Hymenophyllum tunbrigense (L.) Sm., Tunbridge Filmy-fern
Account Summary
Native, scarce or occasional, but locally abundant. Oceanic temperate; widespread and extremely disjunct – a preglacial relict species.
1860; Smith, Rev R.W.; Co Fermanagh.
Throughout the year.
Growth form and preferred habitats
Tunbridge Filmy-fern tends to be the only small, moss-like, mat-forming fern species growing on vertical or near-vertical, permanently and deeply shaded rock faces on mountain slopes and in woodlands, on and under rocks in deep, damp shade.
Identification
The long, flat, overlapping bluish-green fronds easily distinguish H. tunbrigense from the somewhat more common, blackish fronds of H. wilsonii (Wilson's Filmy-fern). Confirmation is often provided by the irregularly toothed margin of the pocket-like indusium covering the spore-sacs, a feature visible with a good hand-lens. The indusia are not always present however when these species are found growing in conditions of very moist heavy shade.
Fermanagh occurrence
In Fermanagh, H. tunbrigense has been recorded in a total of 24 tetrads, 23 of which have post-1975 records. The main areas where it occurs are on Cuilcagh mountain and the sandstone scarps and wooded glens of SW Fermanagh. The isolated 1950 station where it has not been refound was at Annaghmore Glebe Lough.
At some Fermanagh sites, H. tunbrigense colonies can cover several square metres of rock, and sometimes the fern is sufficiently profuse to completely fill rock crevices. Locally, Tunbridge Filmy-fern is almost always found growing on acidic rocks, most frequently on sandstone, but occasionally it may also occur on fairly hard, basic igneous and metamorphic rock types, particularly those providing crevices and with a texture, location and position that enables them to retain moisture for prolonged (or relatively long) periods (Richards & Evans 1972).
H. tunbrigense occurs in close physical proximity with its near relative H. wilsonii right up on the summit ridge of the highest mountain, Cuilcagh (c 600 m). At this site and others on the N-facing slopes of Cuilcagh, on sandstone scarps on the Western Plateau (ie in and around the Lough Navar Forest Park in particular), and in oak and mixed deciduous woodlands nearby (eg the Correl Glen NR), H. tunbrigense is always confined to very sheltered conditions. Typically it grows in deep, shaded hollows under very large overhanging boulders, cliffs or trees, growing on bare rock surfaces or in crevices. It is also found much less frequently on the peaty or uncompacted humus soils of woodland floors. Only rarely does it occur as an epiphyte on the bases of oak or old ash trees in very damp woodland, eg in the Correl Glen, and at the base of the heavily wooded Cliffs of Magho.
Comparison with H. wilsonii
By comparison, H. wilsonii occupies less shaded, somewhat more open and exposed conditions and it is often intermingled and embedded in cushions of moss and leafy liverwort. H. wilsonii does not form large single-species mats to quite the same extent as H. tunbrigense does.
On Cuilcagh, both these species also occur under and around huge, house-sized, rocks on block screes on the northern slopes just below the long, whale-back summit ridge, and again together on further massive rock falls at Cuilcagh Gap, and in similar situations around Lough Atona lower down these same slopes (at c 500 m). There are no trees on any of these heathy moorland slopes, which at this altitude and exposure are dominated by a canopy of ericaceous subshrubs and upland grassland.
Comparative tolerance of desiccation
The normal belief is that of these two species of perfectly frost-hardy filmy-fern, H. wilsonii is better able to tolerate high altitude exposure and the associated risk of desiccation than can H. tunbrigense (Jermy & Camus 1991; Page 1997). In mountain environments in the W of Ireland, rainfall is so very plentiful, frequent and regularly distributed, that droughting of the delicate fronds is not anything like as great a risk as might at first appear. At various sites in Fermanagh drought affected Hymenophyllum plants are occasionally found, particularly affecting the usually more exposed individuals of H. wilsonii, but occasionally also those of H. tunbrigense. The plants may be discovered looking very shrivelled, brown and desiccated, sometimes apparently dead. However, the rhizome and even the fronds have greater powers of recovery than their appearance and structure might suggest, and they can recover surprisingly well from temporary desiccation, or even from the effects of a light heathland fire (Richards & Evans 1972).
Comparative experimental studies by the latter authors have shown that H. tunbrigense suffers the effects of desiccation more immediately than H. wilsonii, but it also recovers more quickly than the latter. However, of the two, H. wilsonii has greater drought resistance, its protoplasm coping better with desiccation and its cells avoiding severe diurnal mechanical stress during drought periods.
A most interesting and rather unexpected finding is that the fronds of both Hymenophyllum species possess the capacity for indeterminate apical growth. This allows individual fronds to survive and continue growing for several years (perhaps four, five, or even more seasons), and thus both species manage to produce sporangia and new indusia in waves, maybe twice a year under favourable growing conditions (Richards & Evans 1972).
Growing as it does in more sheltered and more deeply shaded conditions, it is not really surprising that H. tunbrigense consistently has a lower photosynthetic compensation point than H. wilsonii. On Cuilcagh, the two Hymenophyllum species have found adjacent but distinct habitats, and although their microclimates definitely overlap, obviously they are both able tolerate the prevailing environmental conditions and have found ways of avoiding direct competition.
In the past, doubt has been cast on the finding of H. tunbrigense at altitudes above 460 m in Britain and Ireland (Richards & Evans 1972; Page 1997). However, the local botanists who identified this fern on the ridge of our highest mountain, Cuilcagh, are very familiar with both species of filmy-fern, and I am confident that the identifications are correct. Having emphasised this, H. tunbrigense is recorded only half as frequently in Fermanagh as H. wilsonii, so that of the two, Tunbridge Filmy-fern still clearly has the narrower ecological range (Richards & Evans 1972). While both filmy-fern species grow extremely slowly, c 2.5 cm/yr, H. tunbrigense appears to possess less biological vigour and suffers more from desiccation than H. wilsonii does.
Intolerance of liquid water wetting
One of the interesting facts to emerge from the study of Evans (1964) is that although Hymenophyllum species demand humid conditions, they absolutely do not tolerate being directly wetted with liquid water, for instance, by splashes from streams or waterfalls, which often are the source of the atmospheric humidity they do require. In other words, a rapid change of water content is much more harmful to a filmy-fern than a gradual one (Richards & Evans 1972). Evan's finding appears to directly contradict a statement regarding the habitat of H. tunbrigense made by Page (1997, p. 248), where the latter suggests the fern thrives in the splash zone of cool, permanently tumbling streams. While this is often quoted as one of the preferred habitats of another very much rarer species of filmy-fern, Trichomanes speciosum (Killarney Fern), RSF does not believe he has ever observed Hymenophyllum growing right in the spray zone anywhere in Ireland.
British and Irish occurrence
H. tunbrigense is restricted to a very discontinuous occurrence in the N, W and S of both Ireland and Britain, extending from Cornwall to Skye. In Britain, apart from an outlying group of sites in East Sussex (where it has markedly declined in recent years) and a couple of very scattered sites in NE Yorkshire, S Northumberland and Cheviot (VCs 62, 67 & 68), it is otherwise completely absent from the east of the island (Preston et al. 2002).
European and world occurrence
The distribution of H. tunbrigense in continental European is very sparse and disjunct, even in comparison with its representation in the British Isles. It is known only from a few stations each in France, Luxembourg, N Spain, Italy and the eastern coast of the Black Sea. Beyond this, it does also occur on the Azores, Madeira and the Canaries (Jalas & Suominen 1972, Map 69; Richards & Evans 1972).
Fossil history
Fossil spores of H. tunbrigense have been recorded from the Hoxnian interglacial in Ireland by Watts (1959). This fact, taken together with the species' present-day widely disjunct and sporadic European distribution, undoubtedly confirms it is a relict species. Previously the species had a larger and much more continuous range (probably most recently around the current post-glacial climatic optimum), but it has declined and continues to do so (Richards & Evans 1972). Gradual climatic deterioration, compounded in recent centuries with habitat destruction by man, has resulted in the fragmented distribution of H. tunbrigense we observe in Europe today.
Name
The specific epithet 'tunbrigense' is a Latinised reference to the first known British site, found by Daire a few years previous to Ray's (1686) published report, at High Rocks, Tunbridge Wells, Sussex (for a full history see Evans & Jermy (1962)).
Threats
Some Fermanagh sites were for a time threatened by being overgrown by coniferous forest plantations, but the threat has eased since the trees have been felled and they are not being re-planted.