Wetlands Fen and Raised Bog Characteristics. Threats and Management

Fen

Formation and Characteristics
Fens are an integral stage of ecological succession, the initial development of a fen occurs when freshwater fills a depression in the land surface, soon after a pond or lake has developed, vegetation begins to colonise, this is often a reed swamp. The last ice age created vast changes to the landscape, which majorly influenced the formation of new water bodies and flows, by changing land depressions, forming valleys and transporting base minerals.

The water is fed by mineral rich alkaline groundwater; this provides conditions for typical plant species. As the plants build up and die they create layers of dead organic matter. New vegetation grows on top of the older, slightly decaying matter.
Over time the bed of the lake rises with partially decayed plants. The saturated soils reduce aerobic respiration by bacteria and fungi in the soil, reducing decomposition rates and eventually leading to layers of peat with low mineral content.

At this point the fen is regulated and prevented from turning into a bog by the presence of alkaline ground water passing into the fen. If the water source were to change to rainwater instead of groundwater, the fen would soon convert into a bog and a raised bog can grow on. Once the surface of the peat reaches the water surface it becomes a fen.
Too little rain or seasonal dryness allows air to get into the peat, spurring sudden decomposition, alternatively too much water can wash out hydroxide ions and rainwater acidifies the peat, converting the fen to bog.
Fen development is fragile and can be brief once created as succession continues to Carr and eventually woodland, fen can be maintained where removal of willow and alder by grazing occurs and can often merge into marsh in these areas.

Characteristic Plant Species:

• Flat sedge
• Great fen sedge
• Davall's sedge
• Dioecious sedge; Brown sedge
• Slender sedge
• Flea sedge
• Common spike rush
• Few-flowered spike rush
• Slender spike rush
• Broad-leaved cotton sedge
• Brown bog rush
• Meadow thistle

Rare species:

• Fen Orchid
• Crested buckler

The UK supports a large proportion of fen-land found in Europe, unfortunately fens have declined dramatically over the past century, this is due to several influence factors:

Neglect.
Fens are a stage of ecological succession, without human intervention this habitat would naturally progress to scrub and eventually woodland, drying out in the process.

Loss of habitat from drainage.
Conversion to agriculture and excessive abstraction from aquifers have resulted in a lower water table, subsequently springs and groundwater have a reduced flow and fens do not receive the high water quantities they demand.

Change in water quality.
Abstraction affects the balance of water quality between ground and surface waters, resulting in unstable volumes of chemicals entering the ecosystem and changing the plant community types. This can also occur s from agricultural runoff, (valley fens are increasingly vulnerable to this)

Fragmentation. Continually smaller and more fragmented areas become incredibly vulnerable when several key species are reliant on the habitat.

Fen-lands are dynamic, semi-natural habitats, which require a low level of management (mainly scrub clearance and water level control) to preserve their natural characteristics and to retain an open fen. Fens can support up to 550 different species of plant and in some cases more, they are also an ideal habitat for over half of Britain’s dragonflies, several thousand insects and birds such as bittern, Bearded tit and marsh harrier.

Unsurprisingly most fens have some protective designation, from SSSI/ASSI to the Ramsar Convention and SPA protection from the Birds Directive. Some larger sites are designated NNR and are management with influence from Natural England. Several fen types are listed as priority habitats in the Habitats Directive (transitional mire, poor/rich fen)

Management generally focuses on:

• Restoration of fen to favourable condition
• Reedbed management – prevent over encroachment of reeds that could lead to succession, this often involves “in water” work and aquatic plant removal with hand tools.
• Removal of woody growth can be implemented through mowing and grazing regimes, this prevents invasion of scrub and tree species.
• Management of water levels controls how much water enters and leaves the site, ditches, drains and storage sites allow managers of fens to remove excess water or allow more water in when necessary. The careful balance of water levels keeps the habitat in favourable condition.
• Fen creation is beginning to occur on former abandoned peat workings, this could allow a slow development of new fens to areas where they have ceased to exist in recent years.
• Connection of fragmented fens can occur with the purchase and removal of arable land or forest plantations between two fen sites, these often have extensive drains which could divert water away from the site
• Monitoring and surveys continually provide a deeper understanding of fens and their ecosystems, bird surveys, plant/invertebrate transect counts, water quality testing etc.
• Species recovery for damaged populations with the fen ecology.

Raised Bog

Formation and Characteristics
These bogs form from vegetation which can survive merely on what is provided by rainwater, the raised bogs develop on already existing fens or reed swamps where the peat built up has become deep to the point where vegetations is no longer influenced by alkaline groundwater and develops a more acidic tolerant layer of vegetation: predominantly species of sphagnum moss.

Layers of the sponge like sphagnum moss can hold up to twenty times its own cellular weight in water, providing water for the next generation of moss. As with other waterlogged soils decomposition is reduced and the layers build up increasing the height of the bog over thousands of years. The older sphagnum directly above the fen is well humified and often present with bog cotton, newer layers of peat are relatively un-humified and quite well intact, fresh vegetation continues to grow on the surface.

Raised bogs do not follow the natural contours of the landscape, instead they can be raised, several metres above the local landscape and seem to be domed across its span. The actual surface of the raised bog is an even pattern of hummocks, hollows and pools that harbour microhabitats.

Typical raised bog characteristics

• Water content; Undrained
• Solids: Undrained
• PH: 3.8 – 6.5
• Organic content: 97%
• Inorganic content: 3%
• Peat depth: average of 7.5m (up to 13 metres)
• Annual rainfall: 700-1000mm

Raised bog have declined greatly in recent years with up 94% of natural raised bogs in the UK lost or irreversibly damaged due to the effects of modern life; peat extraction, landfill development, forestry, drainage, pollution, dereliction after previous disruption poor livestock management, built development, atmospheric nitrogen deposition and climate change all contribute to bog destruction and continue to do so.

The mosaic of pools, hummocks and lawns provides habitat a for a variety of flora occurring in micro-habitats, though mostly dominated by sphagnum moss’, undisturbed raised bog surfaces can support species of cross leaved heath, ling, cotton-grass, deer-grass, and sun dews. These tolerant species in waterlogged soils provide a habitat ideal for waders or wildfowl such as Curlew, Hen harrier, Meadow pipit, Skylark and Snipe, invertebrates and other species. The unique wildlife value of raised bogs raises their conservational importance and this has been internationally recognised, as this habitat is now a priority in the UKBAP.

Most bogs are designated or are the process of being designated (SSSI, SAC, SPA, NNR etc) this works to prevent further decline of the habitat. The active management on raised bogs is limited as even slight disturbance, can permanently damage this fragile ecosystem.

Natural Primary Bogs are defined as being sites, which have only received natural disturbance and have reached a natural climatic phase of raised bog. The layered make up of raised bogs has developed over thousands of years to reach this stage, peat removed for whatever use, is irreplaceable.

Raised bogs are affected by three main factors:

Increasing rate of water loss.
A loss of water will result in drying of the bog, accelerated decomposition and shrinking. Managers of bogs need to work towards providing a stable water table and prevent land drainage where it will affect bogs.

Increases in nutrient status.
Nutrient drift or runoff can effect competition between bog species; increased nutrients unbalance the ecosystem, favouring other species over the developed acidic loving plants of a bog. Nutrients can come from fertiliser, pesticide/herbicide, pollutants etc and get into the bogs hydrological cycle.

Loss of vegetation as a natural regulator of water retention.
Raised bogs depend on the layers of living sphagnum to hold water and keep the site continually wet. This is how raised bogs can regularly be higher than the local water table. A loss of this vegetation means water will not be held and the site will eventually dry out, as vegetation becomes less.

Degraded raised bog habitats, occur when active peat formation ceases, this is when no more layers of vegetation are being put down and growth of peat forming vegetation has disappeared. Despite the cease in formation, these sites have an opportunity for restoration with some very careful, long-term management.

If degradation occurs as a result of local forestry and the installation of drains diverting water away from the local hydrological cycle, then removal of trees and blocking of drains could re-wet the site and sphagnum growth may resume. The slow growth of sphagnum makes it very difficult to monitor the success of a rewetting program.

Some species associated with raised bog:

• Dragonflies
• Emerald Damselfly
• Large Red Damselfly
• Blue-tailed Damselfly
• Common Blue Damselfly
• Variable Damselfly
• Azure Damselfly
• Irish Damselfly
• Common Hawker
• Four-spotted Chaser
• Common Darter
• Black Darter

• Moths
• Beautiful Yellow Underwing
• Bordered Grey
• Common Heath
• Drinker
• Emperor moth
• Fox moth
• Grass wave
• Northern Eggar
• Puss moth
• Scallop shell
• Wood tiger

References:
http://www.ukbap.org.uk/UKPlans.aspx?ID=18
http://www.raisedbogrestoration.ie/about/default.asp
http://www.peatlandsni.gov.uk
http://en.wikipedia.org/wiki/Fen
http://www.bnm.ie/files/20061124040538_raised_bogs.pdf.
http://www.greatfen.org.uk/about-management.php