Oreopteris limbosperma (All.) Holub, Lemon-scented Fern
Account Summary
Native, very rare, although easily over-looked. European temperate.
1806; Scott, Prof R.; Co Fermanagh.
June to September.
Growth form and recognition
O. limbosperma is a rhizomatous, deciduous species that can be very readily overlooked, being easily mistaken for young Dryopteris filix-mas (Male-fern) or D. affinis (Scaly Male-fern), although the shorter, more slender, less scaly stipe, the pinnae tapering nearly to the extreme frond base and the naked, marginally set sori are all useful distinguishing features. The young fronds of O. limbosperma also give off a distinctive lemon or orange scent when lightly brushed, although as always, appreciation of this depends very much on the individual's sense of smell (Webb et al. 1996).
Preferred habitats
O. limbosperma is a strongly calcifuge plant of open sunlight, but at the same time it often occupies more or less sheltered situations. In terms of soil, it prefers damp to moist, peaty slopes which are sufficiently steep to create either surface run-off or sub-surface seepage after typically frequent rainfall. Moving groundwater is characteristic of the specific habitat and Lemon-scented Fern is reckoned to be indifferent to lime, and sensitive to both frost and summer heat (Jonsell et al. 2000). This set of growing conditions creates the cool, constantly moist, well-aerated root and rhizome environment which O. limbosperma demands, and as a result the species is most frequently found on rather steep peaty banks beside mountain or moorland streams, or in open, acidic woods at somewhat lower altitudes (Jermy & Camus 1991; Webb et al. 1996; Page 1997).
In Flora Nordica (Volume 1), the authors describe the habitat of this fern as oligotrophic forest and heath, ie nutrient poor, unproductive vegetation in terms of growth rate and biomass (Jonsell et al. 2000, p. 50).
Fermanagh occurrence
This is an exceedingly rare and vulnerable fern in Fermanagh, there currently being very few plants in the county spread over just three widely spaced sites. Previously, there were two other old stations in the county, but the fern is probably extinct in these now. O. limbosperma has been known in Fermanagh from Brookeborough Deerpark since the 1860s, and it is still there, having been refound in 1981 by a waterfall on the edge of the forest. Very sparse colonies, each of a few fronds, have also been discovered in two new sites in the last 30 years: at Tullynanny Lough in the SW of the county (Scannell, M.P.H.; June 1974), and in a ravine beside a stream at Stranahone in the NE (RHN & RSF; August 1994). It has not been seen, however, at West & Tetley's 1899 site, simply listed as "Florencecourt", nor at Praeger's similarly vague site, described as, "two stations on the lower hills W of Church Hill" (Praeger 1904).
Irish occurrence
In N Ireland, Lemon-scented Fern is rare and extremely local, and as with Phegopteris connectilis (Beech Fern), it is very much centred on the Mourne Mountains, Co Down (H38), in a few glens in NE Antrim (H39), and in the Sperrin Mountains and surrounding moorlands of Cos Londonderry and Tyrone (H40 & H36) (Hackney et al. 1992; Northern Ireland Flora Website 2014). In the Republic of Ireland likewise, Lemon-scented Fern is rare and very widely scattered, although it is locally abundant in a few sites in mountainous regions such as Connemara and Wicklow (Jermy et al. 1978; Page 1997; New Atlas).
British occurrence
O. limbosperma is much more frequent in suitable habitats in Britain than in Ireland, the species increasing in abundance as one travels both westwards and northwards. This trend culminates in W Scotland having the greatest abundance of Lemon-scented fern in Britain and Ireland, and probably also on a world scale (Jermy et al. 1978; Page 1997; New Atlas). The New Atlas survey prior to 2000 found that the distribution in upland areas of Britain remains stable, and concluded that many of the losses in lowland areas that occurred before 1930 resulted from the destruction of heathland habitat (T.D. Dines, In: Preston et al. 2002).
European and world occurrence
On mainland Europe, the distribution of O. limbosperma most closely resembles that of Dryopteris dilatata (Broad Buckler-fern), occurring predominantly in NW Europe, more or less continuously from the Pyrenees northwards to western Norway and up to just within the Arctic Circle. It also occurs further south on the Atlantic isles of the Azores and in Madeira. On a world basis, it is regarded as circumpolar, but it only manages this in an extremely disjunct manner (Hultén 1962, Map 144; Jalas & Suominen 1972, Map 72). Despite its very wide but sparse geographic distribution, in terms of abundance W Scotland still very probably represents the world headquarters of what really looks like a very fragmented relict species (Page 1997).
Comparison of Irish and Scottish presence
The very extensive and abundant presence of O. limbosperma in western and central Scotland contrasts so sharply with its rare and sparsely spread occurrence in Ireland, and especially so when compared with N Ireland in particular, that one cannot but wonder at the scale of the difference and ponder on the possible reason or reasons. The climate, the rock structure and geological history of the two regions are strikingly similar and are intimately linked, the geology possibly most obviously so with respect to their Tertiary igneous activity (Whittow 1974, 1977). This being so obviously the case, the speculation by Page (1997, p. 275) on the different representation of O. limbosperma in the two regions seems unlikely to offer an explanation, since in Ireland he suggests poorer drainage in lowland stations might be responsible, and at higher altitudes he believes that there are more base-rich sedimentary rocks than in reality is the case.
As generally happens, however, it is easier to criticise others' explanations than it is to suggest a better alternative! A careful examination of the habitat requirements and tolerances of the species might generate a fresh hypothesis, but we should not ignore the historical factors either, including for instance, differing human population pressures and land management regimes stretching across many centuries. The earlier and almost total deforestation that took place in Ireland, for example, could well be significant, as might differing grazing, tillage and burning patterns. It is difficult to imagine anything that would affect O. limbosperma on quite the scale necessary to account for the enormous difference in its presence evidenced on the New Atlas British and Irish hectad map (Preston et al. 2002).
Names
The genus name 'Oreopteris' is a combination of the Greek words, 'oreos' meaning 'mountain' and 'pteris' meaning 'fern' (Gilbert-Carter 1964). The specific epithet 'limbosperma' is derived from the Latin 'limbatus' meaning 'a border' or 'margin', and 'sperma' meaning 'seed' (or in this case, 'spores'), referring to the characteristic marginal position of the sori on the underside of the frond (Gledhill 1985).
As with Phegopteris connectilis (Beech Fern), Oreopteris limbosperma has moved through six genera over the years and, partly as a result of this has also been given a long sequence of English common names including 'Mountain Fern', 'Mountain Buckler Fern', 'Heath Fern', and in reference to the scented glands, 'Sweet Mountain-fern', 'Scented Fern', 'Lemon-scented Fern', 'Hay-scent Fern' and 'Tea-scent Fern', the latter two names being local to Cumberland (Step & Jackson 1945).
There does not appear to be any tradition of use of this fern or folklore associated with it as the species was not commonly differentiated from the Buckler-ferns, Dryopteris species.
Threats
The minimal populations in Fermanagh could very easily be eliminated by any disturbance of their streamside habitats. This is very clearly a species in urgent need of local habitat management to encourage and support its survival.