Department of Climate Change

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GPO Box 854 Canberra
ACT 2601 Australia
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+61 02 6274 1888
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Key activities

Biomass - plant growth and life cycle analysis

Overview

Plant biomass is a major source and sink of greenhouse gases in the Australian environment. Accumulation and decomposition of biomass can occur over many years - so biomass is estimated annually over a 30 year time period. A range of modelling and mapping techniques are used, providing results at a fine spatial resolution across the continent which are consistent through time. The basic analytical steps for estimating biomass are:

Diagram of biomass estimation

The NCAS targetted specific areas for improvement of data and models:

Implementation

Crop and forest biomass are estimated separately, due to the different nature of models and data.

Crop biomass

Crop biomass is a sink of greenhouse gas, and contributes to the soil carbon pool through decomposition. Crop biomass is estimated through a combination of regional maps of crop types and published crop production statistics.

Crop types are mapped over a 30-year period according to regions based on biogeography, soil type and cropping system. For each region and crop type, data are compiled on yield, the allocation of growth to various crop components (stalks, leaves, grains/fruits and roots), the natural shedding of plant material, land management practices and decomposition - all of which affect both litter and soil carbon pools. The data are mostly derived from published statistics.

Diagram showing crop biomass estimation

Crop biomass estimation: Agricultural yield statistics for
biogeographic/soil regions are used to calculate total biomass for crops and pastures.

Forest biomass

Forest biomass is both a source and a sink of greenhouse gases, and contributes to the soil carbon pool through decomposition. A forest is defined as vegetation with a minimum 20 per cent canopy cover, potentially reaching 2 metres high and a minimum area of 0.2 hectares. Forest biomass is estimated using a combination of forest maps and productivity models.

Forest age and extent is mapped using a time sequence of Landsat satellite imagery at a 25m resolution, for several time periods since 1972. A 'maximum potential biomass' estimate is derived for each 25m land unit from the 3PG productivity model, and applied to a growth formula which takes account of age of forest. The age-based estimate is then partitioned into plant components (stem, branch, bark, leaf and root) using a national vegetation map, before conversion to carbon.

Wood density estimates are used to verify the modelled biomass estimates against volume-based field plot data. Known wood densities are mapped at a broad level, and then used to convert biomass estimates to stem volume estimates. The stem volume estimates are then compared against available field plot data.

The amount of carbon stored in forest litter and the amount of material entering the soil carbon pool are estimated from standard national tables.

Diagram showing forest biomass estimation

Forest biomass estimation: An aged-based productivity index
is used to estimate forest biomass. The productivity index is derived from 3PG, a plant growth model.

Analysis and integration

Crop and forest biomass data are used within the FullCAM modelling framework to provide a 30-year dynamic perspective on the nature and extent of change in land systems across the continent over the period since 1970.

Technical reports

Specific work commissioned by the NCAS on biomass estimation is documented in the following technical reports. Some work was developmental in nature and may have been superceded in the final analysis and integration.