2140 Decalcified fixed dunes with Empetrum nigrum

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.