Synonymy

Peaty moorland lochs West 1910 p.p.; Juncus fluitans consocies Pearsall 1921; Juncus fluitans vegetation Pearsall 1921; Juncus fluitans-Utricularia sociation Spence 1964; Juncus fluitans-Sphagnum subsecundum sociation Spence 1964; Sphagno-Sparganietum angustifolii Tüxen 1937 sensu Schoof-van Pelt 1973 p.p.; Scorpidio-Utricularietum Muller & Gors 1960 sensu Birse 1984 p.p..

Constant species

Juncus bulbosus.

Rare species

Utricularia intermedia.

Physiognomy

Juncus bulbosus is a plant of quite high frequency and some local abundance in a variety of aquatic communities and swamps, but in the vegetation included here it tends to be the most prominent feature along with a range of rather distinctive associates. Stands of this Juncus bulbosus community are of varying cover and luxuriance, with J. bulbosus itself occurring in the forms commonly recorded as var. fluitans, with branches procumbent and rooting at the nodes, or in free-floating masses, often forming dense tangles.

Occasionally, stands have very little else apart from J. bulbosus and no other species occurs frequently throughout the community, but Potamogeton polygonifolius is fairly common, sometimes with quite an extensive cover of floating leaves, particularly in shallower peaty waters, and some stands, often at greater depth and on rather more silty substrates, have P. natans. Then, Scirpus fluitans may be found, frequently hard to see among the floating rush shoots, and there can be some Callitriche hamulata or C. stagnalis. Myriophyllum alterniflorum is characteristic of one kind of Juncus vegetation and may be accompanied by occasional Potamogeton obtusifolius, P. gramineus and P. berchtoldii, though these are not usually present at very high covers.

More striking is the occurrence of various Utricularia and Sparganium spp. Commonest among the former is what has generally been recorded as U. vulgaris, but what is more likely to be the less basiphilous taxon in this aggregate, U. neglecta (Perring 1968). This is very hard to separate from U. vulgaris sensu stricto, especially with vegetative material which is all that has been found in the northern part of Britain where this vegetation is most common. U. minor has also been seen in this community and, very rarely, U. intermedia, although much care needs to be taken to distinguish this from U. ochroleuca.

Synonymy

Nymphaea alba consocies Pearsall 1918; Nymphaea occidentalis consocies Pearsall 1918; Nymphaeetum albae Tansley 1939; Nymphaeetum occidentalis Tansley 1939; Nymphaeetum minoris Vollmar 1947; Nymphaea alba sociations Spence 1964.

Constant species

Nymphaea alba.

Rare species

Nuphar pumila, Nymphoides peltata.

Physiognomy

Nymphaea alba is a native water-lily, but a plant widely introduced to lakes and ponds because of the ornamental character of its large white flowers, appearing above water in mid-summer and attractively set off against the dark green floating pads which are put up from the stout rhizomes some two months earlier (Heslop-Harrison 1955d). Some authorities (Clapham et al. 1962, but not Tutin et al. 1964) distinguish a ssp. occidentalis, to include altogether smaller plants than those reserved for a ssp. alba, but there is continuous variation between the extremes of size, and the taxa have only rarely been separately recorded in the available data. For these reasons, it is difficult to give a precise indication of the natural floristic affinities of the plant or to know how taxonomic differences within it are reflected among the associates. But there seem to be three kinds of floatingleaved vegetation dominated by water-lilies in which N. alba plays a prominent role. Quite commonly, and particularly in the English lowlands, it occurs in abundance among Nuphar lutea, and in this scheme such stands are included within the Nuphar community. In many situations, though, N. lutea is extremely rare among abundant N. alba and it is such vegetation that forms the basis of the Nymphaeetum albae, of which two types have been distinguished. Few other plants occur with any frequency throughout the community, indeed many stands are very species-poor, but Potamogeton natans varies from occasional to very common, and there is quite often some submerged P. obtusifolius. Lemna minor can also be found, although extensive covers of this are best seen as representing mosaics of the Lemnetum minoris among the water-lily vegetation.

Sub-communities

Species-poor sub-community. Very often, N. alba is the only plant in this vegetation, with just occasional P. natans or P. obtusifolius, but the rare Nuphar pumila has been recorded here in some of its Scottish localities and, in lowland England, Nymphoides peltata, another rarity that may have been introduced to parts of its present range (Perring & Walters 1962, Perring 1968).

Synonymy

Caricetum Pearsall 1920 p.p.; Carex vesicaria-Veronica scutellata sociation Spence 1964

Constant species

Carex vesicaria, Equisetum fluviatile, Galium palustre.

Rare species

Carex aquatilis.

Physiognomy

The Caricetum vesicariae is almost always dominated by Carex vesicaria, the tufted shoots of which form an open or closed cover in which there is usually a little emergent Equisetum fluviatile, some scrambling Galium palustre and sometimes abundant Juncus effusus. Indeed, although few other species are frequent throughout, the general appearance of the vegetation is often more like a fen than a swamp.

Sub-communities

Carex vesicaria sub-community. Here are included those rare stands in which C. vesicaria forms a virtually pure and sometimes extensive cover.

Mentha aquatica sub-community. In this sub-community, the C. vesicaria has an understorey of small or scrambling herbs such as Mentha aquatica, Myosotis scorpioides, Galium palustre and, particularly distinctive, Veronica scutellata. There is also usually some Equisetum fluviatile and Juncus effusus and scattered tall herbs such as Filipendula ulmaria, Lythrum salicaria and, sometimes abundantly, Phalaris arundinacea. A variety of other poor-fen and swamp species occur occasionally or rarely, including Carex aquatilis.

Carex rostrata sub-community. The vegetation here is somewhat similar to that of the Mentha sub-community except that Carex rostrata, Potentilla palustris and Menyanthes trifoliata are frequent and sometimes abundant components. Carex aquatilis has also been recorded from this sub-community.

Habitat

The community occurs in open-water transitions on mesotrophic inorganic or peaty substrates of pH 6.0-6.8 around lakes and in slow-moving or standing waters of streams and dykes. The Carex vesicaria and Carex rostrata sub-communities are characteristic of deeper standing waters, up to about 40 cm, whereas the Mentha sub-community typically occurs in drier situations where the water-table varies between 10 cm below and 20 cm above ground. The vegetation is sometimes cattle-grazed.

Zonation and succession

Although stands of the Carex vesicaria sub-community can occur in standing open water, the usual position of the community in marginal zonations is for it to occur inshore of the Caricetum rostratae to which it may grade through the Carex rostrata sub-community. Around such higher reaches and along the banks of dykes, the Mentha sub-community may occur alongside or pass to the Phragmitetum australis or to the Phalaridetum arundinaceae.

Synonymy

Vegetation of nearly stagnant waters Tansley 1911 p.p.

Constant species Ranunculus peltatus.

Physiognomy

The Ranunculetum peltati is dominated by clumps or patches of Ranunculus peltatus, the most variable of the British crowfoots. It can grow as an annual or perennial, submerged in a spreading or erect form, as a frail, floating plant or occur terrestrially, when it is loosely tufted (Haslam 1978, Rich & Rich 1988). Many stands are small and cover can be very sparse, but luxuriant plants develop in congenial situations with maximum growth in early summer.

The Ranunculetum peltati is frequently found in close association with other kinds of aquatic and marginal vegetation, including patches of the Ranunculetum aquatilis, and it can be very difficult to distinguish the two dominants from one another (Holmes 1979, Rich & Rich 1988). In denser stands, however, associates are few and usually not very abundant and such plants as do occur are often amphibious or water-margin herbs. Lemna minor and L. gibba are often found on the surface of the water or moist mud, with L. trisulca occasionally caught among the submerged shoots, and there can be patches of Callitriche spp., C. platycarpa in the stands sampled. Potamogeton natans and Polygonum amphibium are occasionally seen and sometimes there are plants of other Ranunculus spp., such as R. sceleratus, R. trichophyllus and R. circinatus. Later in the season, Glycerio-Sparganion plants often become prominent with patches of Nasturtium officinale, Glyceria fluitans, G. plicata, Apium nodiflorum and Berula erecta.

Habitat

The Ranunculetum peltati is characteristic of the shallows and margins of mesotrophic to quite nutrient-rich waters, occasionally fairly fast-flowing, though usually sluggish or still. It appears to occur through most of southern Britain outside the highland areas, but is especially frequent in the English lowlands. It withstands seasonal or periodic drying out of the habitat and this enables it to persist in fluctuating or ephemeral water bodies.

R. peltatus can be hard to distinguish from R. aquatilis (Holmes 1979, Rich & Rich 1988) and the two taxa are clearly similar in their geographical distribution and ecological preferences, though it has sometimes rather puzzlingly been reported that they do not in fact occur together (Cook 1966). Like the Ranunculetum aquatilis, this vegetation is commonly found growing submerged, and it may penetrate a little deeper (Holmes 1979), although most stands seem to occur in less than 1 m of water.

Synonymy

Scirpus maritimus zone Willis & Davies 1960; Stands dominated by Scirpus maritimus Gimingham 1964; Scirpetum maritimi Packham & Liddle 1970; Brackish water communities Birks 1973 p.p.; Scirpus maritimus nodum Adam 1976.

Constant species

Scirpus maritimus.

Rare species

Juncus subulatus.

Physiognomy

The Scirpetum maritimi is always dominated by Scirpus maritimus which usually forms a tall dense cover, about 60-80 cm high in which up to 10 shoots dm -2 have been recorded (Gimingham 1964tz). No other species is frequent throughout. Pure stands are common and very rarely are there more than ten associates. Most often, there are scattered plants of a wide range of species characteristic of the upper salt-marsh and strandline: Triglochin maritima, Juncus gerardi, Oenanthe lachenalii, Cochlearia anglica, Plantago maritima, Elymus pycnanthus and Apium graveolens are those most commonly encountered. Even in the richer sub-communities, which are distinguished by the constancy of additional species, the associates rarely exceed a combined cover of 30% and most frequently form a patchy cover beneath the canopy of S. maritimus. S. lacustris ssp. tabernaemontani is rare here and never abundant.

Bryophytes are very sparse but there may be a prominent mat of algae over the substrate surface and around the bases of the Scirpus stools. Gimingham (1964a) noted the abundance of Vaucheria spp. in this algal mat in Scottish stands of the community.

The Mediterranean maritime rush, Juncus subulatus, has become naturalised within the Scirpetum maritimi on the young salt-marsh at Berrow in Somerset (Willis & Davies 1960). It successfully competes against S. maritimus there by healthy rhizome extension.

Sub-communities

Sub-community dominated by Scirpus maritimus. In the very species-poor vegetation of this sub-community, S. maritimus occurs alone or with one or two associates of salt-marsh communities and freshwater swamps. Surface algae, Ruppia maritima or tall swamp species such as Phragmites australis or Glyceria maxima occasionally attain local prominence.

Atriplexprostrata sub-community: Halo-Scirpetum maritimi (van Langendonck 1931) Dahl & Hadac 1941. Here, Atriplexprostrata is constant and forms, with the less frequent Puccinellia maritima, rayed Aster tripolium, Triglochin maritima and a variety of upper-marsh species, an open ground cover.

Synonymy

Vegetation of nearly stagnant waters Tansley 1911 P-P-

Constant species Ranunculus aquatilis.

Physiognomy

The Ranunculetum aquatilis is dominated by clumps or patches of Ranunculus aquatilis, a very variable annual or perennial crowfoot, able to grow submerged, when the shoots are spreading to erect, as a delicate floating plant or terrestrially on moist ground, when it is loosely cespitose (Haslam 1978, Rich & Rich 1988). Cover is very variable and many stands are small, but in suitable conditions this species can make luxuriant growth, becoming most abundant in early summer.

The vegetation often grows in intimate association with various other aquatics and amphibious plants, including stands of Ranunculus peltatus, which is morphologically very similar to and readily confused with R. aquatilis (Holmes 1979, Rich & Rich 1988). But the most frequent companions in denser stands are various stoneworts, particularly Callitriche stagnalis and C. obtusangula, patches of which can become prominent with the crowfoot early in the season, and Glycerio-Sparganion herbs, which increase their cover later. Commonest among these latter are smaller Glyceria spp., especially G. fluitans, and Apium nodiflorum, but Nasturtium officinale, Veronica beccabunga and Berula erecta can also be found, and each of these can grow in vigorous patches among the submerged plants or as floating or emergent shoots. Myriophyllum spicatum, Ceratophyllum demersum and Elodea canadensis are sometimes seen, and there can be floating-leaved mats of Potamogeton natans with patches of duckweed thalli caught among the vegetation on the water surface or wet mud.

Habitat

The Ranunculetum aquatilis is typically found in and around the margins of mesotrophic to fairly nutrient

Rich waters, sometimes quite fast-moving, in other cases standing or sluggish. It probably occurs through much of southern Britain outside the highland zone, extending into streams around the upland fringes, but becoming much more common in streams, dykes, canals and pools in the lowlands. It tolerates periodic or seasonal drying and will colonise disturbed or ephemeral water-margin habitats.

The difficulty of identifying R. aquatilis with certainty and in particular the problem of separation from R. peltatus (Holmes 1979, Rich & Rich 1988), makes it hard to define precisely the geographical or environmental limits of this vegetation. It seems to occur through most of the lowlands and the upland fringes, and is concentrated in shallower waters, often much less than 1 m deep and sometimes fluctuating, with fertility that avoids the extremes of impoverishment and marked eutrophication.

Synonymy

Lemnetum minoris (Oberdörfer 1957) Müll. & Gors 1970; Societie van Lemna minor Westhoff & den Held 1969.

Constant species Lemna minor.

Rare species

Azolla filiculoides, Lemna minuscula.

Physiognomy

The Lemnetum minoris consists essentially of floating mats of Lemna minor, the small thalli often very numerous and densely crowded, but usually forming a single layer on the water surface. There are no other constants and, indeed, associates of any kind are often very few, their abundance being local or ephemeral or indicating a shift to different kinds of vegetation. L. gibba, for example, is generally scarce, though it can oust L. minor and mixtures of these plants often presage a development of the Lemnetum gibbae. Then, at a few places in south-east England, the rare alien duckweed L. minuscula has been recorded as an aggressive invader of this vegetation: an introduction from the warmer parts of North and South America, distinguished from our native species by its smaller size and single vein, it seems able to overwhelm other duckweeds, both here and in those other parts of Europe where it has gained a hold (Leslie & Walters 1983). The introduced aquatic fern Azolla filiculoides, a native of the tropical Americas, can also be found in this community, its small branching shoots with their pinkish leaves increasing to enormous abundance from time to time and smothering other floating plants with a thick layer of growth.

A distinctive form of the community has locally prominent L. trisulca beneath the floating mat of duckweed and some stands are characterised by the periodic occurrence of the aquatic liverworts Riccia fluitans and Ricciocarpus natans. Apart from this very modest amount of variety, though, most of the floristic differences among the Lemnetum minoris reflect the range of submerged aquatic and emergent communities with which it is found.

Sub-communities

Typical sub-community. This includes the most speciespoor stands with a generally very abundant, sometimes continuous, cover of L. minor.

Lemna trisulca sub-community. The submerged duckweed L. trisulca can attain considerable abundance beneath rather more open mats of L. minor than usual, with occasional records for a variety of other aquatics such as Potamogeton lucens, P. pectinatus, Callitriche stagnalis, C. hamulata and Myriophyllum spicatum.

Synonymy

Slow-moving water community Tansley 1911; Submerged-leaf association Pallis 1911; Moderate current vegetation Butcher 1933; Submerged community Tansley 1939; Potamogeton filiformis-Chara sociation Spence 1964; Potametum pectinato-perfoliati Den Hartog & Segal 1964; Potamogeton perfoliatus Gesellschaft Oberdorfer 1977.

Constant species

Myriophyllum spicatum, Potamogeton pectinatus.

Rare species

Potamogeton filiformis, P. trichoides, Ranunculus baudotii.

Physiognomy

The Potamogeton pectinatus-Myriophyllum spicatum community includes most of the richer and more diverse pondweed vegetation in which Potamogeton pectinatus and Myriophyllum spicatum play a prominent role. Usually, both these plants are present, often in some abundance by mid-summer, but stands in which just one or the other occurs, together with the characteristic community associates, can also be placed here. P. pectinatus is particularly striking where the vegetation extends into more nutrient-rich, standing or slow-moving waters, where it can make rapid growth in early summer from its overwintering tuberous buds or fruits, forming bushy clumps up to 2 m or so long. M. spicatum can also grow luxuriantly in this community, and tends to perform rather better than P. pectinatus where the flow is somewhat faster or has occasional spates, so it is often the more abundant plant where the vegetation extends into the moderately swift reaches of rivers. M. alterniflorum, however, is very scarce here and only occurs where there is some influence from more basepoor and oligotrophic waters, where the community begins to grade to the Potamogeton-M. alterniflorum vegetation characteristic of such situations. The rare M. verticillatum occurs very locally in stiller waters, mostly in eastern England.

Other Potamogeton spp. occur with varying frequency and abundance in the community and further sampling might allow the definition of additional vegetation types of this general kind in which various of these pondweeds occur with more consistent prominence. Comparison with earlier accounts, however, suggests that local dominance among these associates was once much more widespread than is now the case (Pallis 1911, Tansley 1911, Pearsall 1921, Butcher 1933). P. pusillus, a linear-leaved pondweed like P. pectinatus, though not such a common plant, remains fairly frequent throughout,

Synonymy

Myriophyllum alterniflorum consocies Pearsall 1918; Chara-Myriophyllum alterniflorum sociation Spence 1964.

Constant species

Myriophyllum alterniflorum.

Rare species

Potamogeton filiformis.

Physiognomy

The Myriophylletum alterniflori comprises species-poor vegetation in which Myriophyllum alterniflorum is the obvious dominant, growing thickly in congenial situations, though often sparse and battered in more turbulent waters. No other species occurs frequently, but occasional include Juncus bulbosus, Littorella uniflora, Lobelia dortmanna, Callitriche hamulata, C. stagnalis, Potamogeton natans, P. gramineus, Chara and Nitella spp. and the moss Fontinalis antipyretica. The rare P. filiformis is found among this vegetation at a few sites around the Scottish seaboard.

Habitat

The Myriophylletum is characteristic of lime-poor and less fertile waters, standing to quite swiftly flowing or spatey, in lakes, pools and streams, predominantly in the north and west of Britain.

M. alterniflorum grows best in base-poor waters of low conductivity, draining catchments of acidic rocks, like harder arenaceous sedimentaries or lime-free igneous and metamorphic rocks (Haslam 1978). Stands of this vegetation can be found in the lowland south and east of the country, but the community is very local there, being strongly confined to streams and pools on deposits like the Tertiary sands of the New Forest, and it becomes widespread and common only in the upland fringes of the north and west, where suitable rocks make up the bulk of the landscape. The plant will tolerate quite impoverished conditions, so it can thrive even where the substrates of the catchment are very resistant to erosion, though it tends to occur in peaty waters, such as those draining the extensive tracts of upland blanket mire, only where their dystrophic character is ameliorated a little by the deposition of some mineral material.

M. alterniflorum has a fairly shallow rooting system but, in the sandy to stony beds favoured here, it can gain quite extensive and firm anchorage and its thin, finelycut foliage offers but slight resistance to faster flow or wave-wash (Pearsall 1918, Haslam 1978). The shoots will break with moderate force, but remaining fragments readily regrow, so plants can persist in spatey waters (Haslam 1978).

Synonymy

Ceratophyllum demersum vegetation Pallis 1911, Ellis 1939, Lambert & Jennings 1951, Lambert 1965, ail p.p.; Ceratophylletum demersi (Pop 1962) den Hartog & Segal 1964.

Constant species

Ceratophyllum demersum, Elodea canadensis.

Rare species

Azolla filiculoides, Potamogeton coloratus.

Physiognomy

The Ceratophylletum demersi is characterised by often dense free-floating masses of Ceratophyllum demersum, usually with at least a little, sometimes much, Elodea canadensis. Some stands have few other plants, apart from a patchy floating mat of Lemna minor, but others are richer, with frequent records for Ranunculus circinatus, Callitriche stagnalis and one of the other introduced Canadian pondweeds, E. nuttallii, with L. gibba and Nuphar lutea among the floating element.

Sub-communities

Ranunculus circinatus sub-community. Richer stands of the Ceratophylletum are included here with, in our data, preferentially frequent R. circinatus, one of the easier water-crowfoots to distinguish, with its leaf segments all held rigidly in a single plane (Holmes 1979). Callitriche stagnalis is also very common and this is one of the aquatic communities in which the presence of E. nuttallii has been increasingly recognised (Simpson 1984). Also among the submerged plants is occasional Myriophyllum spicatum, Potamogeton pectinatus, the nationally rare P. coloratus, a species found only very locally and mostly in eastern England, and Sparganium emersum. Lemna minor is scarce but L. gibba and L. trisulca occur, together with occasional Nuphar lutea and Azolla filiculoides.

Lemna minor sub-community. Apart from C. demersum and E. canadensis, L. minor is the only frequent plant in this sub-community.

Habitat

The Ceratophylletum demersi is typical of still or slowmoving, eutrophic waters, mainly in the warmer, southeastern lowlands of Britain. It is a widespread community of sluggish streams, and pools, dykes and canals, and is perhaps becoming increasingly common in such waters because of eutrophication with agricultural run-off.

C. demersum is largely confined to those parts of the country (Perring & Walters 1962) where there is a mean annual maximum temperature in excess of 28 °C (Conolly & Dahl 1970), but it has a widespread range through Europe, except the Arctic regions, and its association with this part of the country may be as much a reflection of its nutrient requirements as any warmth-dependence.

Synonymy

Elodetum Matthews 1914, Tansley 1939; Mediumslow current vegetation Butcher 1933 p.p.; Pflanzengesellschaft mit Elodea canadensis Solinska 1963; Elodeetum canadensis (Pignatti 1953) Passarge 1964; Elodea society Spence 1964.

Constant species

Elodea canadensis.

Physiognomy

The Elodea canadensis community comprises speciespoor vegetation in which this particular kind of North American pondweed has become dominant. The plant overwinters as short unbranched stems or as turions but, by early summer, it can become very abundant, either as free-floating masses or with the shoots rather loosely anchored in the substrate. In such stands, other species are at most only occasional, though quite a variety are represented at low frequency. Commonest among other submerged aquatics are Ceratophyllum demersum, Myriophyllum spicatum, Potamogeton pectinatus, P. crispus, P. perfoliatus, Callitriche stagnalis, C. hamulata, Ranunculus circinatus and R. penicillatus ssp. pseudofluitans. Then, there are quite often some associated mats of duckweeds Lemna minor and L. gibba, with Potamogeton natans and Polygonum amphibium forming an open canopy of floating leaves. Glyceria fluitans shoots sometimes trail in from water margins.

Habitat

The Elodea canadensis community is most characteristic of still to sluggish, nutrient-rich waters, shallow to quite deep, and generally with fine mineral beds. Since the introduction of the plant, some century and a half a go, it has become very widespread through the warmer lowlands of Britain, occurring commonly in ponds and lakes, canals, dykes and slow-moving rivers and streams. Although its overall range seems to have now stabilised, E. canadensis will still actively colonise new water bodies, and the community often makes an early appearance in sites like flooded gravel pits, as well as sometimes quickly returning to waters recovering from pollution.

E. canadensis is native to most of the United States and parts of Canada and was first authentically reported from Great Britain in 1842, having perhaps been introduced on imported timber. Its period of rapid spread started shortly after 1850, when it began to appear in abundance around the Fens, and within a decade it was to be found in most of southern England and parts of the Midlands, extending through nearly all of the English lowlands and into some of southern Scotland by 1880.

Synonymy

Scirpo-Phragmitetum Koch 1926 p.p.; Scirpeto-Phragmitetum medioeuropaeum (Koch 1926) R.Tx. 1941 p.p.

Constant species

Phragmites australis.

Rare species

Cicuta virosa, Utricularia intermedia.

Physiognomy

All the vegetation types included here are characterised by the generally overwhelming dominance of Phragmites australis. However, this is a very polymorphic species and the gross appearance of the vegetation, even with pure stands, can be very variable. Much of this variation is known to be phenotypic adaptation perpetuated in clonal populations and variously described as ‘kinds’, ecotypes, biotypes or biotopes of reed (e.g. Rudescu et al. 1965; Björk 1967; Haslam 1971a, 1972a, Dykyjová 1978a); some is related to genotypic differences (e.g. Björk 1963, 1967; van der Toorn 1972; Riacu et al. 1972; Pazourková 1973: Dykyjová 1978a). Although variation in Britain is said to be less than elsewhere (Haslam 1972a) and presumed to be phenotypic, there has been no systematic study of its extent in this country or of its relationships to the kind of environmental differences that are reflected in the floristics of the various kinds of Phragmitetum. Here, it is possible only to give a very general indication of the reed morphology in each of the sub-communities.

Phragmites is normally a highly gregarious species and individual stands of the community can be very extensive. The vegetation is generally very species-poor and no other species attains even occasional frequency throughout. The Phragmitetum is, however, very variable and individual stands may show marked peculiarities of composition.

Sub-communities

Phragmites australis sub-community: Phragmites communis reedswamp Tansley 1911; Phragmites-Scirpus associes Pearsall 1918 p.p.; Phragmites communis-Sparganium minimum and Phragmites communis-Littorella open sociations and Phragmites communis society Spence 1964; Phragmites monodominant stands Haslam 1971a; Phragmites nodum Daniels 1978; Association of Phragmites australis and Schoenoplectus lacustris Pigott & Wilson 1978 p.p.; Phragmites communis swamp sociation Wheeler 1978; Phragmites australis reedswamp, species-poor variant Meres Survey 1980; Scirpo-Phragmitetum typicum Wheeler 1980; Phragmites communis nodum Adam 1981 p.p.; this sub-community includes pure and very species-poor swamps and reed-beds in which Phragmites is the sole constant. The reed cover can be open or closed but Phragmites is always the most abundant helophyte forming a canopy from about 1-3 m high. Other species can, however, be locally prominent including other swamp dominants such as Typha latifolia, T. angustifolia, Carex riparia, C. acuta, Glyceria maxima, Cladium mariscus, Scirpus lacustris ssp.

The sampling of aquatic vegetation

Interest in the description of aquatic vegetation in Britain began with Tansley (1911), though he willingly admitted to having insufficient data for anything like a systematic scheme of classification. What Types of British Vegetation contains is, in fact, an outline of a few distinctive assemblages of aquatic species grouped by growth form and characteristic of broadly contrasting types of fresh waters, moving and still, the accounts relating for the most part to a small number of particular sites. Especially prominent was the contribution of Pallis (1911) on the aquatics of the Norfolk broads and rivers, in whose account an interest in the hydroserai succession is very prominent.

There are more data on aquatic vegetation in Tansley (1939), assembled from a variety of studies in the intervening years, some on river vegetation (Butcher 1927, 1933) but mostly on ponds and lakes (Walker 1905, Matthews 1914, Godwin 1923), with Esthwaite and other Cumbrian lakes (Pearsall 1918, 1921) now being added to Broadland as classic sites for description of aquatic assemblages, swamps and fens. Among these various accounts, the definition of vegetation types is uneven. Sometimes, there are simply lists of species associated with particular conditions of water depth, quality and speed of movement or substrate character, usually with a DAFOR rating (dominant, abundant, frequent, occasional, rare) to denote relative abundance. In other cases, assemblages are more precisely defined as associes or consocies with dominance playing a large part in distinguishing the more species-poor vegetation types. Again, there is a strong preoccupation with the dynamics of succession.

Since these accounts were published, more phytosociological descriptions of aquatic communities have been very few. In some cases, aquatic vegetation has been sampled and characterised as part of a comprehensive survey of particular areas, like Skye (Birks 1973), where it makes but a minor contribution, and only in Spence (1964) and the work of the Macaulay Institute, as it then was (Birse & Robertson 1976, Birse 1980, 1984), has there been any attempt at more extensive data collection and definition of communities, less strictly phytosociological in the former, more so in the latter.

Synonymy

Caricetum acutiformo-ripariae Soo (1927) 1930 p.p.; Caricetum acutiformo-paniculatae VI. & Van Zinderen Bakker 1942 p.p.; Carex acutiformis fen Lambert 1951 p.p.; Carex acutiformis stands Meres Survey 1980 p.p.

Constant species

Carex acutiformis.

Physiognomy

The Caricetum acutiformis is always dominated by Carex acutiformis forming an open or closed canopy of shoots and arcuate leaves about 1 m tall. No other species is constant but there are usually some scattered tall fen herbs such as Angelica sylvestris and Valeriana officinalis and shorter species like Galium palustre and Mentha aquatica. Other swamp species, e.g. Carexpaniculata, Sparganium erectum and Typha latifolia, may be locally prominent, and Juncus effusus is sometimes abundant. However, many of the occasional reflect the particular floristic context of the often small stands.

Habitat

The community seems to be typical of situations which are, in some respects, similar to those occupied by the Caricetum ripariae. It has been recorded from moderately eutrophic, circumneutral substrates on the margins of slow-moving or standing lowland waters in openwater transitions, in wet hollows within flood-meadows, in ditches and alongside sluggish streams and rivers. Here the water-level may be up to about 20 cm above ground and the substrate pH 6.0-6.8. There is some evidence, however, (e.g. Haslam 1978), that the Caricetum acutiformis is more consistently associated with calcareous habitats than is the Caricetum ripariae'. it occurs, for example, in ditches in fen peat and also on the margins of slow chalk streams.

Zonation and succession

Around more extensive open-water transitions, the community may form swamp which passes gradually to fen in which C. acutiformis remains a prominent component, e.g. some forms of the Peucedano-Phragmitetum, and it was from such situations that Lambert (1951) described her C. acutiformis sere along the Bure valley in Norfolk. At more abrupt water margins, the community occurs in often narrow and fragmentary transitions with the Sparganietum erecti towards deeper and, to landward, the Glycerietum maximae or Phragmitetum australis. Unlike the Caricetum ripariae, this community may also form swampy patches in calcareous floodmeadows and flood-pastures, passing gradually through some form of Calthion community to damp mesotrophic grassland, but, with agricultural improvement, such transitions are becoming more rare. C. acutiformis seems to be quite an aggressive species which may be able to readily invade riverside fields where drains become blocked. It also appears to be able to tolerate cattle grazing (Wheeler 1975).

The appearance of Volume 4 of British Plant Communities brings us near the end of the publication of the results of the National Vegetation Classification. Sustaining commitment and interest through the years has been vital to the continuance of the work and, as coordinator of the NVC and editor of these volumes, I would like to record again my personal gratitude to Donald Pigott and Derek Ratcliffe, the co-chairman of our research team, to John Birks, Andrew Malloch, Michael Proctor and David Shimwell, the research supervisors and to Jacqui Huntley, Elaine Radford, Martin Wigginton and Paul Wilkins for their many and various contributions along the way. More widely, on their behalf, I wish to thank all those others who have assisted in preparing the descriptions of the aquatic, swamp and tell-herb fen vegetation that is included here.

Among the NVC team, it was David Shimwell and Elaine Radford who had particular repsonsibility for producing preliminary accounts of the swamp and fen communities and I was greatly helped by their hard work and experience. For aquatic vegetation, we also benefited from the brief secondment to the research team of Dr Michael Lock who, as well as carrying out some underwater sampling by diving, collated our own data on aquatics and helped with the early characterisation of communities.

We were fortunate, too, while working on these vegetation types to have Mrs Margaret Palmer as the Nominated Officer for the project within the Joint Nature Conservation Committee. She was already playing an important role in developing other methods for the survey of standing waters in Britain and, as well as being given access to the data which a range of projects had produced, we were much informed by her wide experience and through her comments on our work.

Among others involved in those projects, we are especially grateful to Dr Kevin Charman and Messrs Martin Alcock and Clive Doarks. Drs Terry Rowell and Nigel Holmes also made valuable comments on the manuscript at various stages and we had informative discussions, too, with Mr John Ratcliffe and Ms Ros Hattey of the then Welsh Lowland Peatland Survey, Dr John Harvey of the National Trust, the Reverend Gordon Graham of the Durham Flora Project and the late Professor David Spence, whose enthusiastic understanding of aquatic vegetation is greatly missed by all of us.

Synonymy

Caricetum rostratae Rübel 1912 p.p.; Caricetum acutovesicariae comaretosum Passarge 1955; Carex rostrata nodum Daniels 1978 p.p.

Constant species

Carex rostrata, Galium palustre, Menyanthes trifoliata, Potentilla palustris, Calliergon cordifolium/cuspidatum! giganteum.

Rare species

Carex appropinquata, C. aquatilis, C. diandra, Lysimachia thyrsiflora, Peucedanum palustre, Sium latifolium.

Physiognomy

The Potentillo-Caricetum rostratae comprises generally rather species-poor but somewhat varied mixtures of monocotyledons and herbaceous dicotyledons, sometimes developed as a floating mat. Among the monocotyledons, Carex rostrata is the most frequent species throughout but other sedges, such as C. vesicaria, C. nigra, C. elata and, more rarely, C. aquatilis and C. appropinquata, or Eriophorum angustifolium may each on occasion replace it as the dominant. In other cases, Phragmites australis or, less frequently, Juncus effusus or J. acutiflorus may dominate (see Sub-communities below).

A consistent feature of the community, however, is the presence of Potentilla palustris and Menyanthes trifoliata as an open or closed carpet beneath these usually taller species. They can occur in intimate mixtures or as patchy mosaics in which clumps of either attain local dominance and, scattered amongst them, is a variety of herbs, most commonly Galium palustre (which can sprawl extensively), Cardamine pratensis, Epilobium palustre, Mentha aquatica, Myosotis laxa ssp. caespitosa, Caltha palustris, Hydrocotyle vulgaris and Veronica scutellata. Taller species, such as Angelica sylvestris, Cirsium palustre and Valeriana officinalis, are sometimes conspicuous and there is a much greater variety of these in one of the sub-communities. In wetter places, Equisetum fluviatile may be abundant and, here too, there may be scattered clumps of Sparganium erectum or Typha latifolia. Agrostis stolonifera with, less frequently, some A. canina ssp. canina and Poa trivialis, can form extensive and. thick carpets, sometimes semi-submerged.

Almost always, one or more of the larger Calliergon spp., C. cuspidatum, C. cordifolium and C. giganteum, occur and these may form conspicuous patches over the mass of interweaving rhizomes and litter. No other bryophytes are frequent throughout but individual stands may have some Sphagnum squarrosum or S. fimbriatum or other locally abundant species.

Synonymy

Ceratophyllum submersum societie den Hartog 1963.

Constant species

Ceratophyllum submersum, Potamogeton pectinatus, Ranunculus baudotii.

Rare species

Ceratophyllum submersum, Ranunculus baudotii.

Physiognomy

Ceratophyllum submersum is a softer and fresher green plant than the more widely distributed C. demersum, with its finely segmented leaves 3^4- times forked, rather than once or twice divided (Tutin et al. 1964). And, although it can form similar dense masses of submerged, unanchored shoots, it seems generally to be found in this distinctive Ceratophylletum submersi, with amounts of Potamogeton pectinatus and Ranunculus baudotti, a rare water-crowfoot with considerable variation in the degree of development of its floating laminate leaves and the finely divided submerged ones (Holmes 1979). Myriophyllum spicatum is also frequent, though often quite hard to make out among the C. submersum, and there is occasionally some Zannichellia palustris, Callitriche obtusangula and Ranunculus trichophyllus.

Habitat

The Ceratophylletum submersi is characteristic of standing or sluggish, eutrophic and generally brackish waters, and is almost entirely confined to the extreme south-east of England where it is a locally common vegetation type of pools and dykes in coastal and estuarine marshes.

C. submersum has a fairly wide distribution throughout Europe, except towards the extreme north (Tutin et al. 1964), but in Britain its range is very restricted. It has sometimes been confused with C. demersum but, though there are old inland records scattered across the southeastern part of the country, recently confirmed stations lie mostly on or near the coast and, apart from a few localities in the west, the plant appears to be largely confined to the submaritime fringe from Sussex round to Essex (Perring & Walters 1962). Here, it is typically found in the dykes, usually shallow and fairly narrow, that drain the reclaimed marshes, where it can extend into waters that are quite saline: along the north Kent coast, the community has been found in brackish dykes with a mean conductivity of over 5000 /zmho, or a salinity of 2.7% (Charman 1981). Generally, however, there is no tidal movement, the waters being stagnant or very sluggishly draining the surrounding ground.