Growth and degradation of British and Irish blanket mires

Blanket mires extend over approximately 22,500 km 2 of the north and west of the British Isles, and are of both national and international importance. Blanket peat started to form locally in some regions 9400 years ago, but the period of most active spread was between 5100 and 3100 BP. Clearance of forest and scrub by human activity probably triggered peat formation in many areas, though the influence of longer term changes in climate can be detected both in the times of peat initiation and in subsequent peat growth. High-quality blanket mires can be characterized by the presence of distinctive plant communities, well-defined microtopographic differentiation of the mire surface, and a diplotelmic structure to the peat mass. Many blanket mires, however, currently have a modified, damaged, or eroded vegetation cover. British and Irish blanket mires are used for a variety of purposes: year-round grazing (mainly sheep), game stocking (red grouse and red deer), water catchment, plantation forestry, domestic and commercial peat extraction, military training, recreation, and conservation. The associated management practices can have significant effects on the composition and structure of the vegetation, the bird and invertebrate faunas, and the physical and chemical properties of the underlying peat. A major part of the British blanket mire resource is affected and modified by grazing, burning, and drainage; only approximately 4000 km 2 may remain in a near-natural state. An estimated 3500 km 2 is afforested and a further 3500 km 2 is eroded. Atmospheric pollution has been shown to have had significant effects on the blanket mire vegetation only in the Southern Pennines, but substantial areas in Wales, northern England, and southwestern Scotland are potentially at risk from high levels of sulphur and nitrogen deposition. The peat mass is very susceptible to erosion from natural agents: frost, drought, rain, and wind. However, erosion normally occurs only when the plant cover is damaged. Severely eroded peat is found in all blanket mire regions of the British Isles and is most common above 450 to 500 m altitude, where it is associated with the development of gully systems within the peat mass. Erosion has almost certainly increased in intensity in the last 200 years, as land-use pressures have intensified, but there is evidence from a number of regions that active gully systems were present by at least AD 1500-1700. Damage and erosion of the blanket mire vegetation has considerable economic and amenity consequences: loss of grazing land, infilling of reservoirs with sediment and periodic discolouration of water supplies, loss of distinctive wildlife (particularly birds), and impaired recreational value. Techniques are now available for the revegetation and restoration of badly damaged blanket mire, but they are very costly. Government schemes that provide incentives for less-intensive management of blanket mires offer a potentially more viable way of meeting our international conservation obligations.Key words: blanket bog, peat, grazing, pollution, burning, afforestation.