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

110 Joachim Schrautzer et al.
of NNM after rewetting (van Dijk et al. 2004). The simulation results show
that after rewetting wet pastures, a major change occurs in the carbon budget
from a high carbon source to a carbon sink. How far the latter is realistic is
difficult to assess, because peat-forming processes are dependent on the plant
species that develop after rewetting (Roth et. al 1999) and the model WAS-
MOD does not consider these aspects. According to Roth et al. (1999), only
a few hygrophilous species such as Carex elata, Carex acutiformis, or Phragmites
australis are peat-forming species. On the other hand, there are no data
available that allow a reliable calculation of the carbon budget of tall sedge
reeds because it seems to be nearly impossible to measure their belowground
phytomass dynamics (Schrautzer 2004). Nevertheless, it can be assumed that
the long-term development to eutrophic alder carrs will enhance the carbon
storage capacity of rewetted fens (Kutsch et al. 2000). Apart from vegetation
dynamics, the mode of rewetting also influences the carbon budget of the systems.
Asada et al. (2005) demonstrated that dry peatlands might be changed to
carbon sinks after flooding. The authors measured an accumulation of organic
material of 10 × 10 3 kg ha −1 during a period of 9 years in areas with marsh
vegetation.
The simulated denitrification rates of the rewetted ecosystems were relatively
low. This was due to the fact that the simulation was carried out only at the site
level and consequently, only internal processes were represented. However,
rewetting of fens is often aimed at the reduction of nitrate concentrations of
eutrophied surface water by denitrification (Leonardsson et al. 1994). The potential
for denitrification of fen ecosystems is high and therefore the intensity
of this process in rewetted fens depends mainly on the nitrate concentration of
the surface water that is supplied (Davidsson et al. 2002). These aspects were
not considered in our simulation.
5.4.4 Development of Species Richness After Mowing
Without Fertilizing
Many characteristic species of seminatural fens such as small sedge reeds and
mesotrophic wet meadows are missing in the agricultural landscape of northwestern
Europe. Conservation of these species depends on a moderate disturbance
regime to prevent successional changes caused by abandonment (Bakker
and Berendse 1999, Jensen and Schrautzer 1999, Diemer et al. 2001). Moreover,
to avoid the effects of eutrophication on species composition, external
nutrient inputs, which in central Europe usually exceed the natural outputs of
these low-productive systems, have to be controlled (Olde Venterink 2000).
Mowing seems to be the most successful management measure to fulfill these
demands (Grootjans and van Diggelen 1995; see Chapter 6).
The relationship between standing crop, light availability, and the occurrence
of target species reveals that a specific (“target”) biomass production of
the stands is an important prerequisite to maintain or create suitable habitat
conditions for many of these species (cf. Kotowski and van Diggelen 2004).
Many light-demanding species of the Scheuchzerio-Caricetea occur mainly in
stands with maximum standing crop values below 400 g m −2 , but the maintenance
and reestablishment of most Molinietalia species do not require such
low standing crop values. Investigations of Güsewell and Klötzli (1998) and
Olde Venterink (2000) in other northwestern European fens as well as studies in