British & Irish distribution: principally in northern England, northwestern Scotland and western Ireland, with scattered local development elsewhere.
Limestone, fundamentally, is calcium carbonate, formed from deposits of varying degrees of purity, particularly in warm, shallow seas. Many organisms, particularly many types of microorganism, secrete calcium carbonate as a by-product of metabolic activity or for formation of protective coverings, and their remains may make up a significant proportion of the resultant limestone rock.
Many limestones are relatively soft, notably chalk and the oolitic limestones in the south of England, and these limestones weather too quickly to form pavements. However, where the limestone is hard and horizontally-bedded, typically scoured by glacial action, weathering is more pronounced along the joints and then pavements can form. Almost all such pavements in the British Isles are formed from the massive limestones laid down in the Carboniferous era - important examples including the Craven Pennines (notably around Ingleborough) in Yorkshire, Gait Barrows in Lancashire, Hutton Roof Crags in Westmorland (Cumbria) and the Burren in western Ireland. However, the much older Cambrian limestone of north west Scotland is also hard enough for localised pavement formation, as in the vicinity of Durness, West Sutherland.
Limestone is readily dissolved by rainwater that has been acidified by dissolved carbon dioxide or organic acids and this water can act on the rectangular joints, deepening and widening them to form fissures that may become several metres deep. These fissures are commonly termed 'grikes' (originally a Yorkshire word and the word most familiar to ecologists), or in Ireland they are known as 'scailps'. The slabs, usually flat on top, that are left between the grikes, are 'clints'. The type of landscape produced is an example of karst, "terrain in which soluble rocks are altered above and below ground by the dissolving action of water and that bears distinctive characteristics of relief and drainage." (J.N. Jennings, quoted from Huggett, 2003).
Depending on the nature of the limestone and the action of the weather, the clints may remain intact, or else may be broken up to form 'shattered limestone pavements', with fragments of all sizes laying loose on the surface and falling into the fissures as they form.
Any depressions on the clint surfaces will trap rainwater which in turn will allow the growth of algae, notably the cyanobacterium ('blue-green alga'), Nostoc, which secretes protective mucilage which will protect it from rapid drying (though it does survive extreme desiccation in the summer months). Nostoc itself produces the organic acids that further dissolve the limestone by biological weathering to form solution hollows.
It is a necessary part of the concept of a limestone pavement that the rock is exposed (though similar rock geomorphology may exist under soil covering). Disagreements exist on the extent to which some pavements have ever been soil-covered following glacial scouring. However, once joints in the bedrock have opened through solution processes, existing soil and any other drift material is likely to be lost into the grikes as they form, and may indeed be lost to subterranean cave systems.
Soil may, itself, be involved in the initiation of limestone pavement or other karren features. Any acidity in rain percolating through a calcareous (calcium-rich) soil is likely to be neutralised by the bases in the soil, and so the soil protects the rock below. If the soil itself is acidic, not only will this neutralisation not occur, but the acidity will be reinforced by organic acids from the soil. The acid waters will penetrate joints, eventually dissolving large grikes, into which the soil may be washed, exposing the bedrock surface. Since erosion under a wet soil results from continuous corrosive soaking, as distinct from the episodic nature of rainfall events, the rock surface my be eroded and dissected in a more irregular manner, producing bizarre shapes in the eventually exposed rock. Detailed discussion of these processes is given by Trudgill (1985).
The best limestone pavements tend to be upland or on exposed coasts, and combinations of wind and, more particularly, grazing often prevent extensive colonisation of the pavement surfaces by vegetation. On the other hand, the grikes provide a microhabitat that is sheltered, humid and inaccessible to sheep and other large grazing animals. An apparently hostile landscape can contain a rich variety of plant species, including a number that are nationally rare or scarce.
Where thin soils can accumulate on limestone pavements, these are typically rendzina soils with the shallow humus layer grading into a raw mixture of loam and limestone fragments directly above the bedrock, with no zone of deposition as is the case with mature soils. High levels of free calcium carbonate reduce the availability of other cations, leading to deficiencies of iron and other plant nutrients. Thus the habitat is suitable only for those species that can tolerate these conditions, species that in many cases are intolerant of competition from more vigorous plants on richer soils and so rare or of restricted distribution. Many are calcicoles, lime-demanding species (though it is often difficult to distinguish a requirement for high calcium levels from high tolerance and competitive restriction to such habitats).
The thin surface soils are, however, also liable to lose minerals as a result of leaching, the minerals dissolving in rainwater and being lost in surface run-off and transport down into the grikes and bedrock. This can lead to formation of limestone heath, in which lime-intolerant (calcifuge) species such as Heather (Calluna vulgaris) can grow side by side with lime demanding species - the co-existence based partly on different rooting levels, with the calcifuges rooted in the more acidic surface layers.
Within the grikes there can be greater accumulations of leaf-litter, leading to 'woodland' conditions (with or without the trees!), with shade, humidity and accumulations of richer soil. Typical woodland herbs, such as the Primrose (Primula vulgaris) and Dog's Mercury (Mercurialis perennis), can grow well, and nitrogen-loving species such as Stinging Nettle (Urtica dioica) may find a suitable niche.
Pavements usually seem largely treeless, but appearances can be deceptive. Deeper, wider grikes can contain quite mature trees, notably Ash (Fraxinus excelsior). Trees and shrubs remain relatively dwarfed, effectively in a 'bonsai' state, emerging shoots being pruned by wind and grazing animals. Sometimes, in exposed places, a tree or shrub can emerge from a grike but then is forced by the wind to grow prostrate over the rock surface.
Where pavements are protected from grazing and have a degree of shelter from strong winds, scrub can begin to develop. On some pavements this is predominantly Hazel (Corylus avellana), but, as noted above, Ash can be a colonist of grikes and this can continue to grow into relatively large trees. Eventually, as more leaf-litter accumulates on the clint surfaces, they can be colonised by mats of bryophytes, and eventually the whole pavement can become wooded. Notably this has happened at Colt Park Wood, part of the Ribblehead Woods SSSI in Yorkshire, where there is mature ashwood on pavement. Forest of this type fits the 'Fraxinus excelsior-Sorbus aucuparia-Mercurialis perennis woodland' (W9) of the National Vegetation Classification and there is good discussion of the climatic and edaphic factors that characterise this type of forest in Rodwell et al. (1991). The fragmentary 'forest' in the grikes of many open pavements undoubtedly also belongs here.
It should be noted that wooded limestone pavement should be rated DANGEROUS in fieldwork safety assessment. Bryophyte and other vegetation mats close over deep grikes but will not support human weight. Testing of every step is vital.
Not all vegetation is confined to the grikes. Where pavements have eroded to different levels, open vegetation or more continuous turf can develop on the thin soils between larger limestone blocks. Here may be many of the plant rarities, intolerant of the competition from more vigorous and coarser species on deeper soils. Many have bright, showy flowers, contributing aesthetic beauty as well as scientific interest.
Another ecological grouping of plants that can find a niche on limestone pavements are the 'spring ephemerals' (therophytes). The open surfaces of the clints are a hostile habitat in the summer, not only dry but also liable to heat up in the sun to temperatures that will kill much plant growth. (Plants themselves, in the same ambient temperatures, have efficient re-radiation to avoid overheating.)
Depressions and shallow solution hollows in the surface can accumulate a little humus and mosses can grow on these surfaces in the wetter, mainly winter months. Such mosses must be highly tolerant of desiccation and able to revive in wetter weather. However, small annuals, often germinating in autumn in the moss carpets and overwintering as small rosettes, can also survive these conditions by completing their life-cycles early in the year. Large populations of these tiny plants in April and May may have died, dried up and vanished by mid-summer, yet their seeds are surviving in the moss and rubble, ready again for the autumn rains.
Undoubtedly the grike microhabitat suits many invertebrates, but the overall habitat is not notable for specialised vertebrates, except in that pavements, particularly those colonised by scrub, provide refugia for species becoming more restricted by intensive agriculture. The Common Lizard (Lacerta vivipara), while by no means restricted to limestone habitats, does undoubtedly benefit from the basking opportunities provided by the tops of the clints - enhanced by the reflectivity of the limestone rock.
In Ireland, the grikes and crevices of the Burren provide good hunting territory for the Irish Stoat (Mustela ermina hibernica) [a trinomial name as it is a separate subspecies], and the scrubbier areas of the Burren are the stronghold in Ireland for the Pine Martin (Martes martes), though whether the latter can be said to be associated specifically with limestone pavements is doubtful.
A claim for a moth of limestone pavements can be made for the Burren Green (Calamia tridens occidentalis), an attractive bright green moth, known in the British Isles only from the Burren and first discovered there as late as 1949. The food plant is apparently Blue Moor-grass (Sesleria caerulea) (Skinner, 1998), a very characteristic grass of the pavements. A moth of the major limestone masses in northern England is the Least Minor (Photedes captiuncula), which is also on the Burren as a distinct subspecies.
A nicely produced account of Burren wildlife is that of D'Arcy & Hayward (1992), including an authoritative consideration of threats to and conservation of this unique area. However much of the book considers fens, turloughs (temporary lakes) and other features of the Burren limestone that are outside the scope of this present summary of the limestone pavement habitat.
Limestone pavements have long survived because they have been resistant to many human activities. Only where pavements were thin was it possible to clear the surface stone to create more continuous pasture - and indeed some such old pastures themselves are of considerable floristic interest.
However, mechanical breaking and removal of limestone pavements has become more feasible. A number have been lost to agriculture, with farmers not unreasonably wanting to extend their usable land (and to reduce loss of livestock into grikes). Arguably of greater recent impact has been the removal of limestone blocks for horticulture, with weathered (particularly water-worn) limestone considered of great value in landscape gardening schemes, as well as finding ready sales to amateur rock gardeners via garden centres. The garden rockery can represent irreversible damage and destruction of the habitat that gardeners are trying to recreate.
SSSI notifications have provided only weak protection, particularly where 'agricultural use' of land is effectively unchanged. The Wildlife and Countryside Act (1981) has provision for Limestone Pavement Orders (LPOs), whereby statutory protection can be given to 'areas of limestone wholly or partly exposed on the surface of the ground and fissured by natural erosion'. The Act not only prohibits removal of limestone, but also disturbance and damage. The casual visitor to a designated site who takes home a few lumps of limestone in the boot of his car commits an offence, just as does someone involved in commercial clearance.
Limestone pavement is a priority habitat included in the UK Biodiversity Action Plan (see link below).