Published: 2009

‘Working trees’ key to urban resilience?

Mary-Lou Considine

They lower ambient temperatures in summer, reduce flooding, filter pollutants, sequester carbon, provide habitat for birds and animals, and add beauty and value to our homes and neighbourhoods. Yet the trees that grace the streets, backyards, parks and fringes of some Australian cities are beginning to disappear due to drought, high-density development and neglect.

Street trees such as these in Eastwood, Sydney, provide valuable services to governments and local communities.

Credit: ANU

In a recent paper, ‘Urban trees: worth more than they cost’, the University of Melbourne’s Dr Greg Moore estimated that over recent years at least 15 per cent of mature trees in metropolitan Melbourne have suffered drought stress, with some already dead and many close to dying.¹

The rate could be accelerating, with the Melbourne Age reporting in August that the City of Melbourne had lost 900 trees from public spaces last year – triple the normal annual loss – with 40 per cent of the remaining trees under stress due to drought and water restrictions. Adelaide has also reported drought-related tree loss.

Dr Moore says trees are ‘mature infrastructure assets’ providing ecological services that have yet to be properly valued and, once lost, would be difficult to replace.

He estimates the cooling effect of 100 000 mature urban trees – inner city Melbourne has around 70 000 – could save a city 3 million kilowatt hours of electricity annually. This represents around 3600 tonnes of saved carbon emissions, not to mention the 300 million litres of water that would have been used to generate that amount of electricity.

In the nation’s capital, the ACT Government last year commissioned a detailed study by the Australian National University on the value of trees in Canberra’s streets, parks and reserves.

The ANU researchers estimated the 2008 value of ecosystem services provided by Canberra’s 26 million square metres of street tree canopy to be $23.5 million – $6 million saved annually in energy and air conditioning costs, $12 million in pollution reduction, and $5.5 million in stormwater mitigation and reduced infrastructure costs.²

ANU researcher Dr Chris McElhinny points out this figure does not take into account the carbon sequestration and storage value of these trees. As it turns out, because they are relatively young and fast-growing, Canberra’s urban trees have a high sequestration rate – around 0.6 tonnes of carbon per hectare per year, compared to 0.07 tonnes of carbon per hectare per year for the mature native vegetation surrounding Canberra.

‘While 95 per cent of the current carbon stock of the ACT is stored in the non-urban estate (land), its contribution to annual carbon sequestration is disproportionately low – around 52 per cent – compared to the urban estate, which contains just 1 per cent of the ACT’s carbon stock, but accounts for around 48 per cent of the sequestration,’ says Dr McElhinny.

The ACT Government will use the information from the ANU study to develop its climate change strategy and tree-planting plans for the future.

Dr Moore says little scientific research work has been done in Australia on the benefits of urban vegetation and there is even less economic data to inform decisions.

He disagrees with the view that urban trees – and lawns and gardens, for that matter – should be left to die in the wake of severe water restrictions.

A City of Melbourne spokesman says the council ‘works closely with water suppliers and the state government to keep trees as healthy as possible’ (Melbourne has been subject to Stage 3a restrictions since 2007). Additional mulching, subsurface irrigation systems and rainwater collection are included in a recent council proposal to save the city’s landmark trees.

The council wants to ensure the long-term health of Melbourne’s tree stocks by maintaining soil moisture levels at a minimum 40 per cent – the scientifically established ‘trigger point’ below which trees suffer severe water stress. At the moment, levels in some areas are as low as 20 per cent.

‘The water used to maintain trees and landscapes during drought and summer is neither wasted nor lost,’ says Dr Moore. ‘It returns real economic and sustainable value in the years ahead.’

¹ Moore GM (2009) Urban trees: worth more than they cost. Proceedings of the Tenth National Street Tree Symposium. (Eds D Lawry, J Gardner and S Smith) pp. 7–14. University of Adelaide/Waite Arboretum, Adelaide.
² Killy P, Brack C, McElhinny C, Cary G and King K (2008) ‘A carbon sequestration audit of vegetation biomass in the ACT’. ACT Government Report.

Published: 2009

Northern water assessment informs development capacity

Helen Beringen

It either doesn’t rain or it pours up north. Such variability is one of the challenges identified in the first consistent, analytical water assessment of northern Australia released from CSIRO’s Water for a Healthy Country Flagship. The work is informing thinking about the development capacity of the region.

Regional coverage of the Northern Australia Sustainable Yields Project.

Credit: CSIRO

Rainfall and runoff variability are just two components of a comprehensive review of water resources for water policy decisions which form part of the assessments undertaken by the Northern Australia Sustainable Yields Project.

Released by Parliamentary Secretary for Water, Dr Mike Kelly, at the RiverSymposium in Brisbane on 21 September, the study resulted from a March 2008 agreement by Council of Australian Governments (COAG) to extend the CSIRO work on sustainable water yields and availability that had been completed in the catchments of the Murray-Darling Basin.

Dr Kelly said the research would be a valuable resource to inform decisions about the conservation and development of northern Australia’s water resources.

‘From Broome in Western Australia to Cairns in Queensland, the Northern Australia Sustainable Yields (NASY) reports provide important information on current and likely future water availability in northern Australia,’ he said.

Despite popular perceptions that northern Australia has a surplus of water, the research found the extremely seasonal climate with continuously high temperatures meant that the landscape was annually water-limited, with little or no rain for three to six months every year, and very high potential evapotranspiration rates.

‘Northern Australia experiences high rainfall during the wet season, with most falling near the coast and with year to year amounts that are highly variable,’ said project leader Dr Richard Cresswell.

‘Runoff follows a similar pattern to rainfall, with most surface flow approaching the estuaries, with potential inland dam sites receiving less and quite variable amounts of water and suffering very high evaporation rates.

‘The very few river reaches that flow year-round are mostly sustained by localised groundwater discharge and have high cultural, social, ecological and developmental value,’ he said.

‘Groundwater may offer potential for increased extractions for development, though the highly dynamic nature of shallow aquifers, which rapidly fill during the wet season and drain through the dry season, means there is little opportunity to increase this groundwater storage and careful management is required where these groundwaters also provide sources for the few perennial rivers.’

Dr Cresswell said future climate predictions for the north suggest that evapotranspiration was likely to increase while rainfall was likely to be similar to historical levels, which were lower than the last decade, particularly in the west.

Speaking to ABC Rural, Joe Ross, Chair of the federal government’s Northern Australia Land and Water Taskforce (charged with finding new development opportunities in Northern Australia), said the CSIRO report confirmed that expectations for food production in the north have been ‘over the top’, and that there would be limited opportunity for broad-scale agriculture in the tropics.

The Taskforce’s submission to the government later this year will take into account the central findings of the CSIRO teams’ work. ‘It covers not only the amounts of water in northern Australia, but also what impact there’ll be on the livelihoods of the community of northern Australia, in particular, one of the main constituents being Indigenous Australians,’ Mr Ross said.

The Northern Australia Sustainable Yields Project is part of the Australian Government’s Northern Australia Water Futures Assessment (NAWFA), a five-year program to develop an enduring knowledge base to inform decisions about conservation and development of northern Australia’s water resources, so that any development proceeds in an ecologically, culturally and economically sustainable manner.

More information:

Dr Richard Cresswell: Richard.cresswell@csiro.au

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