A poor fen (also known as transitional bog, transitional mire or sedge mire) is a natural wetland habitat, consisting of dense low growth of small sedges and other plants. It develops on wet ground where the water is fairly acidic and has very few plant nutrients.
Poor fen is intermediate between the taller vegetation of fen, which occurs where the water is much less acidic, and the short, mossy vegetation of bog, which is even more acidic.
Poor fen Wikipedia
Poor fen is found where the ground is permanently wet with nutrient-poor water which is somewhat acidic. For example it occurs as the vegetation of flushes (marshy springs on slopes), in places where neutral water enters more acidic bogs, in wet acidic grassland, and sometimes in the flatter parts of flood-meadows where much of the water is ombrotrophic (derived directly from rainfall). It sometimes forms a floating mat over water or very wet peat, making a mobile surface known as quaking bog.
Poor fen is usually grazed by wild animals or livestock, which prevent ecological succession into wet woodland.
In north-western Europe, characteristic plant species of poor fen communities include common sedge (Carex nigra), carnation sedge (Carex panicea), star sedge (Carex echinata), white beak-sedge (Rhynchospora alba) and many other small sedges, as well as other plants such as jointed rush (Juncus articulatus), marsh cinquefoil (Potentilla palustris), bog-bean (Menyanthes trifoliata), bog pimpernel (Anagallis tenella), lesser skullcap (Scutellaria minor) and marsh valerian (Valeriana dioica). Sphagnum mosses also occur, but are not dominant as they would be in bog vegetation. There may also be some grasses, but these are usually inconspicuous amongst the sedges and rushes.