Dune slacks are low-lying areas within dune systems that are seasonally flooded and where nutrient levels are low. They occur primarily on the larger dune systems in the UK, especially in the west and north, where the wetter climate favours their development when compared with the generally warmer and/or drier dune systems of continental Europe. The range of communities found is considerable and depends on the structure of the dune system, the successional stage of the dune slack, the chemical composition of the dune sand, and the prevailing climatic conditions.
Creeping willow is often found in dune slack vegetation and the boundaries between Humid dune slacks and 2170 Dunes with Salix repens ssp. argentea are often diffuse and difficult to define on the ground. While Humid dune slacks include creeping willow, the Annex I type excludes those sites where the species is dominant. It is variously associated with Yorkshire-fog Holcus lanatus andthe bryophytes Campylium stellatum and Calliergon cuspidatum. A further community is typified by silverweed Potentilla anserina and common sedge Carex nigra. In the UK the predominant NVC types include:
SD17 Potentilla anserina – Carex nigra dune-slack community
True dune slacks are fed mainly by rain water and are characterised by a pattern of pronounced annual fluctuation of the water table, related to the landform of the dune system as well as climate and the nature of the underlying sediment – whether porous shingle or impervious clay. Variations in the extent and duration of flooding of the dune surface are very important in determining the vegetation and influence the breeding of aquatic species, including the rare natterjack toad Bufo calamita. Humid dune slacks occur on calcareous sand, where the slack vegetation is similar to that of small sedge mires (mires with low-growing sedges), or on acidic dunes where the vegetation may have affinities to wet heath.
A range of other wetland types, especially swamp, mire and tall herb fen communities, occur on some dunes. These communities are not confined to dunes, although they comprise an important part of the mosaic of vegetation characteristic of dune slack and are dominant at a few dune sites, and can be included in the Annex I type. Some stands of the SD14 Salix– Campylium and SD15 Salix – Calliergon dune-slack communities are charcterised by the prominence of great fen sedge Cladium mariscus, and may be referable to Annex I type 7210 Calcareous fens with Cladium mariscus and species of the Caricion davallianae.
Dune slacks are often rich in plant species, particularly rare and local species. Several species, such as the Annex II 1395 Petalwort Petalophyllum ralfsii, 1903 Fen orchid Liparis loeselii, and round-leaved wintergreen Pyrola rotundifolia, are found mainly in this habitat type.
European status and distribution
Humid dune slacks are widely but locally distributed throughout the coastal zone of the EU.
Mainly owing to the cool wet climate of the UK, Humid dune slacks are a more prominent feature of dunes in the UK than in many other European countries, and the UK has a significant proportion of the EU resource. Dune slacks are widespread but local in the UK and the habitat type exhibits considerable ecological variation.
Barry Links is a virtually intact dune system, composed predominantly of base-poor sand on the east coast of Scotland. The slacks range from species-rich, open types to those with a closed canopy of scrub. The hydrology of the site is well-conserved and successional processes can be seen operating. The site has some morphological similarities to Braunton Burrows, though the range of communities is different owing to the different soil base-status and climate. The Humid dune slacks occur in a complex mosaic with other sand dune habitats, several of which have been proposed as Annex I habitat types in their own right.
Braunton Burrows is one of the largest virtually intact dune sites in the UK, with an exceptionally large area of Humid dune slack vegetation, representing a significant proportion of the national resource. The slacks have formed in base-rich sand and are rich in species such as marsh pennywort Hydrocotyle vulgaris, marsh helleborine Epipactis palustris and round-leaved wintergreen Pyrola rotundifolia. Vegetation types range from those with almost permanent water to those dominated by scrub. The site contains representative examples of most of the communities of base-rich humid slacks in south-west England, namely those characterised by creeping willow Salix repens ssp. argentea, those with bryophytes or those with Yorkshire-fog Holcus lanatus.
In total this site includes almost 100 ha of Humid dune slack, representing the largest area in Wales. Many of these dune slacks are very rich in species, including the rare Annex II species 1903 Fen orchid Liparis loeselii. A number of successional stages are present, ranging from embryonic to fully mature slacks, and the area is also notable for its interesting dune slack – saltmarsh transitions.
Morrich More in north-east Scotland is one of the largest acidic dune sites in the UK. The sequence of development has resulted in the formation of extensive humid slack communities of an acidic character which lie as parallel hollows between the dune ridges and form part of a complex mosaic of dune habitats, several of which have been proposed as Annex I habitat types in their own right. This is the most important acidic dune system in Scotland, owing to its size and the exceptional diversity of the habitats within it.
Studland Dunes is a large acidic dune system in south-west England with well-conserved structure and function. The site has been intensively studied. The structure and function of dune systems are well-represented with dune-building processes still active. These processes have resulted in the formation of acidic humid dune slack communities with a high water table, which lie in the parallel hollows between the dune ridges. In these slacks, acidic fen and reedbeds have developed. Some areas are dominated by grey willow Salix cinerea and birch Betula sp. carr with the very local royal fern Osmunda regalis a conspicuous element. The dune slacks are linked to an area of open fresh water known as the Little Sea.
Kenfig in south Wales contains the most important example of Humid dune slacks in the UK, owing to the extent of the habitat type and the conservation of its structure and function. These calcareous dune slacks are also amongst the most species-rich in the UK, supporting communities dominated by a variety of mosses and a number of rare plants, notably 1903 Fen orchid Liparis loeselii, for which the site is also selected. Some of the dune slacks on the site are still in the early successional stage of development.
Magilligan in Northern Ireland is one of the largest dune systems in the UK. It has an extensive and well-developed series of Humid dune slacks, which contain virtually all of the dune slack vegetation in Northern Ireland. The dunes are of an acidic character. A wide range of slack vegetation occurs on the site, including wet open dune slacks at an early successional stage, slacks dominated by creeping willow Salix repens ssp. argentea and older hollows filled with mire vegetation. The hydrology of the site is little modified and the structure and function of the site are therefore well-conserved.
Dune slacks are particularly well-represented at Sandscale Haws, the largest calcareous dune system in Cumbria. The slacks support a good range of vegetation communities and are very species-rich. Several uncommon species including marsh helleborine Epipactis palustris, dune helleborine Epipactis dunensis and coralroot orchid Corallorhiza trifida occur.
Morfa Harlech a Morfa Dyffryn (Morfa Harlech and Morfa Dyffryn) is one of two sites representative of dune slack vegetation in north Wales. Examples of three different humid dune slack communities have been recorded within the complex. The dune slack vegetation with silverweed Potentilla anserina and common sedge Carex nigra is particularly well-developed.
The slacks within this site are comparatively small and the Yorkshire-fog Holcus lanatus community predominates. The site represents Humid dune slacks on the dry east coast of England and present an extreme of the geographical range and ecological variation of the habitat within the UK. They are calcareous and complement the acidic dune slacks at Winterton – Horsey Dunes, also in eastern England. The dune slack communities occur in association with swamp communities.
North Northumberland Dunes represents a rare example of well-developed dune slack vegetation on the east coast of England. Holy Island contains a number of calcareous, species-rich dune slacks, which support a number of rare species, such as coralroot Corallorhiza trifida, dune helleborine Epipactis leptochila var. dunensis and seaside centaury Centaurium littorale. Active slack formation is continuing at this site and a range of successional stages are present. The humid dune slacks of nearby Ross Links contain vegetation typical of more base-poor conditions. The site as a whole therefore contains an exceptional range of humid dune slack types, including 40 ha of the full range of slack vegetation types characterised by common sedge Carex nigra.
Penhale Dunes in south-west England is an extensive and exposed calcareous dune system where active geomorphological and successional dune processes occur. Humid dune slacks with an interesting flora are well-developed in the northern section where they often form marshy areas or pools. The drier slacks support short, rabbit Oryctolagus cuniculus-grazed turf with species such as silverweed Potentilla anserina, common centaury Centaurium erythraea and pyramidal orchid Anacamptis pyramidalis. The damper slacks are colonised by taller herbs including meadowsweet Filipendula ulmaria, water mint Mentha aquatica, great willowherb Epilobium hirsutum and water horsetail Equisetum palustre. The dune slacks also support a number of uncommon plant species including populations of the Annex II species 1441 shore dock Rumex rupestris for which the site is also selected. Other low-lying wetlands within the site are important for sedge and fern-dominated communities.
The Humid dune slacks at this site are part of a successional transition between a range of dune features, and some have developed from saltmarsh to freshwater habitats after becoming isolated from tidal inundation by sand deposition. There is a range of different communities present, many of which are species-rich. The species present depend on the wetness of the slack, its location within the system and the management history. Some of the drier slacks support a very wide range of species; this has been encouraged by management. The wetter slacks often have more permanent standing water and are composed of stands of sedges and rushes.
Sands of Forvie represents Humid dune slacks in an acidic dune system in north-east Scotland. The dune slacks are mainly of an acidic type, with extensive areas of wet heath, characterised by crowberry Empetrum nigrum and cross-leaved heath Erica tetralix, and transitions to dune heath. Early stages in slack development occur, with the dune slack flora characterised by common sedge Carex nigra and marsh pennywort Hydrocotyle vulgaris. Higher zones have more creeping willow Salix repens ssp. argentea, which tends to invade and replace the wet heath.
Sefton Coast is a large area of predominantly calcareous dune vegetation, containing extensive areas representative of Humid dune slacks in north-west England. Some active slack formation can still be seen and a variety of successional stages are represented. The sequence from foredunes to dune grassland and dune slack is extensive. The site contributes to the range and variation of humid dune slack vegetation, being a large and representative base-rich system towards the northern limit for some humid dune slack communities along the west coast of Britain.
The slacks within Winterton – Horsey Dunes are chiefly of interest because they occur on an extremely base-poor dune system on the dry coast of East Anglia in eastern England. Because of their acidic soils, the dunes support swamp and mire communities, in addition to small areas of typical dune slack vegetation characterised by creeping willow Salix repens ssp. argentea with Calliergon cuspidatum and Yorkshire-fog Holcus lanatus. As a result they represent an extreme of the geographical range and ecological variation of Humid dune slacks within the UK.
Abermenai to Aberffraw Dunes represents Humid dune slacks in north Wales. There are large areas of open dune vegetation and many Humid dune slacks remain, although there have been changes in the water table that are partly attributable to the growth of the commercial forest. The changes have influenced the development of humid dune slacks, which nonetheless retain most the essential features of the habitat type.
SACs where this Annex I habitat is a qualifying feature, but not a primary reason for site selection