Much-loved leafy streets and shady parks in Sydney and Melbourne are in jeopardy, according to new research that found climate change severely threatens the health of more than one-third of tree species in Australia's cities.
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The federally funded study of 1.5 million trees in 29 council areas across Australia found that higher temperatures and urban heat means new tree species may be introduced, existing trees must be given special care and some trees may disappear in certain locations.
More than four in 10 houses in Australia's capital cities have a street tree.
Trees can greatly affect people's experience of a city - providing shade, places for recreation and a sense of place and heritage. They also cool the city, capture rain, slow stormwater and provide habitat for birds and other animals.
But the study found 24 per cent of all public trees, or 35 per cent of tree species, were at high risk from increased temperatures under a business-as-usual scenario in which emissions continue to increase to 2070.
Some 14 per cent of all public trees, or 22 per cent of tree species, were at high risk of increased temperatures if emissions were limited, in line with international commitments, in the years to 2040.
Trees were deemed at high risk when predicted temperatures were warmer than 97.5 per cent of locations where the species is found - making them particularly susceptible to drought, physiological stress and pest and disease outbreaks.
In the City of Sydney, 50 per cent of trees were at high risk under a business-as-usual scenario. They included brush box, rose gums, grey oaks and several eucalypt species.
In the Sydney council area formerly known as Marrickville, now part of the Inner West Council, a business-as-usual scenario put 40 per cent of trees at high risk, including casuarina she-oaks, black locusts and several eucalyptus species.
Some 32 per cent of trees were at high risk under business-as-usual in the City of Melbourne. They included rose apples and several species of elms, oaks and eucalypts.
Melbourne's inner north City of Moreland would see 26 per cent of trees at high risk under a business-as-usual scenario, such as purple-leafed plums, prairie crabapples and the narrow-leaved ash.
Darwin had the highest proportion of trees - 85 per cent - most at high risk if emission levels rose to 2070, while Ballarat had just 1 per cent at high risk.
Risks to trees were posed by both rising global temperatures and the urban "heat island" effect, where localised warming occurs due to dark-coloured and paved surfaces, buildings and the emission of heat from human activities.
The study was conducted by the Clean Air and Urban Landscapes Hub, a consortium of four universities funded by the Department of Environment and Energy.
It said "changes to the composition and the traits of the urban forest will lead to changes in the sense of place and identity of cities."
"Many cities in south-eastern Australia have a strong European colonial heritage expressed in their many broad-leaved deciduous trees that is likely to change under future climates," it said.
Conversely, local native trees helped create unique city identities and connections to natural heritage and traditional Indigenous ownership.
The report said urban forest managers could adapt to increasing temperatures by providing irrigation or improved pest and disease management, improved tree maintenance and selecting trees that are better adapted to future climates.
