Conifer Plantations on Drained Peatlands in Britain: a Net Gain or Loss of Carbon?

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Abstract
It is estimated that British peatlands, excluding lowland fens, contain about 3000 million tonnes of carbon, 76 per cent of which is in deep peats (> 45 cm deep) and 9 per cent of which has been drained and planted with trees. Undisturbed peatlands emit CH 4 but accumulate CO 2 -derived carbon. The net greenhouse effect may be near zero. Peatland drainage virtually stops methane emission and increases CO 2 -carbon loss through aerobic decomposition, but can also increase CO 2 -carbon fixation by the peatland vegetation partly through microbial mineralization of nitrogen, resulting in either a net loss or gain in CO 2 -carbon. Planting conifer forests leads to an accumulation of CO 2 -derived carbon in the trees, wood products, litter and forest soil up to equilibrium values, totalling about 16.7 kg C m −2 for Picea sitchensis, Yield Class 12. Deep and shallow peats in the British uplands contain about 0.47 and 0.80 kg C m −2 per centimetre depth, respectively. Thus, the 16.7 kg C m −2 that is stored by P. sitchensis (Yield Class 12) is equivalent to the carbon stored in about 35.5 cm of deep peat or 20.9 cm of shallow peat. If forests are planted on peats substantially deeper than this, there could be a net loss of CO 2 -carbon in the long term. Scenarios are presented for the time course of CO 2 -carbon gain and loss when peatlands are drained and planted with conifers. If CO 2 loss rates from drained peats are 50–100 g C m −2 a −1 there is likely to be increased carbon storage in the whole system for at least three rotations; but if CO 2 loss rates are 200–300 g C m −2 a −1 increased storage may be restricted to the first rotation, after which there is a net loss of carbon.

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