Synonymy

Floating-leaved vegetation West 1910, Tansley 1911; Polygonum amphibium sociation Spence 1964; Polygonum amphibium Gesellschaft Oberdörfer 1977; Polygonum amphibium Community Birse 1980.

Constant species

Polygonum amphibium.

Rare species

Juncus filiformis.

Physiognomy

Polygonum amphibium is a far-creeping rhizomatous perennial that occurs occasionally in a variety of swamps and mires, and even in some quite dry habitats, but which can assume local dominance in water and around its margins in this Polygonum community. Growing in an aquatic state, the plants are usually rooted in the banks or shores, with shoots trailing through the water for up to 75 cm or so, the longpetioled leaves floating on the surface. Where the waters fluctuate, quite a common feature where this species grows, stands can be found exposed, with the shoots lying unsupported on the ground, and the vegetation can survive considerable periods like this before resubmergence. Indeed, if such water margins are freshly colonised by P. amphibium, the plants can be found in the strikingly different terrestrial form, erect, little branched and with sessile or very shortly stalked leaves (Lousley & Kent 1981), and pure stands of such individuals are obviously indistinguishable floristically from those growing under water.

Although aquatic P. amphibium vegetation is quite often found in close association with a variety of other floating and floating-leaved communities, dense stands frequently have few other species intermixed with them. There is occasionally some Nuphar lutea or Nymphaea alba, Potamogeton natans or Lemna minor, and sometimes trailing shoots of Glyceria fluitans or Callitriche stagnalis can be found. Beneath, there can be sparse plants of Potamogeton obtusifolius, P. perfoliatus or P. gramineus and a little Elodea canadensis, or an open sward of Littorella uniflora, or occasionally emergent Equisetum fluviatile. The rare Northern Montane rush Juncus filiformis occurs in great abundance in this kind of vegetation at its locality in County Durham (Graham 1988), where it is an introduction, perhaps brought in by birds (Perring & Farrell 1977).

Habitat

The Polygonum community is characteristic of the surrounds and shallows of standing to generally slowmoving, sometimes fluctuating, waters, often base-poor and usually only moderately nutrient-rich. It occurs widely through most of the British lowlands and upland fringes, in and around pools, lakes and reservoirs, dykes and canals, streams and rivers, and periodically wet flood-plain hollows.