These are essentially topogenous mires,
familiarly described as fen. Open water
transition mire refers to the shallow edges of both lakes and rivers, which show
colonisation by emergent aquatic vegetation to form swamp communities. An
arbitrary separation has to be made in denning the limits of open water transition
mire, as distinct from open water. The limits drawn here are at the outer edge of the
zone of emergent swamp vegetation, composed mainly of monocotyledons which
rise well above normal water level. Aquatic plants with leaves floating on the
surface, and submerged aquatics are considered to belong to the open water class
of habitats, e.g. macrophytes such as Potamogeton spp., Myriophyllum spp.,
Callitriche spp. and Nymphaea spp., and the majority of freshwater algae. Species
which belong essentially to the open water of rivers and ditches will also be
excluded from the present account.
Flood-plain mire describes lowland alluvial
plains traversed by (usually) sluggish
rivers and subject both to permanently impeded drainage and periodic flooding;
there is development of a similar and sometimes identical range of hydrophilous
vegetation to that of the open water transition mires. Where shallow lakes occur on
alluvial flood plains the two types of mire form a single continuous system. Both
occur especially in the lowlands, where drainage water is from a mature river
system and tends to be base-rich, and they may persist indefinitely only in regions
where the rainfall is too low to allow the development of ombrogenous mire; both
types have frequently been the precursors to raised mires which have developed in
western Britain. The occurrence of oligotrophic nuclei amongst certain of the East
Anglian fens has been interpreted as the initiation of raised mire development.
However, some of these mires probably persist indefinitely because topographic
and hydrologic features of the site and catchment ensure that the surface can never
outgrow the influence of base-rich drainage water. Conversely, the extensive cutting
of certain raised mires, e.g. Shapwick Heath in Somerset and Thorne Waste in
Yorkshire, has created depressions and channels influenced by base-rich water,
with subsequent redevelopment of eutrophic mire vegetation, juxtaposed with acido-
philous communities.
The nutrient status of the water varies
considerably within this class of mires, the
more oligotrophic types being characteristic of the predominantly hard and acidic
ancient rocks of northern and western Britain, whereas the eutrophic types are
found mainly within the base-rich catchments on the younger horizons of the south
and east, especially East Anglia. The parallel climatic contrast, as between
oceanicity and continentality, also enhances the effects of this geological contrast.
Eutrophic examples nevertheless occur in virtually all regions and are well
represented in the central Lowlands and east of Scotland; even in the oceanic
western Highlands and islands there are eutrophic mires of this type associated
with wet ground and lochs on the shell sand machair.
At the oligotrophic end of the series
these topogenous mire communities
intergrade with the still more markedly acidophilous types belonging to
ombrogenous mire, and it is thus convenient to begin description with this.
Compared with raised and blanket mires,
the mire water of the topogenous mires
at the oligotrophic end of the series is slightly less acidic and poor in nutrients, and
is evidently associated with lateral movement or other ground water influence,
giving a different type of vegetation, sometimes known as poor-fen. A Sphagnum
carpet is typically present, but with a different spectrum of species, mainly S.
papillo-sum, S. palustre, S. recurvum, S. cuspidatum, S. fimbriatum, S. squarrosum
and S. subsecundum vars. auriculatum and inundatum, and there are Bryalean
mosses such as Aula-
comnium palustre, Drepanocladus exannulatus
and Acro-cladium stramineum. The
vascular plants forming a layer above the Sphagnum carpet have some species in
common with the previous, more strongly acidophilous range of communities, but
are characterised by dominance of carices, notably Carex rostrata, C. lasiocarpa,
C. nigra, C. echinata, C. curia and C. limosa. C. aquatilis and C. paupercula are
typical plants of northern poor-fen. Other characteristic species include Equisetum
palustre, E. fluviatile, Eriophorum angustifolium, Narthecium ossifragum, Juncus
kochii, Agrostis stolonifera, Molinia caerulea, Caltha palustris, Menyanthes
trifoliata, Potentilla palustris, Ranunculus flammula, Drosera rotundifolia, Vaccinium
oxycoccus, Potamogeton polygonifolius, Viola palustris and Dactylorchis maculata.
Often, the growth of sedges and other plants excludes the Sphagnum spp., and in
some of these mires, the Sphagnum carpet is fragmentary or absent. Where this
type of mire dries out sufficiently to allow colonisation by shrubs, Salix cinerea and
S. aurita are usually the most abundant species. Where Myrica gale is present and
ungrazed, it also may become abundant and form a shrubby growth several feet tall.
Birch commonly invades and becomes more abundant if the surface dries, though
the Sphagnum spp. often continue to form a carpet and Polytrichum commune often
becomes very abundant. Molinia may become dense and Dryopteris carthusiana
often becomes abundant. Alder sometimes appears but is more typically
associated with richer conditions. Some extensive examples of this poor-fen occur
in Scotland (e.g. the Insh Marshes in Inverness-shire), but it is more widespread as
small patches fringing the shallow edges of lakes and tarns.
As nutrient status of the mire water increases
to a level within the lower edge of the
mesotrophic range there are marked floristic changes. If a Sphagnum carpet
persists, it is usually composed of the relatively basiphilous species S. teres, S.
subsecundum, S. contortum, S. warnstorfianum and S. squarrosum. Carex rostrata,
C. nigra and some of the poor-fen species remain abundant, but there is an
addition of others such as C. vesicaria, C. elata, C. paniculata, Phrag-mites
communis, Phalaris arundinacea, Galium palustre, Myosotis secunda, Veronica
scutellata, Lythrum salicaria, Epilobium palustre, Pedicularis palustris, Lychnis flos-
cuculi, Hydrocotyle vulgaris and Senecio aquaticus. The Bryalean mosses typically
include Acrocladium cuspidatum and Mnium seligeri, and the liverworts
Lophocolea bidentata and Mar-chantia polymorpha var. aquatica are sometimes
abundant. Colonising shrubs are again mainly Salix cinerea and S. aurita, but S.
pentandra and Frangula alnus are locally abundant, and alder often appears in the
developing carr. Sphagnum carpet may persist, some of the herbs remain in the
field layer, and there is often an abundance of Juncus acutiflorus, Deschampsia
cespitosa, Angelica sylvestris, Dactyl-orchis fuchsii and Calamagrostis epigejos.
This intermediate kind of fen is widespread in Britain, especially on rock formations
which give moderately fertile but non-calcareous soils.
Where richness of water lies within the
upper range of mesotrophic and passes into
eutrophic, Sphagnum spp.
disappear and the vegetation is often
dominated by vascular plants almost to the
exclusion of bryophytes. The outer colonising zone of this swamp vegetation often
has dense, tall growths of Schoenoplectus lacustris, Phragmites communis, Typha
latifolia and T. angustifolia. The characteristic sedges include Cladium mariscus,
Carex riparia, C. acutiformis, C. diandra, C. disticha, C. acuta, C. elata, C.
pseudocyperus, and other typical vascular plants are Ranunculus lingua, Iris
pseudacorus, Galium uliginosum, Scutellaria galericulata, Oenanthe fistulosa,
Berula erecta, Rumex hydrolapathum, Myosotis scorpioides, M. caespitosa,
Stellaria palustris, Veronica anagallis-aquatica, Bidens cernua, Juncus sub-
nodulosus, Sparganium erectum and Schoenus nigricans. In some mires the grass
Glyceria maxima becomes completely dominant, forming a floating raft.
Communities of the above type are known under the general term of rich-fen, and
are probably best represented in the Norfolk Broads.
In mires where the water is strongly calcareous,
another type of moss carpet often
develops, with a more open growth of vascular plants, and is composed of the
'brown mosses' Campylium stellatum, C. elodes, Acrocladium giganteum,
Scorpidium scorpioides, Drepanocladus revolvens var. inter-medius, Cratoneuron
filicinum, C. commutatum and Ctenidium molluscum. Many of the above named
plants of rich-fen are present, including especially the monocotyledons, but
characteristically there are also additional species such as Parnassia palustris,
Pcdicularis palustris, Valeriana dioica, Epipactis palustris, Dactylorchis
praetermissa (southern), D. purpurella (northern), D. incarnata, Eriophorum
latifolium, Triglochin palustris, Juncus articulatus, Eleocharis quinque-flora, Carex
lepidocarpa, C. pulicaris and C. dioica. Vegetation of this kind is especially
associated with drainage from Chalk and limestone, and is well developed though
seldom extensive from the south to the north of Britain.
The rich-fens show hydroseral development
by the invasion of alder and willows to
form carr. Salix cinerea remains a common constituent, but
other willows
such as S. pentandra, S. viminalis, S. fragilis, S. triandra, S. purpurea and
S.
alba are all locally abundant. In the East Anglian Fens both Frangula alnus and
Rhamnus catharticus have ! locally become abundant and there is a good deal of
Prunus i padus, Ribes sylvestre and R. nigrum. Within the carr, many
• of the swamp species are killed
by shade, and the peat sur-
! face often decomposes to a rather treacherous
open mud.
Some species are able to flourish under
the willows, where
shade is not too dense, e.g. Iris and
Carex paniculata, and
: others are especially
associated with this habitat, e.g.
Thelypteris palustris, Eupatorium
cannabinum and Crepis
• paludosa. Where the water table
falls but there is no invasion : by woody species,
tall herbs such as Filipendula ulmaria, ' Valeriana officinalis, Angelica sylvestris,
Epilobium hirsutum,
• Lythrum salicaria, Lysimachia vulgaris,
Senecio aquaticus and
. Phalaris arundinacea often form dense
growths, and the
i woody climber Solanum dulcamara is sometimes
abundant.
; In the Norfolk Broads and the remaining
fragments of the
\ East Anglian Fenlands, this kind of
succession continually
reduces the extent of the earlier swamp
communities, and
18
has to be kept in check deliberately if
these are not to disappear.
A related vegetation type is the brackish
swamp sometimes found fringing salt
marshes and the upper reaches to estuaries, coastal lagoons, or river valleys which
have recently become sealed off from the sea, as on the Suffolk coast. Typically,
there are dense beds of Phragmites, but in places a mixed community of more or
less halophytic species such as Scirpus maritimus, Juncus maritimus, Carex
otrubae and Oenanthe lachenalii, is mixed with plants of rich fen such as Iris
pseudacorus, Sparganium ramosum, Carex disticha and Dactylorchis spp. The
swamps of the lochs and waterlogged flats on the shell-sand machair, in the Outer
Hebrides, have a good deal in common with the rich-fens of southern England,
except that their predominantly herbaceous swards are mostly shorter in stature.
Vegetation in this type of mire shows
a certain tendency, noted also in the
herbaceous field layer of woodlands, towards development of societies, i.e. patchy
dominance of single species, such as Menyanthes trifoliata and Potentilla palustris,
in a heterogeneous pattern that shows no superficial correspondence to variation in
habitat. This may represent an effect of random spread and chance arrival of
species in an essentially immature and unstable kind of vegetation.
There are especially close relationships
between flood-plain mires and valley mires
and in some cases the differences between the two are rather arbitrary. Flood-plain
mires occupy broad valleys or extensive plains usually with a central stream or river,
whereas valley mires occupy much narrower valleys and often lack an actual open,
flowing stream. Similarly there can be difficulty in separating open water transition
mires from basin mires which still contain a central open pool. However, there is
again usually a difference in size, the basin mires being much smaller and with
more obviously internal drainage. Any one flood-plain or open water transition mire
tends to have vegetation belonging to a single trophic level, and varying mainly
according to wetness of ground, whereas both valley and basin mires tend to show
large variations in trophic level of vegetation within a small area. Oligotrophic nuclei
occur in a few flood-plain or open water transition mires, but strongly acidophilous
Sphagnetum is usually absent, whereas it is very well developed in many valley and
basin mires.
Open water and flood-plain mires have
probably been more extensively destroyed
by man in Britain than those of any other class. This is especially true of the once
vast swamps and marshes of the East Anglian Fenlands, occupying the large
shallow basin of the three main rivers draining to the Wash. Many others have been
reclaimed for agriculture, since the richer types of fen peat give a very fertile soil
when dried out, and a great many have been partly drained or modified by human
activity. By way of compensation, the best remaining examples, in the Norfolk
Broads, owe their present existence mainly to the large-scale cutting of peat during
the Middle Ages, forming extensive hollows which developed numerous hydroseral
complexes from open water to rich-fen and carr.
