In 2009, approximately 7% of Australia’s annual net greenhouse gas emissions reported under the Kyoto Protocol were from the clearing of forest land, and the subsequent release of carbon stored in trees and soil. The quantity of emissions released from the clearing of a particular unit of land depends on the size and density of trees and other vegetation on the site – also known as aboveground biomass.

Forest land is defined under the Kyoto Protocol by Australia as all land of at least 0.2 hectares with a tree height of at least 2 metres and crown canopy cover of 20% or more. This definition covers land as diverse as the thick rainforests of Cape York, the tall Mountain Ash forests of the Victorian highlands, and the widespread open eucalyptus woodlands of the inland. Land clearing in each of these forest types has a significantly different carbon impact.

In order to better understand the contribution of clearing in different forest types to Australia’s total deforestation emissions, estimates of aboveground biomass prior to clearing were generated for all lands in Australia’s Kyoto Protocol deforestation account. The Kyoto deforestation account consists of all areas of land that were forest on 1 January 1990 and have been subsequently cleared through direct human action.

land clearing – an assessment of biomass density

Australia’s national inventory uses a combination of satellite measurements and modelling to estimate emissions from land clearing. The satellite measurements identify the location of land clearing events at a resolution of approximately 25 metres, and biomass modelling (together with the model FullCAM) is used to estimate the age and type of vegetation prior to clearing from data on past land management decisions, soil type and local climate. Emissions from the clearing of vegetation on each unit of land can then be estimated.

The maps in Figures 1a and 1b on the following page display land clearing activity between 1990 and 2009 and estimated plant productivity across Australia. As illustrated, during this period land clearing occurred predominantly in woodland areas of inland Queensland and New South Wales, where plant productivity and forest growth are relatively low.

These data were used with information from the 2009 National Inventory Report (2011 submission) to generate modelled estimates of the initial aboveground biomass density prior to clearing in areas across Australia’s Kyoto deforestation account. The results represent an estimate of the long-term average aboveground biomass density that was present in these areas prior to the first clearing event — this can then be used to assist in the estimation of net emissions or carbon stocks.

australian national greenhouse accountsLand sector reporting

1 AUSTRALIAN NATIONAL GREENHOUSE ACCOUNTS Land clearing – an assessment of biomass density

2 AUSTRALIAN NATIONAL GREENHOUSE ACCOUNTS Land clearing – an assessment of biomass density

However, the distribution also has a long tail extending to much higher values, as well as a ‘bulge’ around 100 to 160 t dm / ha. This is due to clearing events detected in high-productivity areas, where the model predicts higher initial aboveground biomass densities. The lowest value in the distribution is 7 t dm / ha, and the highest value is 647 t dm / ha. The model does not predict any initial aboveground biomass densities outside this range for these areas.

The mean of the distribution occurs at 80 t dm / ha. This represents the modelled average aboveground biomass density prior to first-time clearing in areas within the Kyoto deforestation account. The median is 59 t dm / ha, which is lower than the mean due to the effect of relatively infrequent units of land with high modelled initial biomass densities.

Biomass of regrowing forests

These modelling estimates can be used to assist in the calculation of emissions from land clearing activity, and carbon stocks in forested areas in Australia.

However, this requires additional assumptions regarding belowground biomass densities (tree roots), soil carbon, as well as biomass densities in regenerated forest — the Kyoto deforestation account includes some areas of previously cleared forest, which had regenerated by 1 January 1990 and were cleared again later. Emissions from re-clearing of regenerated forests are typically lower than from first-time clearing, since regenerated forests may not yet have reached their long-term average biomass densities. In the National Inventory Report, these effects of regrowing forests are modelled using FullCAM.

Modelling results

In Figure 2, a frequency distribution of the modelling results is displayed, expressed as the area in the Kyoto deforestation account (in thousands of hectares) at every value of the modelled initial aboveground biomass density (in tonnes of dry matter per hectare, t dm / ha).

As demonstrated, the modelled estimates cover a large range of values. Most estimates are quite low, between around 25 and 85 t dm / ha. This is because most land clearing since 1990 has occurred in areas of relatively sparse forest cover.

Figure 1a:Location (in red) of land clearing events detected between 1990 and 2009, which are included in the Kyoto deforestation account.

Source: National Inventory Report 2009, vol. 3, p. 62.

Figure 1b:Map of the forest productivity index across Australia, which measures plant productivity and forest growth at each location based on variables such as rainfall, temperature and soil type.

Source: Derived from data in the National Inventory Report 2009, described in vol. 2, appendix 7.A. For more information on the forest productivity index, see: Kesteven, J. et al (2004), Developing a National Forest Productivity Model, NCAS Technical Report No. 23, Australian Greenhouse Office.

Figure 2:Distribution of modelled initial aboveground biomass density across Australia’s Kyoto deforestation account.

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Modelled initial aboveground biomass density (t dm / ha)

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