Hymenophyllum wilsonii Hook., Wilson's Filmy-fern

Account Summary

Growth form and preferred habitat

This moss-like, rhizomatous, cushion-forming perennial fern grows in similar habitats to H. tunbrigense (Tunbridge Filmy-fern) on ± vertically erect, damp, shaded, mountain rock crevices and ledges, and on mossy rocks and tree boles in damp woodlands, but while usually in shade it always occupies slightly more open situations than the latter.

Fermanagh occurrence

In Fermanagh, H. wilsonii has been recorded in 34 tetrads (6.4%), 31 of which have post-1975 records. As the tetrad map indicates, it is quite widely distributed on the Western Plateau, Cuilcagh Mountain and Florencecourt. The most isolated recent station is on a scarp at Drumskinny Td in the far north of the county, where RHN found it in May 1990.

Comparison with H. tunbrigense

The fronds of H. wilsonii are generally longer, narrower and more upright than those of H. tunbrigense and in overall appearance they are less flattened and of a deeper shade of olive-green, sometimes rather blackish, or indeed brown if it has recently suffered drought. This little fern is often found growing amongst mosses where its distinctive colour makes it quite easy to spot.

Wilson's Filmy-fern generally forms rather smaller patches than Tunbridge Filmy-fern and sometimes when it is growing through cushions of moss, the fronds are few and quite distant from one another, so it could easily be overlooked or mistaken for one of the larger species of leafy liverwort. The frond is stiffer and of thicker texture than that of H. tunbrigense and, because it is more drought resistant and can thus thrive better in illuminated sites than the latter, it sometimes competes with lichens as well as with mosses and liverworts.

Although the two filmy-ferns occur in very similar habitats and their micro-environments overlap, in any type of habitat where they occur together they typically occupy different vegetation zones. For instance, H. wilsonii occurs higher on the bark of trees and on block screes on mountainsides it occurs in more exposed, better lit sites. Like H. tunbrigense, H. wilsonii is usually absent from the spray zone of waterfalls, but while it normally avoids running or dripping water, it is slightly more tolerant of being wetted in this way and occasionally it is found on rocks which are flooded from time to time (Richards & Evans 1972).

Of the two species, H. wilsonii is better able to withstand relatively prolonged drought. Sometimes, however, drought can result in colonies peeling away from rock or soil surfaces, which may lead either to their destruction (Richards & Evans 1972) or, very occasionally in the W of Ireland, this may serve as a method of vegetative dispersal.

Very little is known about the life-history of Hymenophyllum species, save that they are very slow-growing and that the individual fronds may survive up to about five years. The frequent occurrence of large colonies of Hymenophyllum suggests that they may be long-lived, perhaps surviving in stable environments for centuries. This is clearly an area which would repay further field study.

Irish occurrence

The distribution of Wilson's Filmy-fern overlaps that of Tunbridge Filmy-fern in the Atlantic-influenced N and W of both Britain and Ireland, but it is quite a lot more widespread – recorded in more than twice as many hectads as H. tunbrigense. In Ireland, H. wilsonii is locally abundant in all the main mountain ranges (which with the exception of the Galtees in Tipperary, all happen to be quite coastal), and in addition it is thinly scattered in damp woods throughout the island (Richards & Evans 1972; Jermy et al. 1978).

Survey work for Atlas 2000 in the north of Ireland has proven that H. wilsonii is much more widespread than had been previously considered (NI Flora Web site 2001; New Atlas).

British occurrence

In Britain, the species is again locally abundant in the N & W of the island, but unlike H. tunbrigense it has no south-eastern outlier. Instead, it is confined west of a line from Start Point in Devon to the mouth of the River Tees. Within this area of Great Britain, it does extend further east and much further north than H. tunbrigense does, extending to Shetland (Richards & Evans 1972).

World occurrrence

H. wilsonii has a much more confined world distribution than H. tunbrigense, being largely restricted to W Europe and N Atlantic islands (Hultén & Fries 1986). It is known only from the Faeroes, one station in Iceland plus the more southern Atlantic islands (Azores, Madeira and the Canaries), which are not really part of Europe but where species populations are generally conspecific with European flora. On the European mainland, the distribution of H. wilsonii runs from a broad coastal band of SW Norway plus just four coastal sites in N France (Jalas & Suominen 1972, Map 70; Page 1997; Jonsell et al. 2000).

Names

The genus name 'Hymenopyllum' is a compound of two Greek words, 'umen' meaning 'a membrane or thin skin' and 'phyllon' meaning 'a leaf', a reference of course to the membranous fronds which are a single cell thick (Step & Jackson 1945). The Latinised specific epithet 'wilsonii' commemorates the bryologist, W. Wilson, who in 1830 drew the differences between the two filmy-ferns to the attention of the elder Hooker (Professor, and later Sir William). Hooker named the species but subsequently reunited the two species under H. tunbrigense once more (Richards & Evans 1972). For a full history, see Evans & Jermy (1962).

Threats

Some sites could be threatened by forestry operations.

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