Restoration of mires and wet grasslands - Restoration Ecology - page 119

9.5.2 Restoration of fens and fen meadows

Restoring the hydrological regime

Restoring the hydrological regime is more than just
increasing water levels (rewetting). In groundwater-
fed mires, in particular, increasing water levels may
lead to acidification when the discharge of base-rich
groundwater cannot be restored (van Wirdum et al.
1992). Restoring groundwater-fed mires includes
increasing water levels in the whole surrounding
catchment area, which in most cases is not possible
any more, due to other hydrological claims (farmers,
cities). When groundwater pressure in a mire is still
high, digging of small shallow ditches can be recom-
mended (Grootjans et al. 2002), which can transport
most of the infiltrating rainwater out of the mire.
This prevents the influence of acid rainwater lenses
dominating in the mire (Schot et al. 2004).

It is generally believed that under the present high
rates of N deposition fens and fen meadows cannot
be maintained without a regular mowing regime. The
first thing to do in abandoned hay meadows is to
resume the traditional haymaking. In some cases a com-
bination with extensive grazing is also possible. Sod
cutting is sometimes applied when the topsoil of a fen
meadow has been severely acidified by drainage or
when the vegetation has shifted to a grass-dominated
stage after long-term exposure to high N deposition
from the air.

Resume traditional management

Resuming the traditional management of haymaking
without fertilization has proven a very suitable meas-
ure to restore species-rich meadows in Europe. This
type of management is widely applied in nature
reserves in western Europe where intensive agricul-
tural production is not allowed. In some countries
private or state nature-conservation organizations
carry out the mowing themselves, but more often local
farmers are paid to do the mowing. A success story
is, for instance, the creation in 1965 of the largest
meadow reserve in the Netherlands, the Drentsche Aa
catchment area, where most of the meadows on peat
soils had never been used intensively and where deep
drainage had not been applied on a large scale. Most
of such meadows responded very well to restoration
management after a short period of abandonment. The
increase of target species depended on the amount of
nutrients accumulated during former fertilizer applica-
tion (Bakker & Olff 1995). It may take 5 -15 years
before the nutrient output has reduced the nutrients
stocks to a level in which the productivity of domin-
ant species no longer prevents the establishment of
target species (see Prach et al. 1996). Even more
target species established in former agricultural areas
on mineral soils after topsoil removal, but only when
former hydrological conditions were restored, seed
banks had not been depleted and where dispersal mech-
anisms (flooding) from adjacent nature reserves were
effective (Moen 1990, Jansen et al. 2000).

Nutrient removal

Resuming the traditional management in fen meadows
that have been fertilized intensively in former days
will not lead to rapid success without rewetting and
topsoil removal. This is clearly illustrated for the
Veenkampen experiment (Oomes et al. 1996) where
mowing without fertilization was very unsuccessful,
even after 10 years, despite several target species
being present in the seed bank. Mowing in com-
bination with rewetting was much more successful
and combined with sod cutting it has lead to a re-
appearance of many Red List species with a long
persistent seed bank (Matus et al. 2003). These
species had survived in the soil seed bank for up to
40 years. Under appropriate site conditions, therefore,
activation of seed banks can be beneficial for the sur-
vival of endangered plant species, since the rapid popu-
lation expansions after restoration measures raise the
opportunity to produce a new seed bank (Jansen &
Roelofs 1996). Tallowin and Smith (2001) reported on
successful restoration of a species-rich fen meadow after
topsoil removal combined with seeding and planting
of seedlings of target species.
Shallow sod cutting in former agricultural fields,
drained and fertilized for a long time, is usually
ineffective (Pfadenhauer et al. 2001, Grootjans et al.
2002), since (i) soil degradation has initiated irreversible
changes in the topsoil and (ii) seed banks are depleted
while dispersal mechanisms are ineffective (van