Calcareous
grasslands are distributed over a variety of geological formations,
which are chemically all limestones, ranging from the soft
Cretaceous Chalk through the moderately hard Jurassic examples to
the hard rock of the Carboniferous. Besides occurring over a range
of limestone rock varying physically, calcareous grasslands are
also found on limestone with large amounts of magnesium (the
Mag-nesian Limestone and the dolomitic Durness Limestone), and on
calcareous igneous and metamorphic rocks, including pumice tuff,
dolerite, basalt, andesite, and certain schists and gneisses.
Related communities are also found on soils derived from sand and
chalky boulder clays in the Breck-land, and there are close
relationships with communities of stable calcareous sand dunes on
the coast, notably the shell sand machair of the Outer
Hebrides.
In spite of wide
differences in the physical and litho-logical nature of the
underlying substratum, the soils derived from these varied parent
materials are relatively similar chemically and this common factor
gives the characteristic composition, both in terms of floristics
and structure, to calcareous grasslands. These soils are
characterised by a high pH, usually in the range 6.5-8.5, a high
available calcium content, usually in the range 300-1000 mg
calcium/ 100 g, a high free calcium carbonate content (30-75 %) and
often a high organic matter content (7-20%), the last being
associated with the permanent nature of these grasslands. Rendzinas
are well developed, especially on steeper ground, and there is a
wide range of brown earths.
Local variations in
soil character may be caused by the presence of variable
superficial deposits, such as Clay-with-Flints, glacial gravels and
sand, or loess, which insulate the vegetation from the country
rock, or by different degrees of leaching which may be attributed
to varying aspect, precipitation and steepness of slope. The degree
of variation in the vegetation caused by these local differences in
soils depends largely on the extent to which lime-status is reduced
in the rooting horizon of the soil; thus a thick Clay-with-Flints
deposit over Chalk may support an acidic heath community containing
no calcicoles, while local leaching of a soil derived from impure
Carboniferous Limestone may create, locally, conditions which
enable a calcifuge species such as Potentilla erecta to compete in
an otherwise essentially calcicolous community. In the north and
west, peat often forms over calcareous substrata in stable
situations. Chalk and limestone soils tend to be porous and
therefore dry, but where there is clay or compacted drift derived
from calcareous rocks, marked drainage impedance may result and
give a transition from dry grassland to marsh or even mire (fen).
Minor spatial variations in soil moisture can also give a good deal
of floristic diversity. There are also transitions from calcareous
grasslands (including machair) to sub-maritime grasslands and
heaths where salt spray enriches the soil with other nutrients
besides calcium.
Open rock habitats
have extremely immature soils -except on big ledges - and on
cliffs, screes and pavements they consist largely of variable
pockets containing mixtures of humus and downwashed or windblown
mineral particles. These vestigial soils have rendzina or brown
earth affinities, and are usually developed directly on the
bedrock. The importance of open rocks is that they provide
favourable habitats for species which are suppressed by grazing, or
those which cannot tolerate competition in a closed community,
especially one composed of more luxuriant species. In wet climates,
too, the instability of rock habitats resists leaching and allows
an intimate contact between plant roots and a highly calcareous
substratum. Thin, immature soils over friable chalk or limestone,
and shallow calcareous sandy soils, are also prone to instability
and erosion, again giving a range of open habitats.
Geology is obviously
important in producing variation in the range from calcareous to
non-calcareous grasslands, but other factors are evidently
responsible for much of the variation in botanical composition
within the range of calcareous grasslands. However, for convenience
and ease of reference, the lowland calcareous grasslands have been
subdivided according to the major geological formations, i.e.
Chalk, Jurassic limestones, Carboniferous Limestone, Devonian
limestone, Magnesian Limestone and other calcareous
rocks.
There is some
parallelism between grassland floristics and the nature of the soil
parent material, though the relationship may be largely incidental,
because the major calcareous rock formations show a certain
geographical/climatic separation within Britain, e.g. Chalk in the
south and east, Carboniferous Limestone in the north and west.
Hardness of the parent rock and its effect on the physical nature
of the habitat and soil is evidently a factor of some importance in
determining vegetational differences between these ca
careous formations.
It is possible that more critical pedo-logical investigations may
disclose other differences between the soils of these different
geological formations which prove to be significant for differences
between their associated grasslands. Until this has been shown,
however, these floristic differences are best regarded as the
result mainly of a combination of geographical/climatic, land-use
and chance factors which have produced a variety of distribution
patterns for many plant species of calcareous
grassland.
The calcareous
grasslands are characterised by great floristic diversity not only
in grasses but also in forbs, and therein lies their special
botanical interest. There is, however, growing evidence that this
floristic richness is often associated with a poverty of major soil
nutrients other than calcium, i.e. nitrogen, potassium and
phosphorus, which limits the growth performance and competitive
power of certain species (especially grasses). Addition of manure
and fertilisers can upset this competitive balance and lead to
sward impoverishment as some species increase in cover and
stature.
Despite their wide
geographical variations in floristics, lowland calcareous
grasslands have a large enough number of constant vascular species
to give a certain overall homogeneity and characteristic appearance
to this broad class of vegetation. The constants are recognised
from approximately 1500 1-m2 sample quadrats, consisting of c. 1100
on Chalk, 184 on Jurassic limestones, 170 on Carboniferous
Limestone, 39 on Devonian limestone and only 4 on Mag-nesian
Limestone. These overall constants are: Briza media, Festuca ovina,
F. rubra, Carex flacca, Lotus corniculatus, Plantago lanceolata,
Poterium sanguisorba and Thymus drucei. Other species which reach
high constancy through the range of lowland calcareous grasslands
include Carex caryophyllea, Koeleria cristata, Helictotrichon
pratense, H. pubescens, Agrostis stolonifera, Trisetum flavescens,
Orchis mascula, Leontodon hispidus, Scabiosa columbaria, Campanula
rotundifolia, Helianthemum chamaecistus, Linum catharticum,
Veronica chamaedrys, Ranunculus bulbosus and Polygala vulgaris.
Some less abundant species occur at lower frequency through most of
the five classes of lowland calcareous grassland and so are
characteristic of the formation as a whole; they include Botrychium
lunaria, Thalictrum minus agg., Arabis hirsuta, Viola hirta,
Geranium lucidum, Anthyllis vulneraria, Sedum acre, Saxifraga
tridactylites, Pimpinella saxifraga, Daucus carota, Gentianella
amarella, Acinos arvensis, Calamintha ascendens, Plantago media,
Carlina vulgaris, Coeloglossum viride, Gymnadenia conopsea, Ophrys
apifera, Anacamptis pyramidalis.
Other groups of
local constants can be recognised and may be ecologically
meaningful as well as useful in characterising associations within
this grassland complex. For example, Brachypodium pinnatum, Zerna
erecta and Helianthemum chamaecistus show higher constancy on
Jurassic limestones than on Chalk, although all three are common in
some chalk grasslands; while Dactylis glomerata, Koeleria cristata
and Prunella vulgaris are constant on Chalk but less so on Jurassic
limestones. A number of species of lower constancy and rare species
belong mainly to one or other of the major calcareous rock
formations and so have value as differentia for the various
grassland associations.
Chalk rocks of UK
http://jncc.defra.gov.uk/pdf/V23Chap1.pdf
