Carbon pools and sequestration in forest ecosystems in Britain

British vegetation is estimated to contain 113.8 million tC, 80 per cent of which is in forests and woodlands (91.9 million tC). Sitka spruce plantations, although covering 21.4 per cent of the forest/woodland area, contain only 8.2 per cent of the forest/woodland carbon, because the plantations are young and have an average of only 14.1 tC ha-1. Broadleaved woodlands in Britain have an average of 61.9 tC ha-1 and contain 46.8 per cent of the total carbon in all vegetation. A breakdown is given of the carbon density (tC ha-1) and content of different tree species. A carbon density map of Britain highlights the concentration of carbon in the broadleaved woodlands in southern England and in the large conifer plantations in southern Scotland and northern England. Carbon storage in the trees, products, litter and soil can be evaluated in terms of long-term equilibrium storage or short-term rate of storage. These two components vary among forest types in Britain and globally. Plantations harvested at the time of maximum mean annual increment (MAI) will not store as much carbon as mature, old-growth forests on the same site unless they have long-lasting products and/or are very fast growing. Maximum equilibrium carbon storage is generally achieved by harvesting at the time of maximum MAI when the lifetime of products exceeds the time to maximum MAI. Undisturbed peatlands sequester CO2 and emit CH4, and may be greenhouse neutral. When peatlands are drained and planted with trees, they stop emitting CH4 and store carbon in the trees, forest litter, forest soil and wood products. However, these greenhouse gas 'gains' are offset by the oxidation to CO2 of the peat, and the gains are exceeded by CO2 losses when 20-40 cm depth of peat has been oxidized. Forests in Britain are currently sequestering 1.5-1.7 million tC a-1 in trees, 0.3-0.5 tC a-1 in litter and 0.5 million tC a-1 in wood products, totalling about 2.5 million tC, equivalent to about 1.5 per cent of the carbon currently emitted by burning fossil fuels in the UK. In order to maintain the current forest carbon sink, the forest area needs to continue to expand at about 25 000 ha a-1 of upland conifers or 10 000 ha a-1 of poplars on good land.