Value of Trees in cooling

How can we get the planning decision makers in local and state governments to read and act on this article from today’s Age.

 

It's time literally to go green

• Peter Fisher
• January 23, 2009

 

Cities go green. Photo: Dyson

As air-conditioner sales soar and concrete abounds, the role of trees, grass and other plant life in helping reduce carbon emissions is often overlooked, writes Peter Fisher.

GOVERNMENTS are seriously overlooking what city trees can do to reduce water use and fight climate change. This week's scorchers have certainly focused the mind on how to stay cool. For some years Australia has experienced a staggering growth in the installation of air-conditioners, with sales surging by as much as 10 per cent each year, and some houses having more than one. Whether the looming recession results in less take-up and use, or a resort to electric fans, remains to be seen.

These units are ravenous electricity users, imposing heavy demands on peak-load generating capacity, especially the older, less-efficient models. And they're the most pernicious of black balloons with their own feedback spiral — the more air-conditioners to fight the stifling heat, the more the carbon emissions, the more the planet heats up, and so on.

Adding to their workload is the explosion in hard surfaces — patios, pebble gardens, car parks, walls, roads and buildings — that retain heat, causing an urban heat island effect. That is, areas with excessive hard surfaces can experience a temperature spike.

While this effect has been known for some time, not so well understood is the extent that trees, shrubs, lawns and wall gardens can cool places. Estimates by the Co-operative Research Centre for Irrigation Futures, for example, are that they can lower temperatures by 2-8 degrees because increases in evapotranspiration reduce building energy use by 7-47 per cent.

Then there is shading: A Californian study has found that the average electricity saving per tree due to lower air-conditioning use ranges from 70-90 kilowatt hours a year, with savings even greater at peak times reducing overall energy demand by 10 per cent.

Melbourne has a rich assortment of trees, shrubs and grassed areas — a resource now widely known as the urban forest. Importantly, much is on private land — it's not just in public parks and along roads and nature strips — so that even in a densely settled locality like East Melbourne 60 per cent is in street blocks.

This green infrastructure is a legacy of the dedication of generations of gardeners and plenty of rain. As much time and effort and precious water has been invested in the trees alone, they represent an asset that should not be squandered.

Greg Moore, of Burnley Horticultural College, has calculated that the 100,000 mature trees in inner Melbourne sequester 1 million tonnes of carbon, worth close to $20 million. These trees have the capacity to offset the more extreme temperatures and ferocious winds in prospect while helping to mitigate climate change. As such, they are worthy candidates for inclusion in Australia's emissions accounting.

Last month's rainfall provided some relief for the city's drought-stressed specimens; quite a few of the deciduous varieties had seemed reluctant to set leaf this spring. Many still look sickly, even prematurely colouring — especially the elms. Others appear to have come good but some may still throw in the towel in the hot months ahead.

The longer-term outlook is similarly bleak. Extreme winds, in combination with deluges, which weaken root anchoring, may lead to domino tree falls and withering heat may fry plants and have detrimental effects on pollinators.

Although plants could initially benefit from increasing carbon dioxide, they will wilt if average global temperatures rise by more than a few degrees. They could also be hit by plagues of south-moving insect pests and may face severe competition from rampant weedy species.

In the face of this botanical nightmare, we will need to breed plants that are more sympathetic to climate change — extra pest and disease-resistant, fire-retardant, shade-providing but trees less prone to branch fall; vegetation that can better resist erosion in coastal zones subject to sea-level rise and strong storm surges; and an expanded variety of resilient food plants suitable for backyard plots.

The possibility of harsher water restrictions implies that tap water released to the environment for urban gardens is a luxury we can no longer afford. But, might this be a false economy?

The water saved in power stations due to lower demand for air-conditioning in cities because of shade trees may be greater than that saved from zero or drastically reduced irrigation of suburban gardens and public lands.

Equally, the loss of trees (which began to gather pace last autumn) means we are steadily losing an opportunity to lessen the "carbon footprint" of our city — a typical mature tree can store as much as 10 metric tonnes of carbon. Then there are the other environmental benefits they can provide: stormwater retention, dust settling and air pollution absorption.

This sort of greenery also goes some way towards offsetting what Harvard biologist E. O. Wilson — who popularised the idea that we are biologically drawn to natural landscapes — has called our entry into an Eremozoic age, an age of loneliness, isolated from all other living organisms.

A greater commitment is needed to help people maintain and develop suitable gardens to avoid Melbourne becoming a hot, dusty, desiccated landscape in the decades ahead — something that could be hard to reverse given the difficulty of growing back large trees with reduced rainfall.

Certainly, the trend to treeless, hard-surfaced private spaces should not be encouraged where it translates into an overall diminution in an area's green coverage. Regular, systematic audits of the city's tree canopy are well overdue.

Dr Peter Fisher works in the environment industry, specialising in water and climate change adaptation.