By Graeme Salt
Cities around the world are planting more trees to cope with rising urban heat.
And parts of Melbourne are 18°C more comfortable due to tree planting.
But recent academic research shows trees alone are often not enough. In some cases, the wrong kind of greening can even make streets feel less comfortable on a hot day.
Layered vegetation – where trees are combined with shrubs and ground cover – often cooled cities more effectively than trees alone.
Cities trap heat. Roads, buildings and asphalt absorb solar energy during the day and slowly release it back into the air, especially at night.
This “urban heat island” effect, combined with climate change, is making heatwaves more intense and more dangerous in our cities.
In Melbourne, street trees reduced radiant heat absorbed by pedestrians by more than 18°C, compared with open streets. Even where air temperatures changed only slightly, shaded streets feel substantially cooler.
According to the research, combining trees with shrubs and ground cover often performed better than trees alone, but the benefits depended on how the planting interacted with the local environment.
The Microclimate and Human Thermal Comfort Impacts of Increased Vegetation Complexity in Urban Streets and Greenspaces. Urban Sustainability
Mohammad A. Rahman, Pui Kwan Cheung, Nayanesh Pattnaik, Theresa W. L. Lam, Marie C. Dade, C. Y. Jim, Stephan Pauleit & Stephen J. Livesley
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