The preceding account of mire vegetation
types has indicated a range of floristic
diversity in relation to water nutrient status. Table 23 gives the ecological
distribution of each British vascular mire species according to the major divisions
of water pH and nutrient status, as far as these are known at present. Species
which occur widely in other habitats besides mires are omitted, as are woody
species above the size of medium shrub (i.e. most of the willows) as these belong
more properly to woodland or lowland scrub classifications. The true hydrophytes
belonging essentially to open water are treated under that habitat formation, though
the limits of this group have had to be arbitrarily defined, and species characteristic
of ombrogenous mire pools are included here.
A few general features emerge from this
study of edaphic relationships. Few
species appear to be confined to eutrophic fen, but there is a large group of
broadly basiphilous species whose tolerances/requirements range from
mesotrophic to eutrophic. This could perhaps reflect an inadequacy of data on
water chemistry for some species or a badly chosen boundary between the two
trophic levels, but it suggests that above a certain level, further increase in nutrient
status has no effect on many species. There is a much smaller number of species
confined to oligotrophic mire waters, compared with those in the two higher trophic
levels. Only a few species show a complete range of tolerance to acidity/alkalinity
of mire waters.
There is insufficient information to record
the distribution of each species in relation
to the six main morphological types of mire. However, chemical factors, and thus
species distribution, tend to vary independently of mire morphology, except in
ombrogenous mires (raised and blanket mires), for these have only oligotrophic
conditions. It is more meaningful to separate oligotrophic ombrogenous from
oligotrophic topogenous as there appears to be a real difference between these
two in both chemical and floristic terms; some plants belong to raised and/or
blanket mire, but not to poor-fen, and vice versa, and the controlling differences
may include rate of movement and aeration of the water.
Geographical distribution of mire plants
is also shown in Table 23. There is a
certain degree of parallelism between ecological and geographical distribution of
mire species. The restriction of ombrogenous mire to the west and north ensures
that species confined to or occurring mainly in this class of peatland have a similar
distribution while the strong representation of mesotrophic and eutrophic fen in
southern Britain (especially in East Anglia) is matched by the restriction of some
strongly basiphilous species to this part of the country. Since lowland mires have
been so widely drained and destroyed in the interests of farming and building,
many of their characteristic species have suffered great reduction in range or even
extinction. Many wide-spread mire species are much less common and generally
distributed in the south and east of Britain than in the north and west because of this
effect, and the patchy distribution of some largely reflects the present restricted
occurrence of suitable habitats.
To give examples of this retreat, the
East Anglian Senecio palustris is extinct while
S. paludosus is known in a single ditch in this region: the northern Scheuchzeria
palustris is known only on Rannoch Moor in Scotland; and the once rather
widespread Carex elongata is now quite rare. Other .restricting factors may
operate and it has been suggested that the decline of the rare fern Dryopteris
cristata is partly the result of introgressive hybridisation with D. carthusiana,
accompanied by selection in favour of the hybrid forms.
As an analysis of mire species geographical
distribution .according to presence or
absence in particular regions often fails to indicate distinctive patterns, a different
and more .meaningful treatment has been adopted. Study of mire species
distribution maps suggested that eight main patterns of scatter could be
recognised; these have been chosen subjectively and arbitrarily but nevertheless
enable a useful analysis to be made. These distribution classes (see Table 23) are
defined as follows: . ,...
WIDESPREAD/COMMON
Species occurring over virtually the whole
of Britain, and limited mainly by lack of
suitable habitats. These naturally tend to be species with wide ecological
amplitude, but the group which find their optimum water nutrient status in the upper
oligotrophic to lower mesotrophic range are especially well represented, as waters
of this type are probably the most widespread in Britain. Typical examples are
Caltha palustris, Lychnis flos-cuculi, Juncus articidatus and Phalaris arundinacea.
WIDESPREAD/LOCAL
Species occurring in most districts of
Britain, but much less abundantly than the last
group, and often absent from apparently suitable localities. These appear to show
no particular ecological bias as a group. Typical examples are Ranunculus lingua,
Utricularia vulgaris, Dactylorchis in-carnata, Carex vesicaria and C. acuta.
SCATTERED/VERY LOCAL
Species showing a highly discontinuous
scatter, absent from a great many
apparently suitable localities, but widely spread and showing an irregular pattern
with no single geographical bias. These certainly show no particular ecological bias
as a group. They are Dryopteris cristata, Cicuta virosa, Pyrola rotundifolia,
Naumburgia thyrsiflora, Dactylorchis traun-steineri, Eriophorum gracilc, Cladium
rnariscus, Carex flava, C. elongata and Calamagrostis stricta.
SOUTHERN
Species which become increasingly widespread
and abundant with distance south
in Britain; they belong essentially to England and Wales, but especially southern
England, and though some reach Scotland they are local or rare there.
This group includes a high proportion
belonging to mesotrophic and eutrophic mire,
e.g. Thalictrum flavum, Berula erecta, Oenanthe fistulosa, Rumex hydrolapathum,
Cirsium dissectum, Epipactis palustris, Dactylorchis praetermissa, Carex
pseudocyperus and C. riparia.
OCEANIC
Species occurring mainly in coastal districts,
but especially along the south and/or
west coasts. The most localised is the thermophilous southern oceanic Hypericum
undulatum, only in south-west England and west Wales, which contrasts with the
northern oceanic Drosera anglica, most abundant in the north-west Highlands.
Other characteristic members are Hypericum elodes, Drosera intermedia,
Pinguicula lusitanica, Scutellaria minor, Rhynchospora alba and R.fusca. Osmunda
regalis, Oenanthe crocata, Eleocharis multicaulis and Schoenus nigricans show the
same general pattern but occur more widely away from coastal districts. If the Irish-
American Spiranthes romanzoffiana can be regarded as a mire plant in Britain, it
belongs here. It is perhaps significant that the majority of species in this group grow
also on acidic wet heathland, a characteristic oceanic British vegetational
formation.
EAST ANGLIAN
Species found mainly in eastern England
from Suffolk to the Humber, but especially
in East Anglia. The rare or very local species which occur in very few places
outside this belt are Viola stagnina, Lathyrus palustris, Peucedanum palustre,
Sonchus palustris, Liparis loeselii and Carex appropinquata. More widespread and
plentiful species which could be regarded as southern, but are more abundant in
the Suffolk-Humber region than elsewhere include Thelypteris palustris, Sium
latifolium, Juncus subnodulosus, C. data and Calamagrostis canescens. These are
without exception plants of rich-fen. ..
NORTHERN
Species which become increasingly widespread
and abundant with distance north
in Britain and have their headquarters north of the Midlands, but are confined to
fairly low altitudes. This includes extreme rarities such as Carex buxbaumii, C.
chordorrhiza, Scheuchzeria palustris and Ledum groenlandicum, and local species
such as Listera cordata, C. paupercula, C. pauciflora, C. aquatilis and Corallorhisa
trifida. Most of these could be regarded as submontane as they occur mainly in
upland districts. Many northern species have a wide distribution in Britain, reaching
to the extreme south of England, where they are, however, rare or very local. These
include Vaccinium oxycoccus, Utricularia intermedia, Hammarbya paludosa, Carex
limosa, C. lasiocarpa, C. curia and C. dioica.
Nearly all the northern species so far
mentioned belong to oligotrophic mire,
especially of the topogenous kind (poor-fen), thus reflecting the general edaphic
trend towards increasing prevalence of base-poor waters in northern Britain.
Strongly basiphilous and widespread northern species include Parnassia palustris,
Dactylorchis purpurella, Erio-phorum latifolium and Carex lepidocarpa.
Andromeda polifolia is essentially a northern
species in mainland Europe, but has
a curious British distribution, for it does not reach the Highlands and occurs mainly
in northern England and southern Scotland, where it is locally abundant in
oligotrophic mires (cf. Primula farinosa in calcareous habitats). Calamagrostis
stricta could be regarded as a northern species, but is included in Table 23 as
scattered, local. Eriophorum vaginatum and Trichophorum cespitosum occur in the
extreme south of England but are regarded as northern as they become really
abundant only from Wales northwards.
MONTANE
Species with a northern distribution which
are found mainly at high altitudes and are
most abundant in or confined to the Scottish Highlands. These are necessarily
restricted to the only kinds of mire found at high levels, namely, blanket and
soligenous mires. The montane species of blanket mire are Betula nana,
Arctostaphylos uva-ursi, Arctous alpinus, Vaccinium uliginosum, V. microcarpum,
Empetrum herma-phroditum and Rubus chamaemorus. Those of oligotrophic
soligenous mire are Carex rariflora, C. lachenalii, C. bige-lowii and Siaxifraga
stellaris. The montane species belonging to mesotrophic or eutrophic soligenous
mire are S. hirculus, Thalictrum alpinum, Polygonum viviparum, Kobresia sirn-
pliciuscula, Juncus castaneus, C. atrofusca, C. capillaris, C. microglochin and C.
norvegica. A few species in this group appear to be indifferent to mire water base-
status, e.g. C. saxatilis, C. vaginata, Alopecurus alpinus and Phleum alpinum. Most
of these species of montane soligenous mire are omitted from Table 23 as they
belong to a wider range of habitats, including flushes, rills, springs and rock ledges,
and they are dealt with under the flora of uplands (Chapter 9).
Some northern submontane species also
occur in montane mires up to
considerable elevations, e.g. Vaccinium vitis-idaea and Empetrum nigrum. It is
noteworthy that the northern and montane mire flora has a strong representation of
acidophilous dwarf shrubs (mainly Ericaceae) which belong mainly or entirely to
ombrogenous mire.
Of the 161 peatland species listed in
Table 23 only about half are classified into
phytogeographical elements by J. R. Matthews (1937). A few more species have
been tentatively classified from literature records and added, but distribution of
most of the remainder could not be judged accurately enough and they have been
left blank. These classifiable species give the following representation in the
various elements: oceanic southern (i), oceanic west European (7), continental
southern (2), continental (8), continental northern (43), northern montane (4),
oceanic northern (3), Arctic-subarctic (2), Arctic-alpine (9).
The northern continental element thus
constitutes over half the total list. The
remaining unclassified peatland species would seem to belong either to this or to
the continental element, or to be so widespread in Europe that no definite
distribution pattern is discernible within this continent.
European and British distributions of
mire species show some parallelism but there
are certain discrepancies. Most of the oceanic western species in Europe have a
western and coastal distribution here, but Drosera anglica, D. intermedia,
Rhynchospora fusca and R. alba (all somewhat oceanic and with a western bias in
Britain) are either continental or continental northern in Europe. All the European
Arctic-alpine or Arctic-subarctic species are northern, and most are montane, in
Britain; and the four northern montane species are northern and submontane here.
Several continental northern species show no particular northern bias in Britain, and
many British southern species are widely distributed in Europe.
The British vascular mire flora contains
13 rare species (present in 1-15 10-km grid
squares) fairly well spread through the different ecological and geographical
groupings.
