Preserving Treescapes as Cities Get Hotter

When did you last notice the trees in your neighbourhood? Perhaps it was on a scorching summer afternoon when you took advantage of the cooling shade they provided, or when you stopped to shelter beneath their leafy canopy during a downpour. Was it when you looked up to track the source of a burst of bird calls and realised the branches above you were alive with lorikeets? Or when you stopped to admire a town boulevard painted with living autumnal colours? 

Trees are a vital feature of towns and cities, yet remarkably little is known or documented about how well they cope with the pressures of the urban environment, how that is changing with a warming climate, and how we can help urban trees survive and adapt to those challenging conditions. 

This is where the work of Western Sydney University researchers, Professor Mark Tjoelker and Dr Manuel Esperon-Rodriguez, comes in. At Western’s Hawkesbury Institute for the Environment, they have been developing datasets and guidelines that will help secure a greener, healthier and more sustainable urban treescape.  

"As a nature-based solution to the urban heat-island effect, trees have an outsized impact in urban environments compared to their role in natural environments," Tjoelker says. Their ability to shade heat-absorbing surfaces can have a big effect on urban energy consumption by reducing air-conditioning needs. "Some studies have shown that trees in urban environments can directly reduce energy consumption by 10 or 15%," he says. "That’s a huge potential contribution to getting to net zero carbon emissions."

To help urban landscapers pick the healthiest, most viable saplings at a plant nursery, Tjoelker and colleagues have developed a new national standard, accredited by Standards Australia: AS 2303:2018 Tree Stock for Landscape Use. These national standards exist to ‘ensure the quality and consistency of products and services, ranging from bushfire-resilient building to designing for access and mobility. 

To develop this standard, the research team gathered biometric data on nearly 14,000 container-grown trees in plant nurseries — representing 159 tree species commonly used by urban landscapers across Australia. 

"The standard has a checklist of different criteria that are used to help ensure quality," Tjoelker says. "It will help ensure that the tree has every chance, if planted in the correct location and treated right, to survive, grow and thrive."

That checklist includes questions about whether the volume of the pot is suitable for the height and diameter of the tree, and whether the potted roots have the proper form and are free from defects. "It ensures the right balance between the roots that need to supply water and nutrients to the tree and the above ground portion of the tree," Tjoelker says. "If that’s too small, the tree will really struggle to get a foothold in the landscape."

"Purchasers now have a clear standard to quote when purchasing tree stock and the result of this is a high-quality product being supplied that will have a greater survival rate; this means our cities and towns are going to be greener, safer and cooler," says Hamish Mitchell, the managing director of Specialty Trees, one of the nurseries that uses the new standard. 

Ensuring that young trees have the best possible chance to flourish is just one part of the challenge; equally important is deciding which tree species to plant in a particular location. 

Esperon-Rodriguez has long been fascinated by trees. He began his research career in the mist-cloaked cloud forests of Mexico, studying how tree species in that unique setting are dealing with the changes in climate that are threatening to dry out the lush environment. A post-doctoral opportunity brought him to Australia, and eventually to Western Sydney University and the Which Plant Where research programme: a joint initiative of Western Sydney University, Macquarie University, the NSW Department of Planning and Environment, and Hort Innovation. 

The aim of that five-year project was to build a resource for all those involved in developing urban greenscapes to enable climate-ready decision making and to develop resilient green spaces of the future. To do that, the project team has gathered extensive information from around Australia about what factors are associated with urban tree success or failure, with a particular eye on how climate change is shaping these outcomes.  

"I spent two years contacting people at the councils of more than 200 cities all across Australia, asking if they could give me any species lists or names that they were no longer planting because of climate change, because they knew they were failing," Esperon-Rodriguez says. It turned out that very few city councils had that data recorded in any systematic fashion, which is perhaps unsurprising given the myriad reasons why any particular urban tree might fail: climate and weather, planting techniques, species vulnerability, damage, vandalism or poisoning. 

Esperon-Rodriguez widened his search to a network of people in 14 countries around the world, and still came up empty handed. It showed there was a desperate lack of information, and at a time when it was badly needed. “I decided I wanted to do something about climate change and urban trees and try to help councils to identify the vulnerable and resilient species,” Esperon-Rodriguez says.  

That took the form of a comprehensive assessment of the distribution of more than 3,000 urban tree species in 164 cities globally, which expressed that distribution in terms of those species' tolerance for climatic conditions such as temperature and rainfall. 

It revealed some surprises, including non-native tree species whose original distribution might suggest they weren’t well suited to Australian temperatures but were in fact thriving, and native species that weren’t coping nearly as well as expected. "The models can say this species is vulnerable, but in reality, we can see that the trees are surviving quite well," Esperon-Rodriguez says. 

It’s valuable information for local governments trying to plan for urban treescapes, says Karen Sweeney, an urban forester with the City of Sydney Council. "We want to get 40 to 80 or even 100 years out of some trees, particularly in parks," she says. "In our streets, it’s about making sure that we have good environments and the right adaptive trees to be able to thrive in those harsh environments for the long term."