This geological
formation has a much more northerly and westerly bias in
distribution than the Chalk and Jurassic limestones. The scattered
southernmost occurrences are mainly coastal, around the Bristol
Channel and are isolated from the major, mostly inland, outcrops in
north Wales, the north Midlands and northern England. The variation
in floristics resulting from this geographical and ecological
separation is amplified by the uneven and often rocky nature of the
Carboniferous Limestone terrain, which gives great diversity in
edaphic and micro-climatic conditions. A final factor which makes
for floristic heterogeneity within the grasslands of this rock
formation is the relatively large number of locally abundant
species with a disjunct distribution within the total extent of the
Carboniferous Limestone.
Festuca ovina-F.
rubra swards are again a common type of grassland association, but
both Zerna erecta and Brachypodium pinnatum are southern species
poorly represented on the Carboniferous Limestone. Other southern
constants or characteristic species such as Poterium sanguisorba,
Helianthemum chamaecistus, Hippocrepis comosa, Filipendula
vulgaris, Asperula cynanchica, Orchis morio and Cirsium acaulon
become increasingly discontinuous in distribution with distance
north, and lose constancy, or even presence. The northern and
mainly upland grass Sesleria albicans is usually constant and
locally dominant in limestone grasslands reaching down to sea-level
in northern England, and some of these swards are lowland in
character. There are transitions in this region to submontane types
of calcareous grassland with northern and upland species such as
Anten-naria dioica, Saxifraga hypnoides, Potentilla crantzii,
Galium boreale and Minuartia verna. Farther inland, especially in
the Pennines, these in turn assume a distinctly montane character
by the addition, at higher levels, of Polygonum viviparum,
Cochlearia officinalis ssp. alpina, Draba incana and, very locally,
Gentiana verna, Carex capillaris and Myosotis
alpestris.
The scars, screes
and pavements of the Carboniferous Limestone in various regions
carry distinctive assemblages of plants, including many which have
a need for open, immature habitats with freedom from competition.
As most of these species do not form communities, except in the
loosest sense, they are best mentioned under the account of
flora.
