British distribution: on all coasts but more sparsely distributed in the south and south-east.
World distribution: worldwide but less developed in tropical and subtropical zones (lower wind velocities and damper sand) (Pethick, cited in Packham & Willis, 1997).

Low dunes (a 'spit dune' forming a peninsula) on Holy Island, Northumberland, 1975.

Dunes can develop wherever there is an appreciable amount of blown, dry sand and where the physical nature of the site favours its deposition. The UK Biodiversity Action Plan for Coastal sand dunes (1999) recognises several physical types of dune, classified according to their location:

• bay dune
  where sand is trapped between two headlands;
• spit dune
  forming as a sandy promontory at the mouth of an estuary;
• hindshore dune
  where sand is blown some distance inland over low-lying ground;
• ness dune
  building out from the coast;
• tombolo
  where sand is deposited in a narrow connection between the mainland and an island, or between two islands;
• climbing dune
  where sand is blown up onto a cliff or high ground adjacent to a beach;
• offshore island dune
  where a small island has become the base for sand deposition.

Dune systems of any size will be diverse in terms of topography and plant community structure. Successional processes will be apparent and dune systems should be regarded as dynamic entities. Nevertheless, the interplay between building phases and erosion, varying nutrient status (especially proportion of calcium in the sand), past and present management, local climate and underlying topography may all preclude generalities and simple schemes of succession.

While the nature of dune systems varies considerably, the following components may be present:
 

• The strand line
  Extreme high tides may leave a zone of several metres landward of the normal high water mark. Seaweed and other jetsam will be left stranded along this zone and this material may include seeds of various specialist strandline plants - mostly annual species, often of the genus Atriplex (Oraches). Seeds amongst the litter will be protected from extremes of temperature (midday temperatures may be 20 ºC higher in open sand) and may also be protected from excess evaporation. Germination tends to be in April or May and the development and botanical composition of strandline communities varies considerably from one year to another.
  Development of strandline communities may be markedly affected by human activity, by direct disturbance from holiday makers and other beach users and, perhaps more significantly, by use of beach-cleaning machines. Some species, e.g. Sea Holly (Eryngium maritimum) have shown marked local declines that can be attributed to such disturbance, though some species have shown significant national fluctuations and declines that are difficult to relate to local conditions.
  Colonisation by Sand Couch (Elytrigia juncea) may occur, but growth in this zone tends to be too disturbed or haphazard to be regarded as a direct precursor to dune formation, or at least to any sand accumulation that will survive winter wave action and spring tides.
   
  

  Frosted Orache (Atriplex laciniata), a typical plant of the strandline, Holy Island, Nothumberland, 1975.

  
  

• Fore-dunes
  Above the highest tides, accumulated jetsam and growth of perennial grasses such as Sand Couch (Elytrigia juncea) (= Agropyron junceiforme) and Lyme Grass (Leymus arenarius) (= Elymus arenarius) will trap sand and may initiate dune formation. Ability to withstand temporary burial by sand and ability to grow back though such sand are essential characteristics for pioneer dune grasses. E. juncea can survive annual accretion rates of about 30 cm per year (Ranwell, 1972). Both of these species can withstand high levels of salt spray.
  Colonisation by Marram (Ammophila arenaria) will allow stabilisation of sand at higher accretion rates (see next section).
   
  

  Sand Couch (Elytrigia juncea), on fore-dunes, An Fharaid, Sutherland, 2001. The upright tufts in the background are pioneer plants of Marram (Ammophila arenaria).

  
  

• White dunes (a.k.a. Yellow dunes)
  Marram (Ammophila arenaria) is a large, very tough, tufted perennial grass that can withstand accretion rates of up to 100 cm per year. Together with its closely related American equivalent, A. breviligulata, it is the prime former of high sand hills. Open sand remains visible between and within the tufts, leaving the dunes showing yellow or white at a distance (depending on the colour of the sand and the proportion of shell fragments).
   
  

  Marram (Ammophila arenaria), An Fharaid, Sutherland, 2001.

   
  Sand tends to be blown from the windward face and to accumulate on the leeward side, resulting in:
  1. the characteristic steep windward face and more gently sloping leeward face;
  2. the tendency of the whole sand hill to move distances of one to several metres per year (sometimes engulfing other land or even buildings).
  Sand grains mostly travel in the wind close to the ground, being blown in a series of bounces. This process is known as saltation. The impacts of these grains as they hit the sand push forward other grains, that move by surface creep. Since the lighter grains move by saltation and the heavier grains by surface creep, this can have a significant sorting effect.
   
  Other plants on the high sand hills are usually few, though Leymus arenarius may continue and in northern Britain may be locally co-dominant with Ammophila (higher, more mineral-rich rainfall appears to be the main factor here). Sand Fescue (Festuca arenaria), another specialist dune grass, may spread by long rhizomes and further stabilise the surface where accretion levels are low.
  A moss layer, including Bryum algovicum and the specialist dune species, Tortula ruraliformis (see below), may also form on the sand surface, particularly in the more humid conditions of the lea side of a sand hill. These mosses have the same ability as other dune plants of being able to grow back up through accreting sand, and by doing so may hold together the surface for a depth of a few centimetres. This repeated upward growth to the dune surface is sometimes called 'flotation'.
  The sand hills are always potentially unstable. High accretion rates will overwhelm even Ammophila, and the hills are easily eroded by wind action, forming blowouts, or by even light human trampling. At Gullane, East Lothian, in the 17th century, collection of Ammophila for thatch destabilised the sand hills and resulted in sand engulfing part of the village.
   
  

  White dune, An Fharaid, Sutherland, 2001. The sand is shell sand, hence its dazzling white colour. Strong winds exacerbate human damage here and the dunes are severely eroded.

  
  

• Stabilised dunes
  A transition to dune meadow or dune heath. Ammophila may grow less vigorously and form a sward rather than separate tussocks, while other grasses, such as Sand Fescue (Festuca arenaria) may become more abundant and continue the stabilising process. The ground may be heterogeneous, with open areas remaining, often through rabbit action, and dune mosses such as Tortula ruraliformis (acrocarpous) and Brachythecium albicans (pleurocarpous) may be important. T. ruraliformis is known to be tolerant of the high temperatures (above 60 ºC) that may exist at the sand surface. Further, the twisting and untwisting of the leaves in response to dry and wet conditions, while not unusual in mosses, acts in the case of T. ruraliformis to throw loose sand off the colony surface (see Richardson, 1981).

  
  

• Dune meadow/pasture
  Develops where leaching is not too serious, or where appreciable quantities of shell fragments maintain calcium levels in the soil. Shell-sand machair of N.W. Scotland and the islands comes under this heading.
  Finer grasses have replaced Ammophila and the turf is often floristically rich. Nitrogen-fixing legumes may be significant; they include species of clover (Trifolium), vetch (Vicia) and Bird's-foot Trefoil (Lotus corniculatus). Bryophytes may be abundant in damper areas, typical species being the mosses Pseudoscleropodium purum and Climacium dendroides. Grazing by rabbits or sheep can often be important in maintaining floristically rich sites and preventing dominance of coarser grasses.
   
  Areas of dune pasture now represent an significant component of our remaining 'unimproved' grazed grasslands and can be notable for their fungi. Declining fungal groups, such as the true puffballs and their allies (Lycoperdales), waxcaps (Hygrocybe) and earth-tongues (Geoglossaceae), may be well represented. In conservation terms, such sites may be significant as waxcap grasslands.
   
  

  Above: floristically rich machair on shell sand, An Fharaid, Sutherland, 2001. Below: detail of the machair vegetation, with Wild Carrot (Daucus carota), Harebell (Campanula rotundifolia), Red Clover (Trifolium pratense), etc. Uncommon plants such as Frog Orchid (Coeloglossum viride), Field Gentian (Gentianella campestris) and Northern Felwort (Gentianella septentrionalis) also grow in quantity in this high-grade, unimproved grassland.

  
  

• Grey dune/Dune heath
  Sand loses soluble nutrients quickly and such leached dunes, especially if of low calcium content, are likely to develop into heathland, dominated by Heather (Calluna vulgaris), Bell Heather (Erica cinerea) (dryer sites), Cross-leaved Heath (Erica tetralix) (damper sites) and, sometimes, Crowberry (Empetrum nigrum). Ground lichens of the genus Cladonia may be frequent to abundant and, mostly being grey, form 'Grey dune'. The brown lichen Coelocaulon aculeatum is also typical of such areas.
  Low nutrient conditions inhibit microbial decomposition of organic matter and, consequently, peat formation may occur on damper sites (so further increasing water retention).
  In north east Scotland, some native pine woods have formed on former dune soils. These pine woods are notable for their rare (red data-list) plants and fungi.
   
  

  Dune heath, Galloway, 1993.

  
  

• Dune-slacks
  Depending on the height of the water table, areas between sand hills may be damp or even contain standing water. Often such areas are well defined and constitute 'dune-slacks'. Receiving nutrients leached from surrounding dunes, they may be occupied by lime-loving species ('calcicoles') and can be floristically rich and contain local or national rarities. Orchid species may sometimes be prominent members of dune-slack communities.
  The height of the water table is likely to be important and the following classification was given by Ranwell (1972):

  Semiaquatic habitat	Water table never more than 0.5 m below the soil surface, flooding the surface from autumn to spring or later. Amphibious hydrophytes* represented in the flora. [*hydrophyte: aquatic plant]
  Wet slack habitat	Water table never more than 1 m below soil surface, plants have their roots within reach of adequate moisture supplies at all seasons. Bryophytes particularly abundant, bulk of the flora mesophytic,grass species few.
  Dry slack habitat	Water table lies between 1 and 2 m below the surface at all seasons. Shallower rooted species beyond the influence of the water table, deeper rooted species benefit from its presence in summer drought. Grasses abundant, also lichens if rabbit grazed.
  Dune habitat	Water table never rises above 2 m below the surface, most plant growth independent of it. Xerophytes* and therophytes** are common, vegetation tends to remain open. [*xerophyte: plant adapted to dry places; **therophyte: annual, usually ephemeral species]

  
  Dune slacks may be parabolic, developing from blowouts in the seaward dune ridge, and may form repeating series.
  Erosion of sand hills, often through human activities, may result in build up of sand within the slack. The consequent heightening of the floor of the slack raises it further from the water table, with consequent loss of marsh species. This may pose conservation problems where rich slacks adjoin popular beach areas. However, smaller slacks nearer the sea may in any case be part of a cyclic alternation of building and erosion phases. At Newborough Warren (Anglesey, North Wales), the alternation of Ammophila dunes and Salix repens slacks has been estimated by Ranwell to follow an 80 year cycle.

  A large dune slack, Braunton Burrows NNR, Devon, 1967. Note the erosion of the white dunes beyond the slack. Sand is likely to blow into the slack, bringing about a drying effect, as described above.

  Dune slacks commonly become dominated by low scrub, often Creeping Willow (Salix repens) which rarely exceeds a metre in height, often much less. Larger Salix species, e.g. Common Sallow (S. cinerea) may sometimes colonise the wetter slacks.

  
  

• Dune scrub
  A few shrubby species are capable of invading sand dunes to form scrub. Gorse (Ulex europaeus) is a legume with nitrogen-fixing root nodules (presumably?) and Sea Buckthorn (Hippophae rhamnoides) is an unrelated shrub that also has such nodules. It is native to eastern England and S.E. Scotland, but is widely planted and sometimes out of control elsewhere (see next page). Both species are spiny and, along with Hawthorn (Crataegus monogyna), wild roses (Rosa spp.) and brambles (Rubus spp.), they can form dense, impenetrable thickets. The non-spiny species, Wild Privet (Ligustrum vulgare), is abundant on some dunes (mainly western?) but absent from others. Thickets of Sallow (Salix spp.) may form in slacks, as stated above.

  Gorse (Ulex europaeus), forming dune scrub, Torrs Warren, Wigtownshire, 1977.

  
  

  Above: mixed scrub (Elder, Sambucus nigra, Bramble, Rubus fruticosus, and Gorse, Ulex europaeus), colonising stabilised dunes, Gwithian Towans, Cornwall, 2000. Note the white dunes beyond (seawards). Below: interior of the scrub, showing the yellow lichen, Xanthoria parietina, on the Elder.

  Succession from scrub to woodland might be expected, with trees such as Oak (Quercus robur) or Scots Pine (Pinus sylvestris) able to colonise dune scrub. However, the landward margins of dune systems are typically highly managed, often as golf courses ('links' courses) or as agricultural land, or else native woodland has been replaced by plantations.

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