Asplenium marinum L., Sea Spleenwort

Account Summary

Growth form and preferred habitats

Except at the western extremities of its markedly Atlantic distribution, A. marinum is restricted to a very narrow zone, seldom more than 20-30 m above sea level, where the winter air temperature is ameliorated by warm sea spray from the Gulf Stream. The usual habitat requirements of this glossy, evergreen, singly-pinnate, perennial fern are a cool, moist crevice, sheltered from full sun in summer and entirely frost-free in winter (Page 1997).

British and Irish occurrence

Sea Spleenwort is entirely coastal throughout its British and Irish range. In Britain it stretches almost continuously from the Isle of Wight on the middle of the south coast of England, westwards to Land's End and up most of the west coast through Wales to Shetland. On the east coast of Britain it is again represented from the far north of Scotland as far south as Scarborough, although here it is less continuous and definitely more thinly scattered.

In Ireland, the New Atlas hectad map shows A. marinum is again very well represented on almost all of the S & W coasts, but it is absent from a few stretches of the E coast, mainly from Dublin Bay to Wexford (New Atlas).

Fermanagh occurrence

In view of this information, the solitary Fermanagh site in Lough Navar Forest Park is extremely abnormal, being 17 km inland from the tidal estuary at Ballyshannon and situated at an altitude of 210 m. Originally the plant occurred at two places on the same set of N-facing sandstone cliffs, there being about 20 plants at one site and about 50 at the other (Northridge et al. 1988). On a visit in July 2002, there was a very noticeable decline in the smaller more accessible population, the result of aggressive competition from young Hedera helix (Ivy) stems also present in the crevices. Subsequent visits up to September 2010 showed three plants surviving at the smaller patch; the larger patch was thriving. In addition, two inaccessible plants growing higher up the cliffs were identified through binoculars!

Ecology

The Fermanagh sites being so remote from the sea, it cannot be that A. marinum has an absolute requirement for sodium chloride. The red sandstone of the Lough Navar cliffs has been dolomitized by the vertical percolation of waters rich in calcium and magnesium sulphates, so that mineral replacement has occurred and the rock has become base-enriched. At both Fermanagh sites, A. marinum grows where the top of the cliff overhangs the base to a considerable degree, providing a measure of frost protection, although one would think that this would hardly be sufficient to entirely avoid freezing temperatures at this altitude. Many of the plants grow in a shaded pocket of the cliff while other small individuals grow along fault line crevices. A. ruta-muraria (Wall-rue) grows close to some of the A. marinum plants, but many have no nearby competitors.

The winter-green, coarsely divided, leathery, glossy fronds and stiff, dark purple-brown stipes are readily recognised. It is the opinion of the current author that the thick cuticle of the fleshy frond enables A. marinum to withstand the extreme dryness of this site. A visit to the site in July 2002 found that Ivy had invaded some of the cliff crevices and rendered A. marinum locally extinct.

Fortunately another cliff outcrop further west maintains a vigorous population of the species at its base, and the Ivy stems appear to have grown up and beyond the fern, so that they are unlikely to compete directly for space and light.

Reproduction and possible hybridization

The fronds produce spores abundantly from August onwards and overwinter before decaying the following spring as new fronds are produced (Page 1997). The breakdown in the Fermanagh site of the usual ecological barriers between A. marinum and A. ruta-muraria suggests the possibility of a hybrid, but none has yet been found here. Page (1997) suggests the reason for this failure to hybridize with any other native northern species of Asplenium is that A. marinum's phylogenetic affinities appear to lie with an extensive group of species characteristic of wet forests in the tropics and sub-tropics.

European and world occurrence

The wider distribution of A. marinum is Atlantic-Macaronesian (ie along the W and SW coasts of Europe, plus the island groups of Madeira, Canaries and Azores (Jalas & Suominen 1972, Map 78; Page 1997, p. 72). This, together with its essentially frost-free ecological requirement, tends to support the idea that it associates more closely with Asplenium species of warmer climates than those encountered at present in the British Isles. In their phytogeographical survey of British and Irish plants, Preston and Hill (1997) classified A. marinum as Suboceanic southern-temperate, although they noted that it also occurs in the Southern hemisphere. Hultén and Fries (1986, Map 40) indicate that this refers to presence on the Cape Verde Islands and St. Helena.

Trends in Britain and Ireland

Page (1997) points out that in general A. marinum is much less frequent and less luxuriant in many of its current British and Irish habitats than it was in the 19th century. Past ravages of fern collection, together with slowly changing factors, for example, pollution and contamination of shores, and natural changes in climate, that is, global warming, with respect to which this species appears to be in an extremely delicate balance, are almost certainly involved. Thus this species is under threat, and should on no account be collected.

Names

'Asplenium' is derived from the Greek 'a' meaning 'not' and 'splen', 'splene' or 'splenon' referring to the spleen, alluding to the supposed medicinal properties of the fern genus. The herbal medicinal use is also invoked by the English common name applied to the genus, ‘Spleenwort’ (Hyam & Pankhurst 1995). The Latin specific epithet ‘marinum’ means ‘sea’, an obvious reference to the preferred habitat and hence the English common name ‘Sea Spleenwort’.

Like A. adiantum-nigrum, while A. marinum is widely distributed along coastal sites in both Britain and Ireland, it does not appear ever to have had a genuine folk-name or English common name, only the invented book name, ‘Sea Spleenwort’. Presumably it is too uncommon to have merited folk interest, and I cannot locate any mention of folk use of the plant.

Threats

Part of one of the Fermanagh sites is being overgrown by Hedera helix (Ivy).

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