Fen Management Handbook - Scottish Natural Heritage

Appendix VI – Fen management for bryophytes
In general, favourable management of fens should deliver favourable conditions for fen bryophytes,
although there are obvious exceptions when succession has modified the bryophyte flora of a fen
and management then impacts on the resulting flora. Ecological change and loss of management
appear to be the main causes of the loss of populations of Red Data Book (Church et al, 2001)
and UKBAP species from lowland mires in recent years, especially in East Anglia, although wholescale
habitat destruction through drainage is thought to have caused the extinction of three rich-fen
mosses including Paludella squarrosa in northern England in the late 19th century, and Meesia
triquetra in Ireland as recently as the 1960s.
Key environmental and management factors for mosses and liverworts
Water chemistry
Bryophytes are highly sensitive to changes in water chemistry. The majority of fen bryophytes require
low macro-nutrient (i.e. N, P & K) levels and are intolerant of nutrient enrichment. They vary more in
terms of base tolerance, from low base status (poor-fen) species, through neutral water specialists
to obligate rich-fen species. It is critical that site managers understand the natural water chemistry of
a site and ensure irrigation with nutrient-poor water of suitable base-status.
Kooijman & Bakker (1995) carried out cultivation experiments that showed Sphagnum squarrosum
responding to nutrient enrichment more than S. subnitens, and Scorpidium scorpioides being more
tolerant of mineral-rich groundwater than the two sphagna. The common generalist Calliergonella
cuspidata replaced the fen specialist S. scorpioides as nutrient levels rose, and the base-intolerant
Sphagnum squarrosum replaced Calliergonella when inputs of groundwater declined. Such
transitions can be observed in situ and may be rapid; sometimes resulting in substantial change in
the character of fen features. For example, blocking ditches feeding mineral-rich water into a wetland
can lead to the formation of poor-fen (van Wirdum, 1995). Complete competitive exclusions are
probably rare (Malson & Rydin, 2009), at least among rich-fen bryophytes, but declines in abundance
and vigour can lead to the loss of species to other environmental changes.
Even traditionally managed fens can lose specialist bryophytes because of changes in surrounding
land use or atmospheric N deposition (Bergamini et al., 2009): surveys in 1995 and 2006 of 36
traditionally managed fens in Switzerland showed significant losses in fen specialist bryophytes and
Red-list plants, and a significant increase in vascular plant biomass. N deposition is implicated in
an increase in Sphagnum fallax on bogs (Limpens et al., 2003) and clearly has an impact on fens
as well. Most of lowland Britain is above the critical threshold for N enrichment of fens (www.apis.
ac.uk), making management even more challenging and emphasising the need for restriction of
nutrient inputs within the entire catchment of a fen. Grazing or mowing may be needed to remove
the competitive vascular plants that develop in response to enrichment. The effects of atmospheric
enrichment are often exacerbated by water abstraction (see Case Study 10) and enrichment, as
demonstrated by the loss of the UKBAP liverwort Leiocolea rutheana from four of its five sites in
Norfolk and its decline at the fifth (Church et al., 2001; Swann, 1982).
Hydrology
Changes in hydrological regime can produce unsuitable conditions for key rich-fen mosses. Many
experience fluctuating water levels and periodic inundation in the winter, but a rapid or sustained
increase in water levels can lead to the loss of species such as Calliergon giganteum, which is
replaced in deeper, more nutrient-rich water by C. cordifolium. Buoyant fen rafts may be more
resilient than non-buoyant surfaces, but any rapid change in water levels should be avoided in
favour of a gradual rise. Ditch blocking to raise water levels in one part of a fen may divert enriched
or chemically unsuitable water on to another part and can have unforeseen consequences on the
bryophyte flora.
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