Habitat account - Coastal sand dunes and continental dunes
2140 Decalcified fixed dunes with Empetrum nigrum * Priority feature
Background to selection
|Distribution of SACs/SCIs/cSACs with habitat 2140 Decalcified fixed dunes with Empetrum nigrum. Click image for enlarged map.|
Description and ecological characteristics
Decalcified fixed dunes with Empetrum nigrum represent the later, more mature, stages of the well-marked successional sequence characteristic of sand dunes. Exposure to rainfall over long periods means that there is leaching of the surface layers, causing a loss of calcium carbonate and increased soil acidity. Where the shell content of the sand is low, the original calcium carbonate content of the soil will be low and acidic conditions develop more rapidly. This is particularly the case in the north and west, where a combination of a wetter climate and a more widespread occurrence of silica sand encourages the development of more extensive areas of acidic dune vegetation. In such conditions, dune communities tend to be dominated by heather Calluna vulgaris and crowberry Empetrum nigrum, with the relative abundance of these two species varying with site conditions. In the UK this corresponds to NVC type H11b Calluna vulgaris – Carex arenaria heath, Empetrum nigrum ssp. nigrum sub-community.
The nature of dune heath varies considerably depending on physical conditions, degree of leaching, type of substrate, geographic position and grazing intensity. Fixed dune vegetation tends to occur on the larger dune systems, which have the width to allow it to develop. Decalcified fixed dunes with Empetrum nigrum tend to occur in mosaics with other habitats, depending on local physical and soil conditions, and often occur in association with 2150 Atlantic decalcified fixed dunes, 2130 Fixed dunes with herbaceous vegetation ("grey dunes"), dune heath, wet heath, 2190 Humid dune slacks, and acidic grassland. The last of these forms transitions with dune heath, and in drier conditions acidic grasslands are often precursors to its development.
Grazing helps to maintain the open nature of the vegetation, which would otherwise develop into scrub and woodland through the process of succession. However, it is vulnerable to overgrazing, and planting of trees can lower the water table, which in turn will suppress open dune heath vegetation.
The Annex I habitat types Decalcified fixed dunes with Empetrum nigrum and 2150 Atlantic decalcified fixed dunes (Calluno-Ulicetea) are similar in composition. Decalcified fixed dunes with Empetrum nigrum has a more restricted distribution, being found mainly in Scotland in relatively wetter and more base-poor conditions; 2150 Atlantic decalcified fixed dunes is more widespread, tolerating a wider range of conditions. At some Scottish sites it is very difficult to allocate stands of dune heath to one Annex I habitat type or the other, as the vegetation forms a continuous spectrum of variation within complex habitat mosaics. The two types may also succeed one another in the same location over time, and vegetation intermediate between NVC types H11a Calluna vulgaris – Carex arenaria heath, Erica cinerea sub-community and the Empetrum nigrum ssp. nigrum sub-community (H11b) has been recorded in Scotland.
European status and distribution
Outside the UK, Decalcified fixed dunes with Empetrum nigrum are restricted to coasts northwards from Denmark.
UK status and distribution Click to view UK distribution of this habitat
In the UK, Decalcified fixed dunes with Empetrum nigrum are confined to Scotland, where this type is widespread but local.
Site selection rationale
Sites have been selected to represent the range of geographic and ecological variation within the habitat type. A substantial proportion of the national resource of Decalcified fixed dunes with Empetrum nigrum occurs on a relatively small number of sites, and the largest and most important examples of this priority habitat type have been selected. For the most part, sites selected are also those least-modified by human activities, so that they are inherently stable in terms of structure and function.
|Dornoch Firth and Morrich More||Extra-Regio, Highlands and Islands|
|In this part of the Moray Firth, north-east Scotland, dune vegetation has developed on a coastline that has been generally rising relative to sea level in the 7,000 years since the last glaciation. A combination of leaching, stabilisation and the decreased influence of salt water has produced a sequence of dry, stable dune ridges, interspersed with wet dune hollows. There is a large area of Decalcified fixed dunes with Empetrum nigrum vegetation on this site, occurring in a complicated mosaic of acidic fixed dune vegetation types, principally 2150 Atlantic decalcified fixed dunes (Calluno-Ulicetea). Within this complex of habitats there are examples of dune, saltmarsh and transitional communities that include large populations of several northern dune species, such as Baltic rush Juncus balticus. This is the most important acidic dune site in Scotland because of its size and the exceptional diversity of habitats within it. Despite some localised industrial development, structure and function are well-conserved at this site and accretion is continuing.|
|Sands of Forvie||North Eastern Scotland|
|This large and mature site on the east coast of Scotland includes a sequence of decalcified, fixed dune ridges that have developed over a period of several hundred years. The dune heath complex within the site can, in this case, be confidently defined as Decalcified fixed dunes with Empetrum nigrum, because crowberry Empetrum nigrum is widespread throughout the heath. It is one of the best examples of the type in the UK because of the extent and diversity of the crowberry community and the diversity of transitions to other habitats. The free-draining heath is interspersed with a number of wet hollows in which important acidic examples of Annex I type 2190 Humid dune slacks are present, typically supporting cross-leaved heath Erica tetralix, for which this site is also selected. There are transitions to acidic dune grassland and wet heath, which grades into dune slack.|
SACs/SCIs/cSACs where this Annex I habitat is a qualifying feature, but not a primary reason for site selectionNot applicable.
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