Polystichum aculeatum (L.) Roth, Hard Shield-fern

Account Summary

Growth form and preferred habitats

While P. setiferum (Soft Shield-fern) is quite gregarious and forms large 'shuttlecocks', each composed of numerous fronds, in comparison P. aculeatum rarely occurs as anything more than scattered individual plants. Also, individuals of Hard Shield-fern are typically small to medium-sized, often with just a few fronds per shuttlecock crown.

P. aculeatum usually grows in shade, and it is very often associated with exposed calcareous or other forms of base-rich rock. Of the two shield-ferns, the evergreen P. aculeatum tends to be the more upland in character, growing in damp pockets of soil in a wide range of shaded places where outcrops of limestone rock occur, including stabilized scree, around swallowholes, in deep grykes in limestone pavement, on steep wooded slopes, river banks and by streams and waterfalls.

The hard leathery texture of the fronds of P. aculeatum and their glossy, dark-green colour when mature makes most plants easy to distinguish from Soft Shield-fern. If in any doubt, then the plant is far more likely to be P. setiferum. A useful and reliable characteristic of P. aculeatum is that the ultimate segments of pinnae in the middle of the frond are more or less symmetrical at their base and the innermost pinnule on the upper side of each pinna is usually very much larger than all the rest and it is more deeply divided (An Irish Flora 1996; Page 1997).

Fronds of P. aculeatum expand from the beginning of May onwards and they begin to spore in mid-July. Like Soft Shield-fern, the fronds are wintergreen and they persist longer in their second year than those of P. setiferum (Page 1992).

Fermanagh occurrence

Although, as the tetrad distribution map indicates, this is a rather widely scattered species throughout much of Fermanagh, having been recorded in 130 tetrads, 24.6% of those in the VC, there really are only two areas in Fermanagh where Hard Shield-fern occurs with any notable frequency. These are along damp, shaded riverbanks and roadsides in the Maguiresbridge district and to the north of Kesh. In these two localities, it is more frequent than the generally much more common and widespread P. setiferum.

In addition to its shaded and rocky natural or semi-natural habitats, in Fermanagh Hard Shield-fern is also recorded in urban situations around Enniskillen, and in more rural areas it is often associated with man-made structures such as old walls, bridges, weirs and even along old disused railway lines.

Similar behaviour is described in England, where additional man-made base-rich sites colonised include canal-sides, locks and bridges (Jermy et al. 1978). In SW Scandinavia, Hard Shield-fern occurs on mountain cairns, 'stone-fences' (presumably this refers to our 'dry stone walls'), and along ditches (Jonsell et al. 2000). The leathery fronds of P. aculeatum are considerably more hardy than those of P. setiferum, enabling it to survive in more easterly areas of the British Isles and Europe which suffer heavier winter frost than western areas like Fermanagh.

Irish occurrence

In the past, Hard Shield-fern has been recorded at least once in every Irish vice-county except Mid-Cork (H4) (Scannell & Synnott 1987). While this is a fact, in reality over most of the island the fern is only thinly scattered, seldom frequent, and occurs chiefly in the N & W of Ireland (Jermy et al. 1978; Webb et al. 1996; New Atlas).

British occurrence

In Britain, P. aculeatum is much more widespread than P. setiferum, extending from the English Channel coast to Orkney in the far north (but not Shetland, where it is considered a rare introdution). It is absent, however, from considerable areas of the English East Midlands. It is most common in N England and Scotland, but less common than P. setiferum in SW England and S Wales (Jermy et al. 1978; Jermy & Camus 1991; Stace 1997; Preston et al. 2002). In areas of the British Isles where their distributions overlap, P. aculeatum and P. setiferum meet in suitable habitats intermediate in altitude, and their hybrid, P. × bicknellii (recorded once in Fermanagh), is not uncommon and should be looked for (Preston et al. 2002).

European and world occurrence

In Europe, Hard Shield-fern is widespread but uncommon, predominantly occurring in the W and C temperate regions, extending to 64°N up the coast of SW Norway, and stretching south to the Azores, Majorca, Corsica, the S Peloponnese and the coast of N Africa (Jalas & Suominen 1972, Map 120). Eastwards the fern spreads ever more discontinuously, reaching the Caspian Sea and possibly beyond. Probable related taxa occur in the Himalaya and in Japan (Stace 1997). Hultén (1962, Map 141) and Hultén & Fries (1986, Map 61) treat P. aculeatum in an extremely broad sense, merging it at least in part with P. setiferum and related species, so that their maps and accounts are for once singularly unhelpful.

P. aculeatum is sometimes grown for ornament, and Jones (1987) lists four garden varieties, of which 'Pulcherrimum Gracillimum' is described as "the most beautiful British fern" [in cultivation].

Name

The genus name 'Polystichum' is derived from two Greek words 'polus', 'many', and 'sticos' or 'stichos', 'row' or 'file'. The genus name thus translates as 'many rows', a reference to the regular rows of sori on the fertile frond (Gilbert-Carter 1964). The Latin specific epithet 'aculeatum' means 'having prickles, thorny or prickly' (Gledhill 1985), which in the present author’s opinion, rather overstates the case as the frond does not feel prickly when handled.

Threats

Excessive clearance or tidying of riverbanks.

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