How to Rewild

Fen Marsh and Swamp

Habitat Management Plan

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Fen Marsh and Swamp
Habitat Guide

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Unlike Bog, which is a habitat above the water table, Fen Marsh & Swamp sits below it. As a result, the water flowing into the system contains more minerals, which creates a much more verdant, lush habitat, where healthy trees can grow in the less-waterlogged patches. Some of these sites are wet year-round, while others are only seasonally-inundated, and this has a big impact on the species you’ll find there.

Like Dense Scrub, Fen Marsh & Swamp is often a ‘seral’ zone – a transitional habitat in the process of becoming something else. Reeds are a pioneer species that colonise everything from river margins to saltmarsh, while quaking mires are ponds and lakes in the process of becoming land. This means these habitats move over time, encroaching on open water or being lost as land dries out. In pre-human times, natural processes would have maintained a mosaic landscape, but our ecosystems tend to shift towards dry woodland unless we intervene.

Sub Habitats

Lowland wetlands supplied with both groundwater and rainwater, sitting below the water table. Rich in nutrients due to inflowing runoff.

Wet grassland/swamp dominated by Purple Moor Grass and/or Rushes, typically grazed by livestock.

Upland wet grassland/swamp habitats where water flows in as runoff and through groundwater.

The band of vegetation which grows along the edge of a waterbody, and/or narrow waterbodies which are entirely overgrown by this vegetation – e.g. ditches.

Swamps where the water table is typically at or above ground level, and there are large stands of Common Reed (more specific detail at UKHab website).

Other swampy areas not covered by the above categories, including dry reedbeds.

Value

The nutrient-rich water and soils which are typical of fen habitat typically support dense, green vegetation at the height of spring. However, the category Fen Marsh and Swamp contains myriad conditions, which vary enormously in their value to wildlife, and even non-Priority Habitats, like Aquatic Marginal Vegetation can be of high value when it is in good condition, so there’s no easy rule-of-thumb.

Wetlands in general have high biodiversity, as they are highly-productive ecosystems, because water is not a limiting factor. But waterlogging, drainage, oxygen and nutrient availability, and pollution can impact biodiversity. These habitats, which are often highly-connected to our river system, are vulnerable to invasive species like Himalayan Balsam and Swamp Stonecrop, which can become a near-monoculture, reducing biodiversity significantly.

Seasonally-flooded fens are often valuable bird nesting sites, especially for wading birds which are particularly affected by fox predation. Reedbeds are another habitat which have high value for birds, with this relatively low plant diversity habitat hosting a huge abundance and diversity of roosting and breeding birds, from Starling to Bittern. But, in the absence of livestock (and wild herbivores), these habitats tend to revert to scrub or wet woodland, so management is an intrinsic part of many fenland landscapes.

Lowland Fen
Lowland fen habitats can be extremely productive due to their rich soils and seasonal flooding, such as this site at Wild Ken Hill, Norfolk.

Protect

Many of these habitats are threatened nationally, including Lowland Fens, Purple Moor Grass and Rush Pastures, and Upland Flushes, Fens and Swamps. These have historically been drained, and converted to agricultural land, making their protection and restoration today a high priority for landowners. All fens are dependent on a relatively high water table, so it is important to avoid unintentionally lowering this by digging ditches which connect to a watercourse.

Nevertheless, ditches may become silted up over time and require maintenance; in the wild the only parallel we find with this artificial habitat are the ‘canals’ dug by beavers. These can be over 500m in length, and, similar to ditches, will become colonised with Aquatic Marginal Vegetation over time. ‘Desilting’ mimics the beavers’ maintenance of these ditch habitats, which are important connections between waterways and water bodies across the terrestrial landscape, like aquatic hedgerows. When desilting a ditch, the impact on the height of the water table should be taken into consideration.

Seasonally-flooded fenland habitats are typically maintained in good condition by grazing with cattle (not if sphagnum is dominant), but the livestock must be kept at relatively low density to avoid ‘poaching’ the soil. Poaching is compaction of the ground, which creates a ‘hard pan’ during dry spells and a waterlogged surface (puddles) in wet conditions. While small amounts of poaching can be beneficial around the edges of ponds and across grassland, too much can lead to the dominance of species like Silverweed and Creeping Thistle.

Meanwhile, Reedbeds may require a different kind of management to maintain the open nature of this habitat. These stands of Common Reed, with some other plant species mixed in were traditionally cut to supply thatch for roofing homes. But thatch fell out of favour due to its tendency to catch fire and the regular cost of re-roofing. Now, even the few thatched houses left in the UK are largely supplied by production in Europe. But reedbeds still need maintenance to avoid scrub encroachment, with a 4-10 year cycle of autumn/winter (deep water) or early spring (shallow water) cutting, in sections, much like a coppiced woodland. Cuttings should be removed, and are typically piled up at the edge of this habitat as refuges for amphibians and reptiles, sold to thatchers or burned off if there is an excess of material.

As fens receive nutrients from inflowing water, they may suffer from the effects of pollution, which can come in many different forms. Microplastics, nutrients, chemical and pharmaceutical pollutants, and sediment all flow in from streams or runoff neighbouring land. These can cause damage to sensitive aquatic ecosystems, but mitigating the impacts may be as simple as setting up a treatment wetland at the point where water flows into your land. Read more about creating a treatment wetland.

Ditch with water
Aquatic marginal vegetation is cleared every year from this drainage ditch, improving throughflow, but decreasing biodiversity.

Restore

A degraded fen habitat is typically suffering from either low water levels, high nutrient influx or an invasive species.

Nutrient inflow is already addressed above with treatment wetlands, although if the land itself has been nutrient-enriched, such as with fertiliser, then a strategy should be developed to strip nutrients over time. It may be necessary to strip off some of the topsoil, though this will have a significant medium term carbon footprint. Scarifying is an alternative, which is better for avoiding issues which arise from large-scale removal of grass, like runoff and soil compaction. On less-enriched soils, the grass can be grazed, or mown and the arisings removed and composted offsite.

Low water levels may require more drastic action, though digging ponds can ‘rewet’ the land and create valuable landscape diversity, too. Historic maps can show the existence of ‘ghost ponds’ – pools which were filled in during agricultural ‘improvements’. If these can’t be found, then ponds and scrapes (see below) should be dug at low points across the land. Clustering 2 or 3 together will create a wetland, which has much higher value for biodiversity. 

Mole drains may have been installed below the surface of the soil, and these can be removed, or their draining effect can be mitigated by excavating the margin of a ditch and crushing any pipes found. The impact of the drainage ditch itself can be minimised by installing simple rock weirs at intervals to raise the water level, by dropping brash into the ditch to form an obstruction or by creating a ‘leaky barrier’ with logs. If the project has substantial financial backing, then river restoration (‘rewiggling’) can reconnect a fen to its neighbouring watercourse, raising the water table and restoring a seasonal flooding cycle.

Seasonally-flooded fen habitats have high potential for biodiversity if the right balance of livestock and management is followed, with cattle and Konik ponies most suitable for grazing. These areas have typically been flattened over time for agriculture, or have silted-up, and the restoration of shallow wetland ‘scrapes’ is likely to create high value sites for amphibians and birds. These scrapes replicate the activity of Bison and Wild Boar, which dig their own wallows to access drinking water. If ground-nesting birds are present, then research shows that a structurally-diverse habitat increases the survival of young. With more tussocky grass and shrub, there is more availability of rodent prey, which appears to distract foxes from bird nests, reducing the need for unsightly ‘predator fencing’.

Invasive plants can rapidly colonise Fen Marsh and Swamp habitats, as water enables the flow of floating species and the spread of seeds. Once an invasive plant is established, it takes a lot of work to remove it, but this work can be worthwhile, as it will restore diversity and health to the ecosystem. Identifying invasive species early on is essential, as this is the point at which they are easiest to eradicate. So, it is worth familiarising yourself with the identification of these invasive plants:

  • Floating Pennywort (Hydrocotyle ranunculoides)
  • Giant Hogweed (Heracleum mantegazzianum) – dangerous to touch!
  • Himalayan Balsam (Impatiens glandulifera)
  • Japanese Knotweed (Reynoutria japonica)
  • Swamp Stonecrop (Crassula helmsii)
  • Water Fern (Azolla filiculoides)


If any of the above are identified, check into the relevant legislation, as you may be legally required to report the location under certain circumstances, and/or take action to prevent them from spreading beyond your land.

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