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Equisetum fluviatile L. Water Horsetail

Account Summary

Native, very common, widespread and locally abundant. Circumpolar

boreo-temperate.

1881; Stewart, S.A.; Co Fermanagh.

Throughout the year.

Growth form and preferred habitats

This distinctive, erect, emergent aquatic or semi-aquatic rhizomatous, deciduous horsetail is very variable in size and in degree of branching. It is most commonly and abundantly found in still or slow-moving, shallow water by lakes, ponds and ditches, a habitat where it quite often represents the dominant colony-forming species. Even when dense pure stand communities of Water Horsetail are found, be advised by Wolfe-Murphy et al. (1992), who wrote with bitter experience, that the rhizomes are less robust than those of rather larger emergent species such as Schoenoplectus lacustris (Common Club-rush), and they do NOT form platforms that can bear the weight of the average botanist!

In lakes and ponds, E. fluviatile frequently forms large, dense stands, either pure or accompanied by a very long list of other common emergent wetland species such as Schoenoplectus lacustris, Cladium mariscus (Sword Sedge), Carex rostrata (Bottle Sedge), C. elata (Tufted-sedge), Sparganium erectum (Branched Bur-reed), Glyceria fluitans (Floating Sweet-grass) and Phragmites australis (Common Reed).

E. fluviatile can also persist, but to a lesser extent, in shallows in bays of larger water bodies which by their nature are subject to more water turbulence and wave-induced physical scour. In these circumstances, where Phragmites australis (Common Reed), Schoenoplectus lacustris or Typha latifolia (Bulrush) often represent the deeper water dominants, E. fluviatile regularly replaces Eleocharis palustris (Common Spike-rush) when water is deeper than about 50 cm (Spence 1964). Water Horsetail can also be the dominant species and regularly forms dense, almost pure stands in water 1.5 m deep or deeper (Spence 1964; Wolfe-Murphy et al. 1992; Page 1997).

Like its common relatives, E. arvense (Field Horsetail) and E. palustre (Marsh Horsetail), the rhizome of E. fluviatile runs much deeper in the soil than the underground organs of associated species, thus avoiding most root competition with them, if not altogether.

Identification

Individual stems are readily identified by the large central hollow that occupies between 80 and 90% of the stem diameter. As with several other Equisetum species, when E. fluviatile is in more open, unshaded situations, its stems are typically unbranched, or they bear only short, sparse lateral branches. The latter are usually irregularly whorled near the middle of the aerial length of the stem, but in more shaded situations, for instance in marsh or reed-swamp among fairly dense tall grasses, sedges and rushes, or under trees or scrub in fen carr, Water Horsetail regularly produces regularly whorled lateral branches on the emergent portion of its stems (Preston & Croft 1997; Page 1997, p. 446; Rose 1989, plate 52).

Fermanagh occurrence

Fermanagh has a huge number and variety of lakes, both very large and small, and they exhibit an enormous diversity of water chemistry (Gibson 1988). It is therefore not surprising that the shores of these supply E. fluviatile with plenty of scope for colonisation, and it is by far the most frequently recorded horsetail in the VC. While it has been recorded in 329 Fermanagh tetrads, 62.3% of those in the VC, E. fluviatile is not the most widespread horsetail, a distinction held by E. arvense (Field Horsetail).

Wide ecological tolerances

Water Horsetail is very remarkable for the extreme width of its ecological tolerances with respect to a spectrum of inter-related environmental factors, restricted levels of which typically curtail the growth and govern the occurrence of most other wetland plant species. The tolerances of E. fluviatile include levels of nutrients (from oligotrophic to eutrophic), lime content (from starved to rich), acidity-alkalinity (from pH 4-7.5), light (from full sun to half-shade) and exposure (from still backwater to open, moderately wave-beaten shore). The substrate textures it tolerates vary from clean, firm, mineral sand to silty, smelly, organic mud, deep enough to suck the boots off you!

Water Horsetail can also survive in oxygen-depleted, highly anaerobic, hydrogen sulphide-releasing conditions that exclude many other aquatic species (Grime et al. 1988; Wolfe-Murphy et al. 1992; Preston & Croft 1997). The very large central cavity in the hollow stem of E. fluviatile is considered to be an adaptation allowing air to diffuse downwards to the rhizome, in which latter organ, unusually among Equisetum species, the wide lumen persists. Very possibly it is this unusual morphological property which enables the rhizome of E. fluviatile to penetrate, grow and persist in anaerobic layers of mud (Page 1997).

In N Ireland, Wolfe-Murphy et al. (1992) carried out a detailed lake survey for government from 1988-1991 covering all six counties in the Province. This generated a macrophyte vegetation classification of lake vegetation using the computer program 'Twinspan' (unpublished report to DOE, NI, p. 294). The study found that E. fluviatile was abundant or dominant in eight of the 30 shoreline plant communities these workers defined in N Ireland, indicating the exceptionally wide ecological range of the species. Water Horsetail is common on exposed mud or shallow water in sheltered backwaters of larger lakes, and occasionally so in sluggish rivers and streams, particularly if there is only a minor fluctuation in water levels. It also frequents, in a more scattered manner, the closed turf vegetation of marshes, swamps and Salix-Alder fen-carr habitats, where its status in these plant communities is that of a minor companion species, except when shade seriously restricts the more competitive species around it: reduced light allows rhizomatous E. fluviatile to reassert its vigour and develop a more significant presence (Grime et al. 1988).

Reproduction

Again as is the case in both E. palustre and E. arvense, although E. fluviatile produces vast numbers of cones and spores, conditions for the completion of the full sexual life-cycle are stringent, and prothalii are very seldom observed (Page 1967; Duckett & Duckett 1980). It is thus very probable that increase and dispersal of Water Horsetail is heavily dependent on vegetative reproduction, achieved mainly by fragmentation. Propagation involves either free-floating segments of broken stem, or rhizome fragments. E. fluviatile does not possess tubers (Page 1997).

Herbivory and incidental vegetative propagation

Ducks, geese and other waterfowl feed on the stems of the plant (Fassett 1957), and Coot have also been observed biting off stems and using them for nest building (Praeger 1934). In both these circumstances, the birds were observed deliberately breaking excessive numbers of Equisetum shoots, more of them than they actually used (and in the case of feeding birds, they broke them into many small segments), so that some of the stem pieces could disperse without being used. These stem fragments can develop roots and may thus propagate the plant on suitable wet terrain (Praeger 1934; Page 1997).

British and Irish occurrence

E. fluviatile is a very common, widespread and locally abundant species throughout most of Britain & Ireland occupying a wide variety of habitats. It has declined quite substantially in the last 50 years, due mainly to drainage of smaller wetlands and poor, unsympathetic management of other sites (C. Dixon & T.D. Dines, In Preston et al. 2002).

Environmental pressures and species change

The distribution of Water Horsetail in both Britain and Ireland has undoubtedly been somewhat curtailed in the past century by agricultural drainage and other forms of development. These habitat pressures are most obvious in the most heavily populated and intensively farmed areas in the south of England (Grime et al. 1988; Preston & Croft 1997). At the same time, in other areas of these islands it is possible that E. fluviatile may have benefited from the suppression or demise of other aquatic and marsh species more sensitive to increased eutrophication than it. This is not to suggest that E. fluviatile tolerates extreme levels of organic pollution or sewage-induced accelerated cultural eutrophication. Study of a polluted lake in S Wales showed the species declined noticeably under such circumstances (Wade 1999).

In comparison to crop plants and terrestrial herbs, aquatic macrophytes have not been much studied by scientists working on climatic change. However, in a three year study, Ojala et al. (2002) found growth and reproduction of E. fluviatile was sensitive to a rise of around 2.5 to 3°C.

European and world occurrence

Water Horsetail has a widespread and more or less continuous distribution in boreal and temperate parts of Europe that is very similar to the occurrence of E. palustre. However, the distribution of E. fluviatile thins much more noticeably than E. palustre in the Mediterranean basin (Jalas & Suominen 1972, Map 34). E. fluviatile spreads eastwards from Turkey and the Caucasus, through temperate Asia to N Japan and N America. Only its absence from Greenland prevents it from being circumpolar (Hultén 1962, Map 96; Hultén & Fries 1986, Map 15; Jonsell et al. 2000).

Names

Since it so frequently occurs in muddy ground, E. fluviatile used to bear the name 'E. limosum', 'Mud Horsetail', the Latin specific epithet being derived from 'limosus', meaning, 'of marshy or muddy places'. An earlier name of the plant was 'E. heleocharis', derived from two Greek words, 'helos', meaning 'marsh', and 'karis', meaning, 'charm, grace, or beauty' (Gilbert-Carter 1964; Johnson & Smith 1931). By comparison, the modern specific epithet, 'fluviatile' is derived from the Latin, 'fluviatilis', meaning, 'growing in a river or in running water' (Stearn 1992). In the current writer's opinion, this is an inaccurate and completely misleading indication of the normal habitat of this horsetail, which more often is characterised by still or slow-moving water.

Additional English common names for E. fluviatile include 'Smooth Horsetail', a feature of the stems, said, in the past, to make them acceptable to cattle as food. However, reference to Linnaeus in this connection suggests he was really referring to E. telmateia (Great Horsetail), since the latter was his 'fluviatile' (Grieve 1931; Step & Jackson 1945).

Other interesting local names include 'Paddock Pipes' and variants thereof (eg paddow, paddie and puddock). 'Paddock' is a Scottish name for frogs, making it rather appropriate for this horsetail, but it is devalued by being used for other horsetails (Britten & Holland 1886). Page (1988) mentions the interesting name, 'Trowie Spindles', but offers no details of its origin or derivation.]

Threats

Drainage probably poses the only threat likely to affect this species locally in Fermanagh.