A18 - Ranunculusfluitans Community Ranunculetum Fluitantis Allorge 1922

Summary

Synonymy

Moderate-moderately swift current vegetation Butcher 1933.

Constant species

Ranunculus fluitans.

Physiognomy

The Ranunculetum fluitantis comprises stands of submerged vegetation dominated by clumps of Ranunculus fluitans, sometimes numerous and close-set, in other cases few and sparse. It is generally a perennial plant, with individuals often long-lived though highly plastic in their morphology with just a few shoots, fairly short, in less congenial situations, but growing much more bushy and very long, up to 6 m, where conditions are more favourable, with the fine capillary foliage trailing downstream. Rarely, it can be found in an annual terrestrial form, with much-condensed shoots growing on moist ground (Cook 1966, Holmes 1979, Rich & Rich 1988). It is winter-green, with quite large populations of shoots persisting from one year to the next, but attains its maximum size early in the summer (Haslam 1978).

Few other plants occur with any frequency or abundance in denser stands, but there is sometimes a little Myriophyllum spicatum, M. alterniflorum or Potamogeton perfoliatus with patches of the mosses Fontinalis antipyretica or F. squamosa growing on submerged stones. Elodea canadensis, Lemna minor or L. gibba may occur in slacker waters, and there can be shoots of plants like Glyceria fluitans and Mentha aquatica trailing in from the margins.

Habitat

The Ranunculetum fluitantis is most characteristic of bigger moving waters, often with quite swift flow, and stable, stony beds, usually only moderately fertile and not very base-rich. It is virtually confined to England where it occurs most commonly in faster lowland streams and wide but not too spatey, rivers in the upland fringes.

R. fluitans needs considerable water movement to maintain good growth, and deeper channels, often of 1 m or more, are favoured (Haslam 1978). It will occasionally colonise sluggish waters in larger dykes and drains, though it rarely flowers in such situations (Holmes 1979), and it is very easily eroded from finer, soft substrates. It is much more frequent on consolidated beds, with gravel or pebbles, sometimes larger stones or boulders, and here it can gain a much firmer hold. The long, flexuose shoots, growing in streamlined clumps, offer little resistance to turbulence, and R. fluitans can extend into quite fast-flowing waters. It will persist even in torrents as sparse shoots but will not tolerate very spatey conditions, so in hill steams tends to be best developed in wider places where the bed is more stable.