Carbon accounting

 

Any framework for negotiating emissions reductions or trading carbon credits must be on the basis of scientifically robust, verifiable measurements. There are two principal ways by which the sizes of carbon pools or rates of carbon sequestration are commonly measured.

Measurement of stock change by periodic inventory

The simplest method of assessment involves measuring the difference in carbon stocks between two points in time. Conventional forest mensuration methods are used either to measure or model timber volumes, which are then converted to dry weight by reference to tables of wood specific density, typically in the range of 0.33 to 0.45 t/m3 for softwoods and 0.49 to 0.56 t/m3 for hardwoods in the UK. The carbon content (typically 0.5 tC/t) is then used to convert dry weight to carbon. Estimates derived in this way represent quantities of carbon in the stemwood of trees, either standing or harvested as appropriate. In order to account for carbon in non-stem components as well as stemwood, the estimates are increased by a factor known as a ‘total:merchantable’ ratio or ‘expansion factor’. The value of this factor depends greatly on tree species, stand age, management and environmental conditions. The range of values published for mature trees varies between 1.3 and 1.8.  

In this inventory-based accounting system, leaf biomass, ground vegetation and litter are often not included. The carbon content of the soil, although of great importance, has seldom been included because of difficulties in defining and carrying out cost-effective assessments of soil carbon. Moreover, stock changes that may be small in comparison to total soil carbon stocks are difficult to identify, particularly when uncertainties associated with the measurements are considered. An alternative method to account for changes in soil carbon is to combine inventories of carbon in forest vegetation with estimates of soil carbon produced by models of soil carbon dynamics. Depending on the purpose of the inventory, carbon stocks or stock changes in harvested wood products may or may not be assessed. Accounting for carbon stocks in wood products is impossible for individual stands of trees because of uncertainty over the long-term fate of harvested wood once it has left the forest. At a district, national or global scale, some assessment of the size of, and changes to, the wood products pool may be possible by reference to relevant industrial and domestic statistics.

Inventory-based carbon accounting models

Carbon accounting models applicable to the UK are available, including C-FLOW, CARBINE and C-SORT. C-FLOW was developed by CEH (Edinburgh) and is currently used for national reporting of carbon stock changes associated with forest biomass. CARBINE and C-SORT have been developed by Forest Research, independently, and based on forest growth and yield models, with C-SORT particularly strong in its treatment of wood products, although still under development.

Measurement of flux

An alternative method of assessment is known as the flux-based approach. This approach measures directly the net flow of carbon into or out of the forest. Technology has been developed so that it is now possible to continuously monitor carbon exchange between all the carbon pools in a forest ecosystem and the atmosphere using a measurement technique known as eddy correlation. This approach has been adopted for the measurement of carbon exchange of lowland oak woodland at the Straits Flux Station, which is operated by Forest Research. The advantage of the flux-based approach is that a net ecosystem flux is measured, accounting for all carbon pools, including dead wood and litter and other fractions which prove difficult to measure using stock-change methods. The major drawback of the approach is its cost, and thus the small number of flux stations that have been established to date. In the UK, there are only four long-term flux monitoring stations, while across Europe, there are more than thirty such sites. The choice of site is also limited by area and topography and the measurements are only representative of the species, site and growth stage under investigation. As with the stock-change approach, carbon in the wood products pool associated with a specific stand cannot be accounted for.

Choice of approach to carbon accounting

The stock-change and flux-based approaches are best applied at different scales of space and time. The best method to use depends mainly on the objective of the assessments being made and, crucially, on the geographical scale that needs to be considered and on the resources available. 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. This approach could also be based upon existing forest inventory networks. The flux-based approach works best at providing information on short-term variations in the magnitude of the carbon sink and in quantifying net carbon exchange in forest systems where the individual pools are difficult to measure. A further role is to provide data for the development and calibration of process-based forest growth models. However, the most important role of flux-based assessments is to provide a cross-check for inventory methods across different forestry systems and to provide essential information on how the environment, particularly climate, may modify rates of carbon exchange. A combined approach using the methods described above, together with remote sensing technology (satellite imagery and aerial photography), probably represents the most robust and cost-effective way to monitor carbon stocks and stock changes.