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Types of Grassland

6 May, 2015 - 09:39

Britain’s lowland limestone grasslands share the common geochemical feature that they are composed mainly of rocks rich in calcium carbonate. Also, until recent times they were unenclosed and formed the principal sheep pastures of southern England. These grasslands, which may never have been cultivated or have escaped ploughing for hundreds of years, are often described as semi-natural grasslands. Their distribution in England rests on five main geological formations,

  • Chalk,
  • Oolitic limestone,
  • Carboniferous limestone,
  • Magnesium limestone
  • Devonian limestone.

The most important differences which influence the vegetation they carry appears to be physical rather than chemical. Limestones are harder and frequently form rocky outcrops on hills which may give rise to cliffs, often associated with screes. In some districts there are extensive areas in which landscape features produced through rocks being dissolved by running water are developed (karstic features). It has been postulated that the difference in hardness of the rocks may have been an important factor in determining the present-day flora, the softer limestone hills having been tree-covered in early post-glacial times whereas the harder rocks, especially on cliffs, gorges and similar exposed sites may have remained open throughout the period of forest maximum. This hypothesis suggests that until forest clearance by people in the Neolithic(3000-1850 B.C.) and Bronze Age (1850-550 B.C.) periods some of these sites may have served as refuges for plants which subsequently colonized the open grasslands. Lowland calcareous grasslands are derived from sedimentary deposits of chalk or other types of limestone, e.g. metamorphic mica schist and serpentine. Other base-rich substrates, such as ultrabasic igneous formations and heavy metal contaminated soil support Calaminarian grassland.

The most diverse communities, with characteristic flora including some important lower plant communities, characteristic and specialist invertebrates and birds, occur on shallow rendzina soils over calcareous bedrock. Other quality measures include the presence of short-lived disturbance patches, patches of developing and mature scrub, and areas with continuity of low-intensity grazing.

Large sites tend to occur along escarpments where modern agricultural techniques are not dominant and where soils derived from glacial deposits have not obscured the calcareous bedrock. Large sites, or smaller contiguous sites, are more likely to suppc characteristic species with large ranges and provide opportunities for recolonisation local extinctions occur.

The Lizard peninsula Cornwall's is Britain's most southerly point. Nowhere else in Cornwall can boast such a density of nationally recognised Sites of Special Scientific Interest (SSSI's) or regionally important county geology sites (formery known as RIGS). The rocks on the Lizard are totally different from the rest of Cornwall. The most extensive (20 square miles) is the serpentine which is largest outcrop.

Serpentine and gabbro produce magnesium or calcium rich soils and it is the resulting alkalinity of the soils on these parts of the Lizard that has enabled a large number of quite rare plants to thrive here, such as dropwort, salad burnet, bloody cranesbill and the rare Cornish heath which is only found on The Lizard.