The definition of fen is an area of no permanent high water-table but one subject to seasonal fluctuations with an inflow and outflow of alkaline water. On the Commons it is limited and generally characterised by patches of Reed (Phragmites). They mainly occur in the NE corner and north of hill VI. This type of habitat is extending and in recent years has attracted both Sedge and Reed Warblers. Ragged Robin, Marsh Fragrant Orchids and Marsh Helleborine can be found nearby. In the drier areas Brambles and Willows are becoming established and conservation measures have been taken to control their spread. Small areas of alkaline fen occur sporadically throughout the larger bogs. This is due in part to the central stream water being basic, having a pH value 7.0 to 8.5. Where this water arises initially is uncertain but is possibly in part influenced by the underlying Boulder Clay and/or Chalk and is discharged through springs.
As there is often many species of flowering plant associated with the edges of the reedbeds many butterflies (such as the Brown Argus), beetles and hoverflies are found throughout.
Marsh Fragrant Orchid