Healthy urban trees are known to be a crucial defence against lethal heatwaves, but councils and governments are planting too few saplings, placing them in the wrong places, and failing to water them.
New research published in Nature Communications shows that while street trees often fail to thrive, the solutions to help urban oases flourish are devastatingly simple.
Dr Thami Croeser, one of study’s co-authors, said planting strong and healthy trees that offer canopy protection was increasingly urgent as climate change creates urban heat islands in cities.
But poor planting processes are stunting and killing city street trees, leading them to be ripped out and replaced before they have grown to maturity.
One major factor was the common practice of planting trees on or among crushed rock beneath asphalted footpaths, limiting trees’ capacity to absorb nutrients and water, and establish sufficient root bases to stabilise themselves.
“You see trees growing half as fast and getting half as big when they’re stressed from not having water, and that’s because we’ve just surrounded their roots with asphalt in most urban settings – they’ve got a tiny little one-metre hole and they’re thirsty,” Croeser said.
“They’re pushing through crushed rock that’s dry and hostile. So when trees are unhealthy, they’re half the size, they live shorter [lifespans], and they don’t really perform on [producing] the canopy.”
A study cited by the authors showed trees planted in compacted or paved areas – a common practice in capital cities – suffered 60 per cent deficits in both cooling properties and structural growth.
As capital city councils have committed to increasing canopy cover by as much as 50 per cent of cities between now and 2050, urban forestry programs have grown around the country.
The City of Sydney has set a target of 40 per cent canopy cover by 2050; Melbourne will pursue 40 per cent by 2040; Brisbane is aiming for 50 per cent by 2031, and Perth is aiming for 30 per cent by 2036.
But the targets are being undermined by the turnover of trees being planted and removed. In the City of Melbourne, almost 2000 street trees were removed within 10 metres of development sites between 2008 and 2017, roughly 20 per cent of street trees removed in that time period.
In 2020, German academics showed that densely planted trees could reduce temperatures by 11 degrees, compared with the 4 degrees of cooling offered by sparsely planted trees.
Croeser said cautious approaches to bushfire management – even in major city centres – meant trees were planted with considerable gaps between them, and frequently pruned.
“A tree that’s getting smashed every two years is just constantly trying to recover from injuries rather than growing a nice, healthy canopy,” Croeser said.
The issue was thrown into stark relief at Green Link Bridge in Sydney’s inner west, where dozens of semi-mature banksias lining the land bridge began dying in November.
Balmain MP Kobi Shetty said the irrigation system had failed, the trees were diseased, were the wrong species for the area and were planted too close together.
Transport for NSW will replace the trees in stages, she said.
Meanwhile, Monash-led study published this week in Springer Nature Link found heat exposure contributed to $30 million in health care costs annually in Victoria between 2014 and 2019.
The researchers examined the causes of more than 6.3 million hospital visits across Victoria during the summers of those years, finding that heat exposure was the underlying cause of 10,920 emergency department presentations and 4574 hospitalisations.
The National Climate Risk Assessment last year predicted a significant increase in the number and intensity of heatwaves caused by climate change.
In 2024, scientists reviewed the health records of 104,000 Australians who had experienced cardiovascular events and fatal heart attacks over 10 years.
They found that when people’s health data was cross-checked against the total green space and tree cover they lived among, a 10 per cent increase in tree canopy cover was linked with reduced risks of all-cause mortality, cardiovascular disease mortality, and fatal or non-fatal heart attacks.
Also in 2024, an international study of more than 2.5 million buildings across eight global capital cities found just 3 per cent of buildings in inner Melbourne had adequate neighbourhood tree canopy cover, while in Sydney, 17 per cent of buildings had enough shade from tree canopies.
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Bianca Hall is The Age’s environment and climate reporter, and has worked in a range of roles including as a senior writer, city editor, and in the federal politics bureau in Canberra.Connect via X, Facebook or email.