A new study suggests that planting trees in cities may not cool them as much as we hoped, especially during heatwaves. Researchers at UNSW Sydney found that trees lose much of their cooling power when temperatures get too high, which challenges the effectiveness of current urban greening plans.

City planners have long promoted planting trees as a way to fight rising temperatures. Trees cool the air by releasing water vapor through their leaves—a process known as transpiration. Under normal summer conditions, this can lower temperatures by 1°C to 2°C. But the new study shows that this cooling effect weakens when the weather is extremely hot.

Overestimating cooling

The study found that most climate models overestimated how much trees can cool the air during heatwaves by up to 60%. When temperatures rise above about 34°C, trees start to conserve water by slowing down their sap flow. This reduces transpiration and, as a result, their ability to cool the surrounding air. In some cases, this could even lead to a slight increase in local temperatures.

The researchers used data from a global tree sap flow database and conducted field tests on eucalyptus trees in Sydney. They found that two-thirds of the trees studied provided much less cooling during extreme heat than expected.

These findings have important implications for urban planning. They suggest that simply planting more trees may not be enough to tackle urban heat. There’s also a risk that, during heatwaves, trees could release more chemicals like isoprene, which can worsen air quality when mixed with other pollutants.

Water management

Water management is another key issue. Without enough water, trees can’t cool the air effectively. This is a significant challenge in cities where water is already scarce.

While researchers are working on genetically modifying trees to withstand higher temperatures, these solutions are still at least a decade away. Such trees could survive in temperatures up to 45°C, making them better suited to extreme heat, but they won’t be available anytime soon.

The study is a reminder that using trees to cool cities is more complex than it seems. As global temperatures rise, heatwaves will become more intense, potentially raising urban temperatures by up to 6°C by the end of the century. To be effective, cooling strategies need to be based on up-to-date science to avoid wasted resources and unintended health risks.

In short, while planting trees is still important, it’s crucial to understand their limits. Urban heat solutions must take into account how trees actually behave under extreme conditions and consider the availability of water to ensure their effectiveness.