Equisetum hyemale × variegatum (E.
× trachyodon) A. Braun, Mackay's Horsetail
Account Summary
Native, rare, but locally abundant.
1904; Praeger, R.Ll.; Bunnahone Lough, Lenaghan Td.
Throughout the year.
Growth form and preferred habitats
Plants of this evergreen, rhizomatous hybrid are more robust and more branched than those of E. variegetum, sometimes stretching up to 75 cm tall. This vigour and the fact that the long black teeth on the nodal sheaths are usually very persistent together help to distinguish the hybrid from both its parent species.
The hybrid grows on rocky lakeshores and wooded riverbanks and in Fermanagh is twice as frequent as one of its parents, Equisetum hyemale (Rough Horsetail). It usually occurs some distance from the main concentration of sites of the other parent, E. variegatum (Variegated Horsetail), which seems to require a more calcareous or more base-rich habitat than does the hybrid. E. × trachyodon does occur near E. hyemale at one spot on the upper reaches of the Colebrooke River, and with E. variegatum at Shean Jetty and Magho Jetty along the S shore of Lower Lough Erne, but these are the only exceptions. In the remaining twelve stations, E. × trachyodon avoids both its parents – or perhaps on account of its vegetative vigour, they avoid it!
The Fermanagh plants of this rhizomatous hybrid can form quite large, dense stands, as happens for instance on the Bannagh River and at one spot on Upper Lough Macnean, but it can also occur as just a few straggling, branched stems as on the shore of Lough Lattone. Alternatively, it may be scattered in clumps, as it is along a km or so of the bank of the Colebrooke River below Littlemount Bridge.
In general, the known occurrence of all forms of hybrid horsetails are far from evenly distributed throughout the overlapping portions of their parent species ranges in Europe (Jalas & Suominen 1972). Rather they tend to be thinly scattered, but with distinct local concentrations in certain areas. This is particular the case in western and northern regions of Britain & Ireland (Page & Barker 1985; Page 1997).
Equisetum hybrids in Britain & Ireland
All eight native Equisetum species in the British Isles are involved in producing hybrids, but they are formed strictly between pairs of species within the same subgenus. Six of the British & Irish horsetail species belong to subgenus Equisetum, and two (E. hyemale and E. variegatum) to subgenus Hippochaete. Perhaps surprisingly, only two hybrids from subgenus Equisetum have been found so far in Fermanagh.
Fermanagh occurrence of E. × trachyodon
In the case of the solitary subgenus Hippochaete hybrid, E. × trachyodon, N Ireland undoubtedly has the greatest concentration of known stations for this hybrid possibly anywhere. Within the six-county province, Fermanagh with its 13 main sites and their sometimes many sub-sites has the greatest representation of this hybrid. The Fermanagh tetrad map plots records from 22 post-1975 tetrads, plus two tetrads with older records. The details of the Fermanagh sites where E. x trachyodon has not been refound are: Bunnahone Lough, 1904, Praeger; and Lough Vearty, 1949, MCM & D.
Other Irish occurrences
The Fermanagh coverage is followed by Cos Down (H38), Antrim (H39) and Londonderry (H40), with Tyrone (H36) and Co Armagh (H37) trailing with just one or two sites each (NI Vascular Plant Database 2001). In the Republic of Ireland, Co Cavan (H30) also has one old 1950s record from Gowland, yet so far neither parent species has ever been recorded there (Reilly 2001). Elsewhere there are a further 12 tetrads scattered, mainly in the west, across 8 Irish VCs from Monaghan (H32) and Sligo (H28) to Mid Cork (H4), plus two more inland VCs, N Tipperary (H10) and Kildare (H19) (New Atlas).
Vegetative reproduction and spread
Some of the Fermanagh sites are obviously linked, for instance those along riverbanks, or along lake shores and they may, or may not, represent fragmented clones generated by secondary vegetative spread. Page (1997) reported that small fragments of hybrid shoots root very readily, even after they have floated around for up to ten days, so this vigorous hybrid definitely has a mechanism for increase and local dispersal.
In Fermanagh, we see evidence of local vegetative spread along the Colebrooke River, along the N shore of Upper Lough Macnean and the S shore of Lower Lough Erne. However, other stations are sufficiently remote to certainly represent independent parental hybridisations, and these cases form the majority in Fermanagh.
British occurrence
In comparison with NW Ireland, E. × trachyodon is very poorly represented in Britain, there being only three or four sites in England and six or seven in Scotland (New Atlas).
A strong westerly trend in hybrid horsetail occurrence
All but one of the British stations are in the extreme west and when one examines the distribution of all horsetail hybrids in Britain & Ireland, a westerly trend in their occurrence becomes very obvious. Indeed, if we 'zoom out' to view the whole European picture of hybrid horsetails, the westerly trend appears to be mirrored even at this much larger scale (Page & Barker 1985; Page 1997).
The explanation for this strongly marked trend in distribution presumably lies in the prevailing oceanic climate of the most westerly parts of Britain & Ireland. This is readily summarised as cloudy skies, high rainfall levels that are evenly dispersed throughout the year (ie over 200 wet days) and generally low temperatures with no extremes (ie mild winters and cool summers) (Page & Barker 1985; Page 1997, Maps 6-16; Porley 2001). The described climatic conditions appear to allow horsetail sporophytes of differing species to grow in close proximity to one another. The damp, mild environmental conditions also favour the survival of normally very short-lived horsetail spores, permit prolonged growth of the gametophyte generation and provide a near-constant film of free water, facilitating male gamete transfer conducive to cross-fertilisation (Page & Barker 1985).
The Massenerhebung effect in mountain areas
The ecological overlap of related species in Equisetum and in several other plant groups is probably facilitated by the wonderfully named 'Massenerhebung effect', a German term literally meaning, 'mountain mass elevation effect'. This is a meterological concept that was introduced by A. de Quervain in 1904 to account for the observed tendency for temperature-related parameters such as treeline and snowline to occur at higher elevations in the Central Alps than on their outer limits (Barry 1981). The concept stipulates that climatic and vegetation zones occur at lower altitudes on isolated mountains than they do in mountain blocks of increasing scale (Wardle 1974; Johns 1985). The energy physics relating to this mass-elevation effect are complex, however, and it can only be applied after very careful consideration of the specific local topographical and meteorological factors involved (Barry 1981, pp. 49-50).
Continentality versus Oceanity
The Massenerhebung effect relates to the much more generally applicable climatic concept of Continentality versus Oceanity. Applying either of these two concepts, we find that the low elevation, the small, isolated mass, and the maritime position of British and Irish mountains results in a greatly steepened temperature lapse rate and a marked compression and lowering of our vegetation zones in comparison with continental European uplands, including the Alps (Barry 1981, p. 265). The overall effect of this pattern of climatic variation allows southern species to migrate northwards along western Atlantic coasts, avoiding cold winters. This has produced, for instance, the highly unusual and very famous mix of phytogeographic elements found in the flora of the Burren region, Co Clare (H9). At the same time, montane or alpine plants and animals, with their requirement for cool summer conditions, can descend to lower levels and, in W Ireland, some of these come right down slope, very close to sea level (Praeger 1934, section 67; Webb 1983; Page & Barker 1985; Nelson & Walsh 1991).
Equisetum species mingling
Thus plant species, including sporophytes of our eight native Equisetum species, are often found growing much more closely together in the western parts of Britain & Ireland than they would under more continental growing conditions, where they would commonly be altitudinally, geographically and ecologically separated. Indeed, in the case of some other Fermanagh pteridophytes, colonies of four or more different but closely related species are often found growing intermingled, and ecological and genetic isolating mechanisms no longer apply. This enables the observed increase in frequency of hybridisation.
Reduced competition?
Perhaps we should not ignore the fact that in the case of Ireland, being a more ancient island than Britain, post-glacial species immigration was cut off earlier by sea level rise and thus the species-poor Irish flora presumably presents a less competitive environment than otherwise to newly arrived genetic combinations in the form of the prothalli of Equisetum hybrids. A similar argument would of course apply when comparison is made between the depauperate flora of Britain and that of mainland Europe.
E. × trachyodon is usually remote from its parent species
A comparative analysis of the geographical stations of species in the genus Equisetum and their then known hybrids was made around 1985 by Page and Barker based on updated hectad maps from The Fern Atlas (Jermy et al. 1978). This showed quite clearly that hybrids and their parent species behave quite differently in the two subgenera. In subgenus Equisetum, for every hybrid, examined over the British Isles as a whole, there is an almost 100% coincidence or association between the distribution of hybrid stations and the presence of both parents in the same or immediately adjacent hectad grid-square. The picture for subgenus Hippochaete is quite the opposite and, for E. × trachyodon, both parents were present in the same or adjacent hectads in less than 20% of its sites.
Our finer scale Fermanagh station analysis mentioned above shows the separation of E. × trachyodon and its parent species is even more pronounced than these workers showed. Page & Barker (1985) considered it likely that whilst a small amount of hybridisation may continue to take place in subgenus Hippochaete, the geographical evidence suggested to them that the majority of clones found today are long-established in their particular sites. Indeed, some clones may be very ancient and their parent species appear to have locally died out, or have been ousted by competition with their hybrid progeny.
European and world occurrence
Beyond the shores of Britain & Ireland, E. × trachyodon is locally fairly frequent in Norway, is widely scattered throughout Iceland and occurs in a few provinces in Sweden and Finland. It has also been reported from at least one station in S Greenland, although RSF has heard that other botanists have cast doubt on the identification: one of the parent species, E. hyemale, is said to be unknown there (Böcher et al. 1968; Jonsell et al. 2000). This might not be as significant as it first appears since exactly the same situation applies in Ireland with respect to E. × moorei Newman (E. hyemale × E. ramosissimum) (Moore's Horsetail), since E. ramosissimum Desf. (Branched Horsetail) is absent. In addition to the foregoing, E. × trachyodon is also reported from scattered localities in Czechoslovakia, France, Germany, Switzerland, Hungary and in parts of Russia and temperate N America (Duckett & Page 1985).
Names
The name 'trachyodon' is a combination of two Greek words meaning 'rough teeth' (Gilbert-Carter 1964).]
Threats
Clearing of riverbanks for fishing or agricultural purposes. Locally this is especially problematic along the Colebrooke River.