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

Chapter 5 Succession and Restoration of Drained Fens 107
intensively grazed, dominantly wet pastures) following the introduction of a
moderate grazing regime.
5.4 Discussion
5.4.1 Effects of Land-Use Intensification on Ecosystem Features
Land-use intensity can be approximately equated with disturbance intensity
(Grime 2001). However, here the mode of phytomass destruction indicating
disturbance intensity varied (e.g., deforestation, mowing, or grazing) and other
human impacts such as drainage and fertilization were superimposed. These
complex variants need to be considered when evaluating the impact of disturbance
intensity on ecosystem functioning. The intermediate disturbance hypothesis
(IDH) is one of the most frequently suggested explanations for the
coexistence of plant species in ecological communities (Wilson 1990). This
hypothesis proposes that species richness is low at low and high disturbance,
and is highest at medium disturbance. Species richness in the herbaceous stages
of the land-use intensification sere from tall sedge reeds to wet pastures (Fig. 5.4;
2A through to 5) shows a hump-shape course conforming to the IDH (see also
Gough et al. 1994, Schaffers 2002). The marked decline in species richness
for the retrogressive succession from seminatural fen ecosystems (small sedge
reeds and mesotrophic wet meadows) to wet pastures due to increased disturbance
intensity has been noted in studies by Grootjans et al. (1986), Koerselman
and Verhoeven (1995), and Wassen et al. (1996). However, if species-rich alder
carrs are included, the IDH does not hold.
The set of ecosystem indicators for water and nitrogen budgets reveals no
clear progressive trends for all indicators across the retrogressive stages, but
C-balance and metabolic efficiency decreased. The patterns for indicators other
than C-balance were mainly high to low to higher across the stages and did
not correlate well with species richness. Among all stages, alder carrs had
highest NPP and acted as carbon sinks or had a well-balanced carbon budget.
Measurements concerning the carbon cycle of alder carrs are scarce. Kutsch
et al. (2000) measured a mean NPP of 8002 kg C ha −1 a −1 and a C-balance of
+3550 C kg ha −1 a −1 based upon measurements of all processes of the carbon
cycle for this forest type. These values are about 30% higher than the mean
values given by the simulation with WASMOD. The reason for this difference
might be that the alder carr investigated by Kutsch et al. (2000) was at a relatively
young stage of development.
The simulated NNM of alder carrs corresponded well with measured data.
Döring-Mederake (1991) measured rates between 20 and 91 kg N ha −1 a −1 ,
which are in accordance with the range of our simulated values (25–80 kg
Nha −1 a −1 ). Due to higher simulated N-leaching and denitrification rates,
alder carrs can be considered higher nitrogen sources than tall and small sedge
reeds. Efficiency measures in alder carrs reached the highest values among all
investigated systems.
Simulated NNM, N-leaching, and denitrification rates for tall and small sedge
reeds (2A, 2B, and 3) were low and carbon as well as nitrogen balances were positive
or weakly negative. Measured carbon balances of these systems were not
available but measurements for the nitrogen budget (Koerselman and Verhoeven
1992) agreed with the simulated nitrogen balances.