by Dr Peter Savill
Woodland Heritage is often asked how much carbon dioxide (CO2) might be sequestered, or “locked up”, offset or “fixed” in the wood of trees as they grow.
The accepted international methodology is given in the Draft Guidelines for National Greenhouse Gas Inventories, published in 1994 by the Scientific Assessment Working Group of the Intergovernmental Panel on Climate Change (IPCC), a joint programme of UNEP and the World Meterological Programme. Assumptions for a typical broadleaved woodland in Britain might be:
- the forest grows at an average annual rate of 5 m³/ha/year for 150 years, and thereafter there is no net increase at all, because any additional growth of some trees is offset by the death and decay of others (and consequent release of CO2 into the atmosphere again)
- that one cubic metre of wood, when dry, weighs half a tonne and that the material which makes up wood is approximately 40 per cent carbon
- thus, the average rate of carbon fixation for the first 150 years would be approximately 1 tonne/ha/year (i.e. 40% of 5/2)
As an example, if the woodland occupies an area of 15 hectares it therefore likely to be able to fix (i.e. “offset”) about 15 tonnes of carbon per year for 150 years, or a total of 2250 tonnes. A concern has always been what happens to the fixed carbon once the trees are mature. There are two possibilities:
- If the forest, having built up a biomass of some 750 m³/ha of wood over 150 years – or 150 tonnes of carbon/ha – is left undisturbed, the carbon is likely to remain more or less fixed, with any deaths of trees being offset by the natural establishment of new ones. The new growth and fixation of carbon will balance that released by the decay of dead trees. However, there would be no fixation of additional carbon. To do this, more previously unforested land would have to be planted.
- If the forest is felled, one must remember that the half-life of timber is very short, at about nine months. Most of the fixed carbon returns to the atmosphere quickly because the wood is converted into ephemeral products such as paper and pallets, much of which is subsequently burnt. Only a very small proportion survives in furniture and other high quality products for long periods. To remove fixed carbon permanently from circulation again, it should be stored somewhere where it will not decay, such as redundant coal mines.