Peatlands are ecosystems in which vegetation
of wet ground builds up organic
deposits over the underlying mineral substratum, under conditions of waterlogging
that are usually anaerobic. The depth and degree of decomposition of a peat
deposit vary according to the behaviour of the water regime during the total period
of peat formation at the site. In general, the more complete and permanent the
degree of waterlogging, the lower the rate of humification and more rapid the peat
formation, so that the deepest peats tend to occur on the wettest sites. The
stratigraphy of a peat deposit therefore contains a record of hydrological conditions
during the whole period of its formation, and typically shows variations in rate of
growth. In general, too, the more acidic and base-deficient the water supply, the
slower the humification, and in many situations the onset of peat formation appears
to have been associated with acidification of the ground surface, often
accompanied by a growing independence from mineral-rich ground water.
The differences in vegetation which accompany
these changes in water regime and
nutrient supply are also involved in the differences in rate of peat formation. This is
especially true in regard to changes in abundance of the genus Sphagnum (bog-
moss) which has such an important role in the formation of peat in Britain. The
Cyperaceae also contribute notably to peat formation. Both these are moisture-
loving groups of plants which decrease if water tables fall and the ground surface
begins to dry out.
Peatlands may be classified either according
to their different physical (especially
hydrological) characteristics or their present range of vegetation, or by a
combination of these two inter-related criteria. Nomenclature of peatlands in Britain
has become a little confused because certain terms defined according to one
criterion have acquired meaning for the other through looser usage. The term bog
(in raised bog and blanket bog) has become synonymous with 'peat moss' as
meaning an acidic peatland usually with abundant Sphagnum. However, it has also
been used in valley bog, where conditions are often strongly alkaline. The term fen
has been used to denote the swampy peatlands associated with transitions to open
water or occupying wide flood plains; as these typically support eutrophic
vegetation, fen has also come to indicate a particular range of plant communities,
especially those dominated by tall grasses, sedges and forbs. The term has then
been extended to
similar vegetation in quite different
topographic situations (e.g. valley bog), and
also qualified (as in poor-fen) to indicate certain communities intermediate
between fen and bog. Because of these confusions, it has seemed best to follow
Scandinavian practice, and apply the term mire to all peat-lands, as a simple name
with no further connotation, and to classify these mires first according to their
topographic and hydrological features.
The permanently high water table which
induces peat formation may result from
different environmental conditions. Land forms such as basins, channels and flat-
bottomed valleys naturally collect and hold water which falls as rain or drains into
these situations from elsewhere, so that suitable topography is one basic factor in
mire development. Permeability of the substratum also interacts with topography to
determine degree of waterlogging, and depends largely on the physical and
chemical properties of rocks and their derived soils. Superimposed on both these
conditions is the overall availability of water, determined basically by the balance
between precipitation and evapotranspiration, i.e. effective wetness of climate. The
wetter the climate the wider is the range of topographic situations which allow peat
formation. The strongly oceanic (i.e. cool, humid) conditions of western Britain, and
especially the high oro-graphic rainfall of the uplands, favour the development of
extensive, continuous areas of peat-forming vegetation over most flat or gently
sloping ground.
Although certain types of vegetation show
a particular association with certain
classes of mire, the classification of mire vegetation does not run closely parallel to
the above subdivision of mires based on physical characteristics. Detailed
characterisation of mire vegetation in terms of particular floristic assemblages
(noda) has not yet been comprehensively worked out in Britain, so that a phyto-
sociological classification cannot be given here, and a descriptive system which
takes account of ecological factors is used. The range of vegetational variation
within these peatlands is best analysed in terms of the major controlling factors,
namely, height of the mire water table and chemical composition of the water.
Some plant species have a very limited
range of moisture tolerance/requirement
and thus are good indicators of water table, whereas others with a wider range
occur in a variety of communities from wet to dry. Mires as a whole show a
tendency to an ecological succession whereby open water is colonised by strongly
hydrophilous plants which build up a peat surface until it can be invaded by other,
less hydrophilous species, and so on, until the ground becomes so dry that mire
plants largely disappear and peat formation and upward growth cease. Such
hydroseral development occurs especially in mires containing pools or those
fringing larger bodies of open water, but may become arrested at any stage. In
ombrogenous mires, in particular, upward growth of a Sphagnum carpet usually
draws up the water table, and, though hydroseral development proceeds
continuously, the early, wet stages persist or redevelop and there is at any one time
a mosaic pattern of different stages (the regeneration complex); there is thus a
steady state overall and the mire surface remains generally wet and spongy. A fall
in water table and consequent surface drying, with reduction in hydrophyte cover,
often results from climatic change and human activity, but some ecologists believe
that it may sometimes also represent a natural termination of bog growth with no
change in other conditions. Ombrogenous mires, with a water table often lifted far
above mineral soil level, are especially susceptible to drying out through peat
cutting, draining, burning and grazing, which often tap the edges first and initiate
degeneration of the whole peat system, ending frequently in erosion. As the water
table falls, either laterally along an environmental gradient or through succession in
time at any one place, there is an increasing tendency to invasion by tall shrubs and
trees to form scrub or woodland. Scots pine can invade wet Sphagnum carpets but,
unless there is appreciable drying, the small saplings remain stunted (checked) and
eventually die in this state. Succession to woodland usually depends on the water
table falling appreciably below the mire surface, either through upward growth or
external change in hydrology. The type of woodland which then develops depends
on the chemical nature of the peat and water, and also on geographical location.
