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).