Fen Management Handbook - Scottish Natural Heritage

Management neglect
Hydroseral succession (secondary colonisation), either because of the cessation of management
or because of a major environmental change, is often a threat to fen bryophytes, and fen managers
need to consider whether natural succession is beneficial to their site or not. For example, the boreal
relic rich-fen moss Tomentypnum nitens has now been lost to scrub encroachment or vascular plant
growth from at least 3 of its 10 Welsh sites (K. Birch, pers. comm.).
On the other hand, localised natural succession may enhance a site, perhaps leading to the
reinstatement of ombrotrophic conditions in areas from which they were lost because of past
management, but can lead to acidification of areas of rich-fen and consequent loss of specialist
bryophytes. Crymlyn Bog in south Wales has experienced multiple successions from fen to raised
bog (Hughes & Dumayne-Peaty, 2002), and a vegetation survey in 2009 suggests that part of the
east side of this 280 ha complex of fen and swamp is experiencing the start of another succession.
Not only is scrub encroaching rapidly, but four species of Sphagnum are locally abundant in
communities dominated by Carex paniculata, Phragmites australis, Typha angustifolia and T. latifolia
– combinations not recognised by, for example, Wheeler (1980). The bryophytes being replaced
here are either those of neutral fen or rich-fen. This change in species composition can enrich a local
flora, for example the arrival of at least six species of Sphagnum at Wicken Fen, Cambridgeshire
(Preston, 2008) after years of scrub encroachment, reduced winter flooding and, probably, acid rain.
A recent focus on maintaining/restoring open fen at Wicken, with consequent scrub clearance, pony
grazing and raising of (calcareous) water levels, has caused the loss of several sphagna from their
only Cambridgeshire site. Meanwhile, rich-fen mosses such as Campyliadelphus elodes were seen
in 2008 for the first time since the 1950s.
As always, fen management can have negative impacts as well as positive ones. The alternative
would be to allow a complete transition to carr, which might benefit some bryophytes but would
undoubtedly cause the loss of other rare species. There is no evidence that the epiphyte flora of
British carr is particularly notable, despite its often very high bryophyte biomass, so scrub clearance
on fen sites is likely to be of concern only if the ground flora is of note.
Soligenous mires
Soligenous fens are home to the UKBAP liverworts Barbilophozia kunzeana and Jamesoniella
undulifolia, both of which grow in small quantity through base-tolerant sphagna at a handful of sites,
for example on the Long Mynd in Shropshire. The scarce temperate form of Hamatocaulis vernicosus
(Hedenäs & Eldenäs, 2007) is also found primarily in soligenous mires on the upland edge in Wales
(Bosanquet et al., 2006) and to a lesser extent in England, Scotland and Ireland. A constant supply
of neutral, nutrient-poor water is the main requirement of these species, so management that alters
a site’s hydrology could have a negative impact on them. The hydrology of sites with H. vernicosus
is often rather complex: several of its sites in south Wales are in areas where highly calcareous
water derived from Carboniferous Limestone over 1 km away spreads on to, or bubbles up through,
a peatland, which reduces the pH and provides suitable conditions for Hamatocaulis. Quarrying
or water abstraction some distance from a flush complex can significantly alter its hydrology.
Reduced grazing is currently threatening Hamatocaulis on upland-edge commons in Wales, perhaps
exacerbated by vigorous vegetation growth because of atmospheric N deposition, and continued
light to moderate grazing is necessary to maintain open swards suitable for the notable bryophytes
to grow.
Concluding remarks
It needs to be emphasised that our knowledge of the distribution of fen bryophytes remains
remarkably patchy. Paludella squarrosa was discovered new to Ireland in 1998 (Lockhart, 1999),
Scorpidium turgescens new to England in 2002 (Porley & Hodgetts, 2005), Leiocolea rutheana new
to Scotland in 2001 (Blackstock, 2002), Sphagnum riparium new to Wales in 2005 (Jones et al.,
2006) and more than five new sites for Jamesoniella undulifolia have been found in Cumbria and
Shropshire in the last few years. Because of this, it is vital that managers of rich-fens and transition
mires get a competent bryologist to survey their site before any dramatic changes in management or
hydrology.
Translocation experiments by Malson & Rydin (2007) suggest that it is possible to reintroduce at
least some bryophyte species to restored rich-fens using gametophyte fragments as propagules.
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