Restoration of a large bog remnant in the Netherlands
(Bargerveen) - an eco-hydrological approach
A.P. Grootjans1, A. Jansen & H. Joosten
1Institute
of Energy and Environmental Studies, University of Groningen/Institute of Water
and Wetland Research, Radboud University of Nijmegen, the Netherlands.
Address: IVEM, University of Groningen, Nijenborgh 4, 9747 AG, Groningen, The
Netherlands.
e-mail: [email protected]
SUMMARY
All bog remnants in the Netherlands are protected and in most of them restoration activities have
been carried out already for many years. One remnant that is presently being restored is the
Bargerveen bog reserve (2200 ha) in the northern part of the country. The restoration approach
includes the construction of a large network of dams and dikes to keep precipitation water in the
reserve. The former dome-shape of the bog is reconstructed by creating a system of terraces. Large
hydrological buffer zones have been and are being installed on agricultural fields that formerly
belonged to the bog expanse. The restoration of the bog has been a very costly (100 million euro?)
and long-term enterprise that only in the last decade has led to the re-establishment of Sphagnum
bog vegetation over a substantial area. This paper also discusses some ecological mechanisms that
restrict Sphagnum growth, including the impact of high atmospheric nitrogen deposition and CO2
limitation in large water basins.
INTRODUCTION
Within the Netherlands the Bargerveen Reserve is one of the largest bog remnants and also the one
where restoration is the most promising. The Bargerveen Reserve is a very small remnant (22 km2) of
the former Boertanger Moor (3,000 km2), which once formed the border between Germany and the
Netherlands (Fig. 1). The development of the bog complex was studied in detail by Casparie 1972).
Figure 1.Left: The location of the former Boertanger Moor bog complex (yellow) and the present Bargerveen
(green) on the border of Netherlands and Germany. Centre: The former extent of the Boertanger Moor with the
Bargerveen (red). Right: the present Bargerveen Reserve with the Meerstalblok (red).
Figure 2 shows an impression of the southern part of the former Boertanger Moor, where several
raised bog domes had merged to form one large bog complex. A small river (Runde) was exporting
excess surface water towards the river Hunze. Between the domes a large dystrophic lake (Zwarte
Meer = ‘Black Lake’) was present.
Bog vegetation
Open water
Peat layers
Mineral soils
Figure 2. Impression of the southern part of the former Boertanger Moor consisting of various large bogs that
has merged to one large bog complex.
Because the bogs were dome-shaped, precipitation water flew from the top of the bog through the
acrotelm to lower areas. Infiltration into the mineral underground was almost totally prevented by
the presence of boulder clay layers in the subsoil and the almost impervious lake sediments and
other organic layers with a high resistance to water flow. The bog complex was not only fed by
precipitation water: locally groundwater from surrounding sandy ridges could enter the complex
through so-called ‘hydrological windows’ where the boulder clay layers had eroded away (Casparie &
Streefkerk 1992).
Meerstal
Figure 3. Impression of the cutover and largely reclaimed peatland with in the lower corner the small bog
remnant Meerstalblok before the start of restoration.
Only a very small part of the Dutch side of the Boertanger Moor has not been subject to peat
extraction. This area, the Meerstalblok, was the only place where typical bog vegetation and a small
bog lake (in Dutch ‘meerstal’) had remained before the area was declared a nature reserve in 1969.
The bog remnant is situated 4 meter above the surrounding area, where large scale peat extraction
in the 19th and 20th century has left some small strongly desiccated bog remnants amidst areas with
shallow remaining peat layers that were transformed into pastures and arable fields (figure 3 and 4)
Bog vegetation
Molinia dominated heath
Open water
Agricultural fields
Peat layers
Mineral soils
Figure 4. Impression of the present bog reserve Bargerveen showing restored bog vegetation around the core
reserve (Meerstalblok in the foreground) and large flooded buffer zones on former agricultural land.
Since 1971 a network of smaller and inceasingly large dams and dikes have been built in order to rise
the water levels in the reserve (Fig. 4). Because of the very large differences in height between the
areas with thick remaining peat layers and the cut-over areas, nearly all low lying areas became
almost permanently flooded. In the central part of the reserve the regrowth of Sphagnum and
Eriophorum species was good, but also the grass Molinia coerulea expanded massively after
rewetting. Experimental research in the field and in the laboratory revealed that the vigorous growth
of Molinia was caused by the high nitrogen deposition from the air (Limpens et al. 2003, Thomassen
et al. 2003). In the Bargerveen area the atmospheric N-deposition is 20-40 kg N ha-1yr-1 (Velders et al.
2013).
Figure 5. The central part of the bog reserve Bargerveen showing the restored bog vegetation growing at
almost the same height as the peat dikes that have been built to rise the water level with several meters. Photo
by Andre Jansen, Dec 2014).
This nitrogen originates as NH3 from agricultural areas with intensive cattle breeding and partly as
NOx from industrial areas in Germany and from traffic. Critical loads for bogs are 5–10 kg of N ha-1yr-1
(Bobbink et al. 1998), above which the Sphagnum species are unable to absorb the nitrogen (Lamers
et al. 2000) and nitrogen becomes available for vascular plants such as Molinia caerulea and Betula
pubescens. Above 18 kg of N ha-1yr-1 the vascular plants start to shade the Sphagnum plants to the
extent that their growth is reduced considerably (Limpens 2003).
Experimental research has shown that Sphagnum re-establishes well when after rewetting the
peat either swells up to the new water table or the upper peat layer starts to float, creating high and
stable water levels. In case of open water Sphagnum starts floating when sufficient methane and
carbon dioxide is produced by the underlying substrate (Lamers et al. 2002, Tomassen et al. 2003),
which may consist of peat or even litter formed by grasses or trees. Sphagnum mats keep floating for
a long time in case little decomposed Sphagnum peat is present (Tomassen et al. 2004) or when
slightly calcareous groundwater enters the peat from below and stimulates decomposition (Malmer
& Wallén 1993, Smolders et al. 2003).
In the Bargerveen Reserve the rapid growth of grasses and shrubs has been suppressed by
introducing sheep grazing in the area. This has stimulated the growth of Sphagnum to the extent that
in the best rewetted areas Sphagnum has become dominant and grazing is no longer necessary. The
final touch to rewetting of the central parts of the reserve was given by the establishment of large
buffer zones on agricultural land around the reserves, where very high water levels were installed.
The farm land was bought, mostly by the government, and even a nearby chicken farm was closed
and rebuilt much further away from the reserve. Figure 5 shows that as a result the high peat dikes in
the central parts of the bog remnant are almost overgrown by bog vegetation. However in the lowlying areas with shallow peat layers and with permanent flooding Sphagnum growth is limited.
Sphagnum indeed does grow under water during spring and summer (Fig. 6), but mostly dies-off
within a year.
Nutrient concentrations in the water are high enough to enable Sphagnum growth, but the
submerged mosses do not form floating maps, possibly because the black peat at the bottom of the
large ponds is highly decomposed and produces insufficient CO2 and methane to keep the Sphagnum
mats floating (Tomassen et al. 2004). Furthermore the growth of submerged Sphagnum is not fast
enough to enable the formation of a floating Sphagnum mat on which hummock species could
establish to induce a rapid bog growth. A possible explanation of this poor growth may the limited
CO2 concentration in the water. Patberg et al. (2013) showed that the growth of submerged
Sphagnum is severely limited when CO2 concentrations in the bog water are less than 3000 μmol l-1
(fig. 7).
Figure 6. Submerged Sphagna growing in the water of the low lying cut-over fields with only a shallow layer of
black peat left. (Photo: Andre Jansen).
Figure 7. Concentration of total inorganic carbon (which at the given pH is mostly CO2) in the soil water of
Sphagnum vegetation in small bogs that have regenerated well 25 years after rewetting (shaded) compared to
small bog pools that had poor regeneration over the same period (after Patberg et al. 2013).
In the laboratory Sphagnum can also grow under much lower concentrations when the water is
constantly mixed. In the field surface water, rainwater and possible inflowing calcium poor
groundwater mix much less. The input of CO2-rich groundwater combined with a modest flow of
surface water may increase the availability of CO2 for and thus the growth of submerged Sphagnum
substantially.
The total budget scheduled for the period 2013-2026 for further measures to restore the Bargerveen
bog relict amounts to about 34 million Euro, including 30 million for restoration measures and
hydrological buffer zones and c. 4 million for continued management of the reserve. These funds
come from the Provincial government, the national government, and the European Union.
The hydrological buffer zones surrounding the bog relic are on the Dutch side 500-800m wide. The
German authorities are planning a hydrological buffer zone of 300m wide at the eastern border of
the reserve. The hydrological buffer zones are largely established and planned on cut-over peatlands
with shallow layers of (black) peat that have been intensively fertilized and are extremely rich in
nutrients, in particular phosphate.
Figure 8. Left: deeply drained agricultural fields; right: peripheral hydrological buffer zone to keep the water
levels in the Bargerveen bog reserve high. (Photo: Andre Jansen).
After rewetting these formerly agricultural lands are highly productive and species like Reed
(Phragmites australis) and Common Cattail (Typha latifolia), or Willow (Salix) dominate the
vegetation within a few years. Harvesting these crops is possible with machines that are adapted to
driving in marshes and shallow water (Fig. 9). Such machines are currently used in the nearby brook
valley reserve Drentsche Aa, but not yet in the Bargerveen buffer zones.
Figure 9. Picture of mowing machinery that is able to drive on wetlands and even can mow shallow lakes.
Photos: Christian Fritz).
CONCLUDING REMARKS
Practical experiences and experimental research both in the field and in the laboratory have shown
that the re-establishment of bog vegetation is most successful when the water moves. This
movement can be very slow (as groundwater) but also more rapid as surface water or in the newly
formed acrotelm. Moving water usually has high concentrations of CO2, particularly in case of inflow
of groundwater with low concentrations of bicarbonate. High concentrations of bicarbonate are very
toxic for most Sphagnum species (Smolders et al. 2001, 2003).
In deeply inundated areas over black peat without input of CO2 from inflowing surface or
groundwater, the growth of submerged Sphagnum species is generally limited by too low
concentrations of CO2 (C-limitation). Furthermore, the strongly humified black peat does after
rewetting not produce sufficient methane and carbon dioxide to allow the formation of floating mats
of Sphagnum.
Many bogs have started as groundwater fed fens and the input of groundwater in bogs may still be
locally relevant. Discharging groundwater may not reach the active root zone of a bog vegetation,
but may be very relevant for supplying enough water for the restart of the bog regeneration.
Although it may stimulate the restart of inundated Sphagnum vegetation in flooded areas of the bog
complex. In case of bog restoration it is good to keep that in mind and not focus solely on
precipitation as the source of water.
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