The Role of Vegetation in Coastal Development in Sand Dunes and Salt Marshes

The Role of Vegetation in Coastal Development in Sand Dunes and 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...

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...to tidal creeks. In this way, the ebb and

flow of the tide becomes concentrated along these lines and

colonization by plants is prevented by tidal scour. Salt pans are

small, roughly-circular pools on the surface of the marsh. The

suggestion has been made that the very high salinity of the water

contained in these pools inhibits plant colonization and resultant

infilling. Finally, mention should be made of the human influence on

salt marsh habitats. Once a marsh has built up to such a level that it

is seldom covered by tides, It is relatively simple to enclose it with

a bank or wall, drain it, and replace the natural vegetation with

cultivated pasture. Obviously at that ultimate stage in salt marsh

development, the plant cover is almost wholly a result of human

interference rather than a response to natural conditions.