De Levende Natuur nummer 3 van 2007 (English summary)


Restauratie van natte schraallanden


DLN 2007-3

Restoration of wet meadows in The Netherlands

Schipper, P.C. & B.F. van Tooren

This issue of De Levende Natuur describes the results of 15 years of research on the restoration of wet meadows in The Netherlands. Almost everywhere the possibilities for restoration of the formerly species rich grassland vegetations depends on recovering the former hydrology. In many reserves changes in the regional hydrology prevent the supply of base rich ground water. Especially in brook valleys and in the peat areas the possibilities for regeneration of the former hydrology are poor. The best chances for restoration are in the dunes. On the Wadden islands species rich dune valleys still exist; along the main coast many valleys have been restored although rare species are frequently still absent. In peat areas removing the top soil and creating small ditches in the grasslands can help to remove rain water and to increase the influence of buffered water but the results are generally poor. In brook valleys the same measures are applied. They can be very successful but the original species richness is rarely obtained.

The situation is even worse for insects. They need much variation in the structure of the landscape. Mostly this is absent; the isolated grasslands are surrounded by intensively used agricultural landscapes. For the future the Habitat and Bird Directive can be relevant in assuring a better hydrology in, and especially in the surroundings of, the reserves.

De Pelterheggen, historic irrigation meadows

Ploeg, N. van der

In the Kempen region (southern Netherlands) irrigation of former wet heathlands with base-rich water of the river Meuse has occurred from the second half of the nineteenth century. Irrigation aimed at growing hay for horses and later for cultivation of poplar. The flooding system is ingenious and consists of a supply canal, supply ditches, trenches from which the water flows over the soil surface and discharge ditches, in which the irrigation water is collected and discharged after flooding. Irrigation finished ca 1960, but was reintroduced in 1984 after the area had been acquired as a nature reserve. The former irrigation meadows were overgrown by tall forbs, but irrigation (two to three times a year) and hay making (two times a year) have resulted in the recovery of species-rich fen meadows (alliance Calthion). Therefore, other former irrigation meadows with poplars will be restored to hay meadows after tree cutting and reconstruction of the former irrigation system.

Does surface water spill over trigger eutrophication of fen meadows?

Kemmers, R.H., A.P. Grootjans, M. Bakker, G.J. Baaijens, J. Nijp & G. van Dijk

In The Netherlands many fen meadows are seriously affected by drainage and subsequent acidification. A field experiment was conducted to investigate whether the old farmer's practice of meadow irrigation could be an effective restoration measure to rehabilitate the threatened Calthion communities of these meadows. The main aim of irrigation was to restore the base saturation of the meadow without triggering internal eutrophication by reductive dissolution of iron oxides and phosphorus mobilization. Over a period of six years several short periods of water inlet to a sink was facilitated by an electric pump during winter. The water deposited its silt in the sink and superficially flooded the meadow. A control section was present, which was kept unflooded. In both the flooded and unflooded sections we measured the deposition of silt and organic matter, including some element contents. To unravel the combined effects of lime, iron and organic debris supplied by the inundation water we also conducted a full factorial block experiment. Prior to the inundation experiment crop productivity appeared to be co-limited by potassium and nitrogen. After six years regular winter flooding appeared to have a very positive effect on the vegetation. However, instead of the rehabilitation of a Calthion community a weakly acid small sedge community developed, with only a few characteristic species of the Calthion. The crop productivity increased by supply of dissolved potassium in the flood water and subsequent elevation of the potassium limitation. In the flooded section 178 kg more organic debris was brought in per hectare during a winter period. Significantly more iron oxides, probably originating from iron bound to organic matter deposited in the flooded section. Flooding resulted in an increased proton neutralization capacity and a slightly better pH buffering by import of iron oxides, consuming protons upon anaerobic conditions. Increased levels of iron oxides probably are a key factor in understanding an observed decrease of P mobilization by an increased phosphate adsorption capacity upon flooding. We concluded that prospects exist to apply artificial irrigation as an effort to prevent further acidification of Calthion communities without triggering internal eutrophication.

The most wet grasshoppers of The Netherlands: Stethophyma grossum and Chorthippus montanus

Reemer, M. & R.F.M. Krekels

The management of fen meadows in The Netherlands traditionally focused on the conservation of flora and vegetation. The underlying assumption was that the characteristic fauna of this habitat also benefits by measures taken to protect the flora. Recently, however, conservational and governmental institutions have become increasingly aware of the apparent gaps between the interests of flora and fauna. This awareness triggered the Dutch Province of Gelderland to finance a Species Protection Plan for the Large marsh grasshopper (Stethophyma grossum) and the Water meadow grasshopper (Chorthippus montanus). Both species are on the Red List of threatened species in The Netherlands. This paper describes the ecology and distribution of these grasshoppers in The Netherlands, especially in Gelderland. Possibilities for habitat management are discussed. A key factor in the recommended measures concerns the mowing of the vegetation. Instead of mowing the entire area each year, a system of partial mowing is highly recommended. Yearly a different part of the area (covering 10 - 20 % of the total surface) should be left unmown, in order to provide the grasshoppers and their eggs some shelter during the winter. Other important recommendations include mowing as late in the season as possible (end of August, early September) and removal of superfluous shrubs and trees to avoid overgrowing. The authors believe that these measures will also be beneficial to other fauna of fen meadows.

Fen meadows in the Natura2000-framework

Schipper, P.C. & C.J.S. Aggenbach

Fen meadows, both naturally and semi naturally, occur in seven habitat types. Five species listed in Annex II fully rely on wet meadows. The conservation status of the relevant habitat types is unfavourable.

Implementation of the Habitat- and Bird directive in The Netherlands is done in several steps. The Natura2000 targets document is a policy document of the Dutch Ministry of Agriculture, Nature and Food Quality. It sets out the framework for the designation decisions for the 162 Natura2000 sites and also lays down guidelines for the Natura2000 management plans to be drawn up subsequently. Both are important steps in the implementation of the European Natura2000 network in The Netherlands.

In formulating conservation objectives at site level, account was taken of current quality as well as the options for maintaining or creating a sustainable situation in the long term. Maintenance targets were assigned, for example, if the habitat of a species is already up to standard. Or if it is not possible to improve the ecological conditions any further, or the effort is not counterbalanced by the resulting additional contribution that the site could make towards achieving the Natura2000 target at national level. The choices were based on the analyses and the site-specific information. For example, the KIWA & EGG reports (2005) provide information about opportunities and challenges for habitat types in specific sites.

National legislation and realisation of restoration measures of fen meadows

Elbersen, J. & P.C. de Hullu

European (Bird, Habitat Directive and Water Directive) and national legislation create a responsibility for The Netherlands to protect several habitat types sustainably. For The Netherlands one of the most important types are fen meadows. We conclude that European and national legislation on nature conservation and water management are sufficient to protect this habitat type in a sustainable way. However, the biggest problem is the realisation of restoration measures in- and outside the Natura2000 areas. Measures outside the areas, in particular, can affect private landowners in a negative way. Natura2000 policy and measures might also interfere with local and provincial planning. In The Netherlands the Provinces have the task to realise the Dutch Natura2000 goals. Therefore, adequate provincial supervision is required to really realise the national targets. We have discussed the present situation of the Nature2000 area "Bennekomse Meent" in order to illustrate the complexity of the decision making process.

Restoration of fen meadows on peat soil by top soil removal and rewetting

Grootjans, A.P., R.H. Kemmers, F.H. Everts & E.B. Adema

In The Netherlands much experience exists with restoration of fen meadows and heathland on mineral soils. Sod cutting and even top soil removal of former agricultural soils has also been applied on a large scale. Little experience exists with top soil removal of desiccated peat soils.

Yet, within nature reserves large areas exist with desiccated peat soils where the goals of nature conservation are not met. We monitored three restoration projects aimed at restoring nutrient-poor fen meadows (Calthion palustris and Cirsio-Molinietum) by applying top soil removal combined with rewetting. Two projects are situated in small river valleys on the Drenthian Plateau, while another project is situated in a low-lying polder area in the province of Friesland, which is surrounded by deeply drained agricultural fields.

In the two groundwater fed river valleys the results of the topsoil removal were quite successful. In both cases many target species established after 12 years. There were differences in the speed of recovery. In one project the recovery was retarded for at least 6 years, because the discharge of groundwater was insufficient during that period. After further closing down of drainage ditches combined with relatively wet years the target communities developed rather well. In the acidified fen meadow in the polder areas restoring the discharge of groundwater to the reserve was not possible, since the surface of the peat soils in the surrounding agricultural areas had dropped so much due to long term drainage that the nature reserve had turned into a permanent infiltration area. Flooding with purified surface water (through a helophyte filter) had a positive effect on some target species, but it was insufficient to restore the original fen meadow vegetation. It was recommended that the restoration goal should be adapted to the new conditions. An interesting observation was that in two areas topsoil removal led to the surfacing of a layer which was rich in pyrite (FeS). When conditions were too dry, in particular during summer, this led to oxidation of pyrite and subsequent strong acidification of the new top soil. When the discharge of well-buffered groundwater increased this acidification stopped.

Hoverflies (Diptera, Syrphidae) of fen meadows
Steenis, W. van
Hoverflies are among the most numerous insects in fen meadows. Their life histories differs amongst species, but in their live cycle they do not solely depend on the meadows, but also on various other habitats in the landscape. Vegetation of ditches, water quality and flowering willows are important as well. In The Netherlands hoverflies of fen meadows show a significant decrease in distribution and numbers. This is mainly due to strongly intensified agricultural use of the grasslands. The numbers of hoverflies in existing nature reserves could be increased if the managers would leave about ten percent of the vegetation unmown.

Restoration measures in fen meadows on mineral soils

Jansen, A.J.M., C.J.S. Aggenbach, A.T.W. Eysink & D. van der Hoek

In The Netherlands species-rich fen meadows on mineral soils are endangered by desiccation and subsequent acidification and eutrophication. Restoration measures (forest removal, hay making, sod cutting, rewetting measures such as infill of ditches and discharge of rain water by trenches) were carried out in four fen meadow reserves in the eastern part of The Netherlands. Inside the reserves such measures have resulted in the survival of the fen meadows and their endangered species, while on control locations without measures the number of these species decreased. Hay making retards the deterioration of the species number of slightly desiccated fen meadows. However, on the longer term characteristic, basiphilous species will disappear due to gradual acidification or internal eutrophication. Sod cutting (after tree removal) promotes the restoration of (acidiphilous and calcareous) small-sedge marshes and fen meadows, and the temporal return of rare pioneer species. Sod cutting in combination with rewetting measures has resulted in a (more) prolonged return of endangered species. Measures in or nearby the reserves do not guarantee restoration on the long term when summer water tables have become too low or discharge of base-rich groundwater too limited. Further, the discharge of rain-water by trenches is under such circumstances not successful any longer. Then, hydrological restoration measures outside the reserves are inevitable.

Fen meadows and leaf beetles (Coleoptera: Chrysomelidae)

Beenen, R.

Characteristic species of this habitat in The Netherlands are listed. The threats of some of the vulnerable species, Cryptocephalus decemmaculatus and Longitarsus holsaticus, are discussed. Suggestions for the management of this type of habitat are given. Apart from rotational management, as a solution to the problem of satisfying multifarious conservation priorities, it is necessary to admit a sparse growing of young birch and willow to conserve or rehabilitate populations of Cryptocephalus decemmaculatus.

Restoration measures in strong acidified fen meadows

Beltman, B.G.H.J. & A. Barendregt

In slightly disturbed areas restoration measures are mostly successful. But what are the restoration perspectives of highly degraded fen meadows e.g. strongly acidified fen meadows with a poorly developed seed bank? Such degraded fen meadows are rather common in the lower parts of The Netherlands. The perspective is illustrated for the Ilperveld area, an acidified and eutrophied fen, where the following restoration measures were carried out: improved supply of base-rich surface water, sod cutting and their combination.

On the short-term all measures resulted in an increased species-richness. Moreover some characteristic species (Viola palustris, Hydrocotyle vulgaris) did recover. Merely sod cutting showed to be unsuccessful on the long term, due to the rapid recovery of the acidiphilous bryophyte species Polytrichum commune and Sphagnum phallax, which outcompeted other mosses and phanerogams. The enormous lateral resistance of peat soils against the penetration of surface water, limits the effect of hydrologic measures to small zones of c. 1m along the water courses. Comparable results were found in several other fen areas. Therefore, it is concluded that the combination of hydrological measures and small scale sod cutting leads to the recovery of a higher base status of the soil and therefore (more) species-rich fen meadows in small zones along water courses.

Butterflies of fen meadows

Swaay, C.A.M. van

The Netherlands used to be a very wet country. Characteristic butterflies of fen meadows were widespread. Because of reclamations before 1950 the area of suitable habitat for specialists like Euphydryas aurinia and Lycaena hippothoe already was too small to maintain vitable populations. After 1950 almost all characteristic butterflies were swept out of the Dutch agricultural landscape, and are now restricted to nature reserves. But here the remaining small populations are also threatened by eutrophication and dessiccation.

On the short term the remaining populations must be protected by careful management of the sites. On the longer run sites must be connected to form large core populations.

Restoration of dune slacks

Grootjans, A.P., E.B. Adema, C.J.S. Aggenbach, F.H. Everts & A.J.M. Jansen

During the last decades, dune wetlands (dune slacks) along the Dutch coast have shown a decrease in biodiversity, due to eutrophication, desiccation and in general a lack of natural dynamics of wind and water. We report on the results of three restoration projects where sod cutting has been applied in combination with hydrological measures aimed at restoring groundwater discharge in the wetlands. Two restoration projects were situated in the Dutch Wadden Sea islands (Texel and Terschelling) and one was situated in the mainland dunes of Goeree in the south of The Netherlands. The restoration process has been monitored for 10-16 years.

The most successful project was sod cutting of 35 ha of dune slack vegetation in the Moksloot area of Texel after stopping of the groundwater abstraction in 1993. Hydrological conditions in the slacks ranged from rainwater fed, groundwater fed and surface water fed. In the (isolated) groundwater fed slacks the accumulation of organic matter was very low, the pH was high and the pioneer stage with many rare species was maintained for at least 13 years. The accumulation of organic matter and the vegetation was more eutrophic in the slacks fed by surface water. The pioneer stage was very short, but the species richness was very high. After about ten years the biodiversity declined again and also the rare species disappeared. The rainwater fed slacks showed the same tendency.

In the Goeree project restoration started after the infiltration of polluted surface water, used for the production of drinking water, had stopped. At present the surface water used for the production of drinking water is purified. The soils of this part of the coast are very calcareous and the sod cutting almost immediately led to the restoration of typical dune slack vegetation, especially at the upper parts of the height gradient. Further down eutrophic marsh species, such as Juncus subnodulosus and Phragmites australis expanded again. Apparently much phosphate is still present in the soil and the restoration process is influenced by too high concentrations of nutrients. After 10 years the species richness and typical dune slack species declined and mowing or renewed sod cutting is necessary.

Sod cutting on the island of Terschelling was less successful in a dune slack where hydrological conditions had changed due to planting pine trees in the infiltration areas and also due to retreat of the coast line. During the first 5-8 years removal of the top soil appeared quite successful, but after some 10 years all established target species disappeared again due to intensified summer precipitation. This example showed that restoration measures that were successful in the past may not be successful anymore when climate conditions change, in this case increased summer rains that flooded the slack almost every year.

Fen meadows with many Red List species

Barendregt, A. & B.G.H.J. Beltman

The types of vegetation with wet, buffered and nutrient-poor conditions are discussed for The Netherlands. In common the major species are small sedges (Carex sp.) and a number of grasses (e.g. Molinea caerulea); their biodiversity is rich with famous flowers like orchids, Gentiana pneumonanthe and Parnassia palustris. These vegetation types were dominating the western half of The Netherlands one century ago, but as a result of frequent application of fertilizers they disappeared. In the eastern half of the country these types were distributed in the brook valleys and lower parts in the region where groundwater discharged constantly. Special types are distributed in the wet dune slacks. All these locations have in common that they are wet (especially in the winter), the soil is buffered by calcium from surface water flooding or groundwater discharge, and the absence of nutrients is limiting the biomass production. Those conditions became rare in The Netherlands and consequently these vegetation types; in the end most represented species became Red List species. This issue will discuss several possibilities to restore these types of vegetation.