Atlantic and Baltic moist and wet dune slack

This is a very broad habitat, comprising all open and closed, low to tall, aquatic, marsh and helophyte vegetation of moist to wet depressions in coastal dunes of the Atlantic and Baltic coasts. It comprises dune lakes and ponds with open water and floating or submerged vegetation, pioneer communities on bare shores, calcareous and acidic fen communities, as well as beds of helophytic reeds and sedges. Within the communities of the hygrosere (the succession series in dune slacks) the late succession stages (scrub, forests) and wet heathlands are considered under other habitats (respectively  B1.6a, B1.7a and B1.5a). Also mesic grasslands in coastal dunes are not included in the dune slack habitat, although they may in some cases grow in mosaic with this habitat. On a higher level EUNIS makes a distinction between coastal and inland habitats, but mesic freshwater grasslands in coastal areas are included under inland types, for example grasslands of the Calthion palustris, Nardo-Juncion squarrosi, Cynosurion cristati or Molinion caeruleae. In coastal areas such grasslands are often associated with old, traditionally managed meadows or pastures, and are less characteristic for dynamic dune slacks. In many cases they are found on the transition from dunes towards the inland, in other situations they are part of old, traditional mosaic landscapes (like machairs, see B1.9).

Dune slacks may develop in two ways. In the first place wind activity may blow out sand, resulting in depressions up to the depth level of groundwater. These depressions are known as secondary dune slacks. Primary dune slacks are the result of new formed fore-dunes, which isolate a beach from the influence of the sea and thus create a depression with groundwater influence within two ridges. In the Wadden Sea area a rare, third situation occurs on so-called green beaches, found on the head or tail of barrier islands. Such beaches are irregularly flooded both from the North Sea and from the Wadden Sea direction, and have complex gradients of dune slack, salt marsh and dry dune communities.

The water table in dune slacks fluctuates strongly during the year, with relatively wet conditions in winter and spring, and sometimes extreme dry conditions in summer. Because of these fluctuations peat layers rarely develop. In the Baltic coast the water table of dune slacks fluctuates less, and here in some cases acidic mire communities or even bogs develop in dune slacks.

In very wet conditions the dune slack communities of this habitat may form azonal climax communities, but in most slacks the pioneer and mire communities, as well as sedge and reed beds show succession towards scrub (Salix) and forest (Salix, Betula, Alnus). Mowing or grazing is practiced to conserve species rich, intermediate and young succession stages, for example communities of the alliances Caricion davallianae and Caricion nigrae. However, although such management may slow down succession, due to other changes in this dynamic landscape species composition changes in time. For example coastal erosion or sedimentation will change the size and place of freshwater lenses, and in this way affect the vegetation of dune slacks. In calcareous dune slacks, due to high precipitation rates, decalcification takes places, leading to changes in species composition. For this reason, the species rich calcareous fen communities of the Caricion davallianae are best preserved in dynamic landscapes, where now and then new primary dune slacks are formed. They may survive in older dune slacks, but only in places with a high rate of seepage of calcareous groundwater.

As many of the vegetation types of dune slacks are also found inland, in similar or other habitats, few plant species are restricted to the Atlantic and Baltic dune slacks. Some coastal species have been described as different ecotypes (Parnassia palustris), others have relatively larger populations in coastal areas (Liparis loeselii, Sagina nodosa, Centaurium littorale, Potamogeton coloratus). Only a few species are mainly restricted to coastal areas, like Carex trinervis.

Along the Atlantic coast these dune slacks are found from Norway to northern Spain, with relatively large areas in the broad dunes of Jutland, the Wadden Sea, mainland Netherlands, northern France and southwestern France. In Ireland, England and the Baltic coasts the dune slacks are relatively rare and more widespread. In Spain their occurrences are marginal, while the dune slacks along the Portuguese Atlantic coast belong to the Mediterranean habitat B1.8b.

Indicators of good quality:

• No anthropogenic changes in water table
• Species richness
• Occurrence of rare and endangered species
• Broad diversity in different types of dune slacks and related plant communities
• Forming intact landscape mosaics with and transitions towards dry dune habitats

Synthesis of Red List assessment

Confidence in the assessment

Pressures and threats