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 the precipitation of carbonate of lime at lower levels. As a result of these developments, the nature of the vegetation changes and a more complex plant cover starts to appear on the dunes. The colour of the sand also changes from white to grey as the dunes grow older.
During the transitional stage from young, white dunes to old, grey dunes there is a great danger of erosion and destruction of the dune system. As the marram grass dies off, and while the replacement vegetation is colonising the sand, there is an incomplete and inadequate plant cover to protect the dunes. Thus, either unaided, or helped by the burrowing of rabbits, burning of the grass or human trampling and other r1cimi the wind may start to attack the exposed sand. Erosion may be so great as to tear a gap through the dunes, such a feature being described as a blow-out. The sand thus eroded accumulates down-wind from the dune, where, being fresh, it is usually quickly re-colonised by marram grass and becomes stabilized. Thus, It is common to see large spreads of this grass in thriving condition at the back of old, grey dunes. Good examples are to be seen on the landward side of the main dunes fronting Scolt Head Island on the North Norfolk coast.
Dunes may build up to considerable heights. Many contain 15-20m of sand. Dunes higher than this are nearly always found to be piled up on a foundation of solid rock. For example, dunes at Pennard Burrows in the Gower Peninsula reach a height of 60m, but have been built up on sloping ground at the head of the beach. However, whatever their size and complexity, vegetation inevitably plays an important role in the initial formation, subsequent growth and stabilization of coastal dune systems.
(b) Salt marshes
Salt marshes consist of broad, flat, low-lying expanses of silt and mud deposited by the sea in the sheltered waters of bays and estuaries or on the landward side of spits and bars. Although areas of salt marsh normally support a complex, dendritic system of tidal creeks, and also have small patches of open water on th surface (salt pans), most of their surface is normally covered by a distinctive assemblage of salt-loving plants. Indeed, these plants play a crucial role in both the initial formation and subsequent development of salt marsh areas.
The development of coastal marshes has been summarised by B .W. Sparks He notes that, any foreshore of gentle gradient can be seen to possess Irregularities on its surface, usually in the form of broad ridges and shallow depressions. When such areas are gently flooded by the tide, fine sediment carried In suspension will be deposited over the whole foreshore. However, owing to the fact that ebbing tidal water is confined to the depressions, sediment does not accumulate there to the same extent as on the ridges. Sediment accumulation on the ridges is aided by seaweed and flotsam which hinders the flow of the tide and further encourages deposition. Eventually, when sufficient deposition has occurred, the higher parts of the foreshore will be colonized by vegetation.
The role of salt-loving or halophytic plants in salt marsh development has been exs by J.A. Steers who made a close study of the coastal marshes of North Norfolk. He noted that early colonizers of silt and mud banks in that area include marsh samphire (Salicornia spp.) and herbaceous sea blite (Suaeda maritima). Neither of these plants develops a very thick cover and in winter they are reduced to bare stalks, so that they alone would hardly provide a trap for further sediment. However, they frequently have seaweed entangled in their stems, and in that way check water movement and encourage further sedimentation. In this way the surface of the marsh will grow slowly upwards. Measurements of accretion rates carried out by Steers on the salt marshes, of Scolt Head Island in North Norfolk and on the Dyfi marshes of West Wales showed that accretion is rapid on the low marshes which are covered by every tide, becomes even more rapid once a close cover of plants has been established, and then slows down on the higher marshes which are inundated less frequently by the tide.
With continued deposition and upward growth of salt marshes, the plant cover changes. The distribution of particular species is closely related to the frequency and duration of tidal inundation on different parts of the marsh. As mentioned above, the lowest parts of the marsh which are covered with water for the longest period each day are characterized by Salicornia spp. and Suaeda maritima. At higher levels, sea aster (Aster tripolium) and common marsh grass (Pucinellia maritima ) begin to appear. Higher still, one finds sea pink (Armeria maritima), sand spurrey (Spergularia media) and sea lavender ( limonium humile). At the head of the marsh, around its landward margin, clumps of shrubby sea blite (Suaeda fruticosa) are typical. However, the sequence of plant colonies varies in detail from one area of salt marsh to another. Steers, for example, recorded striking differences between the coastal marshes of the east and west coasts of England and Wales.
Two relatively minor features of salt marsh areas should also be mentioned; namely, tidal creeks and salt pans. During the early stages of marsh formation, areas of non- deposition (the original foreshore depressions) are converted into 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.
