Dryopteris dilatata (Hoffm.) A. Gray, Broad Buckler-fern

Account Summary

Growth form and preferred habitats

The dark-centred scales on the stipe, the down-turned margins of the pinnules and the dark-green colour of the frond readily distinguish D. dilatata from two much rarer Dryopteris species, D. carthusiana (Narrow Buckler-fern) and D. aemula (Hay-scented Buckler-fern).

The typical habitats of this very common deciduous fern are woods, hedgerows and shaded banks, but it also appears in upland areas on open rocky slopes and in rock crevices and, as a weedy species, in more urban and waste ground situations. Thus, like D. filix-mas (Male-fern), it occurs in a large variety of damp, lowland shade or, in upland, more open habitats throughout almost the whole range of altitude. The biology, ecology and distribution of these two extremely common ferns in Britain and Ireland are very similar and their ecological niches clearly overlap considerably at many shared sites. D. dilatata is more frequently found and is the more abundant of the two species on permanently wet, but not waterlogged soils, at pH levels below 5.0, on bogs, or on acidic, moderately fertile, organic substrates in woods, scrub, hedgerows and on the banks of rivers and streams.

In mixed deciduous acid woodland generally dominated by oak, D. dilatata can carpet the damp, shady floor vegetation, and developing from massive, old, more or less upright rhizomes it forms a dense, mid-green sward of fronds up to 1.5 m tall. In old, less disturbed woods of this type, D. dilatata sometimes also grows as an epiphyte in mosses on the rugged bark of the larger trees in the same way that Polypodium species very often, and Blechnum spicant (Hard-fern), occasionally does (Page 1997). Like Male-fern, D. dilatata also invades conifer plantations, most frequently being found along tracks and fire-breaks, and particularly along the sides of ditches and drains associated with these less shaded, better drained conditions.

D. dilatata is perhaps slightly less frequent than D. filix-mas in limestone areas of Fermanagh, although in our wet western oceanic climate, an insulating layer of peat regularly forms over base-rich rocks, and thus plants that are widely regarded as calcifuge can frequently be found also growing in limestone districts.

Of these two common fern species, D. dilatata is also more often found than D. filix-mas in more open sites on higher ground, such as on more or less steep, rocky slopes, stabilised screes and in rock crevices. Relatively dwarfed plants of D. dilatata are abundant in the clefts between rocks for instance on the summit of Cuilcagh, our highest mountain.

D. dilatata is such a rapidly growing and maturing fern that in less natural, urban and disturbed habitats it can also behave like a weed species, colonising crevices in damp brickwork in the manner D. filix-mas sometimes does, but doing so even more effectively than the latter. It is also quite commonly found in less well-tended gardens, growing out of soil on steps, competing with decorative species in tubs and in greenhouse pots. The pronounced reproductive ability and wide range of variation within the species, suggests that D. dilatata is possibly still capable of further increasing its distribution and range of habitats within Britain and Ireland (Grime et al. 1988).

Fermanagh occurrence

D. dilatata is almost ubiquitous throughout the county. It is both the most frequent and the most widespread fern in Fermanagh being present in 462 tetrads, 87.5% of those in the VC. In the wetter Western Plateau uplands of Fermanagh, D. aemula tends to replace D. dilatata both in wet, acidic, shallow rocky ground in shade and also as an epiphyte in oak woods, eg in the Correl Glen NR.

Reproduction

Broad Buckler-fern produces sporing sori on all but the smallest plants, a feature rather different from Male-fern, which instead takes up to six years growth to achieve sporing fertility (Page 1997). Fronds are less frequently wintergreen than those of D. filix-mas, but D. dilatata produces its fresh annual fronds and sporing sori much earlier in the season than Male-fern. Spores are clearly produced in massive quantities, and from the wide range of habitats and geographical spread in the British Isles, dispersal is very efficient. Probably it is only the essential requirement for free moisture to enable the functioning of the delicate prothallial stage which limits the plant and prevents the even more common occurrence of the species.

A field study by Willmot (1985) found that small sporophyte plants of Dryopteris dilatata and D. filix-mas in woodland, developed in cushions of moss. In the several woods he studied, all populations of D. dilatata produced an excess of small, sterile plants over larger, older fertile ones, while the age structure in D. filix-mas populations did the opposite. Several interpretations of this observation are possible, but a likely one is that Broad Buckler-fern either produces more new sporophytes each year, or that members of this vulnerable stage survive better than those of Male-fern under the site conditions studied and are recruited into the mature population more successfully. There appears to be very few field studies of the population behaviour of any fern in the British Isles, and Willmot's work urgently requires to be followed up and emulated with other species.

Vegetative reproduction

D. dilatata also has a greater tendency to carry out vegetative reproduction than D. filix-mas; the rhizome of some plants, typically when they are growing on shallow soil overlying rock, very occasionally produce long, slender, creeping, offset branches which bear a sequence of small crowns each producing new fronds (Page 1982; Grime et al. 1988). Page (1997) believes that this variant is an environmentally induced form, a suggestion which appears very likely the case.

Toxins

As with Male-fern and other Dryopteris species, D. dilatata is intolerant of grazing pressure and contains the toxic substances thiaminase and filixic acid which can cause blindness and, very rarely, the death of cattle which have eaten the rhizome through lack of other more suitable grazing material (Cooper & Johnson 1998).

European occurrence

In Europe, the distribution of D. dilatata is very much more limited than D. filix-mas, being rather confined to the western region of middle temperate latitudes, at the same time thinning considerably towards the Mediterranean. It does however extend northwards along the Atlantic coast of Norway, and just reaches the Arctic Circle near Bodo (Jonsell et al. 2000).

World occurrence

Beyond Europe (in the Florae Europaea sense), D. dilatata only occurs (presumably rarely) in Asia Minor and the Caucasus (Jalas & Suominen 1972, Map 130). Related forms occur in N America, and possibly also in the Far East (Hultén 1958). Forms in Greenland and Iceland were previously recorded as D. dilatata (Böcher et al. 1968; Löve 1983), but these have been reassigned to D. expansa (Northern Buckler-fern), an amphi-Atlantic species which is now regarded as one of the diploid parent species of tetraploid D. dilatata (Kristinsson 1987; Grime et al. 1988; Jonsell et al. 2000).

Name

The genus name 'Dryopteris' was first given by the Greek physician, Pedanius Dioscorides (c. 40-90 AD), to a fern growing on oak trees, and is a compound of the Greek 'dryas' = 'oak', and 'pteris' = 'fern' (Gilbert-Carter 1964). The specific epithet 'dilatata' is Latin meaning 'broad' or 'spread out', and is derived from the past participle of 'dilato', itself from 'latus' meaning 'broad' (Gilbert-Carter 1964). As the species in the past was not differentiated from ferns in general, it has no local English common names nor any folklore (Step & Jackson 1945).

Threats

None.

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