Linking Restoration and Ecological Succession (Springer ... - Inecol

Chapter 5 Succession and Restoration of Drained Fens 111
Canadian (Moore et al. 1989) and British fens (Wheeler and Shaw 1991) confirm
this conclusion. Our results suggest that several species of the Scheuchzerio-
Caricetea such as Carex dioica or Eleocharis quinqueflora even have optimal
standing crop values lower than 200 g m −2 (cf. Schrautzer and Jensen
2006).
To moderate nutrient inputs by reducing standing crop in previously fertilized
fen ecosystems it is important to know which nutrients actually limit aboveground
phytomass production. A comparison of terrestrial wetlands (including
fens) along a transect from western Europe to Siberia has shown that more endangered
species persist under phosphorus-limited than under nitrogen-limited
conditions (Wassen et al. 2005). The authors concluded that despite high Ndeposition
in western Europe, P-enrichment has been more accountable for
the loss of wetland species than N-enrichment. As a consequence, one indispensable
prerequisite for the maintenance of species-rich fens such as small
sedge reeds or mesotrophic wet meadows is to prevent processes that enhance
P-availability. Several studies have shown that in degraded fen ecosystems
mostly potassium and sometimes phosphorus are the most important limiting
nutrients (e.g., Schwartze 1992, Boeye et al. 1997). According to van Duren
et al. (1998), yearly mowing led to a shortage of potassium and a reduction of
aboveground phytomass in stands of drained fens, whereas the nitrogen availability
remained high. A decrease in productivity of previously intensively used
fens after long-term mowing has been observed in several other studies carried
out in northwestern Europe (e.g., Bakker and Olff 1995).
Our literature survey about the success of haymaking experiments in degenerated
fens confirmed that mowing once, or for best results twice a year, is a
useful measure to maintain species-rich systems like mesotrophic wet meadows
or to enhance the species richness of more degenerated systems. More
frequent mowing will decrease species richness. The results of our own field
experiments showed that percentage cover of target species increased after longterm
mowing in all investigated systems. The potential for a reestablishment
of these species seems to be highest in abandoned wet meadows (cf. Hald and
Vinther 2000, Schwartze 2003). This might be explained by the conservation
of long-term persistent seed banks due to the development of a decomposable
litter layer. Previous investigations (Bekker et al. 1998b, Hölzel and Otte 2001)
about the seed longevity of Molinietalia species concluded that many species
only build up transient seed banks. As a consequence, the seed bank is considered
to be unsuitable as a source for the reestablishment of wet meadow
species in degenerated fens after restoration measures. Our seed burial experiments
showed contrary results, with relatively long-term persistence in the peat.
One important reason for these differences might be that the longevity of seeds
was often underestimated using the determination method of Thompson et al.
(1997). According to these authors, the seed longevity of species is classified
as transient if the species occur in the current standing vegetation and could
not be recorded in the soil samples. The latter might be the result of a low seed
density in the soil. Bekker et al. (1997) mentioned that soil samples taken to
analyze seed banks usually cover only 0.05% of the area that is used to record
the current vegetation.
Although our field experiments showed that the abundance and number of
target species increased during succession, other studies revealed no reestablishment
of these species (e.g., Sach 1999, Grootjans et al. 2002). Consequently, the