Blanket mire is the extreme climatic type
of mire and the most oceanic of the whole
European series. It is not only the most extensive type of mire in Britain, but is also
better represented in this country (and Ireland) than in any other part of Europe.
Indeed, few areas in the world show such a spectacular development of
ombrogenous peatland, and many of the best British examples are thus of great
international importance. Blanket mire is confined to the wetter parts of Britain, but
an even distribution of rainfall is more important to its development than total
amount, and the apparent limiting precipitation frequency of 160 wet days (a wet
day = i mm of rain in 24 hours) is probably a meaningful expression of this
relationship with wetness of climate. Effective humidity for plants depends on the
evaporating power of the atmosphere, and the relative coolness and cloudiness of
the oceanic climate are important factors in promoting peat formation.
Blanket mire formation depends also on
a suitable topography and substratum
giving impeded surface drainage and pronounced acidity. It is inhibited by steep
slopes, porous substrata and soil parent materials of high base-status (especially
limestone), but these limitations are increasingly over-ridden as effective wetness
of climate increases. Thus, under the tremendously humid climate of the western
and northern Scottish Highlands and islands, blanket mire mantles all but the
steepest slopes and alpine summits, irrespective of substratum, almost down to
sea-level. While deep deposits are everywhere confined to slopes of less than 15°,
in the very wettest parts of this region, shallow blanket mire occurs on slopes of up
to 30°. Blanket mire is especially characteristic of broad, flat upland watersheds,
but reaches an upper altitudinal limit at about 1070 m in the Cairngorms. It is
extensive in most moorland areas wheifc gentle slopes and large plateaux prevail,
as on Dartmoor, some of the Welsh mountains (especially in the east and south),
the Pennines, Cheviots, Southern Uplands and eastern Highlands. In the south,
blanket mire occurs mainly above 300 m, but shows altitudinal descent with
distance north. Types intermediate between raised and blanket mire occur in
northern England and southern Scotland at 90-300 m, and blanket mire reaches its
most extensive development in
Britain on the great flow country of east
Sutherland and Caithness, where a vast
area (approximately 2500 km2) of plain and gently sloping terrain from o to 500 m
is covered by this type of peatland.
Two major types of blanket mire vegetation
can be recognised each with various
facies. One of these is the predominant type in western situations generally below
460 m and it is usually this vegetation type which intergrades with that of raised
mires. This western blanket mire is characterised by dominance of Trichophorum
spp., Molinia spp. and Eriophorum spp., together with abundance of pools and
Sphagnum-dominated hollows. The second is essentially an upland type, often
dominated by Calluna and E. vaginatum, which occurs mainly above 300 m and is
best developed on the Pennines and eastern uplands of Scotland. Although these
two types can be separated on a broad geographical basis it is also apparent that
in some districts local variations in topography can cause both types to occur in
close proximity. An example is the northern Pennines where the main blanket-mire
vegetation is Calluneto-Eriophoretum. In this area localised pockets of mire, closely
comparable with the western Trichophoreto-Eriophoretum, are found wherever
blanket mire has developed on flat areas such as the hillside terraces on Yoredale
sandstones. Other good examples occur in Y Berwyn of north Wales, and in east
Sutherland this juxtaposition of the two types is associated with two major
morphological mire types, flat valley-side flows and convex watershed flows.
There is strong evidence that closely
similar vegetation types can result from the
interaction between climatic and topographic conditions in different regions. For
example the type of vegetation developed on level blanket mire in parts of the
northern Pennines under moderate rainfall (152 cm per annum) is closely
comparable with that developed on distinctly sloping blanket mire under a higher
rainfall regime (230 cm) in south-west Scotland, whilst flat-lying blanket mire in the
latter area has closer affinities with the oceanic blanket mires of north-west
Scotland. At present there is insufficient information to make a critical appraisal of
the relationship between climate, topography and variations in blanket mire
vegetation. A more detailed investigation is required.
The western Trichophomm-Molinia-Eriophorum
mire is especially well developed in
Scotland but facies of this complex also occur in northern England, Wales and
Dartmoor. In Scotland this type covers all gently sloping blanket mire in the west up
to about 460 m. The flattest and wettest surfaces have a characteristic linear
pattern of pools and hummocks aligned parallel to the surface contours. Pools
range from deep (20-50 cm), bare types containing only algae and open growths of
Menyanthes trifoliata and Eriophorum angustifolium, to others showing varying
degrees of infilling by Sphagnum spp. mainly S. cuspidatum and S. subsecundum
var. auriculatum, often with Eleocharis multi-caulis and Carex limosa. Lawn and low
hummock communities are dominated by S. papillosum (contrasting with the
dominance of S. pulchrum and S. magellanicum in the northern raised mires) and
taller hummocks of S. rubellum are usually abundant. 5. imbricatum probably attains
its greatest abundance on these surfaces, typically forming tall domed hummocks.
The vascular plants are Molinia caerulea, Trichophorum cespitosum, Narthecium
ossifragum, Erica tetralix, Eriophorum angustifolium, E. vaginatum, Drosera
anglica, D. rotundifolia, Myrica gale and Rhynchospora alba. Certain species are
especially characteristic of the northwestern blanket mire such as S. plumulosum,
Campylopus atrovirens and the oceanic liverwort Pleurozia purpurea. Others which
have a wider distribution in Britain are usually associated with higher nutrient status,
e.g. Carex limosa, Eleocharis multicaulis, Melampyrum pratense and Schoenus
nigricans. Betula nana is a frequent component of these mires in northern Scotland
and both Andromeda polifolia and Vaccinium oxycoccus occur in southern districts.
A conspicuous feature of tall hummocks and drying surfaces more generally in the
western blanket mire is the occurrence of dense crowns or patches of the moss
Rhaco-mitrium lanuginosum. As well as constituting a major element of actively
growing mires in the north-west this moss characterises degenerating pool and
hummock systems on the blanket mire in many southern districts including the flat
spurs of Y Berwyn (north Wales) and extensive areas on Dartmoor.
The second blanket mire vegetation complex
is best developed on upland and
eastern plateaux generally above 300 m. The typical community has few pools or
hollows and Sphagnum spp. are mainly of the hummock-forming group (S.
papillosum, S. rubellum, S. capillaceum and S. fuscum). The vascular plant
dominants are usually Calluna and Eriophorum vaginatum whilst the most
characteristic species is the montane Rubus chamaemorus. Molinia caerulea is
normally absent, and Trichophorum cespitosum shows a patchy or local
abundance. Where extensive level areas with a permanently high water table occur
there are well-developed Sphagnum communities with obvious similarity to those of
raised mires and western blanket mire. Transitional types with pronounced pool
systems occur locally, especially in northern Scotland where they form the
watershed flows of Caithness and Sutherland.
Several distinctive facies of this Calluneto-Eriophoretum
can be recognised. A
Sphagnum-rich facies occurs on flat-lying blanket mire which has not been subject
to erosion, and this is especially well represented in the moorlands of the Scottish
Borders. The more typical and most widespread community (perhaps produced
partly by human influence) has a lower cover of Sphagnum, more Vaccinium
myrtillus and V. vitis-idaea, and a greater abundance of hypnaceous mosses, such
as Pleurozium schreberi, Hylo-comium splendens, Plagiothecium undulatum and
Rhytidia-delphus loreus. In some districts, notably parts of the Pennines, heavy
sheep-grazing and repeated burning have not only severely reduced Sphagnum
cover but have also locally eradicated Calluna, to give Eriophorum vaginatum-
dominated blanket mires. In the Peak District, atmospheric pollution has eliminated
Sphagnum and may possibly be involved in the characteristic replacement here of
Calluna by luxuriant Empetrum nigrum.
From their topographic position, the upland
plateau blanket mires are especially
vulnerable to marginal drying and erosion, and the majority show some degree of
erosion by gullying, which may spread to dissect the whole of a mire surface, and
often develops into sheet erosion, involving complete destruction of the whole peat
cover. Such peat wastage leads to the exposure of tree remains which have lain
preserved in the peat for up to thousands of years, and also causes a marked
drying of the adjoining peat surface. This drying is reflected by an increase in dwarf
shrubs (especially Empetrum nigrum and Vaccinium myrtillus) and locally of
Rhacomitrium lanuginosum and/or lichens. However, in the Scottish Highlands
(especially the east) there are distinctive facies of undisturbed blanket mire rich in
both dwarf shrubs and lichens of the Cladonia sylvatica group. The dwarf shrubs
here include montane species such as Betula nana, Arctostaphylos uva-ursi,
Arctous alpinus, V. uliginosum and E. hermaphroditum (which replaces Calluna at
high levels). Finally, in northern Scotland, particularly Shetland, an unusual facies
includes Erica cinerea and Carex binervis.
Although blanket mires are not floristically
rich, their vegetation is thus diverse, and
the wide range of surface structure is a most interesting feature. The patterned
mires of western and northern Scotland are regarded as especially important
scientifically. Yet despite the great extent of blanket mire in Britain, few areas
appear to have escaped some degree of modification by man. Those which have -
perhaps largely by chance - are outstandingly valuable. Damage has been caused
mainly by heavy grazing and burning, but during the last 30 years subsidised hill
farm improvement has resulted in extensive draining of blanket mires. Commercial
afforestation has destroyed many other areas, though in places where particularly
wet areas have been left undisturbed among the new forests, they now have a
higher degree of protection than before, as in the Kielder Forest area of northern
England. Large-scale working of the blanket mire peat as a source of power, on the
great flows of Caithness, was abandoned as uneconomic in this country. However,
the steady destruction of both raised and blanket mires for this purpose in Ireland
enhances the conservation value of the comparable examples remaining in Britain.
The more distinctive types of blanket
mire association are listed in the
classification of upland vegetation, p. 297.
