A LEVEL GEOGRAPHY ESSAY: DUNES and MARSHESEXAMINE THE ROLE OF VEGETATION IN COASTAL DEVELOPMENT

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A LEVEL GEOGRAPHY ESSAY: DUNES and MARSHES

EXAMINE THE ROLE OF VEGETATION IN COASTAL DEVELOPMENT IN THEFOLLOWING HABITATS.         (a) SAND DUNES  

(b) SALT-MARSHES

(a) Sand Dunes

Before considering the role of vegetation In the development of coastal sand dunes, it may be useful to summarise the general conditions which are conducive to dune formation In order to place the biological processes involved in a broader context. Factors which encourage dune formation include prevailing onshore winds blowing across a wide inter tidal zone which allows the top sand to dry out between tides and to be set in motion by the wind. There should also be some feature at the head of the beach to trap the sand being driven onshore by the wind. This might be patches of coarse shingle or drifted debris or existing vegetation just beyond the high water mark. Ideally, there should be an absence of vegetation on those parts of the beach from which sand is being moved, and active colonisation by plants at the head of the beach where dunes are accumulating. Obviously, not all of these conditions win be met in every area of dune formation, but they indicate the optimum conditions for sand dune accumulation and growth.

The initial accumulation of sand at the head of a beach is often around a clump of coarse grass, such as sea couch grass (Agropyrum junceum). The next stage, provided that the incipient dune is not destroyed as fast as it accumulates, is the colonisation by marram grass (Ammophilia arenaria). Marram grass possesses a deep branching system of roots which effectively bind the dune together. Furthermore, the plant itself only survives as long as a fresh supply of sand is constantly added to the dune. Once this supply of fresh sand ceases, the marram will die out and will be replaced by other species of plants. Other characteristic plants at the marram stage include sea-holly (Eryngjum maritimum), sea spurge (Euphorbia paralias), and sea sandwort (Arenaria pepliodes). In this early stage, the sand of the dunes has not undergone any particular change, and hence the dunes are sometimes referred to as white dunes.

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The supply of sand may be cut off by the accumulation of shingle in front of the developing dune. On this new shingle a further dune may start to form, thus cutting off the supply of sand to the earlier dune. With the reduction of a supply of moving sand, the marram grass will become rather patchy and replaced by lichens, mosses and other plants. At the same time, the decay of earlier plants will start to give a little humus to the sand. If there is shell material in the sand, some solution may take place and lead to ...

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