Carbon accounting
How to calculate rates of carbon emissions and sequestration
When governments or regulators get involved in negotiations about greenhouse gas emissions or carbon trading, they rely on scientifically robust, verifiable measurements of the value and volume of carbon produced or sequestered by human activities.
There are three ways in which carbon levels are accounted for in forestry:
- Periodic, direct measurements of carbon in forestry stock
- Inventory-based carbon accounting models
- Direct carbon flux measurements
The best method to use depends mainly on the objectives of the assessment, its geographical scale and the resources available to carry out the evaluation. The most robust and cost-effective carbon stock accounts will typically combine all three approaches alongside remote sensing technology (satellite imagery and aerial photography).
Measurement of carbon levels in forestry stock
The simplest method of assessment looks at how carbon stocks have changed between two points in time. Conventional forest mensuration methods estimate timber volumes, which then convert to dry weight – and hence carbon – using reference tables.
Problems associated with carbon stock accounting:
- Carbon content in non-stem components are calculated using an ‘expansion factor’, also known as a ‘total:merchantable’ ratio, which depends greatly on tree species, stand age, management and environmental conditions
- Leaf biomass, ground vegetation and litter are often ignored
- Carbon content of the soil is seldom included because it is difficult to define and carry out cost-effective assessments of soil carbon
- Small changes in soil carbon are difficult to measure in comparison to total soil carbon
- Carbon stocks or stock changes in harvested wood products may or may not be assessed, depending on the purpose of the inventory
- Accounting for carbon stocks in wood products is impossible for individual stands of trees – the fate of harvested wood cannot be tracked once it has left the forest
- At a district, national or global scale, the size of, and changes to, the wood products pool is only possible using available statistics
An inventory-based approach, particularly if used to assess carbon stocks or sequestration in woody biomass only, can be used to cover large land areas and a variety of species and site conditions.
Carbon accounting models
Several carbon accounting models are applicable to the UK. They use theoretical and empirically derived models of carbon flows through the forestry value chain.
- C-FLOW - developed by CEH (Edinburgh), currently used for national reporting of carbon stock changes associated with forest biomass
- CARBINE – developed my Forest Research, based on forest growth and yield models
- C-SORT – under development, focusing on carbon sequestration in wood products
Carbon flux
Flux-based carbon accounting directly measures the flow of carbon into and out of the forest. State-of-the-art sensors using a technique called eddy correlation to continuously monitor carbon exchange between all the carbon pools in a forest ecosystem and the atmosphere.
Forest Research uses this technique to measure carbon exchange in lowland oak woodland at the Straits Flux Station.
Benefits and drawbacks:
- Net flux measurements – the ecosystem’s entire carbon pool is covered, including dead wood, litter and other fractions that are too difficult to measure using stock-change methods.
- Cost – the equipment is expensive so only a small number of flux stations are currently online (four long-term flux monitoring stations in the UK, 30 sites across Europe)
- Limited site suitability – site topography make flux monitoring difficult at many sites
- Species specificity – the measurements are specific to the species, site and growth stage under investigation, so carbon in the wood products pool associated with a specific stand cannot be estimated
Flux-based calculations are ideal for delivering information on short-term variations in the magnitude of the carbon sink and in quantifying net carbon exchange in forest systems where the individual carbon pools are difficult to measure. Carbon flux studies also provide an essential validation for inventory methods across different forestry systems.