By Gweneth Leigh
It's not easy growing up in the centre of the city. Traffic congestion, constant noise, the glare of streetlights and cramped living quarters can make for a hard life – especially if you're a tree. Lucky are those planted in pocket parks and playgrounds, where there is room for roots to wriggle wide and limbs to stretch skywards unhindered. But those in the road verge and median strip are in for a rough ride, where dallying with underground pipes, overhead powerlines or blocking important views is disciplined through amputation – or worse.
This is a story our toughened river peppermint gums (Eucalyptus elata) know all too well, the third-generation planting of Northbourne Avenue's median strip.
Canberra's urban forest has more than 750,000 trees, including more than 300,000 street trees.Credit: Lannon Harley
Once numbering more than 800, these quickly disappearing street kids of Canberra's urban forest should be saluted for their service. Their leaves have acted as huge filters, catching particles like soot, pollen and dust. The trunks, branches and root systems have been a giant green vacuum, sucking up and storing carbon dioxide. The broad crowns have provided a green respite for the overtired eyes of computer-bound office workers; relief during hot days through shady lunchtime walks, as well as habitat for birdlife and other fauna. This green corridor has formed an important part of Canberra's broader urban forest – the "engine room" of our urban ecosystems – vital to the health and well-being of our communities.
At just a touch over 30 years old, these river peppermints would be hitting their stride in their native woodland habitat. However, life on the street is hard and ages anyone prematurely, tree or person alike. Unlike tree counterparts in Haig Park – where privileged Monterey pines and Roman cypresses have grown for twice as long in 19 unblemished hectares – these street kids were raised in a world hemmed with physical constraints and heaped with high expectations of performance.
Northbourne's river peppermints: without room for their roots to breathe and expand, trees can suffocate and struggle to survive.Credit: Elesa Lee
Early life was good to begin with, being pampered and primped with personal irrigation lines and customised watering schedules. But as an adolescent tree grows, so does its desire to spread roots, particularly into rich crumbly soil that provides air, water and nutrients. These roots also anchor the tree by spreading far and wide rather than deep; up to 90 per cent of a tree's rooting structure is found within the top 60 centimetres of the soil profile. Without this room to breathe and expand, trees can suffocate and struggle to survive – and be more prone to falling down during storms.
Crumbly soils are not the friend of roads like Northbourne. Air pockets would have been squeezed out to stabilize the driving surface, creating a barrier impenetrable by tree roots. While root systems in Haig Park have the ability to spread to more than 500 square metres (typical for a forest tree), the underground anchoring system of our Street Kids are puny in comparison. They were never allowed to explore beyond the front yard. Adding to these challenges, heat soaked up by surrounding asphalt, concrete and buildings during the summer months keeps temperatures elevated at night, the period when forests (and humans) typically have a chance to cool down and recover. These warmer temperatures extend the working life of insect pests, who survive into the winter months and become another threat to these already vulnerable trees.
Considering these environmental stresses, it's unsurprising that more than half of the trees within the Northbourne Avenue median were lost between 2010 and 2014 due to failing health and storm damage.
Fortunately, when the going gets tough, trees are social creatures that will look after each other. University of British Columbia studies have indicated that trees in forests communicate through far-reaching underground networks using mycorrhizal fungi in the soil to connect their roots and exchange information and goods. Scientists have described these networks like an airline route map, with older trees acting as hubs. This "wood-wide web" allows trees to convey warning of insect attacks, as well as deliver nutrients and water to seedlings and stressed neighbouring trees, regardless of the species. Studies have also indicated trees can communicate by scent, producing compounds that fend off attack by predators (such as pumping bad-tasting toxins into leaves). These compounds also notify nearby trees to follow suit or risk getting attacked as well.
The Northbourne trees are dying more quickly than expected, and will be removed by the end of this month.Credit: Karleen Minney
The author of The Hidden Life of Trees, Peter Wohlleben, points out there are advantages to working together through this cooperative economy. On its own, a tree is vulnerable to the extremes of weather, climate and environmental factors. But a forest can create an ecosystem that curbs temperature extremes, generates humidity and promotes storage of water and nutrients. When gaps arise in the canopy, this weakens the forest defence by making it easier for storms to uproot exposed trees and dry out the forest floor. These challenges are likely to have influenced the decision to accelerate tree removals along Northbourne to finish by the end of this month. Together we stand, but divided we fall.
Of course, these street kids are among hundreds of thousands of trees that comprise Canberra's urban forest. The ACT government manages more than 750,000 trees in our urban space, and more than 300,000 of these are street trees. It is the largest urban forest in Australia under the jurisdiction of one government agency – close to twice as many urban street and park trees per resident when compared to our city council counterparts in Melbourne and Brisbane. While this green infrastructure is a huge asset, it also poses major management challenges. That's especially true when many of the original trees are now reaching the end of their life cycle simultaneously and need to be replaced, like those on Northbourne.
These issues are not unique to Canberra. In 2012, the City of Melbourne implemented an urban forest strategy that significantly changed the way urban forests are managed. The plan was the city's effort to repair the damage that 13 years of drought had inflicted on 40 per cent of the city's significant trees. Urban forest precinct plans were developed, each focusing on performance-based street outcomes – such as canopy cover, forest diversity, vegetation health, biodiversity and community engagement – rather than prescribing specific species for each location.
What makes Melbourne's strategy unique is the way it engages citizen co-design and involvement. Each of the 70,000 trees in the City of Melbourne has a unique ID number linked to an online map – the Urban Forest Visual – which provides information on each tree's botanical identity, location, health and life expectancy. As a way to empower the community, each tree is also assigned an email address, which allows people to email the tree and report issues such as tree decline, vandalism and dropping branches. An unexpected outcome of the project has been a flood of love letters from around the world from people thanking the trees for their hard work ... as well as the occasional apology from dog owners whose pet had raised an appreciative leg.
More than half of the trees within the Northbourne Avenue median were lost between 2010 and 2014 due to failing health and storm damage.
As a community passionate about our trees, I imagine such a strategy in Canberra would have produced eulogies for our river peppermints that waxed long and lyrical. Meanwhile, the fourth generation of Northbourne's street kids – Eucalyptus mannifera (brittle gum) – are waiting patiently at Yarralumla nursery, awaiting orders. These newest recruits for the frontline of Canberra's urban forest have the added challenge of sharing space with light-rail infrastructure, an expanding urban realm and a globally warming climate. Not an environment for the faint of leaf or root.
Which makes it so important to ensure this critical piece of green infrastructure is provided in a way that gives these infant trees the space to do what comes naturally: to grow, explore, mingle and intuitively lean on one another when the going gets tough. As humans, we have much to learn from them.
Gweneth Leigh is a writer and landscape architect.