Blanket Bog

Blanket bog is a wet peatland habitat that dominates much of upland Britain. Although this is a globally restricted habitat type, confined to oceanic climates, it is one of the most extensive remaining semi-natural habitats in the UK. Examples can be found from Devon in south-west England to the Shetland Isles in the north. These bogs are of international importance, forming ‘type’ examples for this globally-restricted habitat.

Blanket bog is an open habitat almost entirely restricted to the uplands in England and Wales, but which descends to sea level in parts of Scotland. It is characteristically underlain by an expansive ‘blanket’ layer of peat. This develops because the climate is sufficiently cool and damp to allow peat-forming plants to grow – the litter of which decomposes very slowly under the permanently water-logged conditions and gradually accumulates into a layer of peat. The peat depth and time over which it has accumulated are very variable – usually it is between 0.5–3 m thick and dates back 5–6,000 years. The main causes of the spread of blanket bog are debated – although in some areas this initiated following clearance of the original forest cover by man, this co-incided with a general natural cooling in climate conditions.

Within upland ‘blanket mire’ landscapes, a wide variety of hydrological and geochemical conditions can be found. Blanket bogs are fed only by rainwater, i.e. they are ombrotrophic mires. Elsewhere, other habitats occur including transitional mires and quaking bogs; minerotrophic (groundwater-fed) poor-fen, flush or swamp; a range of oligotrophic water bodies (whose catchment is largely or entirely blanket bog); relatively small areas of heath and grassland on better drained slopes; and numerous streams and rivers which drain blanket mire landscapes.

The challenging blanket mire environment offers a competitive advantage to the relatively limited range of species which are adapted to such conditions. However, natural bogs characteristically possess a surface pattern or microtopography of hummocks, ridges, hollows or pools that provide a distinctive range of habitats for three broad groups of plants to exploit:

• The main bog species – which dominate the wetter ridges in various combinations and largely create the small-scale topography of the bog surface – and include various Sphagnum bog mosses, common/hare's tail cotton-grass Eriophorum angustifolium/vaginatum, cross-leaved heath Erica tetralix, and deer-grass Trichophorum cespitosum;
• Dry-humid heath species – such as bilberry/blaeberry Vaccinium myrtillus, crowberry Empetrum nigrum, heather Calluna vulgaris, and round-leaved sundew Drosera rotundifolia that maintain a presence on hummocks and drier ridges – and outlier plants of small-sedge fen – such as bogbean Menyanthes trifoliata, bog sedge Carex limosa, and many-stalked spike-rush Eleocharis multicaulist that compete with some success in hollows and pools;
• Regional, blanket bog specialists – including cloudberry Rubus chamaemorus that is mostly confined to high altitude bogs; alpine bearberry Arctostaphylos alpinus on northern bogs; black bog-rush Schoenus nigricans as an ombrotrophic species on western bogs; and the woolly hair moss Racomitrium lanuginosum which largely replaces the role of Sphagnum bog mosses in the north and west and particularly on the Scottish Western Isles.

These combine in various forms to create a suite of specialised plant communities, including:

• bog pool communities characterised by the mosses Sphagnum auriculatum/cuspidatum/recurvum or common cottongrass;
• wet heath communities characterised by deergrass and cross-leaved heath;
• blanket, raised and valley mire communities, variously characterised by beaked sedge Carex rostrata, bog asphodel Narthecium ossifragum, cross-leaved heath, deergrass, hare's tail cottongrass, heather, purple moor grass Molinia caerulea, tormentil Potentilla erecta, or the mosses Calliergon cuspidatum/giganteum and Sphagnum papilliosum/recurvum/squarrosum/warnstorfii;
• soakway communities characterised by marsh St John's wort Hypericum elodes and bog pondweed Potamogeton polygonifolius
• tall herb fen communities characterised by beaked sedge and marsh cinquefoil Potentilla palustre.

Blanket bogs support a distinctive and diverse array of terrestrial and aquatic animals. These include:

• important breeding wader assemblages of golden plover, dunlin, and greenshank;
• other unusual breeding birds, such as the red-necked phalarope and red-throated diver;
• charismatic bird predators, such as the short-eared owl, merlin and hen harrier;
• dragonflies and damselflies (e.g. the rare blue hawker Aeshna caerulea);
• various diving beetles (e.g. the rare relict Oreodites alpinus and rare caddis larva Nemotaulius punctatolineatus);
• butterflies (including the large and small heath Coenonympha tullia, C. pamphillu);
• many moth species (from the large emperor moth Saturnia pavonia to a numerous micro-moth species);
• an array of spiders (including the large, bog raft spider Dolomedes fimbriatus).