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= Fen - (EDIT NOTE: text struck out was from the original text) = From Wikipedia, the free encyclopedia

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This article is about the type of wetland. For other uses, see Fen (disambiguation).

Not to be confused with Fenn. Avaste Fen, Estonia View of Wicken Fen showing vegetation typical of a fen in the foreground and carr vegetation featuring trees and bushes in the background A fen is one of the main types of wetlands, the others being grassy marshes, forested swamps, and peaty bogs. Along with bogs, fens are a kind of mire. Fens are minerotrophic peatlands, usually fed by mineral-rich surface water or groundwater. Although fens are groundwater fed, their water chemistry is also influenced by precipitation. Fens also play a crucial role in the global cycling of nutrients, such as carbon and phosphorous. Fens are usually dominated by grasses and sedges, and typically have brown mosses. Fens frequently have a high diversity of other plant species including carnivorous plants such as Pinguicula. T he distribution of individual species of fen plants is often closely connected to water regimes and nutrient concentrations. Fens have been damaged in the past by land drainage, and also by peat cutting. Some are now being carefully restored with modern management methods. The principal challenges are to restore natural water flow regimes, to maintain the quality of water, and to prevent invasion by woody plants.

Fens vs. Bogs
Fens are distinguished from bogs, which are acidic, low in minerals, and usually dominated by sedges and shrubs, along with abundant mosses in the genus Sphagnum. Bogs also tend to exist on dome-shaped landmasses where they receive almost all of their usually-abundant moisture from rainfall, whereas fens appear on slopes, flats, or depressions and are fed by surface and underground water in addition to rain. Additionally, bogs tend to have overall lower species richness in comparison to fens. Bogs are also ombrotrophic.

Fen Chemistry
Fens are characterized by their distinct water chemistry, which is neutral pH to alkaline, with relatively high dissolved ion concentrations, particularly of base cations (Mg, Ca, Fe), but low concentration of plant nutrients. Continuous input of groundwater into fens maintains a stable water table throughout the course of a year, which helps to maintain the defining characteristics of fens, like their morphology and chemistry.

The organic soils of poor fens are composed of peat. As with all peatlands, the rate of plant decomposition is slower than organic matter accumulation under wetland conditions. As the peat surface thickens, the plant community changes in response to lower nutrient availability, and increasingly acidic and anaerobic chemistry.

The chemistry of a fen is dependent on the peat and groundwater chemistry. In comparison to ombrotrophic peats, the minerotrophic peats found in fens have higher total element content. Generally, peat and water chemistry, specifically cation concentrations, are related to one another. The cation exchange capacity of a peat is very high. The cations present on the exchange sites in the peat can potentially be removed, thereby adds them to the water. However, not all cations exchange equally. Divalent cations, such as and, are highly retained on the exchange sites. This results in higher concentrations of calcium and magnesium in the peat, but lower concentrations in the water.

In addition to peat chemistry, the surface water chemistry of a fen is partially controlled by the topography and parent materials of the landscape. For example, a fen that is positioned on a steep slope and has calcite precipitation occurring will have a high alkalinity and a high specific conductance, which is a measure of the water's ability to conduct an electrical current. In contrast, a fen with a shallow slope and bedrock covered with mosses will have lower specific conductance and higher mineral richness.

Carbon
Peatlands, in general, play a critical role in the carbon cycle, as they serve as both a sink for atmospheric carbon dioxide and contribute methane  to the atmosphere. To create more space for horticultural use, peatlands have been drained and had peat extracted, impacting not only the peatland ecosystem, but also the global carbon cycle. The loss of peat inhibits the peatlands' ability to sequester from the atmosphere, leaving it to serve as a net source of C through emissions. Fens emit more to the atmosphere than bogs, due to differences in vegetation types and the level of the water table. A higher water table is generally related to reducing conditions, allowing to be reduced to, which can then be emitted to the atmosphere. In contrast, a low water table and unsaturated peat is associated with low fluxes in. Low emissions occur since the peat is unsaturated with water, allowing the to diffuse through the peat and become oxidized.

Phosphorous
Phosphorous cycling in fens is controlled by physical characteristics of the peat, as well as microbial activity. Peat, characteristically, has a high organic matter content. For the nutrients to be usable by plants, the organic matter must be decomposed. However, only a small percentage of organic matter decomposes in fens, which results in low nutrient content, including phosphorous. The level and stability of the water table drives phosphorous availability, with a high or unstable water table leading to leaching of phosphorous. Overall, there is a net loss of phosphorous in fens, which is exacerbated by the removal of vegetation of humans. If phosphorous is lost in high enough quantities, then it could trigger the formation of algal blooms in the fens.

Nutrient Availability
Depending on plant nutrient availability, a fen can be classified into one of three main classes: poor, moderate, or rich. A poor fen is has low nutrient availability, whereas a rich fen has high nutrient availability. Nutrient availability is governed by the chemistry and flow of water in the fen. The gradient in nutrient availability leads to variation in the vegetation present in each type of fen. Surface water concentrations of and bicarbonate  gradually decrease along the gradient of rich to poor fens.

Vegetation
Carr is the northern European equivalent of the wooded swamp of the southeastern United States, also known in the United Kingdom as wet woodland. It is a fens overgrown with generally small trees of species such as willow (Salix spp.) or alder (Alnus spp.). In general, fens may change in composition as peat accumulates. A list of species found in a fen can therefore cover a range of species from those remaining from the earlier stage in the successional development to the pioneers of the succeeding stage.

Poor fens are covered with peat-forming sphagnum mosses, such as Sphagnum angustifolium, Sphagnum fallax and Sphagnum magellanicum, whereas other brown mosses can also be frequent, such as Polytrichum strictum. Mosses combine with a high abundance of sedges, such as Carex canescens, Carex echinata, Carex nigra, Carex lasiocarpa, Eriophorum scheuchzeri (white cottongrass) and Trichophorum cespitosum (tufted bulrush). Other abundant plants include Andromeda polifolia (bog-rosemary), Betula nana (dwarf birch), Dactylorhiza maculata (heath-spotted orchid), Eriophorum vaginatum (hare's-tail cottongrass), Potentilla erecta (common cinquefoil), and Vaccinium oxycoccos (bog cranberry).

Natural disturbance factors influencing poor fens include fire, flooding, windthrow, and insects. Similar to grazing, a natural fire regime can prevent succession to woodland. In the absence of fire, a thick layer of leaf litter can encroach on the open mire which in turn stifles fen vegetation.

Rich fens have a specialized and species rich flora, which often consists of orchids, sedges, and mosses. The rich fens are often classified into the categories moderately rich fen and extremely rich fen. The extremely rich fens have a higher pH and more vascular plants.

Wildlife
Poor fens are usually grazed by wild animals or livestock, which prevents ecological succession to wet woodland.[Citation needed]

Threats
Historically, rich fens have been used as meadows and pastures, but the practices ceased during the 20th century. Many wetland areas, among them rich fens, were drained in the 19th and 20th century to create new agricultural land or to increase productivity in the forestry. There are several other threats that might cause degradation and loss of rich fens, such as acidification, eutrophication and overgrowth by trees and bushes because of lack of management. Habitat fragmentation is an ongoing threat that leads to reduced connectivity within the landscape. Small and isolated fragments hold, in general, fewer species than larger and less isolated areas, predominately because it is more difficult for a species to disperse between isolated fragments.