Fens are created in areas where open water
accumulates, including depressions in
bedrock or glacial drift. A fen may develop directly as such, but typically is the result
of infilling a lake or even a semi-enclosed arm of the sea, evolving, therefore, from
an aquatic marginal wetland. The process of vegetation growth will lead to infilling
of the aquatic component of the wetland and gradual terrestrialisation through a
series of suc-cessional stages, each supporting distinctive plant assemblages.
Fen development from a lake
The various stages of the infilling of
a lake, known as hydroseres. Infilling proceeds
from the edge of the lake; submerged macrophytes grow in the littoral zone,
producing detritus and facilitating sedimentation by trapping and retaining silt within
their roots and rhizomes. When the water is shallow enough, emergent species,
such as Phragmites, which can establish in water up to 1 m deep, will invade, often
forming large stands. These are very productive, producing large quantities of
detritus which accelerate deposition even further until, as sedimentation rises
above the water table, species typical of fens, such as sedges, begin to encroach.
At this stage, the sediment, although waterlogged, is not generally immersed.
Sedge peat will begin to accumulate, marking the transition of the wetland from a
fringe wetland to a fen.
Once the fen has developed, wetland trees
such as willow and alder will establish,
increasing detritus inputs and, at the same time, lowering the water table through
elevated rates of transpiration. The former wetland, therefore, becomes a fully
terrestrial environment.
This sequence of hydroseres, in which
each facilitates development of the next by
modifying sediment levels, would appear to be the most likely course that wetland
succession should take, but, in reality, the successional process is much more
complex, and the sequence is by no means the same in different sites. It may be
predicted that the inevitable conclusion of wetland succession is fully terrestrial
vegetation, but this scenario, though widely quoted, is not borne out by the
evidence. The initial stage, in which reedswamp develops from submerged or
aquatic macrophytes, occurs commonly, but reedswamp, in turn, can be replaced
by open sedge fen, carr woodland (formed by wetland-tolerant trees growing
directly on peat) or Sphagnum bog.
A series of successional pathways can
be identified, but almost all culminate in
Sphagnum bog and it has been concluded that the true climax of hydroseres in the
British Isles is not terrestrial woodland but ombrogenous Sphagnum bog.
