Fen is an ecological term borrowed from the fen country of eastern England to describe a rather ill-defined group of wetland habitats. Various ecologists have used the term fen in several different ways and based their definitions on different criteria including the height of the water-table, the pH of the water or the nutrient status of the water. The term is therefore rather vague, but in general usage refers to a wet habitat in which the water table lies at or close to the gound surface and in which the water itself is more nutrient-rich, or base-rich, than that of a bog. But the terms can overlap in such expressions as “alkaline bog”!
In practice fen is used to describe the kind of marshy vegetation found on lowland lakeshores, or which gradually develops on silt and peat in lake basins. In the latter case, over a long period of time, the fen vegetation comes to completely fill in the lake and open water entirely disappears. The lake basin is gradually filled with deposits of mud, silt and fen peat (incompletely decomposed plant fragments, like that of bog peat, but composed of different species).
Some trees - alder (Alnus glutinosa) and sallow (Salix cinerea, S. aurita and other Salix spp.), can invade fen and form quite dense woodland called fen carr, or willow-alder carr. Birch (Betula pubescens) can also invade at a later stage. Good examples of very wet alder-willow woodland are Rea's Wood near Antrim (a site for the local summer snowflake, Leucojum aestivum) and Hollymount Nature Reserve near Downpatrick (the only Northern Ireland site for water-violet, Hottonia palustris).
Similar, but less species-rich wet woodland will form on old cut-out raised bogs, and this is a common feature of the Northern Ireland lowland landscape. Good examples are found in the vicinity of Lough Neagh, especially in its south-east corner. These are sometimes called the 'Lough Neagh Fens' but they are not fens in the strict sense.
N.B. Some of these species are local to a limited number of sites only.