Environmental Engineering Reference
In-Depth Information
cipitation (
external eutrophication
). Atmospheric nitro-
gen and sulphur deposition have a very negative effect
on almost all low-productivity wetland ecosystems.
The concept of critical N load has been developed to
indicate the annual amount of N deposition above
which there is a realistic risk that essential functions of
an ecosystem may be impaired (Achermann & Bobbink
2003 ).
Critical nitrogen loads
for bogs are 5-10 kg of N
ha
− 1
yr
− 1
, and for mesotrophic (unmanaged) terrestri-
alization fens they are 20-35 kg (Bobbink
et al
. 1998 ).
When the deposition of N increases, the
Sphagnum
species start to accumulate N in amino acids, or
mineral N is no longer absorbed and becomes available
for vascular plants such as
Molinia caerulea
and
Betula
pubescens
(Lamers
et al
. 2000 ).
nature development projects now in fact explicitly aim
at designing and creating an entirely new wetland
ecosystem.
In what follows, we will discuss restoration experi-
ences, and lessons learned, in western Europe and else-
where, with a focus on damaged nature reserves or
abandoned peat extraction areas.
16.4.1
Restoration of rainwater-fed bogs
In north-western Europe and in North America, many
restoration and rehabilitation efforts include attempts
to restart peat growth in large bog complexes where
peat has been extracted for commercial use (Wheeler
et al
. 2002 ; Money
et al
. 2009 ). Reinitiation of
Sphag-
num
growth in such fl at surfaces has proven to be very
diffi cult (Vermeer & Joosten 1992). Essential for succes-
ful bog restoration is to re-create a new functional
acrotelm (Money
et al
. 2009). Without that, peat
formation cannot occur. So, the fi rst step is to create
good conditions for very active
Sphagnum
growth. In
degraded bog remnants, these conditions are unfava-
vourable due to (1) too-high water table fl uctuations
(>25 cm), (2) high atmospheric N deposition and (3)
low dissolved inorganic carbon (DIC) concentrations in
the pore water.
16.4 RESTORATION APPROACHES,
SUCCESSES AND FAILURES
There is a growing awareness that
rewetting
(i.e.
increasing water levels) in drained peatlands not only
is benefi cial for protecting
biodiversity
but also
reduces greenhouse gas emissions. New economic
uses of peatlands, such as biomass cultivation for
biofuel, are now emerging, since many peatlands have
become unsuitable for modern agricultural production
(Pfadenhauer & Grootjans 1999). In many peat areas,
in Europe in particular, the maintenance costs and
subsidies paid by taxpayers are simply too high, and the
revenues too small, to allow this type of land use on
peat soils to continue.
For biodiversity conservation and other environ-
mental objectives, rewetting should preferably be done
with anoxic, unpolluted groundwater, because this will
conserve the peat and prevent further emissions much
more than rewetting with surface water (Smolders
et al
. 2010). Rewetting can be achieved by construct-
ing dams or fi lling in drainage ditches. Terminating
groundwater abstraction activities will also lead to
rewetting.
When restoration to the natural state is no longer
possible,
rehabilitation
of some wetland functions is
a more appropriate and realistic goal (e.g. Wheeler &
Shaw 1995). When a wetland is damaged beyond a
certain
threshold
of repair, rehabilitation efforts may
lead to remodelling the system towards a state that
probably never existed before, but is far preferable to
sheer abandonment. In areas that have been aban-
doned by farmers, industries or other land users, most
Too - h igh w ater t able fl uctuations
Water table fl uctuations in degraded bog remnants are
usually too large for
Sphagnum
growth, due to the too-
low storage capacity (specifi c yield) of the remaining
peat layers and because of water losses to surrounding
(drained) agricultural areas. Restoration measures in
such cases usually consist of increasing water levels
in the bog to over the surface by building large dams
in and around the bogs. Water table fl uctuations can
be largely reduced by such measures, but large parts
of the bog may become permanently inundated,
which can limit
Sphagnum
growth due to light limita-
tion and lack of carbon dioxide. Humic acids from the
remaining peat substrate lead to a condition known as
dystrophic (i.e. water that is poor in nutrients and
contains a high concentration of humic acid, with
limited penetration of light). Light limitation can thus
hamper the growth of submerged
Sphagnum
when
water depth exceeds 0.5 m (Wheeler & Shaw 1995).
Shallow fl ooding, however, often results in strong fl uc-
tuations of the water level and the drying out of the
