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Carbon in Australia's Forests
Forests both sequester (absorb) and emit (release) greenhouse gases. They sequester carbon dioxide from the atmosphere during plant photosynthesis and emit it, along with other greenhouse gases, by respiration and the decay or burning of plant material. In Australia, forests sequester more carbon than they emit, thereby helping to offset the country's total greenhouse gas emissions.
Carbon storage in native forests
Trees in Australia's native forests hold about 6.56 billion tonnes of carbon in their biomass (trunks, branches, leaves and roots); eucalypt woodlands and open forests contain the most (Figure 1). This carbon store is equivalent to keeping 24 billion tonnes of carbon dioxide out of the atmosphere, or 46 years worth of Australia's total 2005 net emissions from all sources. It is also equivalent to about 3.3% of all the carbon emissions arising from human activities worldwide since 1800.
Australia's forest soils are another important carbon sink: they contain about 5.51 billion tonnes of carbon, equivalent to about 39 years worth of Australia's total 2005 net greenhouse gas emissions from all sources. Casuarina and callitris woodlands contain the highest proportion of soil carbon and tropical eucalypt woodlands the lowest.
Figure 1: Carbon stored in total tree biomass (above aground plus roots) of native forest, by vegetation group, 2004
The amount of carbon held in forests varies over time, affected by temperature and rainfall, species composition, the forest management regime, natural and human-induced disturbances such as fire and harvesting, the incidence of diseases and increases or decreases in forest area. This variation might be small compared to the total store, but it is still significant in yearly carbon accounting.
Australia's commercial native forests, plantations and wood products sequestered a net amount of 56.5 million tonnes of carbon dioxide in 2005, thereby offsetting total greenhouse gas emissions by nearly 10%. Native forests sequestered an equivalent of 5.5% of total emissions, tree plantations took in a further 3.5%, and just under 1% was stored in wood products such as house frames and furniture.
Emissions from land clearing
Conversely, land clearing for agriculture and urban development remains a significant (although declining) contributor to Australia's greenhouse gas emissions. The Intergovernmental Panel on Climate Change estimates that, globally, the land use, land-use change and forestry sector is responsible for about 20% of total carbon dioxide emissions each year, largely because of the emissions caused by the clearing and burning of forests and other vegetation. Australian emissions from land clearing, mainly for agriculture and urban development, declined from an estimated 70 million tonnes of carbon dioxide in 2002 to 53.3 million tonnes in 2005, which was more than 9% of total national greenhouse gas emissions. The rate of land clearing in Australia is likely to continue to decrease, with consequent further reductions in greenhouse gas emissions.
Overall, Australia's land use, land-use change and forestry sector sequestered slightly more greenhouse gases (about 3.2 million tonnes of carbon dioxide) than it emitted in 2005, the only sector to do so (Figure 2).
Figure 2: Australia's net greenhouse emissions, by sector or subsector, 2005
In recent years, policies have been developed to increase the carbon sequestration role of Australian forests. Several states have enacted legislation establishing property rights over carbon sequestered in tree plantations, allowing owners to trade it. In addition, the Australian Government has announced a 'cap and trade' carbon emissions trading scheme, beginning in 2010, which should help promote the nationwide trading of carbon, including that sequestered by forests.
AGO (2005). Australian Methodology for the Estimation of Greenhouse Gas Emissions and Sinks, Australian Greenhouse Office, Canberra.
AGO (2007). National Inventory Report 2005. Volume 1, Australian Greenhouse Office, Canberra. IPCC (2007). Climate Change 2007: Synthesis Report, Intergovernmental Panel on Climate Change, Geneva, Switzerland.
MBAC Consulting (2007). Forestry Tasmania's Carbon Sequestration Position, Forestry Tasmania, Hobart. MIG (2008). Australia's State of the Forests Report 2008, Montreal Process Implementation Group for Australia, Canberra.
MIG (2008). Criterion 5 - Maintenance of forest contribution to global carbon cycles, State of the Forests Report 2008. Montreal Process Implementation Group for Australia, Canberra.
Ranatunga K, Keenan R, Wullschleger S, Post W and Tharp M (2008). Effects of harvest management practices on forest biomass and soil carbon in eucalypt forests in New South Wales, Australia: Simulations with the forest succession model LINKAGES. Forest Ecology and Management (in press).