Lichens of Lowland Grassland (including Dune Grassland and Maritime Coastal Clifftops)
1. Introduction
Some lowland grassland is important for lichens and other cryptogams, although they are seldom as luxuriant or conspicuous as for example on rocks, heaths or woodland. Lichen interest is usually greatest in situations where the grassland is under stress as they are generally poor competitors with vascular plants and the more vigorous bryophytes. Situations where this may occur include chalk grassland on steep slopes, grassland developed on areas of disturbed ground, exposed coastal cliff tops and headlands and blown sand overlying rock or shingle (usually in coastal sites). For a detailed account of the lichen flora of chalk grassland see Gilbert (1993).
2. The Grassland Habitat
A common problem for the non-specialist is in recognising when the cryptogamic component of grassland is of interest or is indeed present. As a guide, if the following features are present, there is a good potential that the cryptogamic interest may be high:
- The grassland is very closely grazed, perhaps by rabbit and sheep, is on calcareous soils but not exclusively so, with bare open ground, perhaps stony, interspersed with tussocks of fine grasses (eg. Festuca ovina) and herbs, particularly annuals and species such as Hieracium pilosella, Lotus corniculatus, Thymus spp. and Rumex acetosella. This may intergrade with heath and is often called grass-heath.
- There are rock exposures, with crevices, scree, and partially embedded stones or rock fragments in the turf. Thin skeletal soils overlying rocks, perhaps remaining damp, are often very good for cryptogams.
- The habitat has experienced disturbance from farming, mineral extraction, military activities, constructional work or rabbits. The disturbance has often been accompanied by nutrient depletion and the bringing of fresh subsoil up to the surface.
- The topography is varied with anthills, banks or earthworks, coombes, coastal rocky headlands providing a range of aspects, thin soils with low vascular plant cover on exposed sunny slopes, and sheltered and north facing with more closed vegetation.
A survey of volume 3 of British Plant Communities (Rodwell 1992) lists 95 bryophyte and lichen taxa associated with lowland grasslands. However, as cryptogams were not consistently recorded in baseline data, the true total is probably much greater.
3. Microhabitats
Open bare ground is probably the most crucial requisite for cryptogams, and this is usually well represented in the calcareous (or calcicolous) grassland communities CG1, CG2, CG7, the acid (or calcifugous) grassland community U1, maritime communities MC5 and MC8, and some sand dune communities, SD. The cryptogamic floras of calcicolous grasslands tend to be more diverse than those in calcifugous grasslands and equally distinctive.
Anthills are typically occupied by a suite of bryophytes including Ceratodon purpureus, Bryum spp., Polytrichum spp., Pleuridium spp., and in some calcicolous grasslands, Rhodobryum roseum, a distinctive and relatively robust moss capable of co-existing with vascular plants. Anthills that have been disturbed by woodpeckers or badgers frequently provide a niche for mosses such as Microbryum rectum and M. curvicolle, particularly on sheltered north sides of the mound. This same microhabitat is also colonised by a suite of lichens including Bacidia muscorum, B. sabuletorum, Catapyrenium, squamulosum, Collema tenax, Leptogium teretiusculum, Peltigera rufescens and Polyblastia gelatinosa.
Small, partially embedded chalk and limestone stones in grasslands are important in providing niches for lichens. Flints can be extremely important habitat for lichens with 41 species recorded on them at Porton Down, though generally sites with over 25 species are exceptional. Large flints, well-bedded in the turf, appear to be the richest substratum. The total number of lichens recorded from flint in chalk grassland is 78. Chalk pebbles can also be important with a total of 65 species although only a few of these, for example, Verrucaria spp. dominate the community. The diverse cryptogamic flora of the CG9, in part due to its upland locus and hence wetter climate, is also to a large extent due to exposures of bedrock, ledges and scree.
Stony ground with a thin mantle of soil and/or blown sand, which is relatively free of vascular plants, supports a range of lichens such as Placidium spp., Fulgensia fulgens, Diploschistes muscorum, Heterodermia leucomela, Psora decipiens, Squamarina cartilaginea, Toninia sedifolia, Collema sp., and Peltigera spp. These often occur near the sea where soils are drought-prone and under maritime influence. Crevices between outcrops are particularly rich for lichens.
4. Aspect
Shaded chalk pebbles in north facing situations can hold populations of lichens which are otherwise very rare, for example at Heyshott Down in Sussex where a remarkable community occurs on pebbles at the foot of a steep north facing slope in a long disused quarry.
On southern aspects or on flat ground, where unproductive soils are periodically drought affected and exposed to higher levels of insolation, a quite different cryptogamic flora develops, typically found in CG1, CG2, CG7, U1, but also frequently in CG3 and CG5 grasslands. This is the favoured topography for the Ditrichum flexicaule – Diploschistes muscorum sub-community of the CG7 with its characteristic lichens. A combination of micro-climate and unproductive soils provide sufficient open ground for the lichen community can develop. It is generally associated with a suite of mosses including ephemerals such as Encalypta spp. and perennial species such as Homalothecium lutescens, Campyliadelphus chrysophyllus, Ctenidium molluscum, and more locally, Entodon concinnus and Abietinella abietina var. abietina in calcicolous grasslands.
Ancient earthworks and banks, with thin soils and open species-rich turf, are often good for lichens. The variation in topography, aspect and position on slope provides numerous niches. Indeed, ancient earthworks are often the richest areas for cryptogams in grasslands, especially in calcicolous communities.
Earthworks as recent as those created in the Second World War are often excellent for lichens especially where the surface soil was bulldozed down to bare chalk.
Old mine tips often provide toxic conditions or unproductive soils which are important for lichens including a suite of species which prefer soils with heavy metals.
The scrub-grassland ecotone is also important for lichens, particularly in calcicolous grasslands. Shaded chalk fragments also support a lichen flora with species such as Collemopsidium monense, Arthopyrenia saxicola and Thelidium species. Scrub itself is also often rich in epiphytes, particularly elder scrub.
5. Threats
The greatest threats are those which tip the balance away from the stress which is limiting the competition by vascular plants. In this context air pollution is also thought to be damaging to the cryptogamic flora, despite the large buffering capacity of calcareous soils. There is evidence from Dutch chalk grasslands that nitrogen deposition has led to the impoverishment of the bryophyte and lichen floras (During & Willems 1986). This is also probably true in Breckland (Hitch & Lambley, 1996), (Lambley, P.W., Wolseley, P.A. James, P.W. (2004)). Wolseley has also observed similar in pacts on Fulgensia in coastal grassland in Pembrokeshire (Lambley, P.W., Wolseley, P.A. James, P.W. (2004)). The effects may be indirect with nitrogen inputs increasing soil productivity and thus leading to more vigorous vascular plant growth. There may also be more direct effects since many grassland cryptogams are known to be sensitive to pollutants.
Invasive plants and scrub both native and non native also pose a threat usually in situations where grazing pressure is reduced or ceases. This crowds out the short grassland and open ground and may also change the microclimate. Cotoneaster spp are are a particular problem in some limestone grasslands. In the very unusual dune grassland and maritime heath on Tresco in the Scilly Islands non-native flowering plants eg Agapanthus and Mesembryanthemum spp. are threatening a terrestrial Lobarion community.
Extreme trampling through over-grazing and excessive visitor pressure can also be damaging (but Management Guidance).
6. References
Gilbert, O. L. (1993). The lichens of chalk grassland. Lichenologist, 25: 379-414
Rodwell, J. (1992). British Plant Communities: Grasslands. CUP