Geography Sand Dune Succession A “psammosere” is the technical term for this environment . We are focusing on the landform rather than the biological make-up of the dunes but some knowledge of plant types is needed to explain dune succession.

Critical factors for sand dune creation: What is needed for sand dunes to be created? Source of abundant sand Relatively consistent wind Water level fluctuation dune growth is accelerated by high water levels Vegetation to foster sand accumulation plants trap and stabilize the sand

Sand on the move There are three ways wind can move sand: Suspension Moves the finest grains 1% Impact creep Moves the heaviest grains 4% Saltation Moves intermediate grains 95% Saltation- bounces Suspension- suspended in air Creep- nudged by saltating grains 6

Sand dunes are divided into different ecological zones Back Dunes or Dune Forests Interdunal wetlands or troughs Foredunes Beaches Interdunal- between dunes 7

Vegetation to foster sand accumulation Pioneer grasses’ fibrous root systems grow rapidly, binding sand together and stabilizing the dune Vegetation catches sand 8

Dune formation The sand moves until it meets a slight obstruction, such as a clump of grass, which deflects the wind and allows the sand grains to drop. Thus a slight mound or hummock is created. Wind moves sand up to the top of the pile until the pile is so steep it collapses under its own weight and falls down the leeward side called the slip face.

Migrating dunes The collapsing sand comes to rest when it reaches just the right steepness to keep the dune stable. This angle is called the angle of repose and is usually 30-34 degrees . The repeating cycle of sand inching up the windward side to the dune crest and falling down the leeward side allows the dune to inch forward in the direction the wind blows.

Aerial view of a sand dune system youngest dunes oldest dunes

A transect across a sand dune system X Y Reveals variations in relief and vegetation ………..

A transect across a sand dune system X Y

Sand dunes follow a set layout. Copy this diagram to act as a key for your notes.

The beach ends at the strand line The beach ends at the strand line. This is the line highest up from the water where the material deposited by the sea is stranded. 1 The plants here are tolerant of the harsh conditions- strong winds, sand constantly shifting, salty spray, high alkalinity. There is little evidence of any soil. Plants here are pioneers;- sea sandwort sea rocket saltwort.

Embryo dunes are the smallest hummocks of sand Embryo dunes are the smallest hummocks of sand. They create a tiny area of shelter from the on-shore wind. It is here that the first pioneer plant may grow. They are very easily destroyed. 2 The plants here are only slightly better off than at the strand. They have still to put up with very poor conditions. Soil can only be distinguished using chemical analysis. Plants here are;-sand couch grass sea rocket Lyme grass.

Embryo and Fore Dunes: the plants The plants which grow here have adaptations which allow them to grow in a difficult environment : Sandwort • waxy leaves to retain moisture and withstand winds Frosted orache Saltwort Sea couch • prostrate (low) habit to avoid strong winds • deep tap roots to obtain available moisture Sea rocket • high salt tolerance

Fore dunes are starting to look like proper dunes, though they are still very small. There is an increase in the amount of humus in the sand, and a little less saltiness in the environment. 3 The plants here are modifying their environment and a wider variety of plants can be found here as a result. Their roots are helping to bind the sand grains together and this allows them to get more water. Plants here are;-sand sedge grass Sea holly Marram grass.

4 Yellow or main dunes are substantial dunes. They can be up to 10m tall. There is a much higher amount of humus, nutrients and it is more acidic. The younger dunes create much more shelter. The plants here are less tough in many respects, although it is still not a kind environment. Marram grass is the dominant plant, and its long tap roots bind soil and find water deep down. Plants here are;-sand fescue grass Ragwort Marram grass

Yellow Dunes: the environment Click on photo to reveal annotations above the level of high tides ‘Soil’ slightly less alkaline and more water retentive Some humus forming reduced wind speeds Surface continually blown away and replenished with fresh sand pH 6.5 %OM 0.23

Yellow Dunes: the plants The dominant plant species is Marram grass: Salt tolerant Thrives on being buried by sand Inrolled leaves to reduce moisture loss Long tap roots Underground rhizomes stabilise the sand Other plants such as Ragwort, Red fescue and Sand sedge begin to appear

5 Fixed dunes are often called grey dunes. They are lower and more consolidated than the yellow dunes. They have troughs between the ridges of sandy soil- called slacks. Plants here are;-sand sedge grass trefoil buckthorn heather lichen This area supports plants that require more shelter and more acidic conditions. Most need more moisture, and find it in the slacks; more specialist plants appear, like reeds,rushes and creeping willow.

sheltered by higher, seaward dunes sand no longer accumulating Grey Dunes: the environment Click on photo to reveal annotations sheltered by higher, seaward dunes lower pH little mobile sand higher humus content sand no longer accumulating a more closed vegetation community in which marram grass is no longer able to compete pH 5 - 6 %OM 1.0

Dune Slacks: the environment Click on photo to reveal annotations occur in low lying hollows between dune ridges relief intersects the water table soil acidic water table high – especially in winter pH 4 - 5 %OM 8.5

6 This is the last sere of the plant succession. The land here is damp, sheltered, acidic, rich in nutrients and humus. Plants here are true pioneers;-heathland plants gorse Ash, rowan, alder, birch and spruce trees The plants here are as evolved as they are going to get- unless man or storms modify their environment again CLIMAX has been reached !

Dune Heath/Woodland: the environment Click on photo to reveal annotations maritime influence is minimal well sheltered from winds acidic soil nutrient rich soil has high organic matter content pH 4 % OM 12.1

BIOSPHERE SUCCESSION

Management of sand dunes

Sand Dune Succession