14 October 2019

Native trees best suited to beat Canberra's intensifying heat, says report

| Ian Bushnell
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Oak trees provide a cooling canopy in summer. Photo: Michelle Kroll, Region Media.

The heat is on Canberra’s much-loved deciduous trees with a new report rating native varieties as the best species to cope with the ACT’s changing climate, with the kurrajong on top.

Commissioned by the ACT Government, the Urban Tree Species report from the ANU Fenner School of Research identified the best tree species to improve Canberra’s urban tree canopy and adapt to rising temperatures.

It comes after the release last month of Canberra’s Living Infrastructure Plan, alongside the ACT’s Climate Change Strategy, which outlined how the city can be cooled in a warming climate, including increasing urban tree canopy cover from 21 per cent to 30 per cent.

Using climate change models and species-specific data, the report reviewed the Transport Canberra and City Services (TCCS) tree list to determine which species will survive and thrive in Canberra’s climate change future, ranking species according to their climate suitability, capacity as street trees and advising which species are more suitable in certain scenarios.

It assessed and ranked tree species on a range of climate factors including drought tolerance, frost tolerance, extreme heat tolerance, weed potential and allergen potential.

The report assessed 211 tree species used in Canberra’s urban spaces and two lists, one climate-weighted, of top 50 species are dominated by native species, including eucalypts such as yellow box.

There are no exotic deciduous species in the top 10, but various drought-tolerant oaks and liquidamber are listed as well as the indigenous deciduous white cedar.

Other species include various pines and cypresses, crape myrtles, casuarina and acacia. It suggests trialling other native trees such as lemon-scented gum, spotted gum, wilga and silky oak, and even the purple flowering jacaranda which does well in Dubbo.

Despite its resilience and shady canopy, the report recommends popular oriental plane trees be used sparingly due to allergenic pollen.

The report selected trees as best to cope in a Canberra which is forecast to have a climate more like Dubbo, with temperatures up to 4 degrees Celsius higher by 2090. Although rainfall is tipped to remain steady, it will be delivered more in storm bursts than in soaking falls, where water is more likely to run off, particularly in paved environments.

The authors also rule out the mass irrigation of trees across the Canberra urban forest but argue for better planting methods and conditions, and greater stormwater retention.

They also argue for a greater diversity of plantings.

Associate Professor Cris Brack and Shane Rattenbury

Associate Professor Cris Brack and Climate Change Minister Shane Rattenbury.

The report warns that climate change, combined with urban heat island effects and the stress of built environments, may lead to large-scale tree and urban forest failure.

“In south-east Australian cities, the combination of heat output from built infrastructure and climate change-related variability in rainfall and temperature regimes mean that tree decline associated with increasing urban drought severity and frequency is a principal concern for urban tree managers,” it says.

While the report clearly favours native species, it says there are no trees on the TCCS list that are not suitable for Canberra.

Climate Change Minister Shane Rattenbury said the government wanted to plant a lot more trees to protect the city against rising temperatures.

“We need to make sure we’re planting the right ones,” he said. “It’s quite an important report and will help shape the future of this city in terms of what trees are being planted.”

Mr Rattenbury said the Government had not made an explicit decision to turn away from exotic deciduous species to give the urban forest a more native bent, saying the city needed different species in different places to serve different purposes.

“Many of the trees are reaching the end of their natural lives so we’re going to see change over the coming decades, having the community more involved will make that change easier,” he said.

He said the Government did want to get rid of exotic species but some won’t survive the heat.

“Canberrans really love that autumn landscape in the city. It’s not that we want to get rid of it, that’s where we’ll have to work very carefully. The report does identify some of those deciduous species that remain viable in Canberra. Some won’t because they won’t cope in the heat. They’re some of the decisions we’ll have to take,” he said.

Mr Rattenbury said the tree program was expensive and he would like to see more of the community involved at a neighbourhood level to determine what species are planted and how they are maintained, including watering.

We’re more likely to see more eucalypts like these. Photo: Michelle Kroll

He also flagged more onerous conditions for developers to ensure there is decent tree cover and that they are maintained for a time post-development.

“Ideas are being discussed, how do we keep them alive, and make the program more affordable,” he said. “I don’t think it’s the Government’s sole responsibility to deliver it,” Mr Rattenbury said.

One of the authors, Associate Professor Cris Brack, said there would be a mix of natives and exotics that would survive, and the more diversity the better.

But he said perceptions that all eucalypts were not as cooling as exotics were, dropped limbs and were more of a fire hazard were misplaced.

“There is a perception that eucalypts don’t shade as much as exotics, but they do have very important cooling and climate mitigation effects,” he said.

“Eucalypts don’t give much shade at midday but the really big cooling effect is evapotranspiration or how the water moves through the leaves and cools down. Shading is one component but evapotranspiration is a major component. Maybe midday is not the time we need the shade, maybe its morning and late afternoon, when we’re being active.”

He said there were 800 species of eucalypt so it would be a matter of improved tree selection.

Eucalypts in streetscapes were not a fire risk, but he said different species should be planted on the urban interface.

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George Watling10:59 pm 01 May 20

In the case gums vs exotic tree’s fire risk and cooling effects common sense is wrong.

+ Its been proven that exotic trees are the true fire risk. In 2013 study undertaken by researchers from the University of Technology Sydney it was demonstrated that:
– the dried leaves of exotics catch fire much more quickly than the dry leaves of gum trees,
– green leaves on gum trees are no more combustible then green leaves on non-Aussie exotics like oaks, ash trees, and maples.

+ Regarding the cooling effects of gum trees vs exotics there are a number of studies that clearly demonstrate that gums are as cooling, and in some case more cooling, then non-Aussie exotics you prefer.

In a 2016 CSIRO Victorian field study that compared the shading and cooling effects of gums to maples CSIRO scientists found that the maples included in study could not provide the same amount shade as the gums included in the study.

While the gums in the study were able to block 86% of the solar radiation hitting their canopies from hitting the ground the maples were only able to block between 70% and 33% of the solar radiation hitting their canopies from hitting the ground.

The title of the 2013 UTS study is the ‘Differences in Leaf Flammability, Leaf Traits and Flammability-Trait Relationships between Native and Exotic Plant Species of Dry Sclerophyll Forest’. It was published on 18 November 2013.

The 2016 CSIRO study’s title is ‘Greening the West – spatially optimised tree plantings to minimise urban heat island effects’. It was published in July 2016. Its authors are Oswald Marinonia, Mike Battagliab, Matt Beatya. At the time of publishing Oswald, Mike and Matt were working for the CSIRO’s divisions of Land and Water and Agriculture.

The problem with gum trees is they burn ferociously in the canopy contributing to ember spreads. So they add tremendous risk to suburban areas during our hotter times. The advantage of deciduous trees is felt immediately in the suburbs with lower temperatures and fire resistance. But the ACT Government is bound by ideology, not common sense.

George Watling2:27 pm 27 Apr 20

In the case gums vs exotic tree’s fire risk and cooling effects common sense is wrong.

+ Its been proven that exotic trees are the true fire risk. In 2013 study undertaken by researchers from the University of Technology Sydney it was demonstrated that:
– the dried leaves of exotics catch fire much more quickly than the dry leaves of gum trees,
– green leaves on gum trees are no more combustible then green leaves on non-Aussie exotics like oaks, ash trees, and maples.

+ Regarding the cooling effects of gum trees vs exotics there are a number of studies that clearly demonstrate that gums are as cooling, and in some case more cooling, then non-Aussie exotics you prefer.

In a 2016 CSIRO Victorian field study that compared the shading and cooling effects of gums to maples CSIRO scientists found that the maples included in study could not provide the same amount shade as the gums included in the study.

While the gums in the study were able to block 86% of the solar radiation hitting their canopies from hitting the ground the maples were only able to block between 70% and 33% of the solar radiation hitting their canopies from hitting the ground.

The title of the 2013 UTS study is the ‘Differences in Leaf Flammability, Leaf Traits and Flammability-Trait Relationships between Native and Exotic Plant Species of Dry Sclerophyll Forest’. It was published on 18 November 2013.

The 2016 CSIRO study’s title is ‘Greening the West – spatially optimised tree plantings to minimise urban heat island effects’. It was published in July 2016. Its authors are Oswald Marinonia, Mike Battagliab, Matt Beatya. At the time of publishing Oswald, Mike and Matt were working for the CSIRO’s divisions of Land and Water and Agriculture.

Capital Retro10:29 am 18 Oct 19

Why is the native tree Grevillea robusta (Silky Oak) always overlooked in these debates?

liberalsocialist7:08 pm 15 Oct 19

Much prefer deciduous trees – and so do most people. Have a look at what people consider ‘premium’ suburbs with ‘leafy’ streets – whether in Canberra, Melbourne or Sydney. They’re full of Maples, Oaks, Cherry Blossoms, Liquid Ambers… anything but the gum tree’s that will take decades to mature along Northbourne, provide little shade, drop branches on trams and cars and, if they do attract native fauna, puts that fauna in the middle of two of the busiest sets of lanes in Canberra. Well done!

Stephen Saunders7:15 am 15 Oct 19

Sorry, Prof Brack, as you can already see, your “facts” and “evidence” bounce straight off Australians’ irrational hatred of Australian flora and fauna.

They can’t cope with eight syllables in “evapotranspiration”. They’d much rather be cleaning up acorns and oak-leaves all year.

liberalsocialist7:04 pm 15 Oct 19

Have you ever considered that a lot of people prefer the look of a lot of non-Australian species? They have richer colours with deeper greens and reds primarily. I’m one of them.

Just because you like them, doesn’t make anyone preferring non-Australian varieties as ‘irrational’. That in itself is irrational thinking at its best.

It’s pretty obvious that native trees are more resilient, but did they even consider that they provide poor shade in summer?
There’s nothing like a solid row of deciduous trees for cooling a street

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