Major Australian cities fail international tree canopy goals: study

Major Australian cities fail international tree canopy goals: study

New research measuring access to nature for eight major global cities, including central Melbourne and Sydney, found most have inadequate canopy cover despite access to an abundance of trees.

Just 3% of buildings in Melbourne’s innermost suburbs had adequate neighbourhood canopy cover, despite 44% of them having views of at least three trees.

Central Sydney fared better, although only 17% of city buildings were shaded enough despite 84% having views of at least three trees.  

Lead researcher Dr Thami Croeser said better canopy cover was urgently needed to cool down our cities.

“We know depression, anxiety, obesity and heatstroke are more prevalent in urban areas lacking access to shady tree canopy and green open spaces,” said Croeser, from the Centre for Urban Research. 

The study, published in Nature Communications, looked at over 2.5 million buildings across eight cities using an emerging sustainable cities measure, known as the ‘3-30-300' rule.  

The rule states every house, school and workplace should have a view of at least three trees, be in a neighbourhood with at least 30% canopy cover, and be within 300 metres of a park.   

Map of central Melbourne showing buildings that have adequate shade (marked in dark blue) to buildings with inadequate shade (marked in red). Map of central Melbourne showing buildings that have adequate shade (marked in dark blue) to buildings with inadequate shade (marked in red). Credit: Cobra Groeninzicht

While most buildings across the eight cities had views of at least three trees, the study found almost all cities fell short on the 30% canopy cover test. 

Only Seattle and Singapore passed the 30% canopy benchmark, with 45% and 75% of buildings in these cities enjoying adequate shade, respectively. 

Croeser said trees can give poor canopy cover if they are planted in conditions that stunt their growth, such as asphalt covering roots and reducing access to rainwater. 

“Too often, we put trees last in our streetscapes,” Croeser said. 

“We plant them in very tough conditions, and then the moment there’s a conflict, they’re out. Whether it’s for construction access, a resident complaining or an underground cable, we’re very quick to remove street trees and replace them with saplings.

“Trees in urban environments are also pruned very heavily, so not many trees get the opportunity to grow into big old canopy trees except in a few lucky areas.” 

Croeser said canopy cover doesn’t just increase cooling, it can also reduce flood risk as well as benefit mental and physical health and support urban biodiversity. 

“Studies say we actually need at least 40% canopy cover to substantially lower daytime air temperatures, so the ‘30’ metric is the bare minimum; most buildings we studied don't even reach that goal,” he said. 

Croeser said he would like to see state and federal government take charge of a canopy taskforce to retrofit streets to prioritise canopy cover. 

“Councils are already strapped for funding and resources, so we need government to step in to fund and fast track the process to make an impact on canopy goals,” he said. 

Tree canopy in a residential area. Tree canopy in a residential area. Credit: Thami Croeser

A new nature access metric with potential

While the ‘3-30-300' rule, devised by Dutch urban forestry expert Professor Cecil Konijnendijk, is still relatively new in Australia, it is gaining momentum internationally, with at least six cities in Europe, the US and Canada implementing the measure in their urban forestry strategies.

Konijnendijk said he devised the benchmark to help set a ‘bare minimum’ for nature in cities. 

“Getting more parks and trees into cities is complicated work, and I realised that a simple metric could take the mystery out of it and set a proper benchmark based on evidence,” he said.  

“I reviewed decades of science linking nature to human health – and found that views to nature, canopy cover and parks are all really essential if we want to be mentally healthy, physically active and safe from heatwave impacts.”

Technical University of Munich Professor Wolfgang Weisser said metrics benchmarking neighbourhoods’ green infrastructure against human wellbeing were rare.

“Some of the metrics we use now are not really sufficient whereas the ‘3-30-300' metric really demands that nature is brought to the areas that people actually live and work in,” Weisser said.

“A municipality with almost treeless streets and a few large, well-forested parks may score well on aggregated metrics of canopy and per-capita greenery but will be exposed as inadequate by the highly local ‘3’ and ‘30’ requirements.” 

The researchers collaborated with Dutch firm Cobra Groeninzicht (Green Insights) to visualise the results.

“With the techniques our team has developed, we can calculate the ‘3-30-300’ benchmark for any city in the world,” said Dirk Voets, Senior Advisor Geospatial at Cobra Groeninzicht.

“Acute canopy deficits in global cities exposed by the 3-30-300 benchmark for urban nature”, with Thami Croeser, Roshan Sharma, Wolfgang Weisser and Sarah Bekessy, is published in Nature Communications. (DOI: 10.1038/s41467-024-53402-2)

 

Story: Shu Shu Zheng

20 November 2024

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