Fen Management Handbook - Scottish Natural Heritage

3.5 Factors determining fen type
3.5.1 Water Movement
In common with bogs, most fens accumulate peat as plants die back but do not fully
decay. Peat accumulation is greatest where the ground is permanently waterlogged
and with little water movement.
3.5.2 Acidity
Some of the wide variation in the vegetation of different types of fen is associated
with acidity or alkalinity, which is measured by pH (the concentration of hydrogen
ions) and the presence of bicarbonate and particular metallic ions such as calcium
and magnesium.
– ‘Poor fens’ occur mainly in the uplands or associated with lowland heaths, where
the water is derived from base-poor rock such as sandstones and granites. They
are characterised by short vegetation with a high proportion of bog (Sphagnum)
mosses and acid water (pH of 5.5 or less).
– ‘Base-rich fens’ are fed by mineral-enriched calcareous waters (pH 5.5 or more)
and tend to support a wider diversity of plant and animal communities than
those fed by base-poor water. These fens are mainly confined to the lowlands or
upland areas with localised occurrences of base-rich rocks such as limestone.
They can be extremely species-rich, providing a habitat for around a third of our
native flora, more than half the UK’s species of dragonflies and several thousand
other insect species, as well as being an important habitat for a range of aquatic
beetles.
3.5.3 Mineral content
Fens are distinct from bogs in that the water which feeds fens has passed over or
through soil and rock, in the process becoming charged with mineral salts dissolved
from the rock and soil. This contact with rocks is known as ‘residency time’. The
concentration of minerals dissolved, which in turn determines the type of plants that
will grow in the fen, can be affected by the quantity of water passing through.
A fascinating feature of some calcareous fens is the development of tufa, which
is associated with springs where groundwater rich in calcium bicarbonate comes
to the surface. On contact with the air, carbon dioxide is lost from the water and a
hard deposit of calcium carbonate is formed as stony grey tufa.
Tufa forming on
Scorpidium cossonii,
Bryum pseudotriquetrum
and sedge litter in a
spring-fed calcareous
fen, Trefonen Marshes,
Shropshire (I. Diack).
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