Fen Management Handbook - Scottish Natural Heritage

Management objectives for a wetland site can rarely be set in isolation from the
surrounding land. It is possible, for example, that wetting up a fen may cause
wetting up in adjacent non-fen land, especially if this land was originally part of
the same wetland. The consequences of proposed management, particularly
installation or removal of structures which may influence water level, should be
carefully considered and neighbouring land owners/managers consulted at the
earliest opportunity. See Section 11: Fens and People.
5.8.5 Defining a target hydrological regime
Defining an ideal or target hydrological regime for a particular fen can be
considered at various levels depending on available resources, level of
understanding or available information and the level of detail required.
– At the most basic level, for example for fen management undertaken through
agri-environment schemes, defining a target hydrological regime involves
determining which areas of a site (linked to the ecology) should be ‘wet’ i.e.
with soil water levels within 10 cm of the ground surface during the summer
months, and which should not fall more than 30 cm below ground surface for
any significant period of time (say one month). This could be based upon local
knowledge or on published guidance, such as the eco-hydrological guidelines
described below.
– At a more complex level, it may be necessary to identify the key water transfer
mechanisms for the site and some simple spatial (i.e. site or location specific)
and/or temporal (seasonal or time specific) criteria for the operation of these.
For example, perennial flow might be required from a set of springs within a
sloping fen, or complete inundation by overbank flooding might be required
annually for a tall-herb fen on a floodplain.
– At the highest level, for example more ambitious fen management schemes or
those which might affect a public water supply, a lot more detail will be required,
but identification of the precise hydrological requirements of many species and
habitats is limited by lack of research or confinement of current research to a
limited geographical area.
Historically, most wetland management was decided on the basis of expert
judgement or reactive management. Recently, driven primarily by the obligation
to conserve wetlands in a number of European Community Directives (such as
92/43/EEC: Conservation of Natural Habitats and of Wild Fauna and Flora),
ecohydrological guidelines have been developed for lowland wetland plant
communities (Wheeler et al, 2004), including wet grassland, fen and mire, ditch
and swamp communities, and wet woodlands (Barsoum et al, 2005), covering the
different types of wet woodland habitat.
Ecohydrological guidelines are based on the NVC system for plant communities
(see Appendix IV). For example, for S24 (Phragmites australis–Peucedanum
palustre) tall-herb fen, Wheeler et al (2004) include:
90
Optimal water levels. Summer water level is typically around 15 cm below
ground level, but deeper levels may be a perfectly natural feature of some sites.
The sub-community most often associated with a water level at or near the
surface all year round (S24e) on average supports the greatest number of rare
species. Winter inundation is a natural feature of many S24 stands.
Sub-optimal or damaging water levels. Strongly sub-surface winter and
summer water tables are outside the normal range of this community; peat
drying and degradation would lead to development of rank fen rapidly becoming
wooded without management. Very wet sites with widespread summer