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Mat-forming herb with an extensively branched, horizontal system of rhizomes reaching some centimetres deep in the substrate and with vertical branches to aerial shoots. The rhizomes sometimes form very large, loose colonies or mats, often many tens of metres in diameter. Aerial shoots annual, articulate (both stems and branches), in two imperfectly separated shoot generations within one season (i.e., imperfect shoot dimorphy). Reproductive shoots with a single terminal spike appear in the spring and early summer. These shoots are initially unbranched, ascending to erect, brown (non-assimilating) or green (assimilating), but often produce branches at the base when the apical part with the spike withers (therefore ‘imperfect’). The vegetative shoots appearing later in the season are much longer, procumbent or ascending, richly branched, and green (assimilating). On these shoots, the branches are often longer than the stem, and the apical ca. 1/3 of the stem is mostly without branches and whip-like.
Reproductive shoots 4–8 cm long, slender, usually 1–1.5 mm broad. Vegetative shoots 8–17(20) cm long or even longer in sites with much nutrients, slender, ca. 2 mm broad. The branching takes place at the joints, at the base of the leaf whorls (see below), potentially with as many branches as there are leaves in the whorls and ribs on the stem, 3–4(5), but often some branches are missing and making the whorls irregular. Branches with 3(4) sharp angles and 3(4) leaves at the end of each joint.
Leaves as a fused sheath with free teeth (as many as ribs) in whorls on reproductive and vegetative main shoots and on branches. Teeth on main shoots narrowly triangular, 1.5–2 times as long as wide, brown with broad, white hyaline margins, on branches very small, narrowly triangular, green and with or without a very narrow white margin.
INFLORESCENCE AND FLOWER
Spikes oblong or ovoid, 5–12 mm, with sporangiophores (transformed leaves carrying sporangia) in 6–8 dense whorls. Sporangiophores peltate with an angular end plate participating in a compact outer surface of the spike until the spores are mature and the spike opens with slits between the sporangiophores to let the spores loose. Each sporangiophore carries several oblong sporangia in a whorl on the inner surface of the plate. Sporangia open by longitudinal slits. Spores globose, green, each with four long, club-shaped bands (hapters), stretching out in dry weather, making the spore mass fluffy and wind dispersed in such conditions. In moist weather the hapters curve inwards making the spore mass immobile.
Sexual reproduction by spores; very efficient local vegetative reproduction by rhizomes, often forming large carpets that easily are fragmented.
Efficient wind dispersal of spores due to small size and the hapters. Downstream dispersal of rhizome fragments is possible.
The genus Equisetum is immediately recognized by the articulate stems and branches and the characteristic spike with angled sporangiophores.
Equisetum arvense is distinguished from the two other species in Svalbard – E. scirpoides and E. variegatum – especially in that the former has annual dimorphic shoots (two different shoot generations), the two latter perennial monomorphic shoots. Even if this difference is very marked, it has often been overlooked and there have been numerous mistakes in identifications. Some additional characters should be consulted. Firstly, E. arvense has branched aerial shoots, mostly yellowish green, the two others simple shoots with dark green colour. Furthermore, the aerial shoots of E. arvense are soft, with intact whorls of teeth on the main stems, whereas those of E. scirpoides and E. variegatum are firm, with withered teeth (often as a bicolorous band in black and white).
There has been and probably still may be some discussion whether two races of E. arvense are present in Svalbard. Characters that can be used for distinguishing ssp. arvense from ssp. alpestre are: In ssp. arvense the vegetative shoots are stout (> 2 mm) with 4–18 smooth ribs and have a short, simple apical part; branches are mostly 4-angled; teeth on vegetative main shoots are 2–4 times as long as broad and with very narrow hyaline margins; reproductive shoots almost never branching (i.e., perfect shoot dimorphy); and spikes are large (> 15 mm). In ssp. alpestre the vegetative shoots are slender (1–2 mm) with 3–4(5) often rough ribs and have a long (ca. 1/3 of shoot) and whip-like, simple apical part; branches are mostly 3-angled; teeth on vegetative main shoots are 1.5–2 times as long as broad and with broad hyaline margins; reproductive shoots regularly produce branches at the base after spore maturation (i.e., imperfect shoot dimorphy); and spikes are smaller (5–12 mm). The widespread form in Svalbard for sure falls within ssp. alpestre; presence of ssp. arvense in Svalbard is not yet documented by voucher specimens
A common species of many moist and wet environments, in shallow marshes, on sediment plains, in shallow moss tundra, snowbeds, screes with some ground water, bird cliff meadows, moist heaths and other meadows. The species is associated with fine-grained soils but also grows on coarser substrates as long as water is easily available. It is a very common plant in disturbed sites, either disturbed by natural means (land slides, erosion) or by man (road verges, ditches, abandoned mining places). The intensely green colour of the dense mats of Equisetum arvense, seen from far away, often tells that some prior disturbance has taken place.
Present in all zones and sections and probably quite common on all major islands and some minor ones. Not recorded from Hopen or Kong Karls Land.
Circumpolar distribution. Common throughout the Arctic, reaching south in temperate mountains, in Europe to the C European mountains.
The two main subjects which have been discussed concerning Equisetum arvense, in Svalbard are the validity of the two proposed subspecies and whether both occur there. Many authors have doubted whether the two subspecies can be kept apart consistently. As seen from Comparison (above), there are several and assumed independently inherited diagnostic characters. In preparation for Flora Nordica, Benjamin Øllgaard was very sceptical to acceptance of subspecies but ended up with acceptance and with the differential characters above (Øllgaard 2000). Schönswetter et al. (2001) reached the same conclusion but Schönswetter & Elven in Elven & Murray (2008) changed the name of the subspecies from ssp. boreale to ssp. alpestre because the name ‘boreale’, based by Brongniart in 1833 on a plant from Sitka in coastal Alaska, refers to a morph of ssp. arvense. The criteria have been applied on an extensive material from C Europe (P. Schönswetter), Scandinavia (R. Elven), Iceland (H. Solstad & R. Elven), and Svalbard (here), and works well in all these areas. There is currently acceptance for the existence of a morphologically distinct, consistent and widespread arctic-alpine subspecies rather than just local alpine and arctic modifications.
Tall-grown and coarse plants in the settlements of Svalbard (especially in Longyearbyen) have been suspected belonging to ssp. arvense. They are much taller than the tundra plants with significantly longer shoots and more regular branches than in the plants found outside the settlements, and the reproductive shoots often wither away without producing branches. However, in the other characters used for separation, the plants we have inspected still conform to ssp. alpestre. We cannot exclude presence of ssp. arvense some place in Svalbard, and we can certainly not exclude future introduction. Principally for this reason, we have listed the differential characters between the subspecies above.
Elven, R. & Murray, D.F. 2008. New combinations in the Panarctic vascular plant flora. – Journal of the Botanical Research Institute of Texas 2: 433–446.
Øllgaard, B. 2000. Equisetaceae. – In: Jonsell, B. (ed.), Flora Nordica. 1. Lycopodiaceae – Polygonaceae: 17–27.
Schönswetter, P., Schneeweiss, G.M., Wittman, H., Tribsch, A. & Wiedermann, M. 2001. Equisetum arvense subsp. boreale auct. eur. (Equisetaceae) – ein bisher übersehenes, arktisch–alpines Florenelement der Alpen. – Neilreichia 1: 149–164.