Lichens of Wales

Strigula stigmatella var. stigmatella

synonyms

S.alpestris (Vezda) Hafellner (2002); S.stigmatella var. alpestris (Vezda) Coppins (1980)

Description & Identification

This unprepossessing crustose lichen has a thin, smear-like, pale greenish-grey to grey-white crustose thallus (becoming greyer on long-storage in the herbarium) dotted with half-immersed, small black perithecia, c. 0.3mm diam., sometimes with a slightly paler zone around the ostiole apex, which are at first covered by a thin thalline layer. The thallus can sometimes appear immersed in the bark. Accurate identification requires critical and careful microscopic examination of the photobiont, asci and ascospores (see ‘similar species’ below).

Similar Species

Microscopic examination to establish the identity of the photobiont (Trentepohlia), the form of the asci (thickened tholus with ocular chamber) and the ascospore-type (fusiform, (3-)7(-(9)-septate) is essential in order to distinguish it from various similar-looking genera, such as Anisomeridium, Celothelium, Porina, Rhaphidicyrtis & Thelenella, and other species of Strigula.

Habitats

S.stigmatella grows as continuous, spreading, inconspicuous patches over smooth, moist, base-rich bark and bryophytes thereon (in the Welsh collection over bark, the liverwort Metzgeria furcata and the moss Neckera complanata) on the trunks of broadleaved trees in wet, undisturbed ancient woodland. In the mountains of N.Wales and Scotland morphs of S.stigmatella recently reduced into synonymy and now treated at varietal level as var. alpestris by Roux & Serusiaux (2004), but which have been considered a full, separate species by some lichen taxonomists [S.alpestris (Vezda) Hafellner (2002)], occur locally on mossy, base-rich, igneous rock, between c. 600-850m altitude. As well as the montane habitat these morphs have a thicker thallus, more immersed perithecia and perhaps subtle anatomical differences in the development of the involucrellum covering the perithecia.

Distribution

The few epiphytic records indicate a southern-western ‘L-shaped’ distribution in Britain, where apart from the single Welsh site S.stigmatella has been recorded as a corticole only from a very few sites in western Scotland (v.c.105, Wester Ross), an unlocalised 19th century specimen from Sussex and from one site (on Quercus, Great Wood, Francis Rose, 1970, determined T.D.V.Swinscow) in the New Forest, Hampshire (Church et al., 1996; Brewis et al., 1996). The montane muscicolous morph S.stigmatella var. alpestris occurs rarely on outcrops of basic volcanic rock on Cadair Idris, v.c.48, Merionethshire, and in Cwm Uchaf and Cwm Idwal, Snowdonia, v.c.49, Caernarfonshire. The species is widespread in Europe, with a stronghold in the Carpathian Fagus forests, and also occurs in N.America and Australia (Coppins & Orange, 2009). 

Records from Wales

S.stigmatella has been found but once growing epiphytically in Wales, on an old Quercus in a relic fragment of upland broadleaved woodland at Hafod, near Pont-rhyd-y-groes, E of Aberystwyth, in 2000 (Chambers, 2000).  

Notes on individual locations

Hafod, near Pont-rhyd-y-groes, v.c.46, Cardiganshire: a few tiny patches, 1-2cm across, on the damp, mossy, base-rich bark of an ancient Quercus, on the W and SW sides of the buttressed trunk, in a relic fragment of old-woodland surrounded by conifer plantations on a steep, bouldery, N-facing wooded slope at the top of Allt Dihanog above the S side of the Afon Ystwyth at Hafod, GR22(SN)76-72-, alt 270m, February 2000.Coll. S.P.Chambers. Confirmed by B.J.Coppins. Voucher specimen in herb. SPC. New to Wales. The tree also had on its base the ancient-woodland indictor lichens Mycobilimbia epixanthoides, M.pilularis (syn. Biatora sphaeroides) and Porina rosei. Mossy boulders and rock outcrops in this small but intact and rich Oakwood fragment have much Wilson’s Filmy-fern, Hymenophyllum wilsonii, and the rare lichen Opegrapha fumosa occurs on another ancient Oak (one of two trees for the species in v.c.46), while the nearby stream-gorge has Graphina pauciloculata on Corylus avellana.

The fragments of Celtic rainforest on Allt Dihanog owe their survival and present-day existence to successful lobbying in the 1960s-70s from the then Nature Conservancy Council for the protection of Red Kites nesting in the trees (Dr Roger Bray, pers. comm.). These enclaves of ancient woodland would otherwise have been converted, like much of Hafod, to commercial conifer forestry. The destruction of native broadleaved woodland at Hafod by the Forestry Commission in those dark, Sitka Spruce-fixated days was nothing short of an environmental disaster, and probably extirpated a number of notable species, including Leptogium burgessii and Usnea articulata, collected by Sir James Edward Smith, the founder of the Linnaean Society and friend of Thomas Johnes, who visited Hafod to study the lichens on the Johnesian estate in the late 1700s. The rotting remains of large Oak stumps scattered through the conifer plantations along the hillside reveal the scale of the butchery. 

Threats and Status
  • Loss of host tree from inadvertent felling or from natural events, e.g. tree being blown down in a storm. The Hafod Trust has recently re-opened the historic walks originally set out by Thomas Johnes around the estate. The woodland on Allt Dihanog is above the Gentleman’s walk and appears to be at no obvious risk. Forestry Commission Wales and the Hafod Trust have a general awareness of the importance of localities for lichens at Hafod, but recent path reconstruction work was carried out without consulting the county lichen recorder.

  • Changes in bark characteristics (increased or decreased shade, alterations in moisture retention, drying, pH, nutrient-loading etc) from changes in site management, seral development and/or atmospheric pollutants. Eutrophication and/or acidification of bark from atmospheric nitrogen compounds present insidious threats, since the species requires bark with a relatively high pH, and although the woodland is sheltered and remote from local sources of pollution (except those perhaps originating from forestry harvesting operations) high national background levels of N-compounds are of concern, especially in high rainfall areas where wet deposition adds to cumulative loading.

  • On the Wales Lichen Red List S.stigmatella var. stigmatella is classed as Critically Endangered (CR, D) and on the British Red List as Endangered (E) with Internationally Responsibility (IR). The species is also Nationally Rare, s.42 NERC listed and the Hafod population represents c. 33% of the British population. If S.stigmatella no longer survives in the New Forest then Hafod is the southern limit of its distribution in Britain. Allt Dihanog lies within the Elenydd SSSI.

  • The genus has not yet been studied using modern molecular methods, but if the alpine entity proved phylogenetically distinct at the species level then epiphytic occurrences would assume greater conservation significance.

Proposed Actions

S.stigmatella has not been seen or searched for on Allt Dihanog since its discovery in 2000. A detailed survey of the woodland area is recommended to re-find the tree, assess the current status of the species and to search other trees in the vicinity, as the collection was not identified in the field.

The native Oakwood is surrounded by conifer plantation. Consultation with Forestry Commission Wales (or successor body) is needed to discuss management issues and secure the future of the site.

UK BAP Actions for Welsh lichens:

  1. Monitor habitat condition, at least every 6 years, at extant sites to ensure continuity of old broad-leaf trees, maintenance of current light and humidity levels and that the population is not subjected to over-shading (e.g. Ivy, invasive shrubs) and modify management accordingly.

  2. Encourage survey of old broad-leaved trees at historic and other suitable sites and incorporate new sites into the monitoring and protection framework

NBN Taxon Key
NHMSYS0001499163
References

Brewis, A, Bowman, P. & Rose, F. (1996) The Flora of Hampshire. Harley Books, in association with the Hampshire & Isle of Wight Wildlife Trust.

Chambers, S.P. (2000) In New, Interesting and Rare British Lichen Records, Bulletin of the British Lichen Society, No. 86, Summer 2000.

Church, J.M., Coppins, B.J., Gilbert, O.L., James, P.W. & Stewart, N.F. (1996) Red Data Books of Britain and Ireland: lichens. Volume 1: Britain. Joint Nature Conservation Committee, Peterborough.

Account Author
Steve Chambers