Environmental Engineering Reference
In-Depth Information
Most traditional nature-conservation organizations
aim to rehabilitate damaged mires or closely related
systems. Wheeler and Shaw (1995b) described such
activities as repairing a system. Such activities con-
sist of raising the water tables by removing drainage
ditches, terminating groundwater-abstraction facilities,
reinstalling traditional management, etc. Sometimes
part of the threatened vegetation type is transplanted
to another site. When a wetland system is damaged
beyond repair, restoration may end in rebuilding the
system to a state that probably has never existed before.
Most large nature-development projects are actually
close to (re)building a new wetland ecosystem in
areas that have been abandoned by agriculture,
industries or other land users. Rehabilitation of a
former stage of ecosystem development is often not
an option any more, since nothing of the former
stages has remained.
et al.
(2001) and Lamers
et al.
(2002) showed that
Sphagnum
growth is stimulated by addition of small
amounts of HCO
3
−
. The HCO
3
−
dissociates into CO
2
and
provides the
Sphagnum
with an additional C source.
Concentrations between 0 and 1 mM stimulated the
growth of
Sphagnum
, while concentrations above
1 mM restricted
Sphagnum
growth. Limpens
et al.
(2003) showed that high atmospheric N deposition
(30 -35 kg of N ha
−1
yr
−1
) reduces the growth of ombro-
trophic
Sphagnum
species. Tomassen
et al.
(2004a) also
showed that elevated atmospheric N deposition could
favour the growth of
M. caerulea
and
B. pubescens
.
Above 18 kg of N ha
−1
yr
−1
Sphagnum
species cannot
take up all the NH
4
+
from the soil solution and the
nitrogen becomes available for vascular plants, which
start to shade the
Sphagnum
plants, thus reducing
their growth considerably. Water-table fluctuations
in large bog remnants are usually sub-optimal for
Sphagnum
growth, due to the absence of a flexible
acrotelm. Furthermore, water tables around the bog
remnants are usually very low due to agricultural
drainage. Building large dams in and around the bog
remnants can decrease the water-table fluctuations.
To summarize, the restoration in large bog remnants
in north-west Europe is usually slow due to high water-
table fluctuations (> 25 cm), high atmospheric N
deposition, a very low pH and low CO
2
concentrations
in the pore water.
Kettle-hole restoration was quite successful in
north-east Germany (Jeschke & Paulson 2001). After
felling woods and closing drainage ditches,
Sphagnum
growth proceeded rapidly and within 5 -10 years
extensive
Sphagnum
carpets developed. Rapid regen-
eration of
Sphagnum
growth was also observed in
small bogs within a Dutch heathland area (Grootjans
et al.
2003). Here the closing of drainage ditches not
only raised the water tables in the small bogs, but
it also stimulated water flow above still-existing
water-impervious organic B horizons (consisting of a
layer of fine, dispersed organic material that has been
transported by water movement to deeper layers).
These thin, organic layers connected several small bogs
(Plate 9.4), providing them with additional water with
a high CO
2
concentration. These shallow flows over
and through organic soil layers not only reduced
water-table fluctuations and increased the pH, but also
provided the sphagna with an additional C source (elev-
ated CO
2
concentrations in the flowing water).
9.5.1 Restoration of rainwater-fed bogs
In north-west Europe and in North America restora-
tion efforts include attempts to restart peat growth
in the large bog complexes where peat has been
extracted for commercial use. Restoration of such flat
surfaces has proven to be very difficult. Simply
rewetting remnants of large bog complexes usually does
not result in renewed
Sphagnum
growth (Vermeer
& Joosten 1992). Shallow flooding often results in much
fluctuation in water levels, and the topsoil dries out
again in dry years, unless renewed
Sphagnum
growth
is able to start from floating rafts of vegetation
(Tomassen
et al.
2004b). Wheeler and Shaw (1995b)
and Wheeler
et al.
(2002) present an overview of tech-
niques and approaches to bog restoration in north-
west Europe and North America. Often we can see a
rapid growth of
Sphagnum
species in early stages of
bog formation, particularly in small peat-cut pits
(Lütke-Twenhöven 1992), but the development of
late-successional, bog-building sphagna is often very
poor or non-existent. Several studies showed that in
the first stages of peat regeneration
Sphagnum
plants
need support of tussock-forming species, such as
Eriophorum vaginatum
or from perennial species
such as relatively tall
Carex
species that can form
many erect shoots, without shading the
Sphagnum
plant
too much (Pfadenhauer & Klötzli 1996). Smolders
