Gilbert (1993) has shown that the richest lichen sites are centred on areas that have been disturbed by man in the last two centuries or by natural events like landslips and sand blows. The continuation of some form of disturbance is therefore essential for the continuation of the lichen interest and also to counter the potential enriching effect from nitrogen deposition.
Grazing is the other factor which maintains the cryptogamic interest of most lowland grasslands. Bryophytes and lichens are generally unpalatable to herbivores. Most species are intolerant of shading and there has been a marked decline in lichens and bryophytes as a result of scrub and coarse grass encroachment as fewer grasslands are grazed. This was demonstrated in Breckland (Hitch & Lambley, 1996). Scrub may also alter the microclimate. Many of the species have poor dispersal abilities and following resumption of grazing, return only slowly, if at all. Rabbits have been critical in maintaining open turf conditions in many parts of Britain, providing a refugium for bryophytes and lichens. Rabbit activity, unless leading to excessive disturbance, is therefore of benefit to cryptogams. Any drop in rabbit numbers even if temporary is a potential threat to the cryptogamic grassland flora.
1. Management best practice
1.1 Survey
A survey by a competent lichenologist/bryologist will need to be undertaken before any change of management on a site, where it is known or believed that cryptogams including lichens are or may be important. This should then enable appropriate management to be put in place.
1.2 Grazing
Rabbits are ideal grazers and will provide a more open turf by selective grazing. Directing and controlling rabbit grazing however can be a problem, though at Weeting Heath (Norfolk Wildlife Trust) an area of grass-heath which has been fenced has been successful managed by rabbits for over 30 years.
Sheep are also desirable in most situations on calcicolous and calcifugous grasslands. Timing is not critical, but is generally better in summer, particularly on the least productive soils. Sheep produce a fine turf and cause minimal damage through trampling. This is particularly important with respect to foliose lichens, which, under dry conditions, can be damaged by the larger-hoofed cattle. However stocking rates can be critical and there are examples of lambs gathering in pits and destroying rich lichen floras on the slopes through trampling in spring. With especially important areas it may be necessary to fence them off in the lambing season.
On maritime cliffs especially small hardy breeds of pony appear to be effective at controlling scrub and keeping grasslands open. In these situations they are much better than cattle.
Cattle are however better than no grazing animals at all, and mixed grazing often achieves the objectives of maintaining a short open turf. Stocking rates need to be low and not all breeds of cattle are suitable, but sheep should be the sole grazers of CG7 grassland in Breckland. It is often important to include features such as old pits within a grazing regime to ensure that they remain free from scrub.
Grazing should aim to produce a turf of around 2cm in height, although on north facing and sheltered slopes the turf can be up to 5cm. The aim should be to graze some areas harder, most easily achieved by rabbits. Clearly it will be necessary to ensure that this does not conflict with the wider objectives of the grassland. On mesotrophic grasslands where the cryptogamic interest is slow the management should be directed towards maintaining the particular vegetation community or other feature.
1.3 Turf stripping
An objective of managing dry grasslands should be to provide a mixed patchwork of vegetated and bare ground. This will occur naturally on thin drought prone soils, where competition from vascular plants is low, slippage occurs on steep slopes, and grazing and disturbance are found. The creation of bare ground and its timing and siting should be considered if natural occurrences are limited, or to provide suitable conditions for specific plants. The optimal proportion of bare ground will depend on the particular interest and may vary considerably between sites. However, evidence from the Breckland is that scraping off the turf and surface soil down to the sub-soil is preferable to rotovating.
1.4 Trampling
Trampling by humans seems to produce more suitable conditions for many lichens which benefit from the resulting combination of compacted soil and open turf which results. Light to moderate trampling is therefore acceptable but needs monitoring to ensure it does not damage the carpet of turf to the extent of breaking it up.
2. Management bad practice
2.1 Fertiliser application
This raises the productivity of the soil and gives competitive advantage to vascular plants. In this situation only those bryophytes with high relative growth rates will survive or establish, leading to a rapid loss of the more interesting grassland specialists. The effect of herbicides applied to control vascular plants is complex; some bryophytes may benefit, and a flora resembling that initially developing after fire damage, comprising bryophyte weed species, has been reported (Brown, 1992). This has been attributed in part to reduced competition from vascular plants.
2.2 Burning
This is generally harmful to cryptogams. Although it is useful to eliminate litter accumulation, the consequent nutrient in put will raise the productivity of the soil to the detriment of the bryophytes and lichens.
2.3 Scrub removal
Control may be an important facet of management on a grassland. Care should be taken not to eliminate scrub that supports notable epiphytes.
References
Brown, D. H. (1992). Impact of agriculture on bryophytes and lichens. In: Bates, J. W. & Farmer, A. M. eds. Bryophytes and lichens in a changing environment. Oxford: Clarendon Press. pp.258-283.
During, H.J., Willems, J. H. (1986). The impoverishment of the bryophyte and lichen flora of the Dutch chalk grasslands in the thirty years 1953-1958. Biological Conservation 36:143-158.
Gilbert, O. L. (1993). The lichens of chalk grassland. Lichenologist, 25: 379-414.
Hitch, C. J. B., Lambley, P. W. (1996). The lichens of Breckland and the effects of afforestation. In Thetford Forest Park. The Ecology of a Pine Forest. Ratcliffe, P and Claridge, J (eds) pp. 58-66. Technical Paper 13, Forestry Commission.
Lambley, P.W., Wolseley, P.A. James, P.W. (2004). The impact of a changing pollution climate on the conservation of lichen-rich sites in England and the UK. In Lichens in a changing pollution environment.
Lambley, P.W. and Wolseley, P.A. (eds) pp107-116. English Nature Research Report 525. Rodwell, J. S. (1992) (ed.) British Plant Communities Volume 3: grasslands and montane communities. Cambridge: Cambridge University Press