Australian planners and city aficionados often joke that, no matter what the problem is, the answer is always “more street trees”. Like most jokes, this is half true. While not a panacea, here are nine key reasons street trees can help achieve more than a handful urban planning and public health goals.
Cost-benefit analyses can help Australian cities generate evidence to combat anti-tree myths and prejudices, such as:
(a) trees shed leaves and flowers which then rot on footpaths causing them to become slippery
(b) trees attract bugs and possums which then infest homes
(c) trees attract lightning putting adjacent homes in danger
(d) tree branches can detach during storms and kill people or damage roofs
While trees do shed leaves, their benefits far outweigh any organic street debris that may materialise in their shadows.
The US-based website, called iTree, offers a series of tools to estimate the economic benefits of trees.
It found that each tree has a different economic price tag – in the ballpark of a figure like $20,000 worth of environmental and aesthetic benefits over its lifetime – and finding that for every dollar spent on a tree, an estimated A$5.93 worth of benefits are returned.
Urban tree and forest inequity
Tree coverage is not just an environmental but also a social justice issue.
Research suggests that disparities in tree coverage – which are common – are mainly predicated on local incomes. While the wealthy live in leafy suburbs, near parks and urban forests, the poor are more often pushed toward greyer and more polluted areas.
Dorina Pojani (one of the authors), has been involved in two studies on this topic, set in Europe and Australia, and led by University of Queensland honour’s students Megan Buckland and Sean Patton respectively.
To combat this inequity, cities should compile and/or update granular databases of their street trees.
Each city should have a public street tree map, similar to New York’s, which includes information such as tree species, tree age, tree size, tree health, street width, verge size, planter type, and adjacent land use.
Databases like these are already widespread across the world, and are invaluable to measure geographic tree coverage inequality.
To convert this data-gathering into practice, local governments should standardise tree planting guidelines and allocate generous funding for street tree provision and maintenance.
Australian cities should follow in the footsteps of warm Southern European cities such as Madrid and Milan and prioritise tree planting in currently underprovided areas.
Given that mature trees are more beneficial than young saplings, the sooner we start planting across our cities, the better.
Here are nine key benefits to planting trees in our cities:
1. No tan is worth dying for
The first goal relates to protection from sun exposure.
The sun produces ultraviolet (UV) radiation with different wavelengths (UVA, UVB, UVC). UVA and UVB rays penetrate the ozone layer exposing people to skin cancer and burns. According to the Cancer Council, nearly 70 per cent of Australians are destined to develop skin cancer by the time they reach 70.
Reducing sun exposure across the life course is key to lowering this risk. And street trees, especially leafy ones, are one way to provide protection from the sun.
2. Clearing up the air
Another goal is alleviating air pollution.
Carbon monoxide, particulate matter, nitrogen oxide and sulphur dioxide are major contributors to respiratory diseases. Air pollutants come from industries, vehicles, wood burning, and bushfires.
Trees have been shown to improve air quality by removing certain gases and particulate matter from the atmosphere, converting carbon dioxide to oxygen, and increasing air dispersion.
3. None like it hot
A third goal is a reduction in heat waves through shading and evapotranspiration.
In the scorching Australian summer, heatwaves account for more deaths than all other natural disasters combined. Heat stress occurs when air temperature, radiation, humidity, and wind all interact to increase the human body temperature. The negative health effects of heat stress range from mild discomfort to life-threatening medical conditions, such as heatstroke.
Especially concerning are heat extremes in cities, which is where nearly 90 per cent of Australians live. Here heat is further exacerbated by the Urban Heat Island (UHI) effect. The denser parts of cities are typically hotter, owing to excessive impervious surfaces and insufficient green coverage.
Meanwhile, urban trees are known to help cool the air, by a few degrees Celsius at least, thus reducing heat-related ambulance calls.
4. Air conditioning? Not a fan
Due to excessive heat, along bare streets and suburbs, energy expenditures for air conditioning are higher in summer.
Air conditioning costs can take up a significant portion of the household budget. Far from a luxury, air conditioning is a necessity in Australian cities that are both hot and poorly designed from a climatic perspective.
5. Green is the warmest colour
Visual pollution can also be mitigated by trees.
Studies have shown that green space is beneficial for individual and community mental health. Natural environments with trees, grass, and water create a physiological reaction which helps alleviate stress and depression.
MRI scans show that people living closer to trees and greenery enjoy a higher integrity of the amygdala, which is a part of the brain that helps us cope with stress. Forest-based therapies are known to help patients with major depression disorders. And psychological benefits increase in step with the variety of species in urban green spaces.
At the community level, there is a relationship between urban tree exposure and social cohesion whereby people living in greener areas feel more connected and have a stronger sense of belonging and trust.
6. Shaded routes to transport
Street trees have a role to play in transport and walkability. They are, in fact, a pedestrian infrastructure asset.
If local routes leading to public transport stops are shaded by tree canopies, more people may be encouraged to take public transport – even during the hot Australian summers. Walking can be a sole mode but can also serve as a “first/last mile mode” – in other words, used to reach a transit stop from one’s home or office.
Shaded routes to transit stops are particularly important for low socioeconomic or car-free persons who rely on buses, trams, and trains to get around.
7. Hail the trees
Urban trees intercept and absorb rainfall, and as such are a useful tool in sustainable stormwater management. This is a key consideration in a country like Australia, which is prone to storms and floods.
8. Keeping fit
People are more likely to walk – for transport or leisure – if their route is well shaded.
Studies have shown that physical activity, such as walking, cycling and outdoor play, is higher in people living near trees. Even among children, the body weight is lower in areas with more trees and urban forests.
9. Peace and quiet
Noise pollution, particularly from traffic, is known to negatively affect people, reducing their quality of life. Moreover, diseases such as hypertension and hearing loss have been associated with noise.
While studies are not extensive, there is some evidence to suggest that trees can help reduce noise. A review showed that large belts of dense vegetation can serve as an effective noise barrier.
How many trees equal a human life?
Trees are beneficial to human health, life, and wellbeing – but how much is a life worth?
Along the same lines, how many trees equal a human life?
Some may respond that life, both human and plant-based, is invaluable. Yet, governments and insurance companies put a price on these “line items” all the time.
The Australian Government, for one, has determined that the value of a human life is $5.1 million – based on Australians’ estimated willingness to pay to prevent the loss of life.
To put these findings into perspective: Brisbane’s entire budget of $19.73 million allocated to tree planting in 2020 would have been justified on heat death prevention alone if it were to save a net present value of four lives.
In other words, every net present value life saved justifies around 25 per cent of the annual tree planting budget.