Llanelli Naturalists Newsletter - Winter 1992/1993
Alan Orange

If you stand in Dinefwr Deer Park. looking across the pond to the old castle and the flood plain of the Tywi, you will be looking at one of the most beautiful views in Wales. If you are a archeologist, you will soon realise that the lichen florae is also something out of the ordinary. The medieyal aspect of the view, with its rough grassland, ancient oak trees and fallow deer, are clues to the special nature of the lichen flora.

The importance of the Deer Park for lichens was recognised in 1972, and subsequent visits by lichen specialists have continued to add notable species of lichen to the list. Every tree in the Park has now been surveyed for lichens, and some of the results can be correlated with a recently introduced system of tree tagging.

The total number of species recorded on trees and wood in the Deer Park now stands at 142, of which about 50 are rare in Wales or in Britain as a whole (see table). Many of the notable species are considered to be indicators of sites with a long history of ecological continuity - in this case meaning that mature trees haye been present continuously for hundreds of years.

The Deer Park is a relic of the type of woodland management known as pasture woodland, which has the dual purpose of timber production and grazing of animals. Before man began to have a major impact on the natural vegetation of the British isles, most of the land would have been covered by forest. Studies into the probable composition of this forest (the 'wildwood' as it has been called), suggest that some examples of pasture-woodland may be among the closest approximations to the wildwood which we have left, with abundant ancient trees, dead wood, and with glades allowing light to penetrate to the ground.

Lichens found mainly in sites with a long history of ecological continuity are known as old-forest species: they are slow to colonise new habitats and may require niches found only on old trees. They are not found in coppiced woodland, even when of ancient origin, because they eannot suryive the frequent felling.

The likelihood of ancient origin and ecological continuity can be estimated using the Revised Index of Ecological Continuity (RIEC) devised by Rose (1976), where the RIEC is given by n/20 x 100, where n is the number of old-forest species from a published list of 30 species. Dinefwr scores well on this index, with an RIEC of 85. A similar, more recent index, intended to refer mainly to conservation value is the New Index of Ecological Continuity (Rose 1992), which uses a list of 70 species. Dinefwr scores 29 on this Index.

Some of the trees in the Park derive from plantings in the late 18th and early 19th centuries, but the south-west corner contains semi-natural woodland. This part of the Park is the stronghold of seyeral species which prefer shelter or shade: Lecanactis subabietina prefers the deeply shaded recesses at the base of oak trunks, coating the dry bark with a pale grey film dotted with white pin head-sized spore-producing organs called pycnidia.

Dinefwr experiences some degree of air pollution despite its rural position. Although the precise effects of each pollutant on lichens in the field are still inadequately known, acid rain appears to be an important influence at Dinefwr. as in most of Wales. Lobaria pulmonaria, usually a large and distinctive old-forest lichen, can be found on only one oak in the Park, and appears small and unhealthy. This species contains a blue-green alga (cyanobacterium) as its partner in the symbiosis, and such species are thought to be particularly sensitive to the effects of acid rain. The bark of oak is naturally rather acidic, and additional acidity from air pollution can easily make the bark unsuitable for the growth of many lichens. The potentially large lichens Sticta sylvatica and S, limbata also occur only on single oaks.

Oak is oyerwhelmingly the most important tree for lichens in the Park, but ash and sycamore are also important because of their less easily acidified bark. Beech, horse- and sweet-chestnut and lime are of little importance at Dinefwr, although beech may be a valuable lichen tree in some parts of the country.

Many of the interesting lichens are small, and need to be searched for carefully. In Gyalideopsis muscicola (see figure), a good lens is needed to see the minute dark spiny structures sprouting from a thin grey film (the thallus) coating moss on a few sheltered tree trunks. The structures shed long narrow spores from their undersides. This is a rare species in Britain.

It is scarcely practicable to search every square inch of bark, but fortunately it is often possible to spot a potentially good tree, and also potentially good areas of a single trunk. This may be by means of larger species indicating rich bark (e.g. the mosses Hypnum cupressiforme var. resupinatum and Homalothecium sericeum), or by less easily communicated features of bark colour and texture. The oak trees in the park vary considerably in their value for lichens, good trees sometimes occurring near to poor trees. The reasons for this type of variation can only be guessed at at present. Possibly there are genetic differences between the trees, or subtle environmental factors operating. Interesting lichens can sometimes be found only in a narrow zone on the trunk, where conditions of pH, or lack of competition from large mosses, make conditions particularly suitable. The dry side of the trunk that is sheltered from rain has its own specialised community of lichens able to absorb moisture from the air.

Dead wood should by now be well known as an important habitat for several groups of organisms, but it is a particularly threatened substrate in most woodland. Hard, slowly decaying wood is best for lichens. A large oak log in the Deer Park, which is still the only site for the lichen Lecidea turgidula in the whole of South- and Mid-Wales, narrowly escaped becoming firewood in the 1980's.

The Deer Park is now a National Trust property. The future of the lichen flora is dependent on a combination of management, and external factors such as pollution. Maintenance of the numbers of ancient trees and of dead wood, and maintenance of shelter and humidity without excessive shading, are of great importance. Tidying up 'dying' trees and dead wood must be resisted. The beauty of the Park is largely dependent on its 'natural' appearance, and correct management of the natural history need not conflict with the landscape interest.

Further Reading

Harding, P.T. & Rose, F. (1986) Pasture woodlands in Lowland Britain A review of their importance for wildlife conservation. Abbots Ripton: Institute of Terrestrial Ecology.

Rose, F. (1976) Lichenological indicators of age and environmental continuity in woodlands. In Brown, D.H., Hawksworth, D.L. & Bailey, R.H. (eds.) Lichenology;  Progress and Problems.. London: Academic Press.

Rose, F. (1992) Temperate forest management: its effects on bryophyte and lichen floras and habitats. In Bates, 1. W. & Farmer, A. h1. (eds.) Bryophtes and Lichens in a Changing Environment. Oxford: Clarendon Press.

Table 1. List of some notable species recorded from Dinefwr on bark or wood, indicating species used in calculation of the RIEC and NIEC. In the RIEC, any Pyrenula spp., and in the NIEC any Chaenotheca spp., score one altogether.