Examine the role of vegetation in the initiation and morphological development of coastal dune systems.

Even a sparse vegetation cover will weaken the power of wind, shelter the ground surface and trap sand grains (Wolfe & Nickling 1993). Dune plants are xenomorphic, i.e. they have adapted to tolerating dryness. They have long roots that reach down to the lower, humid sand layers, and their stems can survive bending by the wind.

As you can see from the diagram above (Figure 2), the sand dunes are divided into zones. Different vegetation species grow in each of the zones, in relation to how long the sand deposits have been there. The degree of stability is very important in determining which plants become established. Embryonic dunes are nearest to the sea and have few plants growing on them except lyme (Leymus arenarius) and marram grass (Ammophila arenaria) (Figure 3). This is a coarse, long-leaved grass found only on sand dunes, where it is abundant and an important part of the structure and ecology of the dune systems. It binds loose, wind-blown sand with a network of branching rhizomes. Its ability to do this means that it is perfectly equipped to cope with deposition rates of up to 1m of sand per year. The growth of marram is directly related to the rate of influx of sand. As more sand accumulates, the embryo dunes join to form fore dunes which can reach a height of 5m. Fore dunes are also often known as yellow dunes, on account of the amount of bare sand yet to be colonised. Landwards of the fore dune, the increasing amount of vegetation means that sand movement is more restricted, however, as the dunes are higher, the velocity of the wind is much greater. The dune eventually reaches a point where these strong winds remove as much sand as they deposit. As the sand is still mobile, marram grass is the dominant species although grasses such as red fescue (Festuca rubra) (Figure 4) and sand sedge (Carex arenaria) (Figure 5) invade (Hansom 1990). Landwards of the yellow dune, the dunes become increasingly grey as humus and bacteria from plants and animals are added and the dune is occupied by grey-coloured lichens, mosses and low lying shrubs such as buckthorn (Figure 6). These grey dunes can reach heights of 10-30m before the supply of fresh sand is cut off because of their increasing distance from the beach (Waugh 1995). 

Almost all dune systems are subject to erosion, whether it is a natural threat or one created by human activity. Some of the natural threats include falling water tables. In dune slacks, temporary and sometimes permanent ponds form. Here, the ecosystem is stressed due to the seasonal changes in the habitat, so therefore only specialised species are able to survive. Water tables tend to vary from year to year, but a particularly long period of low water tables would put too much stress on the ecosystem and many specialised species would be unable to cope, and be replaced with coarse vegetation and scrub. Another natural threat is grazing. Grazing is needed to maintain fixed dune communities and prevent the dunes developing into scrub and woodland. However, if the dunes are over-grazed by livestock and rabbits, then this can have damaging effects, but if the dunes are under-grazed, the scrub and woodland species will begin to grow. Humans cause interference as sand dunes are very popular with tourists. Holiday makers may walk through them or park their cars on them. The dunes can cope with pedestrians to an extent, but cars and walkways from car parks to the beach cause a great deal of damage. Embryo dunes are formed when sand is blown up the beach and trapped by pioneer species of plant such as marram grass.  When pedestrians continually walk in these areas these very important plant species may be destroyed and sand is no longer trapped. Failure of the dune vegetation to recover from any damage, may result in dune erosion due to the effects of onshore winds mobilising bare sand in the form of blowouts. Movement of sand landward results in erosion of the beach/dune system. The blowouts that induce this mode of erosion may in turn initiate further degradation within landward dunes as drifting sand buries and kills vegetation in its path.

Appendix 1

Bibliography

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Hansom, J. D. (1990). Coasts. Cambridge: Cambridge University Press

Waugh, David (1995). Geography an Integrated Approach 2nd Edition. Surrey: Nelson

Wolfe, Stephen A. & William G. Nickling (1993). Progress in Physical Geography

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