By Gabrielle Stannus
With a PhD in the impacts of climate on vegetation and trees and a Diploma of Arboriculture, University of Adelaide Adjunct Lecturer, Dr Stefan Caddy-Retalic is well placed to speak on the impact of climate change on our urban trees. Similarly qualified, TREENET Director, Dr Tim Johnson shares his passion for improving urban forests in Adelaide and in cities and towns across Australia.
Dr Caddy-Retalic is Regional Coordinator of Resilient South, a partnership between the South Australian Government and the Cities of Holdfast Bay, Marion, Mitcham, and Onkaparinga. Together with the University of Adelaide, Resilient South is driving the Future Trees Project, which seeks to better understand the vulnerability of Adelaide’s urban forest to climate impacts, and how its climate resilience can be improved. A key part of bolstering resilience is to identify which tree species can be planted to replace those that struggle under a harsher climate. With funding from Green Adelaide, the University of Adelaide, Wellbeing SA and SA Power Networks, the Future Trees Project is mapping the diversity and demographics of Adelaide’s urban forest. Over the last six months, Dr Caddy-Retalic, Waite Arboretum Curator Dr Kate Delaporte and their team have assembled a database of around 550,000 trees across 19 Adelaide councils.
“Our first order of business was to benchmark what trees we have planted and find out what our urban forest looks like, because no one has done that work before. What we found is that there is a heavy reliance on a handful of species like Callery pears (Pyrus calleryana), jacarandas (Jacaranda mimosifolia), red gums (Eucalyptus camaldulensis), and South Australian blue gums (Eucalyptus leucoxylon). Some councils or suburbs also focus on varieties which have heritage values, like plane trees (Platanus x acerifolia) in the City of Adelaide, and white cedars (Melia azedarach) in the inner south.
“We plant some trees prolifically and that is a bit of a worry, particularly given that Adelaide is facing a climate future that is warmer, drier and with many more hot days and droughts. A lot of our exotic trees are from places that are wetter than Adelaide. For example, white cedars are from the tropics and plane trees are from temperate northern hemisphere, and it is unlikely they will be able to establish and thrive as well as they have in the past. Queensland box (Lophostemon confertus) and jacarandas (Jacaranda mimosifolia) are more durable and can cope with dry conditions for a while, however prolonged drought and heat waves are likely to be a problem for even those species,” explains Dr Caddy-Retalic.
“As we move into periods where we have several days in a row of over 35 or 40 degrees, heatwaves and droughts will cause problems for those species. Trees draw in moisture from the soil and transpire moisture through their leaves. In prolonged extended drought, trees run out of soil moisture and experience embolisms, air bubbles occurring in their water vessels (xylem), disrupting water transport, equivalent to a stroke in a human. This can cause large parts of the crown or the entire tree to die because they lose that hydraulic pressure meaning they are unable to supply any water to their leaves. As our climate becomes hotter and drier, the health of these trees decreases. And when we get that big heatwave, you could lose a huge cohort of trees all at once, particularly in those suburbs where 20-30% of the urban forest might be one species.
“We are currently seeking funding for the next stage of the Future Trees Project, to do some wide scale tree planting experiments to trial new varieties of trees that will cope with hotter and drier conditions. We are very keen to trial species that are not closely related to the common species we already grow here. This will help us spread the risk across a wide variety of tree types, and limit the risk posed by both climate and pests and disease. Around 40% of Adelaide’s trees are in the Myrtaceae family (which includes eucalypts, lophostemons and bottle brushes), and are affected by pathogens such as phytophthora and myrtle rust. We suspect these diseases are going to be mobilised by climate change, and that the health of trees and their vulnerability is going to be lowered. So, we really need to look at trying to boost diversity to limit the risk of canopy loss.
“Adelaide is transitioning from a temperate Mediterranean climate to a semi-arid climate, so is at the leading edge of the capital cities in terms of climate risk.
The work we are doing is important to identify which tree species we will need to be planting in the future.
“We really need to look for varieties of trees that are going to be able to provide shade while also being able to thrive in our climate. It is a great idea to be looking at trees from drier places. However, we need to be mindful that just because a place is drier, it does not necessarily mean that it is more suitable to where you are now. I have been doing a little bit of looking into this and contacting people in places like Egypt, Syria, and parts of South America to ask what they are growing in their cities. The scary thing is that a lot of them are saying that they are growing the same trees as us, so we might need to get a bit more creative.
“Shade cast by a tree can be up to 10ºC cooler than shade cast by any other structure, like a building or a shade sail. As our cities heat up, we are more reliant on those trees for cooling and liveability. We are also going to have to be deliberate in making space for these trees. Councils around the country are working to plant as many trees as they can, but they are running out of planting spaces because there is only so much public land, much of which is already heavily encumbered by powerlines, gas mains and the like. Keeping our cities green is going to need a lot of work,” Dr Caddy-Retalic warns.
Dr Tim Johnson knows all too well the perils of placing trees in urban environments. Having worked in local government and horticulture for over three decades, Dr Johnson has also completed a civil engineering PhD on tree roots and permeable paving, researching methods to deflect roots around infrastructure without impact. He is also active as Director of TREENET, the national, not-for-profit research and education organisation founded in 1997 and dedicated to improving Australia’s urban forests.
“Trees are the biggest, longest lived, and in many ways, the most magnificent organisms on the planet. And they kind of get under your skin; they are so fundamental to nature. Trees are the skeleton we build urban nature around. They are so fundamental to bringing everything together, from subgrade under the infrastructure deep in the ground, to the atmosphere. They bring all the resources together and support nature from the top of their canopy to the bottom of their roots,” says Dr Johnson.
Urban Forest Literature database
“TREENET’s charter is to help the community to ‘do’ urban trees better,” continues Dr Johnson. His organisation supports numerous urban forest research and education initiatives for communities and practitioners, including the Australian Urban Forest Literature database. This free online platform holds hundreds of TREENET’s symposium papers, videos, and other publications related to trees on topics ranging from climate change to law, accessible to anyone without requiring a log-in.
Urban Tree Trials Database
Similarly, TREENET’s Urban Tree Trials Database provides examples of some of the many tree species growing across Australia. You can use this free database to access, share or contribute to tree species or trials information. Whilst providing an online demonstration of this database, Dr Johnson showed me the record of a dwarf apple myrtle (Angophora hispida), a species he observed growing on the slopes of Kunanyi/Mount Wellington in Hobart, Tasmania. “We know this angophora can handle warmer climates than halfway up Mount Wellington, so maybe that is a good one to plant in Tasmania for the future. Plant a few more. See how they go!” Dr Johnson enthused.
Dr Johnson encourages urban forest practitioners and community members to share their observations in this database as more records shared helps practitioners learn from the experience of others. For example, having species flagged nationally that are not necessarily restricted to botanic gardens provides more practical options for planting in a climate-constrained future. “If you look at the urban heat island effect, our cities have been several degrees warmer than surrounding areas for decades and trees have survived in them. Botanic gardens are generally cooler than city centres, so trees in urban areas are probably more relevant than examples in botanic gardens,” says Dr Johnson about planting trees now for the future.
24th National Street Tree Symposium
If you would like to know more about the impact of climate change on our country’s street trees, check out TREENET’s national conference, one of Australia’s premier urban forest events. This year’s 24th National Street Tree Symposium is taking place in Adelaide from 7-8 September 2023 around the theme of ‘Spaces for trees: Places for people’. Notable speakers include Professor Stefan Arndt from the University of Melbourne who will be speaking on urban tree species selection for future climates.
Future Trees Project: A report covering the first stage of this project will be released shortly on the Resilient South website, https://www.resilientsouth.com/future-trees
Further information and resources
CSIRO’s Climate analogue tool
Trees may not be able to outpace climate change by themselves given the biogeographic barriers restricting their movement, e.g., oceans, mountain ranges, catchment divides. However, with the helping hand of humans, trees may be able to move into new climate analogues. ‘Analogues’ are places that are currently experiencing a climate that another place might experience in the future. CSIRO’s climate analogue tool matches the proposed future climate of a region of interest with the current climate experienced in another region using annual average rainfall and maximum temperature (within set tolerances). The results from applying changes to annual temperature and rainfall settings in the analogues explorer captures sites of broadly similar annual maximum temperature and water balance. Remember though that whilst climate analogues are useful in helping us imagine possible climate futures, they are only approximates. There are several factors that they do not consider including topography, frost days, solar radiation etc.
Climate change in Australia – search for ‘climate analogues’: https://www.climatechangeinaustralia.gov.au/en/
Climate Futures for Tasmania
It is not often that I share resources purely Tasmanian in focus (sorry mainlanders!). However, as a designer and geography professional based on the Apple Isle, I highly recommend the work of the University of Tasmania’s Climate Futures group. Over the last decade, this group has conducted research into the impacts of climate change on Tasmania’s weather, water catchments, agriculture, and climate extremes, including floods and wind damage. Climate Futures’ work has informed a range of low and high emission greenhouse gas emission scenario datasets, hosted by the Tasmania Government on its free LISTmap application. These datasets extend from the current period to the 2070-2099 period, and cover ‘Annual Rainfall Change’, ‘Frost Risk’, monthly average rainfall (Ave Rainfall ‘Month’), monthly minimum and maximum average temperatures (Max Ave Temp ‘Month’, Min Ave Temp ‘Month”), ‘Growing Degree Days [Oct-Apr]’, ‘Chill Hours’, and ‘Frost Days’.
So, if you are a landscape designer or horticultural professional lucky enough to be based in Tasmania, check out the following websites and start mapping your next project’s climate future.
Climate Futures for Tasmania: www.climatefutures.org.au/projects/climate-futures-tasmania/
M: 0400 431 277