UK Lowland Wetland Habitat Types & Characteristics

 

Sphagnum moss growing on an upland flush near to Dartmeet, Dartmoor © JNCCLowland Raised Bog

Raised bogs in the lowlands of Britain occur on elevated deposits of peat and receive mineral nutrients principally from precipitation. They are referred to as ombrotrophic (rain-fed) mires.

 

This habitat is very acidic and nutrient-poor. It is also poorly drained and water-logged. Because of these conditions the decomposition of plant material is strongly inhibited. This results in an accumulation of peat, which, over thousands of years, can become many metres thick. The building peat raises the bog surface upwards to form a gently-curving dome from which the term 'raised' bog is derived. This dome can grow to 10m in height or so and separates the bog from the underlying water-table, minimising the inflow of ground-water. Bogs in which peat is being accumulated are referred to as ‘Active’, whereas those in which peat formation is at a temporary standstill (normally because of drainage) are referred to as ‘Degraded’.

 

Bog-myrtle flowers © Ian Strachan, Scottish Natural HeritageThe vegetation of lowland bogs is very distinctive. It includes a range of specialised wetland plants and varies depending on the pattern of surface pools, hummocks and lawns. Usually various colourful Sphagnum mosses predominate (e.g. Sphagnum auriculatum, S. cuspidatum, S. magellanicum, S. papillosum, S. recurvum). These are highly absorbent and produce a characteristically 'spongy-feel' to the bog surface. Other typical plants include bog asphodel Narthecium ossifragum, bogbean Menyanthes trifoliata, bog-myrtle Myrica gale, cotton grasses Eriophorum angustifoium and E. vaginatum, cross-leaved heath Erica tetralix, deer-grass Scirpus cespitosus, heather Calluna vulgaris, purple moor-grass Molinia caerulea, round-leaved sundew Drosera rotundifolia, and the white beak-sedge Rhynchospora alba.

 

Long leaved or Oblong leaved sundew © Ian Strachan, Scottish Natural HeritageRaised bogs are home to numerous scarce lowland plant species, such as the bog mosses Sphagnum pulchrum and S. imbricatum, uncommon bog rosemary Andromeda polifolia, cranberry Vaccinium oxycoccos, and the great and oblong leaved sundews Drosera anglica, D. intermedia. The vegetation is greatly affected by peat-cutting and drainage, which tend to dry out the bog, encouraging purple moor-grass and heather and allowing birch and pine trees to invade.

 

Lowland raised bogs form an important refuge for many wetland species. These include various breeding waders, wildfowl and birds of prey, such as curlew, snipe, hen harrier, hobby and merlin. Many species of invertebrates occur, including beetles, butterflies, caddis flies, damselflies, dragonflies, mites, moths, spiders and springtails. Some of these are highly localised in the lowlands, including the large heath butterfly Coenonympha tullia subspecies davus, the bog bush cricket Metrioptera brachyptera, the mire pill beetle Curimopsis nigrita, the white-faced darter Leucorrhinia dubia, and Britain’s rarest caddis fly Hagenella clathrata.

 

Bogbean growing amongst poor-fen sedge vegetation at Newbridge Marsh, Dartmoor © JNCCLowland Fen

Lowland fens are minerotrophic peatlands (i.e. their nutrients come from ground water as well as rain water), that are at least periodically waterlogged. Although they are underlain by peat, decomposition tends to be relatively high and so the peat depth is shallow and there is no peat dome (as with raised bogs). Fens are complex and dynamic systems; they frequently form complex mosaics with a number of associated habitat types, including wet woodland (fen carr), reedbed, lowland heathland and lowland meadow.

 

Two broad types of fen can be distinguished. Topogenous fens, where water movement is generally vertical – these include basin fens and floodplain fens – and soligenous fens where water movement is predominantly lateral – these include valley mires, springs and flushes in the lowlands, mires associated with springs, rills and flushes in the uplands, trackways and ladder fens in blanket bogs, and laggs of raised bogs. In addition, fens are also described as poor-fens or rich-fens (see below).

 

Marsh cinquefoil © Helen Baker/JNCCPoor-fens

Poor-fens have low to moderate fertility and are fed by acid water (pH <5) derived from base-poor rocks or glacial till, such as sandstones, granites or sand and gravel. They occur mainly in the uplands or in association with lowland heaths. Their vegetation is characteristically species-poor, with a moderate to high cover of Sphagnum bog mosses (mainly Sphagnum cuspidatum, S. palustre, S. recurvum, S. squarrosum) and sedges (especially Bottle sedge Carex rostrata), with a scattered and an impoverished layer of herbs including, for example, common cotton-grass Eriophorum angustifolium, devil's-bit scabious Succisa pratensis and marsh cinquefoil Potentilla palustris. In some instances Sphagnum can be scarce and various Carex sedges dominate in a more herb-rich sward with species like bogbean Menyanthes trifoliate and marsh bedstraw Galium palustre present.

 

Rich-fens

Rich-fens are fed by alkaline, mineral-enriched, calcareous waters (pH >5). They are mainly confined to the lowlands, but also occur in the uplands where there are localised pockets of base-rich rocks. The vegetation is normally relatively species-rich. It includes mire vegetation dominated by a range of Carex sedges, growing over a variable carpet of mosses and mixed in with various vascular plants (such as bogbean, common butterwort Pinguicula vulgaris, common cotton-grass, lesser spearwort Ranunculus flammula, marsh cinquefoil, marsh bedstraw, marsh lousewort Pedicularis palustris, marsh marigold Caltha palustris, marsh thistle Cirsium palustre, and ragged robin Lychnis flos-cuculi). Other rich-fen mire communities are dominated by black bog-rush Schoenus nigricans, growing mixed with blunt-flowered rush Juncus subnodulosus and purple moor-grass Molinia caerulea, cross-leaved heath Erica tetralix, bog asphodel Narthecium ossifragum and a mixed cover of mosses with Sphagnum as a consistent feature.

Ragged robin flower © Ian Strachan, Scottish Natural Heritage

 

Rich-fens also support mixed tall-herb fen communities in which Common reed Phragmites australis is predominant. These usually include a range of tall herbs, such as hemp agrimony Eupatorium cannabinum, marsh bedstraw Galium palustre, meadowsweet Filipendula ulmaria, milk parsley Peucedanum palustre, purple loosestrife Lythrum salicaria, and yellow loosestrife Lysimachia vulgaris. Much less common is tall, saw-sedge Cladium mariscus dominated, species-poor, swamp vegetation. Some rich-fens in the south-east of England support a kind of species-rich fen-meadow, which is dominated by purple moor-grass with an admix of carnation sedge Carex panicea, devil's-bit scabious, marsh bird's-foot trefoil Lotus uliginosus, marsh thistle and tormentil Potentilla erecta.

 

Fen animals

Swallowtail butterfly © copyright D. Healy

Fens provide an invaluable habitat for a large number of wetland animals. The invertebrate and bird fauna are particularly rich. Numerous species of aquatic beetle, damselfly, dragonfly, fly, moth, snail and spider are associated with fens. Rare invertebrates associated with fens include Desmoulin’s whorl snail Vertigo moulinsiana, the fen raft spider Dolomedes plantarius, the lesser water measurer Hydrometra gracilenta, the Norfolk Hawker dragonfly Aeshna isosceles, the Pashford pot beetle Cryptocephalus exiguus, the reed leopard moth Phragmataecia castaneae, and the swallowtail butterfly Papilio machaon. The bird fauna includes species such as bearded-tit, bittern, Ceti’s warbler, little egret, marsh harrier, marsh warbler, spotted crake and water rail. All of Britain’s amphibian species occur in lowland fens.