Ecology of Sand Dunes
There are six main physical types of dunes which rely on the position of the system in relation to the direction of the shoreline. Three of these being spits, offshore islands and nesses project towards the sea and are generally extending seawards. They are characterstic where the prevailing wind is offshore so in England would be the East Coast. The other three types which are bay dunes, hind shore dunes and machair are found found where the prevailing wind is onshore so a constantly being driven back on to the land behind the shore by the wind and are found on the west coast.
Dunes are known as sub-maritime coastal habitats as they are not plagued by occasional flooding and are not very salty. Dunes can very in size greatly with the height increasing over 25 meters on some coasts and can vary greatly in width and length.
Dune vegetation varies according to stability, moisture content and calcium carbonate content of the sand the dune is made up of. In order for vegetation on the dunes to grow plant succession has to occur. Marram grass colonises the dune (fore dunes) and spreads its tuberous roots just under the surface of the sand, and forms an underground web that helps hold the sand in place. Though marram grass stabilizes the soil for its own survival, an inadvertent side effect is that it makes it possible for other kinds of vegetation to begin to take hold. Other plants take advantage of the increased stability of the soil surface and start to colonize areas the marram grass helped make safe
Embryo Dune Formation
Sand is blown onshore from exposed beaches at low tide. As the wind slows due to obstacles on the land being met the sand is deposited. Large amount of sand are deposited just beyond the highest water mark. This is due to the friction occurring where drift line vegetation is growing and by the accumulating sand deposits present. Eventually low sand hills build up on the shoreline and slowly move inaland.
These embryo dunes become colonized by Agropyron Junceforme which can tolerate saline apray, posses’ a large root system allowing more binding of the sand and rapid growth to avoid burial by the sand. The embryo dune vegetation increases sand deposition and they hills increase in size and advance further inland and develop into first fore and then yellow dunes.
Fore Dunes and Yellow Dunes
Above the driftline and embryo dunes there is often a seaward strip of low dunes with an open growth of plants still tolerant of short immersion during especially high tides. These fore dunes grade into main dunes further inland where Marram grass is the main colonist encouraging the growth of the dunes upwards. This grass is and efficient dune builder but does not form a layer of litter or bind the sand surface so later erosion occurs.
As the colonists have made the sand more stable forther vegetation now grows such as Calystegia soldanella. This grows in the sand between the marram tussocks. Many species of plans which find a coastal niche also appear as open sand colonists. Some of these grow on mainly calcerous dunes, but the effect of varying lime content of the sand becomes more obvious as a closed vegetation develops and stabilization is accompanied by leaching.
Fixed or Grey Dunes
A vegetation of low growing grasses, lichens and mosses are present at this stage. Where contains less lime the characterstic species which occurs amongst the marram are red and sheeps fescue. Lichens such as Cladonia impexa also may become abundant
Dune Heath and Dune Pasture
The oldest and most stable parts of the non-calcerous dunes are eventually invaded by dwarf shrubs. This forms a heath which approaches that of many inland lowland areas with sandy soil. Lichen heaths also develop with species such as cladonia impexa growing in great abundance in dwarf shrubs, it is also possible for bracken to become abundant in this area and large areas of it may develop.
During the early stages of closed sward formation marram may remain very abundant, but with further development of the succession it gradually thins out and eventually disappears. The development of the dwarf shrub heath is also marked by the sand surface becoming acidic and the formation of an acidic humus layer.
Further development involves a thickening of the grassy swardm extension of these herbs into the moss and lichens, and the dissapearance of species such as marram grass. Rabbits can be numerous on these undes and their grazing can produce a close cropped species rich area hardly distinguishable from that on many inland grasslands.