years away from becoming a reality.

He stresses the research doesn’t mean that tree planting initiatives aren’t important for heat mitigation, but says large-scale planting policies require careful consideration of the science.

“The research isn’t against greenery,  greenery is very positive,” he says. “But we have to respect the physiology of trees. We can’t ask trees to perform  operations they are not able to do,  there are limitations.

“If we don’t respect the science, we don’t really solve the problem.”