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Evolution of Sand Dunes

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Where do sand dunes occur?

The obvious answer would be "next to the sea, just inland from the high water mark."  However, they can also occur along the shores of large lakes, e.g. Lake Erie, Canada, and of course in the desert. This website only examines coastal sand dunes.  The ideal conditions for the formation of coastal sand dunes include:

Large sandy beach exposed at low tide

Frequent strong winds from the sea

Macrotidal range, i.e. a big difference between high water and low water levels (over 4 metres)

Because of this, sand dunes are rare in tropical areas where winds are usually light, and in enclosed seas, e.g. the Mediterranean, where tidal range is small.  Sand dunes are most common in coastal regions of the world's temperate belt.

What are sand dunes like?

Most sand dune systems appear chaotic at first sight, but on close inspection most have fairly clear topographic and ecological zones.  These are well documented in textbooks form the basis for the various models of psammoseres.  A psammosere is the succession of vegetation that occurs in sand dunes given a long enough period of time.  Most textbook models make the assumption that the age of sand dunes increases with distance inland from the high water mark and show seres (vegetation stages) running from the pioneer community on the strandline (high water mark - HWM) through to the climax vegetation some distance inland.

The diagram shows the common features of a typical sand dune system.  If you move the mouse along the base of the dunes from left to right the images on the right will change giving you a visual impression of each part of the system.

Although, every sand dune system is unique, most will include the zones shown on the diagram.  The typical features of these zones (as they appear in the British Isles) are described below.

Next to each description is a thumbnail image.  Click on it to see it at its full size, then click the back button on the browser to return to the description.

Strandline and embryo dunes

This zone is on and just above the HWM, a place where debris such as dead seaweed, shells and drift wood tend to collect.  On large beaches the position of the HWM can vary a lot depending on the size of the tide.  During a spring tide the HWM will be a lot further up the beach than it will be during a neap tide. This means that some of the debris deposited by spring tides will not be affected by the sea again until a tide of similar size occurs.  Because of this the uppermost part of the beach can be 'dry' for weeks and the wind can blow the sand around. Sand drifts occur where debris causes an obstruction to the wind.  Sand is deposited in the lee (downwind side) of any obstruction. sdSandDrifts2.jpg (536683 bytes)
Sand drifts are usually only a few centimeters high, but this does mean that they can stick up above water level even during most spring tides.  This added protection from the sea encourages the first hardy plants of the pioneer community, e.g. sea rocket sdSeaRocket1.jpg (128839 bytes)
Plants of the pioneer community, still have to withstand very difficult conditions, e.g. high winds, lack of nutrients, salt spray and having their roots in salty water.  In severe conditions such as when a storm surge occurs, they may even get covered in salt water.  During dry summer weather drought can also be a problem. Other common plants are sea beat and sea orache. sdBeatandOrache.jpg (236537 bytes)
In some places, the first colonizers are grasses.  In Britain, sand couch grass is often the first grass to appear above the HWM.  It looks similar to lyme grass, having flat leaves, but does not grow so tall.  Its leaves are also narrower (<1cm), softer and more flexible. sdSandCouch1.jpg (143114 bytes)
Another grass which occurs very near to the HWM is lyme grass. This has broad flat leaves (up to 2cm width) with distinct ridges running along their length.  These ridges are extremely sharp and if you slide the the leave between thumb and finger you will cut yourself!! Lyme Grass has distinctive large flower heads and the plant retains moisture by having a siliceous cuticle (surface layer) to its leaves. sdLymeGrass1.jpg (59290 bytes)


Once formed the embryo dunes trap further sand and grow larger becoming foredunes.  As these reach heights of around 1 to 2 metres, they are no longer flooded by the sea even during spring tides and severe weather.  This enables more vegetation to survive but the build up of sand is usually too rapid for some of the low lying pioneer species such as sea beat, sea orache and sea rocket, which begin to die out.  Sand couch and lyme grasses still occur, but marram grass often starts to come in at this stage. sdForedunes1.jpg (168224 bytes)

Mobile dunes

These are the usually the largest of the dunes and are also often referred to as yellow dunes or the main dune ridges.  Dunes can reach heights in excess of 100m, but 10 to 20m is more usual.  The dune crests are well above the water table and sand is very free draining, so only plants which are well adapted to 'drought' can survive here.  Marram grass usually becomes the dominant species. sdYellowDunes2.jpg (56072 bytes)
Marram grass has many adaptations which allow it to survive in these conditions.  It has long pointed leaves which appear to be circular in cross section, at first sight.  In fact, the leave is curved into a tight 'C' shape, with the open side of the 'C' facing down wind.  This minimizes the surface area exposed to the wind and reduces water loss by transpiration.  It copes well with inundation by sand. As the dune grows, marram converts any buried leaf area into roots, which can can reach astounding depths of several metres.  As the roots are so long marram can continue to extract water even after a long dry period. sdMarram1.gif (9940 bytes)
In some places slope aspect affects vegetation cover and species diversity.  On the seaward side vegetation cover is less as sand is regularly blown up from the beach (see the 1st image in this section).  On the lee side, cover and species diversity increase as the conditions are less hostile (see the picture to the right). sdYellowDunes1.jpg (79989 bytes)
The continued growth of dunes is often checked by blowouts.  These occur when the wind causes erosion of the dune rather than deposition.  They can take place naturally due to severe weather events, but many of the blowouts that we see around the British coast have occurred due to degradation of of the vegetation caused by trampling and footpath erosion.  Blowouts can be identified by the presence of exposed marram roots and the deposition of new sand. sdBlowout1.jpg (111725 bytes)

Slacks and their origins

Slacks are the low areas between the dunes.  They have a number of possible origins.  Most smaller slacks are excavated by the wind which is funneled between the dune ridges.  Slacks are roughly classified as wet or dry, depending on how close to the water table they are.  Those at an intermediate stage are referred to as drying. cross section of wet and dry slacks
Larger slacks may be the result of large scale blow outs and the formation of parabolic dune systems.  In these cases the slack may well be bounded by dunes which form ridges at right angles to the coast.  If you look at the diagram you will begin to understand that the age of dunes does not show a smooth increase with distance from the HWM.  Parabolic dunes are commonplace.

sdParabolicDunes1.gif (10705 bytes)

Occasionally, some large slacks are actually former areas of the upper foreshore which have been cut off from the sea by the development of dunes further down the beach, e.g. where dunes develop on growing spits or bars. Slacks formed in this way can be referred to as cut-off slacks.  Good examples of this can be seen at Holkham Gap (TF890495) in Norfolk, where the process is not yet complete, and at Aberffraw dunes on Anglesey (360685), where it is complete.

sdCutOffSlack1.gif (8460 bytes)

In cases like this there is no simple increase in age with distance inland from the HWM, e.g. between A and B the wet slack is actually younger than the dunes which between it and the sea.  Between C and D the range of dunes closest to the sea is of the same age as the wet slack. Therefore we would not find an orderly series of seres if we were to carry out a transect. Nor would we expect to see a gradual decline in pH with distance from the sea.

sdCutOffSlack2.gif (8468 bytes)

Dry slacks occurring within the main dune ridge system are usually small in size and consist of bare sand and a few scattered drought resistant plants, e.g. sea spurge and sea holly. 

Larger dry slacks occur where cut-off slacks have dried out, producing 'links'. Links areas have a much greater cover of low growing vegetation, usually referred to as dune heath. Dune systems and in particular links areas seem to have greater species diversity on the southern and western coasts of Britain where severe frost and snow are rare. Rest Harrow is a typical plant of the dune heath. Not all dune systems have well developed links. Excellent examples can be seen at Aberffraw in Anglesey.

sdSeaHolly.jpg (182075 bytes)

sdSeaSpurge.jpg (772752 bytes)

sdRestHarrow.jpg (211206 bytes)

Wet slacks are usually the most heavily vegetated parts of dune systems, and can show great biodiversity of plant species. pH values are usually lower (i.e. less alkaline) due to the presence of rotting plant litter and the release of humic acid.  The organic / nutrient content of the 'soil' is significantly higher than in the dune ridges, encouraging a greater variety of plants.  Creeping willow is a typical plant in the milder and wetter southern and western parts of Britain. sdCrwillow.jpg (290080 bytes)
Sea buckthorn is another typical plant, but in many cases it has been introduced by humans to stabilize dune systems.  It is a vigorous plant and can quickly dominate the plant community leading to a loss of species diversity.  For, this reason, in some 'managed'  reserves (e.g. Ainsdale Sand Dunes, Sefton, UK) attempts are made to remove this plant. sdSeaBuckthorn1.jpg (317339 bytes)
Many rarer species occur in wet slacks, e.g. marsh helleborine and grass of parnassus.  Especially in western Britain, species diversity is wet slacks is high as they represent a fairly early stage of the succession.  With the passage of time certain species, e.g. creeping willow, can become dominant, reducing the diversity.

A word of warning for those of you visit such sites: many of the plants are protected species and picking them can result in a hefty fine and a court appearance!

For a more detailed description of the flora of wet slacks see the Ainsdale Case Study.

sdMarshHelleborine.jpg (327606 bytes)

sdGrassOfParnassus1.jpg (79122 bytes)

Slacks which are further inland are usually older, have thicker soils, lower pH values, higher nutrient content, etc.  For this reason, many other plants can occur including plants that are not specifically adapted to sand dune systems, e.g. buttercups, clovers, daisies, etc. sdOldDryingSlack.jpg (241254 bytes)