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Fen are groundwater-fed, base-rich habitats.
They arise where rainwater permeates spongy calcium-rich rocks, resulting in mineral-rich water. Calcium carbonate dissolves to produce bicarbonate, which acts as a pH buffer, keeping the pH neutral.
Where the water table in the ground intercepts the ground level the water comes out as springs or seeps into the fen.
So fens are wet, ground-fed, base-rich habitats. The seeping water moves slowly so the soil surface has little oxygen- is anaerobic- so the vegetation rots slowly forming peat. This, in turn, means fens are low in the key nutrients of nitrogen, phosphate and potassium, as these are locked up in the peat. The calcium can also bind the available phosphorus reducing the fertility further. The fen community includes specialist plants that tolerate low oxygen conditions, calcium carbonate deposits and low nutrient availability.
The diversity in geology, hydrology, topology, fertility and disturbance results in a mix of open water, low habitats, dense vegetation and woodland, which provides a huge diversity of habitats and species.
Cutting vegetation and grazing no doubt have kept habitats open whilst peat digging has created many habitats we now consider important.
Once drained, these well irrigated, base-rich, high organic content soils are attractive for farming. Hence less than 1% of fens remain. The remaining sites are complex to manage. Maintaining the hydrology, water quality and disturbance is key.
Filmed at Thelnetham Fen, Suffolk Wildlife Trust, reserve.
Thanks to Rowena Langston for guidance.