1. Where are Lowland Rocks?
1.1 Lichens can be conspicuous inhabitants of rocky habitats. These can be natural outcrops or erratic boulders, or they can be man-made exposures in quarries, rail and road cuttings, etc. Further lichen communities can exist on rockery stones and decorative boulders in gardens and estates. Lowland rock exposures are important for extending the geographical range for many lichen species. In particular, lichens common in the north and west of Britain are enabled to inhabit small areas of south and east Britain where lowland rock exposures are present. Man-made rocky substrata further enable extension of the range of many lichens (see this volume, chapter 14). However, many lowland rock exposures are scarce or of very limited area and are deserving of publicity and protection.
1.2 Lowland rocks have seldom been specifically written about, though information can be gleaned from Lichenological field excursions reported in The Lichenologist. Detailed reports occur in Pentecost and Rose (1985) dealing with the Wealden sandstones, Hawksworth (1969) in the Lichen Flora of Derbyshire, Bowen (1976) for Dorset, Fletcher (1979) for parts of Leicestershire. James et. al. (1977) gave a useful summary of lowland rock lichen communities.
1.3 These lowland rock lichen communities may be regarded as natural when they have developed over long periods of time on naturally exposed outcrops. Communities on man-made exposures are also important, and can probably be regarded as semi-natural as the lichens here are opportunists taking advantage of mans' provision of appropriate substrata.
1.4 Although this chapter is devoted to lowland rocks, they may occur in the highland zone, north and west of the line between the rivers Tees and Exe). However, it could be expected that the environmental factors and conditions influencing low-altitude rock lichens in the 'Highland' zone may be somewhat different to those in lowland Britain. This would be particularly true of temperature and rainfall.
2. Types of Lowland Rocks
2.1 Natural rock habitats. It is unusual for a rocky site to be entirely devoid of lichens. Bare rocks still occur in the air-polluted Midlands and industrial North, where rock exposures remain blackened by soot which seems to inhibit lichen colonization. Many lowland sites have rocky cliffs resulting from natural phenomena such as water erosion. These may be low outcrops, as in the Sussex Weald and Derbyshire edges, or can achieve great heights in the Avon, Cheddar and Derbyshire gorges.
Many on the Pennine fringes occupy hill tops, as in Charnwood Forest, Shropshire and Staffordshire, where they exhibit post-glacial frost shattering, and have many crevices and overhangs available for exploitation by lichens. Glacial-erratic boulders are frequent in the highland north and west, and on lowland sites in Wales and the South West, but tend to be buried by glacial drift in the lowlands. Prehistoric structures such as Henges, and natural Sarsen stones, because of their great age, have lichen populations indistinguishable from those on natural exposures and are often dealt with as natural outcrops. Shingle lichens occur on river beds and seashores.
2.2 It should be noted that many habitats dealt with elsewhere in this volume may include rocks when in the lowlands.
2.3 Semi-natural outcrops, are made or altered by man, but have become colonized naturally by lichens. These include quarries, which become richer with time as dust leaches away and the rock surfaces weather. Frequently, relics of natural exposures may be found around quarry workings. Often these relics can be identified from their lichens. Rocks exposed at the sides of roads and in railway cuttings often have notable lichen populations. Shingle and ballast in these places may be even richer.
2.4 Gardens often contain lichen-rich rockery and other ornamental stonework. These can become lichen-rich when old, as in old estates. A great variety of rock type may exists at these sites, giving a wide variety of lichens. It is important to note that man-made habitats extend natural lichen populations into the lowland zone, as for example in Essex, where natural rocks are lacking.
3. Relation to Soil Habitats
3.1 Lowland sites containing rocks usually have a mosaic of soil habitats intermixed with those predominantly of rock. Soil inhabiting lichens may penetrate rock crevices and spread onto rock surfaces. Similarly, some saxicolous lichens may spread onto soil. The role of lichens in soil formation received some attention in the early years of this century. Briefly, Fry (1927) demonstrated that lichens can exfoliate rock surfaces by a process of alternate shrinking and expansion due to drying out. This mechanical process will prise off rock flakes. Foliose lichens are known to accumulate particles beneath their lobes, principally of dead lichen and other plant material, forming a humus, with sand particles from the rock below or blown in by wind. Many invertebrates take advantage of this sheltered soil layer (Gerson 1973). The soil accumulated by lichens can become thick enough to support seedlings and eventually the entire rock surface can be buried. This is seen at its most rapid in woodland or below bracken.
3.2 Chemical weathering of rocks by lichens also takes place (see Monuments and Urban Habitats).
3.3 Rock type greatly influences the types of lichen communities present. Briefly, siliceous rocks, of granite, slate, quartzite, etc., support epilithic lichens containing foliose Parmelia spp., Umbilicaria spp., Porpidia tuberculosa, Lecanora rupicola, L. polytropa, etc. The presence of Lasallia pustulata, Parmelia incurva, P. disjuncta and Umbilicaria deusta, make the site particularly valuable. Soil lichens generally include Coelocaulon aculeatum and many Cladonia spp., but C. luteoalba may be directly on the rock surface. Calcareous rocks include chalk, limestone, oolite and some mudstones. They support mostly crustose lichens, particularly orange Caloplaca, usually in sun. Peculiar to these rock types are endolithic lichens where the thalli live within the rock surface. These are usually visible only by their black fruiting bodies, the perithecia. Examples include species of Verrucaria, Thelidium, Arthopyrenia, etc. Calcareous soils between rocks may be occupied by foliose species of Solorina spp., Squamarina crassa, Collema spp., and Leptogium spp. These often spread onto rock surfaces.
3.4 Stony soils occur around rocks and will bear a mixture of species typical of rocks admixed with terricolous lichens. Generally they are less rich than larger rocks which are more stable in time.
3.5 Rocks found in woodland can have spectacular lichen populations when they are neither too shaded nor over-buried in leaves. Here may be found Sphaerophorus in the N and W., and large thalli of Cladonia. Many of these sites once bore Lobaria and Sticta up to the early 19th Century, but these are now extinct in rocky sites SE of the highland line because of air pollution and drainage.
Lowland rock lichens are relatively neglected and threats to them are generally unrecognised.
4.1 Agriculture. This may be the greatest threat in modern conditions. Rocks in agricultural areas are affected by dusts raised by ploughing, where the nutrients raised inhibit the natural flora and encourage a pollution-tolerant flora. In extreme cases, the rocks may be entirely covered by algae. Fertilizer and herbicidal or pesticidal sprays and dusts can transfer to rocks as droplets or as airborne dust. Again, the nutrient status of the rock surface is greatly altered, promoting only those species which tolerate or prefer raised nutrient levels. Slurry dispersal also affects rock surfaces, as do the dung of cattle and horses. The nearby presence of intensive animal husbandry, especially pig farming, exerts a simplifying effect on lichen communities on trees (Benfield 1994), and this is probably similar on rocks.
4.2 Scrub development. Neglect of management of fields surrounding lowland rocks generally results in development of scrub. This is usually of Gorse (Ulex spp.), Elder (Sambucus niger), Blackthorn (Prunus spinosa), Hawthorn (Crataegus monogyna), etc. The result is over-shading of rock surfaces and their burial beneath leaf litter. Nutrient enrichment through water runoff from branches also affects lichens. Finally, increased shelter will diminish lichens which need rapid drying out by wind and sun.
4.3 Shading. Shading can occur from scrub and tree development, especially when planted too near to rocks. Construction or restoration of long-neglected walls can also cause over shading.
4.4 Bracken (Pteridium aquilinum). This is a problem on deeper soils. However, its effect on lichens is often over-exaggerated. Bracken can give shelter from wind and sun in the summer when abrasion is at its height. It can also inhibit visitors and their trampling damage. In the winter bracken dies down, letting in light at the time when lichens are growing. Provided rocks are not covered by dead fronds, the lichens may be unaffected. A greater problem lies in control of Bracken by sprays and fire, when other organisms are being conserved. There is no traceable information on the effect of herbicide sprays on lichens. But Asulam seems to be harmless unless mixed with wetting agent.
4.5 Gorse (Ulex spp.). This occurs on shallower soils than Bracken. As Gorse does not die down effectively in winter, its shading can become a serious threat. But its control by fire is also a threat to lichens on rocks. However, old Gorse which has become 'leggy' confers a degree of shelter to a site, and can bear interesting epiphytic lichens.
4.6 Amenity. Lowland rocks, particularly on hill tops, are popular with visitors, especially near to population centres. Physical abrasion of rock surfaces by trampling or use as seats, completely eradicates lichens. Rock climbing has also had a serious effect, eradicating lichens from popular lowland outcrops. Occasional accidental fires among Gorse and Bracken devastate saxicolous communities.
4.7 Grazing and trampling. Natural grazing and trampling, usually by Rabbits, is useful for maintaining a very short grass sward. Only isolated rocks tend to be affected by trampling when intensively used as Rabbit scrapes. Sheep tend to avoid rocks, provided their grazing density is controlled. But they will follow rocks when grassland has become overlong or colonized by Bramble and Gorse. Cattle and Horses are a more serious influence through their copious manuring, especially on flat surfaces. They also raise nitrogen levels through urine.
4.8 Removal of shelter. Sudden removal of scrub may affect lichens which could tolerate the previous conditions. Such a site would be opened to wind and sun and abrasion.
5. Management Proposals
5.1 In many areas of SE Britian, lowland rocks are a rare habitat and their importance for biodiversity locally, is considerable. The above account concentrates on threats as a guide to what features to consider when managing a site with lowland rock lichen interest. It depends on this interest being effectively established. Therefore, it is essential that site managers have the lichens thoroughly surveyed and their importance assessed on a regional basis. This allows a degree of management prioritization to be established.
5.2 Following the assessment of the lichen importance of a site the management plan can be divided into two, a plan for lichen conservation and a plan for lichen enhancement. Lichen conservation can follow the simple 'what to avoid guidelines' presented above. It is difficult to be prescriptive however. We simply do not know enough about the effects of some management activities to advise on how much action to take. Generally it is advised that management be done in stages. The effects of an activity should be monitored, preferably photographically, and evaluated before further steps are taken.
5.3 Enhancement of lowland rock lichen communities is rarely considered. Most site managers are content to let nature take care of itself. Generally it is advised that appropriate substrata should be made available and left free from the types of disturbance outlined above. Introduction of new substrata could usefully be considered, but it has to be remembered that lichen establishment by natural agencies can be very slow. The most interesting communities seem to have been developed over time spans of hundreds of years. Newly exposed rock surfaces take several years to develop the simplest of communities. It has been advocated that the process could be speeded up by watering the surface occasionally with dilute cow or horse manure ! The possibility of artificially establishing lichens on a conservation scale has rarely been advocated. However, it would be a simple matter to put foliose or fruticose thalli through a kitchen blender and water surfaces with that slurry.
Benfield, B. (1994) The impact of agriculture on epiphytic lichens at Plymtree, East Devon. Lichenologist 26: 91-94.
Bowen, H.J.M. (1976) The lichen flora of Dorset. Lichenologist, 8: 1-33. Fry, E.J. (1927). The mechanical action of crustaceous lichens on substrata of shale, schist, gneiss limestone and obsidian. Ann. Bot. 41: 437-460.
Fletcher, A. (1979) Lichens. In Evans I.M. (ed) North-East Leicestershire Coalfield survey report. Leicestershire Museums Service.
Gerson, U. (1973) Lichen-Arthropod associations. Lichenologist, 5: 434-433.
Hawksworth ,D.L. (1969) The Lichen Flora of Derbyshire. Lichenologist, 4: 105-193.
Jackson, A., Flanagan, M. (1997) Conservation of Cryptogams in the Weald. Royal Botanic Gardens, Kew.
James, P.W. , Hawksworth, D.L., Rose, F. (1977) Lichens communities in the British Isles: A preliminary conspectus. In Seaward, M.R.D. (ed) Lichen Ecology, 298-413.
Pentecost, A., Rose, F. (1985) Changes in the cryptogam flora of the Wealden sandrocks. J.Linn. Soc. Bot. 90: 217-230.