Topogenous mire develops in enclosed waterlogged
depressions which have
become colonised by peat-forming vegetation and which show no obvious
unidirectional movement of water at the mire surface. The essentially level and
usually stagnant water table is a fundamental feature and affords the major
distinction from valley mire. Such sites occur most frequently in areas of glacial
deposition where local irregularities of relief such as kames and kettle-holes
provide the necessary geomorphological environment of hollows with enclosed
drainage. In northern and western areas such mire pockets have largely been
obliterated by the general growth of ombrogenous mire over the whole area, so that
basin mires are most characteristic of lowland Britain. They are, however, found
mainly in the Midlands, northern England and southern Scotland, where suitable
fluvio-glacial topography is best developed. Basin mires are usually small in area
(less than 50 ha) but are often very deep in relation to their size, and some are
underlain by deposits of Late-glacial age.
Mires of this type vary from oligotrophic
to eutrophic, and although there is a
preponderance of the former this is more a reflection of the inherent successional
tendency towards an oligotrophic surface than an actual prevalence of base-poor
ground water. Usually, either the whole basin becomes filled with peat or a floating
raft becomes established across the original water-body. As the surface of the mire
grows above the direct influence of ground water, conditions favouring the
development of central oligotrophic communities are initiated. In such mires,
relatively base-rich fades are restricted to the margins, where succession to fen
carr often occurs. Analogy with raised mire is clear, but basin mire development is
determined and restricted by the hydromorphology of the basin. Furthermore, in
view of the limited extent of such mires and the relatively dry climate under which
many are developing, it is doubtful if the central oligotrophic communities can be
regarded as truly ombrogenous. More probably the mire water budget is
supplemented to some extent by ground water which is effectively buffered during
movement through the acidic peat-body.
The vegetation of basin mire is composed
very largely of communities already
described under raised mire or the topogenous series from poor to rich fen. Open
spongy areas of Sphagnum lawn communities are most typical of basin mires, but
a distinction is made between those sites exhibiting varied Sphagnum
communities (including S. rubellum, S. magellanicum and S. papillosum) with
numerous leafy liverworts and a hummocky surface, in association with Erica
tetralix, Vaccinium oxycoccus and Andromeda polifolia, and those which show a
floristically poorer Sphagnum lawn association dominated largely by 5. recurvum
and Erio-phorum angustifolium. This latter type (which is apparently spreading in
some sites) may be a result of eutrophication, and the connection with disturbance
is shown by its usual occurrence in old peat-cuttings. The more varied type of
Sphagnum surface often has hollows and hummocks and very closely resembles
the surface of many undamaged raised mires. A common feature of many
oligotrophic basin mires is their relatively recent colonisation by Scots pine often as
a result of local seeding from plantations, but with persistent depauperate checked
growth where the water table is high. Though tiny by comparison, these pine-grown
mires bear a striking resemblance in general appearance to the great forest mires
of Scandinavia.
Poor-fen of Sphagnum lawn and Carex layer
(mainly C. rostrata-C. lasiocarpa)
occurs in some basin mires. Strongly eutrophic basin mires are infrequent, being
limited by the chance association of calcareous rocks and soils in conjunction with
a suitable topographical environment. Instead of Sphagnum lawns, there are
typically 'brown moss' carpets and succession to willow carr is a feature of such
mires, the vegetation being of the rich-fen type. Some basin mires show a range of
vegetation types from strongly acidophilous Sphagnum surface, through poor-fen to
rich-fen, and there is seldom uniformity of water conditions over the whole surface.
Where small lakes and tarns in enclosed basins are undergoing marginal
succession, the resulting open water transition mire may eventually extend over the
whole surface, and would then be regarded as a basin mire. Clarepool Moss in
Shropshire, although regarded as a basin mire, still contains a large pool
representing the last stage of infilling of an original lake.
