Context
Bogs are wetlands that support vegetation that is usually
peat-forming and which receive mineral nutrients principally from
precipitation rather than ground water. This is referred to
as ombrotrophic (rain-fed) mire. Two major bog types are
identified, namely raised bog and blanket bog. These two
types, are for the most part, fairly distinctive, but they are
extremes of what can be considered an ecological continuum and
intermediate (or mixed) types occur.
The vegetation of bogs which have
not been modified by surface drying and aeration or heavy grazing
is dominated by acidophilous species, such as bog-mosses
Sphagnum spp., cottongrass Eriophorum spp. and
cross-leaved heath Erica tetralix. The water-table
on these types of bogs is usually at or just below the
surface.
Raised bogs are elevated deposits of
peat in the lowlands. They are divided into active bogs, in
which the peat is still being added to, and degraded bogs in which
peat formation is at least temporarily at a standstill. The
difference is reflected in the Natura 2000 features: 'active raised
bog' and 'degraded bog still capable of natural
regeneration'. The Natura 2000 feature 'depressions in peat
substrates (Rhynchosporion)' may also be found on lowland
raised bog.
In lowland areas with predominantly
acid substrata there are examples of valley and basin mires that
receive acid surface seepage, which gives rise to vegetation
similar to that of bogs. However, these types are covered in
the
Fens
and marshes reporting category.
SSSIs can be notified if they
qualify under criteria outlined in Section 4 of
Chapter 8
Bogs of the
Guidelines for Selection of Biological
SSSIs. In Northern Ireland, ASSIs are selected on a very
similar basis – the
Guidelines
for the Selection of Biological ASSIs in Northern Ireland is an
addendum to the SSSI guidelines rather than an
alternative.
Interpretation
Only a fifth (21%) of lowland raised
bogs reported are in favourable condition. This is well below
the average for terrestrial habitats, all habitats, or all features
combined. The proportion of features on A/SSSIs and SAC sites
reported in favourable condition is similar, with 22% and 19%
respectively. More of the unfavourable SAC features reported
are recovering (52%) than on A/SSSIs (35%). This no
doubtreflects the additional resources aimed at achieving
favourable condition on Natura 2000 sites. No lowland raised
bogs in England are in favourable condition, but 44% are
unfavourable-recovering.
The main causes of unfavourable
condition are water management (presumably, drainage) and lack of
remedial management (neglect). It is important to recognise
that water management (drainage) affects bog condition when carried
out within the designated site and around its borders within a
marginal area known as the hydrological protection zone.
Other important causes include invasive species (e.g. birch
Betula spp.) and under-grazing. Development carried
out under planning permission may have been under-recorded, as it
should include commercial peat extraction. It may be that the
activity is split between this category and
extraction/removal. Peat extraction is still a cause of
unfavourable condition on about 750 ha of lowland bog in England;
most of this is on two pSAC sites.
Air pollution is only cited in a few
cases. This is likely to be an under-estimate, as most
surveyors would attribute the effects of it to other causes, such
as drainage. Critical loads of sulphur are still being
exceeded for some lowland raised bogs, and are predicted to do so
at least to 2010. Dry deposition of ammonia is still very
high in most parts of England, Wales and Northern Ireland.
Bisulphite has an inhibitory effect on some Sphagnum moss
species, and deposition of nitrogen encourages rank competitors
such as the purple-moor grass Molinia caerulea.