Lowland Fen Habitats (definition)

Durham Lowland Priority Habitats
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Lowland Fen Habitat Definition

The definition of lowland for the Durham BAP is all land outside the North Pennines Natural Area. For the purposes of the Durham BAP, lowland fen is taken to include all wetland habitats apart from the following which are covered under other HAPs:

  • aquatic (wholly or predominantly submerged) vegetation
  • blanket bog (including bog pools) & wet heath
  • fen carr (wet woodland)
  • wet grassland (lowland meadows & pastures)
  • fen meadow (purple moor-grass and rush pasture)

Identification and mapping

Wetlands can have complex patterns of vegetation and the different types of lowland fen often merge into each other. This is especially apparent in wet depressions and around the margins of ponds where there is a gradual transition between very wet conditions and much drier conditions higher up.

Lowland fen by its nature is often dynamic, with vegetation fluctuating in relation to changing water levels throughout the seasons or from year to year. Some vegetation types, in particular those representing earlier successional stages can be particularly short-lived and dynamic. Even the most accurate maps record the vegetation only at one particular point in time and on some sites the vegetation types will either be different or will occupy slightly different area from one year to the next.

The topographical situation largely determines the hydrology of the habitat which influences the type of fen vegetation that can develop.

The main topographical types of Lowland Fen found in Durham are:

  • Springs and flushes – areas where water comes to the surface from below and seeps over the ground
  • Valley fen – wetland adjacent to a stream or river and sometimes on the wettest part of the valley sides
  • Floodplain fen – flat river valleys that flood regularly
  • Basin fen – wet depressions whose water comes from precipitation, or from a high groundwater level, without water significant flowing through and out
  • Open water transition fen – wetland that develops on the margins of water bodies other than rivers and streams, e.g. ponds, lakes or reservoirs

Swamp

Swamps are a species-poor type of vegetation, usually with one species that is dominant and only a small number of associated species at much lower cover. The dominant species usually has tubular or narrow, strap-shaped leaves and is usually a tall sedge, grass, rush other monocot or horsetail (Note that ‘club-rushes’ and ‘spike-rushes’ are really types of sedges rather than rushes). In swamps, the base of the dominant plant is usually submerged and there is often open water visible between the stems. Compared to other types of fen vegetation, swamps are overall, significantly more species-poor and have significantly fewer axiophytes. However, although they are species-poor in terms of plants, swamps may be significant habitats for specialist invertebrates and other types of wildlife.

There are two sub-types of swamp, tall swamp and short swamp.

Tall swamps have a tall dominant species, more than 50cm tall and often much taller. They include the following NVC communities (those with the highest nature conservation interest are in bold):

Carex paniculata (greater tussock-sedge) swampGlyceria maxima (reed sweet-grass) swamp, Carex riparia (greater pond-sedge) swamp, Carex acutiformis (lesser pond-sedge) swamp, Schoenoplectus lacustris (common club-rush) swamp, Carex rostrata (bottle sedge) swamp, Equisetum fluviatile (water horsetail) swamp, Carex vesicaria (bladder sedge) swamp, Typha latifolia (bulrush) swamp, Typha angustifolia (lesser bulrush) swamp, Sparganium erectum (branched bur-reed) swamp, Schoenoplectus tabernaemontani (grey club-rush) swamp, Bolboschoenus maritimus (sea club-rush) swamp, Iris pseudacorus (yellow iris) swamp, Scirpus sylvaticus (wood club-rush) swampPhalaris arundinacea (reed canary-grass) swamp

Short swamps have a dominant species that is usually much less than 50cm tall or else floats on the water’s surface. They include the following NVC communities:

Eleocharis palustris (common spike-rush) swamp, Glyceria fluitans (floating sweet-grass) water-margin vegetation, Juncus inflexus/Lemma trisulca (hard rush/ivy-leaved duckweed) aquatic/swamp transition.

Tall herb fen

Tall herb fen includes various mixtures of tall wetland herbs such as meadowsweet, great willowherb, wild angelica, creeping thistle, common nettle and water mint. It is usually represented by not very grassy or rushy vegetation but sometimes soft-rush, tufted hair-grass and hard-rush occur relatively frequently, usually at relatively low cover.

There are two sub-types of tall herb fen; tall herb-dominated and tall herb/swamp transition.

Tall herb-dominated are dominated by tall (over 1m) herbs, usually with little input from any of the dominant species of swamps or from other grasses, sedges or rushes. They include the following NVC communities (those with the highest nature conservation interest are in bold):

Epilobium hirsutum (great willowherb) tall herb fen, Filipendula ulmaria-Angelica sylvestris (meadowsweet/wild angelica), tall herb/species-rich wet grassland transition (MG4-related)

Tall herb/swamp transition represents a transitional category between true swamp and tall herb fen. They are transitional in terms of the overall density of the vegetation, the species composition, the physiognomy and hydrology. The stands are often moderately dense with some water visible between the plant stems, but not to the same extent as in true swamp. The flora is usually a mixture of one or two of the typical species that dominate in true swamps with several tall herb species competing with it for cover. Species-richness is significantly higher than in swamp habitat and similar to the figures for the vegetation types dominated by tall herbs. They include the following NVC communities (those with the highest nature conservation interest are in bold):

Carex rostrata – Potentilla palustris (bottle sedge/marsh cinquefoil) tall herb fen, Phragmites australis – Urtica dioica (common reed/common nettle) tall herb fen, Sparganium erectum (branched bur-reed) swamp, Mentha aquatic (water mint) sub-community, Carex acutiformis (lesser pond sedge) tall herb fen/swamp transition, Typha latifolia (bulrush) tall herb fen/swamp transition, Bolboschoenus maritimus (sea club-rush) tall herb fen/swamp transition.

Short Fen Vegetation

Tall herbs and tall swamp species are rare or absent from short fen vegetation. Usually the vegetation is a mixture of one or more of rushes, sedges, mosses and short to medium-sized wetland herbs. Most of these types of vegetation have very little if any grass, which helps separate them from wet grasslands and purple moor-grass & rush-pasture.

There are three sub-types of short fen vegetation; Moss-dominated spring/flush, small sedge fen and vegetation of temporary wetlands and marginal areas with fluctuating water levels.

Moss-dominated spring/flush is dominated by a moss Palustriella commutata in a spring/flush situation. All associated species will be at a much lower cover.

Small sedge fen mainly occurs in flushes, but can also be present in basin mires or open water transition fens. The vegetation is usually relatively low (<25cm). It includes small sedge poor fen which consists of open mixtures of small sedges with grasses, rushes and wetland herbs suited to acidic conditions, small sedge intermediate fen and small sedge rich fen which is more species-rich and includes species that prefer calcareous conditions.

Vegetation types of temporary wetlands and marginal areas with fluctuating water levels are normally made up of one or more of mosses, rushes, medium-sized perennial wetland herbs and small annual wetland herbs. Sedges are conspicuous by their absence and grasses are also uncommon.

NOTE: For descriptions of the indivudal vegetation types that make up each of the Lowland Fen habitats see Appendix 3 “Fen Classification for lowland East Durham” Ptyxis April 2009”

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