We need urban trees more than ever – here's how to save them from extreme heat

• And we’ve just watched the Northern Hemisphere swelter through their summer, making July 2023 Earth’s hottest month on record.
• We studied the effects of extreme heat on urban trees in Western Sydney during Australia’s record-breaking summer of 2019–20.
• So we hold grave concerns for the survival of both native Australian and exotic species in our urban forest.
• Read more:
  Climate change threatens up to 100% of trees in Australian cities, and most urban species worldwide

Trees during heatwaves in Sydney

• Those most vulnerable to heatwaves included both native Australian and exotic species.
• Some trees died, including red maple (Acer rubrum), tulip poplar (Liriodendron tulipifera), coast banksia (Banksia integrifolia) and water gum (Tristaniopsis laurina).
• Read more:
  Without urgent action, these are the street trees unlikely to survive climate change

Why are some species more vulnerable?

• For example, species with large, thin leaves are particularly vulnerable.
• Thin leaves are less able to buffer against overheating on hot, sunny days when the wind lulls.
• Our research found most urban tree species –- even those under drought stress –- opened their pores to cool leaves on hot summer days.
• This results in rapid water loss but may help prevent tree leaves from scorching.

Why is water so important during heatwaves?

• We found water loss was higher than predicted during heatwaves for all plants.
• In urban trees, leaves reached lethal temperatures of 49–50℃ for species with the lowest rates of water loss.
• But when species with low rates of water loss had access to water, there was little heat damage or scorched leaves.

Preserving our natural air conditioners

• That means urban greening programs need to find ways to provide trees with enough water when rainfall is unreliable.
• Cities need trees now more than ever, as these natural air conditioners take the edge off the extremes.
• David S Ellsworth receives funding from the Australian Research Council, the New South Wales Government, Hort Innovation, and the Herman Slade Foundation.