Page 3947 - Week 11 - Tuesday, 15 Sept 2009
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most effective absorbers, whereas older trees absorb carbon more slowly but in ever-increasing amounts.
The urban forest should not just be looked at in terms of its ability to sequester carbon, as the report states that the urban forest provides many benefits beyond sequestration. Indeed, urban trees reduce air pollution, they reduce the amount of stormwater runoff and they provide cooling to our homes, our offices and our buildings—indeed, the whole microclimate of our suburbs. The report projects that the cooling properties of urban trees provided a reduction in energy use valued at $23 million in 2008. In a city where the vast majority of energy use is in the heating and cooling of buildings, this is a very important issue.
The report also looked at the effects of the 2003 bushfires. The 2003 fire emitted 2.8 million tonnes of carbon, or about 11 per cent of the estimated carbon stock. The report highlights that, because of that fire, it may take 100 years to reach 95 per cent of pre-fire carbon stocks.
The report also looks specifically at the arboretum. This has been the focus of much discussion by other members. The report showed that the arboretum will produce carbon up to 2015 as a consequence of removing the pines. But then it will become a net sequesterer, peaking at 800 tonnes in 2025 and absorbing carbon for around 200 years, becoming a 70,000-tonne carbon sink. This is, by the report’s own admission, based on conservative growth assumptions in relation to the planting of the remaining 200 hectares at the arboretum site due to trees being widely spaced and some species being imperfectly suited to our environment.
It is further noted that the pines removed from the arboretum site are too immature to be used as wood products and will, therefore, be mulched to reduce weed infestation and help conserve water at other locations. To not do so would require additional mulch being purchased if the pines were not chipped.
The report also found that the practice of wood chipping or mulching trees removed from the urban forest promotes the decay of timber, resulting in a faster rate of emission. However, it should be noted that mulching, of course, reduces water consumption and weed infestation; so it does have other benefits that need to be carefully balanced. This one example shows the complexity of the policy challenge that is brought to the fore with climate change. As a government, as policy makers, we are forever trying to bring together the various elements to ensure that we are a good land manager as well as responsible in trying to keep our carbon emissions at a minimum.
Staff of Parks, Conservation and Lands are looking at additional research work to further measure and analyse the carbon sequestration potential of trees in rebuilding our urban forest, to make sure we influence the optimal age-class structure of trees that we plant, so that the urban forest has a large proportion of trees in a semi-mature state, compared to our existing assemblage of over-mature trees. These are difficult policy issues and only through a mature debate within government, with our research partners and with the community and by analysing emerging areas of research will we be able to make sustainable policy decisions.
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