In addition to the general criteria for
assessment of key sites described in Chapter
2, certain criteria have been used in particular to evaluate the scientific quality of
peatland sites. These are as follows.
(i) Degree of intactness of the mire structure
in terms of oth morphological features
and associated vegetation zones. This applies especially to raised mires, where
marginal encroachment by peat-cutting or other disturbance has often destroyed
the characteristic marginal features of lagg and rand, and produced an artificial
edge to the mire, e.g. East Flanders Moss, Perthshire. Erosion of peat, either by
gullying or surface stripping, under the influence of water, wind and frost, produces
very direct damage to a mire surface and profound changes to the vegetation.
Enlargement of pools by marginal scouring is also usually a symptom of
retrogression of a mire surface. Retrogressive changes of this kind lower the value
of a mire system.
(2) Lack of disturbance of the mire hydrology.
Such disturbance is often associated
with (i) above but may occur also when there is no alteration in gross morphology of
a mire, as when the water table is lowered by drainage operations or erosion some
distance from the mire itself. Some flood-plain mires have been so much affected
by reclamation of surrounding peatland that they are no longer viable as mires
unless the water table is artificially maintained by pumping. In these circumstances,
a mire has to have very high intrinsic scientific value to be rated as of national
importance. Burning of the surface vegetation may also help to lower the water
table, and occasionally subterranean erosion may do so without first modifying the
surface structure.
Sites with a catchment which can be readily
protected are valuable, as in the case
of valley mires set in an area of acidic heathland.
Changes to the water table result from
physical disturbance, but disturbance to mire
hydrology also includes chemical modification. This applies particularly to eutro-
phication resulting from run-off of chemical fertilisers (especially nitrogenous kinds)
from agricultural land, and from pollution by sewage. This problem is generally more
serious in the case of open water-bodies, but it affects a number of basin, valley,
open water transition and flood-plain mires, and in some instances appears to have
caused virtually irreversible loss of quality.
(3) Lack of modification of the vegetation,
i.e. naturalness. Vegetational changes
invariably follow disturbance to mire morphology and hydrology, though there may
be a time lag. Any changes which involve lowering of the water table cause
increase in cover of less hydrophilous mire species at the expense of more
hydrophilous species. In oligotrophic mires this usually involves reduction in
Sphagnum cover and increase in abundance and performance of vascular plants
such as Eriophorum vaginatum, Trichophorum cespitosum and Calluna vulgaris. In
western Scotland, Rhacomitrium lanuginosum increases greatly in drying blanket
mire and may become dominant. In the Highlands, the more local montane dwarf
shrubs such as Betula nana, Arctostaphylos uva-ursi and Arctous alpinus decrease
as a result of burning; and in all regions burning accompanied by heavy grazing
tends to suppress the dwarf shrubs as a whole. Severe peat erosion may be
accompanied by profound vegetational change, giving dry heath-like communities
on the residual peat, and there may be re-colonisation and succession on eroded
ground to restore a vegetation cover. In many mires, surface drying is the prelude to
invasion of shrubs and trees, to give eventually a carr or woodland.
While these various forms of modification
give new types of vegetation and thus
increase diversity, there is a strong tendency to regard this derived vegetation as
much less important than the original types, and to select accordingly for mires
showing the least modification. Most derived communities are incidentally
represented (often repeatedly) during the selection of sites for other features.
Occasionally it happens that severe disturbance leads to renewal of surface mire
growth, as when peat-cutting is so extensive that it produces large, water-filled
hollows in which hydrophilous communities regenerate spontaneously. This has
happened at Moorthwaite Moss, Cumberland, where superb Sphag-netum has
regenerated after most of the original mire surface was removed. Undisturbed
Sphagnetum is highly valued in whatever type of mire it occurs.
(4) Diversity in nutrient status of mire
water within a site is regarded as an
interesting and valuable feature and is especially characteristic of valley mires.
Calcareous mires with ' brown moss' carpet and low-medium herb sward have
been highly rated, as a rare and fragile type. Open water transition and flood-plain
mires which are contiguous with important open waters (both lakes and rivers) are
regarded s more valuable than those with little or no open water; this is another
aspect of diversity.
(5) Representation of individual plant
species characteristic of the particular mire
type, i.e. floristic richness. This links with (3) above, as species may be lost through
anthropogenic modification of the vegetation. Small areas of undamaged mire
may, however, lack certain species largely as a result of chance, as smallness of
size reduces chances of colonisation, e.g. Abbots Moss, Cheshire. Species/area
effects of this kind are especially likely in districts where the extent of mire
vegetation has always been limited.
(6) Stratigraphical importance of the
site. The peat and any underlying mineral
substrata are an integral part of a mire system, and have provided the bulk of the
material for the unravelling of the vegetational and climatic history of the Late
Quaternary Period. The most valuable deposits are those with the greatest time
span, and examples which extend back to the early Late-glacial (Late Weichselian)
Period are especially valued. The deposits under some mires have become
classic Quaternary sites and there is a need to preserve the most important against
the need for further investigations of their sequences. Since destruction of a mire
often involves the removal or erosion of the deposits underlying the surface,
National Nature Reserve (NNR) status is usually necessary to secure conservation
of such sites. In a few cases, the Quaternary historical value of a site greatly
outweighs its living surface interest, but in many mires the two run more or less
parallel.
