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An area of wetland vegetation that receives its water by both rainfall and ground water flow (rheotrophic), and in which the summer water table is at or below the surface of the sediment. Fens are peat-forming ecosystems (mires) and can be divided into rich fens, which are usually neutral to acid in their reaction and are often fed by streams draining limestone rocks, or poor fens, in which case their water is slightly acid and the concentration of dissolved nutrients is low. Some fens are erroneously given the label bog (e.g. string bogs and valley bogs), a term that should be reserved for ombrotrophic mires.

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