Raised mires occur on more or less level
flood plains of mature river systems, and
more especially on alluvial deposits of estuaries, where they owe their origin to the
absence of slope which first favoured extensive development of topo-genous mire
(i.e. open water transition or flood-plain mire). With upward growth, and possibly
influenced by changing climatic conditions, the original mesotrophic or eutrophic
mire vegetation became increasingly isolated from and independent of ground
water influence, with the result that vegetation of truly ombrogenous character
became established and has persisted. Some of the most extensive British raised
mires cover an area of 20 km2 or more (e.g. Flanders Moss, Perthshire; Lochar
Moss, Dumfries-shire; Thorne Waste, Yorkshire), though the best active examples
now are much smaller. The mire expanse is typically bounded by clearly defined
features including a drier, steeply sloping margin (rand) and an adjacent stream
course (lagg) with richer water which limits the lateral extension of acido-philous
communities. Although raised mires are characterised by a convex profile the
surface gradient from the centre to the edges is very low (often less than i m in 0.5
km) so that run-off rates are low and these mires do not depend on such a high
precipitation/evaporation ratio as blanket mires. One of the most extensive raised
mires, Thorne Waste, has developed in an area of very low rainfall (58 cm per
annum) and remnants of raised mires are known from other low rainfall areas, e.g.
Huntingdonshire and Norfolk, so that rainfall does not appear to be limiting in
Britain.
Raised mires have a very localised distribution
in Britain, reflecting a dependence
on suitable geomorphological conditions for their formation. The majority occur on
coastal lowlands especially adjoining large estuaries, but some occur in areas of
suitable topography, usually broad river valleys, far inland. Important areas of
coastal raised mires include the Somerset Levels, estuaries around Cardigan Bay
and Morecambe Bay, the Vale of Trent, the Solway, the upper Forth valley and
isolated situations along the western seaboard of Scotland. In all cases these mires
lie below 15 m. The inland raised mires are less numerous and vary considerably in
altitude. Some of the largest examples are in Wales and the Welsh border land
(e.g. Fenns Moss 90 m, Cors Goch glan Teifi 170 m and Rhos Goch 270 m) whilst
smaller examples occur at higher altitudes elsewhere, e.g. Malham Tarn Moss at
380 m in the Craven Pennines. Raised mire is increasingly replaced towards the
north-west of Britain by blanket mire, which smothers plain and slope alike, and in
some northern areas where morphological features are not clear, the distinctions
between blanket and raised mires become somewhat tenuous with intermediate
types occurring frequently. This is especially true of Southern Scotland where
discrete raised mires occur on flat ground at low altitudes, but similar mires merge
into the surrounding blanket mire at higher levels (i.e. above 60 m). The
ombrogenous mires of such areas may be viewed as a continuous series, the
components of which vary according to the relative importance of climatic and
topographic factors in any one situation.
Raised mire vegetation varies considerably
throughout Britain. The main
differences correspond to the type and degree of surface relief developed on any
particular site and the relative height of the water table. Climate is the most
important single factor determining type of surface micro-topography, but the size
of any single raised mire also seems to be important. The usual type of raised mire
in lowland Britain has a large scale but shallow hummock-hollow mosaic within a
predominantly Spagnum-covered surface (e.g. Cors Fochno, Cardiganshire, and
Glasson Moss, Cumberland). These are extensive mires over a kilometre in
diameter developed in areas of moderate rainfall (76-114 cm per annum). Smaller
raised mires developed in similar climatic conditions often have a more
pronounced surface
microtopography with steep-sided pools
set in a relatively dry mire surface which is
dominated by Calluna or Erica tetralix (e.g. Rhos Goch, Radnor; Wem Moss,
Shropshire). Such features are accentuated in areas of low and infrequent rainfall
(Penmanshiel Moor, Berwickshire). At the other extreme are the oceanic raised
mires of western Scotland (e.g. Claish Moss, Argyll) with very pronounced surface
patterns consisting of extensive shallow pools separated by narrow waterlogged
ridges, the vegetation being relatively hydrophilous compared with that of more
southerly sites. This type occurs in areas of high rainfall (150-230 cm with over 200
wet days per annum).
In addition to this variation in surface
morphology there are broad geographical
differences in floristics such as the abundance of Rhacomitrium lanuginosum,
Pleurozia purpurea and Carex limosa in northern oceanic situations and
dominance of Vaccinium oxycoccus and Andromeda polifolia in more southerly
sites. Probably no plant species is confined to British raised mires, though the rare
moss Dicranum bergeri is a characteristic species. Communities of A. polifolia
with Sphagnum magellanicum and Rhynchospora alba with S. pulchrum are best
developed in raised mire, though all four species are abundant in other types. The
northern oceanic raised mires of Europe are characterised by an association of R.
fusca and S. pulchrum, but this is rare in Britain.
Marginal communities such as Myrica-Molinia
mire and Betula-Carex carr form an
integral part of raised mire systems. Raised mires differ from valley, basin and
blanket mires in that large-scale soligenous water tracks and associated
communities do not occur on the central mire surface, though these mires have
numerous small water tracks between adjacent pool systems, which are best
developed on the oceanic raised mires with a pronounced surface pattern such as
Claish Moss.
In the more normal development of raised
mire in Britain the mire expanse is
typically covered with a Sphagnum-dominated vegetation showing a gently
undulating surface with a large-scale hummock and hollow pattern, usually of small
vertical amplitude. The hollows seldom contain open water and are usually spongy
carpets of Sphagnum spp. which need constant saturation, such as S. cuspidatum
and S. pulchrum, which grade into flat 'lawns' and then low hummocks, through the
appearance and increase of other species, notably S. magellanicum. Vascular
plants typical of the wettest Sphagnum communities include Eriophorum angusti-
folium, Rhynchospora alba, Drosera anglica, D. rotundifolia and Narthecium
ossifragum, whilst Andromeda polifolia and Vaccinium oxycoccus are often
abundant on lawns and low hummocks, and undisturbed Sphagnetum contains a
variety of small leafy liverworts. Upward growth of the low hummocks is associated
with the appearance of S. rubellum, and a wider range of vascular plants such as
Eriophorum vaginatum, Trichophorum cespitosum, Calluna vulgaris and Erica
tetralix, which increase in cover and luxuriance with height above the water table.
Some species have a wide vertical range in the hollow and hummock system, but
the tallest and driest hummocks are quite different from the hollows and lawns in
appearance - Sphagnum spp. are largely replaced by tussocky growths of
Eriophorum vaginatum and Trichophorum with small bushes of Calluna, and there is
often an abundance of heathland mosses and lichens, such as Pleurozium
schreberi, Cladonia impexa and C. sylvatica.
The more sloping and therefore better
drained margin (rand) of these raised mires
has a drier vegetation resembling that of the central surface hummocks, with the
vascular plants more important than Sphagnum spp., and often an abundance of
species such as Myrica gale and Molinia caerulea. Various forms of human
disturbance (e.g. peat-cutting, burning) which lower the water table also reduce
Sphagnum cover although S. compaction and 5. tenellum may be favoured by
conditions following a fire. Drying of the surface promotes increased growth and
tussock or bush formation in the vascular plants, which increase to dominance. The
vegetation then comes to resemble wet heath, or even, when drying is severe, dry
heath. The marginal rand where the water table is usually lower than in the mire
centre is particularly susceptible to this disturbance and drying. Old peat-cuttings,
characteristic of raised mire edges, are often recolonised to form pockets of
Sphagnetum with other hydrophilous mire plants, but are typically dominated by S.
recurvum, which seldom occurs in undisturbed central mire communities. The
original lagg bounding the outer edge of a raised mire has usually been destroyed
or completely modified in British examples. Where it remains, there is usually a
heterogeneous vegetation ranging from wet Molinia grassland or poor-fen to birch-
willow carr.
On the oceanic raised mires along the
western Scottish seaboard the vegetation is
quite distinct and more akin to that of western blanket mire. The mire expanse has
a pronounced surface pattern of linear pools and ridges aligned parallel to the
surface contours. The pools usually hold open water and there is a greater vertical
amplitude between hollow and hummock than on the more southerly raised mires.
The pools range from deep, bare types containing only algae and open growths of
Menyanthes trifoliata, Eriophorum angustifolium and Carex limosa to others
showing varying degrees of infilling by Sphagnum spp. mainly S. cuspidatum and
S. subsecundum var. auriculatum. Lawn and low hummock communities are
dominated by S. magellanicum, S. papillosum and S. pulchrum whilst the taller
hummocks are composed of S. rubellum and Rhacomitrium lanuginosum. The
oceanic liverwort Pleurozia purpurea is constantly present and the moss
Campylopus atrovirens often occurs in association with algae-covered peat
surfaces. The vascular plants of the lawns and hummocks are similar to those of
more southerly raised mires but Molinia caerulea is abundant whilst Vaccinium
oxycoccus and Andromeda polifolia are absent. A feature of these northern raised
mires is the abundance of Rhynchospora fusca and Eleocharis multicaulis in
shallow pools. The rand is largely dominated by Molinia with Myrica gale whilst lagg
channels are occupied by tussocky Molinietum.
Many raised mires have been totally reclaimed
by agricultural improvement; others
have been considerably modified by drainage attempts, resulting often in an
extensive development of birch and pine woodland, especially around the mire
edges, as at Kirkconnell Flow, Kirkcudbrightshire. At Holme Fen, Huntingdonshire,
virtually the whole of the original raised mire has gone over to mature birchwood,
and the cut-over surface of the huge Thorne Waste in Yorkshire shows extensive
birch colonisation. Peat exploitation has taken place on many sites both on a local
domestic scale (often only peripherally) and more extensively in the case of
commercial peat winning - an industry which has evolved mainly over the last 25
years. Even the few remaining actively growing raised mires are largely covered
with a vegetational mosaic which reflects varying degrees of drying and
modification resulting from disturbance (mainly fire and drainage).
