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- Winning water formula
- Why we need the urban forest
- View from the top
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- A lifetime for societies
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Up Close
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5 of the best
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My Town
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Why we need the Urban Forest
 Trees are a quintessential component of urban infrastructure – forming streetscapes and adorning public lands, softening private homes and commercial properties.
So much hangs on the continued existence of the ‘urban forest’ they form in partnership with understorey plants. Together they beautify landscapes, sequester carbon, feed people (reducing food-miles), support wildlife, return nutrients to soils by decomposition of leaves and wood, recycle water and reduce stormwater run-off.
They also contribute to our mental and physical wellbeing in ways we are only just beginning to understand.
Yet local media and political parties that frequently canvass the need to build and maintain infrastructure have persistently failed to include – indeed seem to have under-emphasised – the role of this green infrastructure.
A disconnect exists in the public mind between the notion of vegetation as an ingredient of business and technology parks, roadsides and public spaces like airports and the role this biomass plays in a climate-change agenda.
In contrast, research in the United States has recognised the value of urban greenery. A study (Figure 1) looking at the benefits of tree planting in California has shown that the average annual electricity saving is 70-90kWh per tree (a similar metric for cool temperate cities like Melbourne and Adelaide would probably average 30kWh per tree), with savings even greater at peak times, reducing overall energy demand by 10%.
A conservative ‘back of the envelope’ calculation might estimate two major shade trees per hectare. Assuming an average built-out radius of 20km for Melbourne, that’s an area of 1.2 million hectares, amounting to 2.4 million shade trees.
With 30kWh per annum saved for every shade tree, this equates to 72 million kWh in total each year. Further, given that for every kWh produced from a brown coal-fired power station there is an emission of 1.2kg of CO2, then 86 million kg or 86,000 tonnes of CO2 per annum is avoided. A higher figure would possibly apply to Sydney – with a larger area and 70kWh saved per shade tree – 200,000t of CO2 avoided.
In addition, as coal-fired power stations use, on average, 95L of water per kWh, that 72 million kWh figure for Melbourne equates to 6840ML or 7GL saved per annum – water that will have to be found elsewhere if the urban forest dies out.
Michael Archer and Bob Beale in their book Going Native write that every city and town would benefit from a broader overview – not only to co-ordinate their own plantings for maximum environmental and social impact, but to ensure that their plans link up with those of neighbouring regions and areas farther afield.
Frequently lost sight of, however, are private contributions to the urban forest as illustrated by an aerial photograph of a single block in East Melbourne (Figure 2). This shows that even in a high-density area, upwards of 60% of the tree coverage can be sited on private land, underscoring the importance of private ownership of this vital feature of the urban landscape.
THE LONG DRY
Meanwhile, hot and dry weather has taken its toll on many large trees – 15% according to some estimates in Melbourne – a situation undoubtedly worsened by the tightening of restrictions on the watering of gardens.
And, despite the respite of recent rains, the longer-term outlook remains bleak for urban greenery. Effects recently identified by IPCC2, an international body charged with studying climate change, would profoundly influence the survival capacity of the urban forest.
For example, extreme winds in combination with deluges (that weaken anchoring) may lead to ‘domino’ tree falls, and withering heat may desiccate plants and have detrimental effects on pollinators. The threat of litigation in the event of limb and/or tree falls may also lead many to clear out such vegetative hazards.
Plants could also initially experience a boost with increased CO2 but then wilt as the average global temperature increase pushes through 3.4°C. They are also likely to become stressed by plagues of (south-moving) insect pests, leading to an increased incidence of disease, and could set seedless due to the loss of pollinators. Many will face severe competition from rampant weedy species.
Among key adaptation responses are breeding hardier plants sympathetic to climate transition and attuned to alterations in climatic zones (practical research is proceeding here), trees that are fire retardant and shade providing but less prone to branch fall, vegetation that can better resist erosion in coastal zones subject to rising sea levels, and an expanded variety of food plants suitable for backyard plots.
WATER RESTRICTIONS
As framed, water restrictions tend to assign a low or zero value of (potable) water released to the environment. Unfortunately, this ignores the ability of urban vegetation to lessen the carbon footprint of our cities and even foster rainfall (a tree can absorb and store a tonne of CO2 as it grows to maturity), and indeed avoid positive feedback from otherwise wilted landscapes.
Manchester University’s Adaptation Strategies for Climate Change in the Urban Environment project has found increasing green space in cities by 10% reduces surface temperatures by 4°C due to water evaporating into the air from leaves and other vegetation.
With the advent of harsher restrictions, greywater has become the stopgap publicly available water source for gardens. Yet, given the lack of control over what can be used in washbasins, showers and baths, etc, there is
the prospect of long-term damage to trees
and shrubs.
A recent international study has found that greywater can contain deadly plant viruses, and antibacterials like Triclosan – often found in toothpaste and soap – that may biodegrade into persistent toxic compounds. In the circumstances, a much more exacting pre-treatment seems necessary.
WARRIORS AGAINST CLIMATE CHANGE
It’s not too harsh to suggest that recent public policy – urban consolidation – has downgraded the contribution back yards can make to an urban forest, leading to an obvious conclusion that a greater commitment, at least at the local level, is needed to help people maintain or develop suitable gardens in furtherance of climate mitigation and adaptation.
The recent trend to treeless, minimal-plant gardens should certainly not be encouraged where it translates into an overall diminution in an area’s biomass coverage.
Now that climate change has at last attained top billing as a public issue, the reduced carbon footprint deriving from the urban forest should no longer be underplayed. 
Dr Peter Fisher is an environment industry specialist and teacher of water management at Central Queensland University. He is also a member of the NatClim team. For more information on NatClim see the April qtr.
This material is derived, in part, from work commissioned by the Nursery and Garden Industry Victoria (NGIV) and Hortwise Pty Ltd |
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