Habitat account - Rocky habitats and caves


8240 Limestone pavements  * Priority feature

Background to selection

Distribution of SACs/SCIs/cSACs with habitat 8240 Limestone pavements.  Click image for enlarged map.
Distribution of SACs/SCIs/cSACs with habitat 8240 Limestone pavements. Click image for enlarged map.

Description and ecological characteristics

 

Limestone pavements are outcrops of rock, typically horizontal or gently inclined, although a few are steeply inclined. The surface has been dissolved by water over millions of years into ‘paving blocks’, known as clints, with a complex reticulate pattern of crevices, known as grikes, between them. A range of calcareous rock, heath, grassland, scrub and woodland NVC types can occur on limestone pavement. The vegetation of limestone pavements is unusual because of the combinations of floristic elements, including woodland and woodland edge species, such as hart’s-tongue Phyllitis scolopendrium and dog’s mercury Mercurialis perennis. On the clint surfaces or the upper walls of the grikes there are plants of rocky habitats, such as wall-rue Asplenium ruta-muraria and maidenhair spleenwort Asplenium trichomanes. The grikes provide a shady, humid environment favouring woodland plants.

 

Grazing pressure is a key factor in determining ecological variation in limestone pavements. Where grazing pressure is low, woodland may cover the pavement and woodland vegetation may mask the limestone surface. Here only the massive areas of pavement may be exposed as clearings. Where there is heavy grazing pressure, vegetation may be found only in the grikes, but, where grazing is lighter, dwarf trees, herbs and ferns may protrude from the grikes. Grikes that are about 60 cm deep provide shelter without unduly limiting light and are usually the best floristically.

 

One rare species, the rigid buckler-fern Dryopteris submontana, has its main centre of population in limestone pavement and, in common with two other rare species, dark-red helleborine Epipactis atrorubens and angular Solomon’s-seal Polygonatum odoratum, flourishes in the low- to mid-altitude pavements. Other rare species, such as baneberry Actaea spicata and green spleenwort Asplenium viride, occur in more montane pavements.

European status and distribution

 

Within the Atlantic Biogeographical Region, Limestone pavements occur in the UK, Ireland and Sweden. Elsewhere in Europe there are believed to be only fragmentary occurrences in high alpine limestone areas.

UK status and distribution Click to view UK distribution of this habitat

 

In the UK limestone pavement is uncommon, covering only 2,000 ha. Nevertheless it is widespread, being found from south Wales to northern Scotland on three different limestone formations – the Carboniferous in England, Wales and Northern Ireland, and the Dalradian and Durness (Cambro-Ordovician) in Scotland. The most extensive limestone pavements occur on the Carboniferous limestone of northern England, from Morecambe Bay to the Pennines. Elsewhere, the exposures in Wales and Northern Ireland and the Dalradian of Scotland are of limited extent. However, those on the Durness limestone of north-west Scotland are valuable because they represent an extremely unusual geological and floristic variant.

Site selection rationale

 

Sites have been selected to reflect the UK’s special responsibility for this priority habitat type. Larger sites have been favoured because they usually contain the widest range of plant communities. Selection takes account of the distribution of the habitat, and geographic variation. Altitudinal range is also significant, as there are significant differences between pavements close to sea level and those in montane areas. Another factor in ecological variation is the level of grazing, and the SAC series contains a range of land use types, from woodland to sheep pastures. All of the selected sites have a well-developed clint and grike structure, with associated vegetation patterning. Several sites provide representation of rare plants.


Site accounts

Asby Complex Cumbria
Asby is one of four sites representing Limestone pavements in the north of England. It has been selected because of its size and its well-developed flora of species typical of more montane pavements and sheep-grazed pastures. Most of the pavements contain dog’s mercury Mercurialis perennis and wall lettuce Mycelis muralis, but in the main the herb flora is restricted, perhaps reflecting exposure to grazing sheep over many decades. The grikes provide a niche for a varied assemblage of ferns. Green spleenwort Asplenium viride, wall-rue Asplenium ruta-muraria, maidenhair spleenwort Asplenium trichomanes, brittle bladder-fern Cystopteris fragilis, male-fern Dryopteris filix-mas, hard shield-fern Polystichum aculeatum and hart’s-tongue Phyllitis scolopendrium occur in most pavements, with limestone fern Gymnocarpium robertianum and rigid buckler-fern Dryopteris submontana in some pavements. Where grazing is less intensive, the flora is more diverse and trees and shrubs grow beyond the confines of the grikes.
Craven Limestone Complex North Yorkshire
Craven is one of four sites representing Limestone pavements in northern England. It is selected on the basis of its size and as an example of mid-altitude pavement. There is a wide range of transitions to other habitats, including 6210 semi-natural dry grasslands, 7230 Alkaline fens and 9180 Tilio-Acerion forests. Despite being accessible to grazing sheep, these pavements provide a refuge for downy currant Ribes spicatum and, occasionally, alpine cinquefoil Potentilla crantzii and baneberry Actaea spicata.
Durness Highlands and Islands
This site on Cambro–Ordovician Durness limestone is the most north-westerly occurrence of Limestone pavements in the UK. It is one of five sites representing the northern variant of this habitat type. Because of its location it supports a diverse flora rich in northern and arctic-alpine species. The site supports communities with a maritime element, similar to those on Strath, with burnet rose Rosa pimpinellifolia, stone bramble Rubus saxatilis, mountain avens Dryas octopetala, dark-red helleborine Epipactis atrorubens and black spleenwort Asplenium adiantum-nigrum. Both lesser meadow-rue Thalictrum minus and alpine meadow-rue Thalictrum alpinum occur here. Further inland, brittle bladder-fern Cystopteris fragilis and hard shield-fern Polystichum aculeatum flourish.
Inchnadamph Highlands and Islands
Inchnadamph is one of four sites representing Limestone pavements on Cambro–Ordovician Durness limestone in north-west Scotland. It is one of the most floristically-rich limestone pavement sites in Scotland. The communities have some species in common with the more maritime communities on Strath, including burnet rose Rosa pimpinellifolia and stone bramble Rubus saxatilis, but are montane in character. For example, holly-fern Polystichum lonchitis is abundant and whortle-leaved willow Salix myrsinites is notable here as it occurs on limestone pavement. When S. myrsinites is abundant, the habitat is referable to Annex I type 4080 Sub-Arctic Salix spp. scrub, for which the site is also selected.
Ingleborough Complex North Yorkshire
Ingleborough is one of four sites in northern England representing Limestone pavements on Carboniferous limestone. It has the most extensive series of Limestone pavements in the UK, varying from moderate altitude to montane in character (300-640 m). The pavements range from those where grazing is completely excluded (Colt Park Wood National Nature Reserve), to some where grazing is restricted (pavements amidst cattle-grazed pastures) and others within common land intensively grazed by sheep. Characteristic species include baneberry Actaea spicata (more abundant here than elsewhere), great bellflower Campanula latifolia, found only here as a limestone pavement species, lily-of-the-valley Convallaria majalis, marsh hawk’s-beard Crepis paludosa, wall lettuce Mycelis muralis, lesser meadow-rue Thalictrum minus and mountain melick Melica nutans. Among the ferns, green spleenwort Asplenium viride, brittle bladder-fern Cystopteris fragilis and hard shield-fern Polystichum aculeatum occur on most pavements. Rigid buckler-fern Dryopteris submontana and limestone fern Gymnocarpium robertianum are widespread but much less abundant than at Morecambe Bay Pavements. Dog’s mercury Mercurialis perennis and wood sorrel Oxalis acetosella occur on most pavements.
Morecambe Bay Pavements Cumbria, Lancashire
This is one of four sites in northern England representing Limestone pavements on Carboniferous limestone. This site provides an example of lowland pavements that range from low to moderate altitudes (up to 274 m). Some of the pavements form woodland clearings that are sheltered and warm up quickly in spring. The pavement flora is here at its most diverse and, where grazing is absent, can be seen at its best because plant growth is not confined to the grikes. Trees and shrubs, including yew Taxus baccata, juniper Juniperus communis, buckthorn Rhamnus cathartica, hazel Corylus avellana, small-leaved lime Tilia cordata and ash Fraxinus excelsior, grow above the pavement surface. Some pavements lie within sheep pasture but are for the most part lightly grazed. Rustyback Ceterach officinarum is restricted to pavements that form sheltered woodland clearings. Other ferns occurring on the site include the nationally scarce rigid buckler-fern Dryopteris submontana, which is abundant on Hutton Roof Crags, and limestone fern Gymnocarpium robertianum. These pavements tend to be rich in herbs, with lily-of-the-valley Convallaria majalis, dark-red helleborine Epipactis atrorubens, pale St John’s-wort Hypericum montanum, ploughman’s-spikenard Inula conyzae, angular Solomon’s-seal Polygonatum odoratum, wood-sage Teucrium scorodonia, lesser meadow-rue Thalictrum minus and hairy violet Viola hirta achieving their best representation in limestone pavement here.
Rassal Highlands and Islands
Rassal is one of four sites representing Limestone pavements on Cambro–Ordovician Durness limestone in north-west Scotland. In terms of the extent of the pavements, within Scotland Rassal is second only to Strath. Although the pavements are less well-developed structurally than on Strath, they occur up to a higher altitude (230-380 m) and have a more montane flora, similar to that at Inchnadamph. Notable northern and montane species occurring in the grikes include holly-fern Polystichum lonchitis (especially abundant), whortle-leaved willow Salix myrsinites, stone bramble Rubus saxatilis, green spleenwort Asplenium viride, globe-flower Trollius europaeus and rock sedge Carex rupestris. Stunted trees and shrubs, mostly downy birch Betula pubescens, rowan Sorbus aucuparia, ash Fraxinus excelsior and holly Ilex aquifolium are confined to rock crevices by browsing. Mountain avens Dryas octopetala occurs on thin soil on the clints and grades into 6170 Alpine and subalpine calcareous grasslands. Patches of S. myrsinites scrub are referable to 4080 Sub-Arctic Salix spp. scrub, while 9180 Tilio-Acerion forests of slopes, screes and ravines is also developed on part of the same exposure of limestone pavement. These are all Annex I habitats for which the site is also selected.
Strath Highlands and Islands
Strath is one of four sites representing Limestone pavements on Cambro–Ordovician Durness limestone in north-west Scotland. It is the most extensive and floristically rich limestone pavement in Scotland and represents a more maritime variant of the habitat type. The pavements are found at a range of altitudes from close to sea level up to 280 m. There is a maritime influence across the whole site. Some of the species on this site, including tutsan Hypericum androsaemum, dark-red helleborine Epipactis atrorubens, herb-Paris Paris quadrifolia, burnet rose Rosa pimpinellifolia, stone bramble Rubus saxatilis, wood-sage Teucrium scorodonia and black spleenwort Asplenium adiantum-nigrum, are more southern species, characteristic of Morecambe Bay Pavements near the coast. Indeed, one pavement supports the characteristic southern scrub woodland of hazel Corylus avellana. Other characteristic species, such as melancholy thistle Cirsium heterophyllum, globeflower Trollius europaeus and mountain avens Dryas octopetala, are distinctly northern. Perhaps as a result of their more northerly latitude, vernal species such as ramsons Allium ursinum, wood anemone Anemone nemorosa, primrose Primula vulgaris and common dog-violet Viola riviniana are more common here than in southern coastal pavements.
West Fermanagh Scarplands Northern Ireland
The limestone pavement within the West Fermanagh Scarplands occurs on Carboniferous limestone. Although development has not been deep (possibly due to the intensity of glacial action), it is the most extensive area of this distinctive habitat in Northern Ireland. The grikes provide a niche for a varied assemblage of ferns and higher plants. Wall-rue Asplenium ruta-muraria, black spleenwort A. trichomanes, brittle bladder-fern Cystopteris fragilis and hart’s-tongue Phyllitis scolopendrium are widespread, as are herb-Robert Geranium robertianum, wood-sorrel Oxalis acetosella and hazel Corylus avellana. The clint surfaces support typical calcareous grassland.

SACs/SCIs/cSACs where this Annex I habitat is a qualifying feature, but not a primary reason for site selection

Moor House - Upper Teesdale Cumbria, Tees Valley and Durham
Tulach Hill and Glen Fender Meadows Eastern Scotland
 

Many designated sites are on private land: the listing of a site in these pages does not imply any right of public access.