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Equisetum pratense Ehrh., Shady Horsetail

Account Summary

Native, very rare. Circumpolar boreal-montane.

1904; Praeger, R.Ll.; scarp behind Poulaphouca cliffs, Western Plateau.

June to November.

Growth form and preferred habitats

E. pratense is a rhizomatous perennial with two forms of aerial shoot (vegetative and sporing), neither of which overwinter. The species is a northern one with an overall distribution markedly circumpolar boreal-montane or arctic-alpine (Stewart et al. 1994; Page 1982 & 1997). In Ireland it is a rare Irish Red Data Book species, confined to the northern province of Ulster, where it is most frequent in the glens of Antrim (H39), although even here it is really rare and local (Praeger & Megaw 1938; Curtis & McGough 1988, pp. 93-4; Hackney et al. 1992). E. pratense is rarely recorded, perhaps in part because it is not very distinctive in appearance and it often occurs as scattered, diminutive individuals composing small, diffuse colonies. In order to locate it, field recorders first need to encounter a good specimen colony and get to know its particular habitat requirements. Thus it could easily enough be overlooked and under-recorded.

In texture and colour, the shoots of E. pratense most closely resemble the delicate ones of E. sylvaticum (Wood Horsetail), although unlike the latter, the slender primary lateral branches are themselves unbranched. The flat tops of mature shoots are quite distinctive in appearance.

Like the majority of other horsetail species, it is a plant of well-drained slopes (including upland glens and talus slopes below cliffs), where silty or sandy soil is flushed with, or subject to seepage of, base- and mineral-rich groundwater, and where shade, shelter and proximity to running water is sufficient to keep humidity high and prevent any possibility of desiccation (Stewart et al. 1994). A certain amount of winter flooding or mild scouring of base-rich E. pratense sites near streams and rivers may also be significant, producing surface instability and shallow erosion of the wet soil, thus minimising the burden of competing vascular plant species while simultaneously providing plentiful silica (Page 1988). In Fermanagh, this could particularly be the case at the Cladagh River Glen site. To some extent, therefore, Shade Horsetail may be considered a pioneer species, growing best in situations where relatively open conditions are maintained by these types of naturally operating factors. Both of the Fermanagh sites are relatively lowland, but the species is reputed to reach 850 m in Glen Coe in Scotland.

Fermanagh occurrence

In Fermanagh, E. pratense is found in only five tetrads and is the rarest horsetail species in the VC. It is twice as rare as E. hyemale (Rough Horsetail), which is known in twelve post-1975 tetrads), and much less frequent than two of our three hybrid horsetails, E. × litorale (Shore Horsetail) and E. × trachyodon (Mackay's Horsetail). In fact, there are just two main sites (with subsites), in the county for Shady Horsetail in shaded, moist woods and scarps, but they are rather different from one another.

In the Cladagh River Glen (also referred to as the 'Marble Arch Glen'), E. pratense grows in one large patch on a moist, sloping bank in the light shade of an ash dominated woodland. It also has a number of smaller colonies nearby, along the bank of the river either side of the adjacent path. At the second more extensive site on the scarp top of the Magho cliffs and the talus slope below that overlook Lower Lough Erne, Shady Horsetail occurs thinly scattered within the canopy of a six km long stretch of wet, mixed deciduous woodland, growing here amongst Luzula sylvatica (Great Wood-rush) and Stellaria holostea (Greater Stitchwort).

Even when failing to cone and restricted to an entirely vegetative condition, the persistence of E. pratense is quite remarkable. Records of Shady Horsetail at Co Antrim sites (H39) extend back up to a century or more. The plant has not, however, been refound in Fermanagh at Praeger's original 1904 site, which he (in the days before grid references) rather loosely described as being amongst rank heather on an open moorland scarp, behind (ie south of) Poulaphouca cliffs – a name rather widely printed across several cliff ranges on the one inch OS map (Praeger 1904). Nevertheless, the species persists on adjacent scarp tops and scarp woodland along the Cliffs of Magho and, indeed, if one can call it such, the species has its Fermanagh headquarters here.

Sterility and changing winter temperatures

In Fermanagh, in common with elsewhere in Britain & Ireland in recent years, observation suggests that E. pratense is sterile and cones only very rarely and spasmodically, if at all. E. pratense appears to rely almost exclusively on vegetative reproduction for its increase and survival, and on rhizomatous creep for any local spread that it manages to achieve. The species is, however, long persistent in the vegetative state in Britain and Ireland, most of the sites known a century or more remaining extant (Stewart et al. 1994).

As Page (1982 & 1997) points out, herbarium specimens from the 19th century indicate that E. pratense used to cone abundantly, and possibly did so regularly. However, it is possible to imagine that field botanists may have favoured or restricted their collecting to 'complete', ie sporing specimens, thus unintentionally distorting the frequency of cones in the herbarium record.

Since the great majority of Shady Horsetail sites in Britain & Ireland are fairly remote, they are left relatively undisturbed by man's activities. In the absence of human disturbance, the near sterility of modern clones suggests some other environmental factor(s) controlling cone induction has changed sufficiently during the last 100 - 200 years to prevent spore production. The most likely rapidly changing environmental factor that might affect horsetail fertility is winter temperature. Since about 1850 AD there has been a gradual climatic warming in Britain & Ireland producing milder winters. This trend began just prior to current CO2 and other greenhouse gas-induced global warming associated with the industrial revolution. Even slight climate warming may have provided the significant biological threshold beyond which E. pratense fails to produce sporing cones (Page 1997).

Other evidence of species decline

In Fermanagh, E. pratense is at the very southern edge of its W European geographical range and, because of its poor to negligible reproductive capacity in modern times, Page (1982 & 1997) fears that the species is in the process of dying out in Britain & Ireland. Species living at the extreme limits of their distribution are often represented by small, scattered, isolated populations, which frequently are either completely sterile or are reproductively weak. Compared to populations in the central parts of the species distribution, they tend to occur, as E. pratense now does here, in smaller numbers and in a much more restricted range of habitat conditions than occurs elsewhere within their overall distribution. These features indicate such species are suffering from both poor competitive ability and a lack of recruitment into fresh suitable sites.

In genetic terms, small, isolated populations inevitably lose vigour over extended periods of time, gradually losing genetically variability (sometimes referred to as 'genetic erosion'), as a consequence of restricted gene flow and 'genetic drift' (ie the tendency for gene alleles to fix within small populations at random, or even somewhat against selective forces) (Richards 1997, p. 46-9).

These processes associated with inbreeding lead to the accumulation of homozygous gene alleles and deleterious recessive mutants but, in polyploid species, such as most pteridophytes, these processes may develop very slowly indeed. Thus we cannot involve them in the much more rapidly occurring loss of fertility that appears to be the case in E. pratense populations.

As a northern-montane or circumpolar boreal-montane species, E. pratense is also likely to find the current rather dramatic rapid climatic warming in NW Europe unfavourable to its continued survival in these islands, a situation leading to slow decline and eventual extinction as Chris Page (1982 & 1997) has predicted. Thus E. pratense in Britain & Ireland already demonstrates several of the features associated with genetically weakened species populations (reproductively inadequate, small, scattered, more or less isolated populations lacking recruitment, displaying poor competitive ability). When this is compounded with the unsettling effects of an unfavourable warming climate, plus the thinning effects of random mortality inevitable in all small populations, with regret the current writer is forced to agree with Page's depressing verdict.

Along with other arctic-alpine, circumboreal and northern-montane plants, E. pratense must be recognised as a relict species in our latitudes of past cooler environments, and as such it is doomed to local extinction, probably in the not very distant future (Briggs & Walters 1997, pp. 411-419).

British occurrence

In England, E. pratense is already completely confined to a few scattered stations in the northern Pennines, but it is much more widespread in Scotland, typically occurring as small patches in the lower valleys in the Highlands and islands, but occasionally found on more open upland moorland where drainage and spring water flushes provide nutrient enrichment of the turf (Jermy & Camus 1991; Stewart et al. 1994; Page 1997).

European and world occurrence

In Europe, E. pratense has a very pronounced circumpolar boreal-montane range that is distinctive from any other European horsetail. It ranges from Iceland, north to within the Arctic Circle in Scandinavia, and south to the C European mountains and the Caucasus. However, it is completely absent from both the French Alps and the Pyrenees (Jalas & Suominen 1972, Map 37; Jonsell et al. 2000, p. 21). The distribution then stretches eastwards across most of N Asia to Manchuria, N Japan and across N America, south to about 40o N. It is absent, however, from Greenland and from much of W Europe (Hultén 1962, Map 83; Hultén & Fries 1986, Map 18; Jonsell et al. 2000).

Names

E. pratense is neither distinctive enough nor sufficiently common to have been given local English common names, 'Shady Horsetail' or 'Shade Horsetail' being mere invented book names. The Latin specific epithet, 'pratense', means 'growing in meadows' (Gilbert-Carter 1964), which fits the behaviour of the species in Scandinavia and undoubtedly in other northern parts of its range.

Threats

Both Fermanagh sites are protected; the Cladagh River Glen is a National Nature Reserve and part of the Magho cliffs is a Forest Nature Reserve. They could still be vulnerable to grazing or trampling.