Cyclical Destruction And Regeneration In Coastal Habitats

Coastal vegetation faces disturbance from rising sea levels and human disturbance on unprecedented levels. Under natural conditions the rich variety of coastal communities and the easy dispersal of propagules by sea has in the past enabled the plant communities of maritime habitats to recover from sea level changes and other natural disturbances.

Probably the greatest danger to maritime vegetation will be the measures employed by human populations to save themselves from sea inundations, including sea walls, groynes and other artificial barriers. Ifallowed to find their own equilibrium with changes in sea level, plants can be effective protectors of coastal margins. Cliffs have a rigidity of their own. Sand dune and slack systems are biologically rich, with foredunes, yellow dunes, grey dunes and slacks. Collectively, this constitutes a well-adapted coastal defence system. Provided the detritus of the sea, seaweed and other flotsam, is not removed and there is an adequate supply of sand, most sandy beaches can repair storm damage. However, resistant as they are to frontal attack from the ocean, these natural coastal defences are nevertheless vulnerable to an attack from the rear through windblow and sometimes river erosion as well as other unexpected alterations in their environment. Although sand dune vegetation is highly drought resistant the water table must be accessible. Falling water tables, removal of expected resources such as seaweed, truncation oftheir natural development by roads and golf courses, all contribute to weakening the resilience of dune systems to withstand physical disturbance, whether from human interference or natural disasters. The conservation of biodiversity in dune systems therefore requires more than just ensuring the physical preservation of the front line of dunes. Equally, over protection can also result in dominance of a few aggressive species with the result that diversity is lost. For long-term preservation of dune systems and their biota a careful balance has to be struck between ensuring adequate physical protection and enough disturbance to enable the natural cycles of denudation and recol-onization to take place without causing a loss of species diversity.

Coastal systems are inherently unstable, for as soon as they are consolidated by the sand-retaining grasses these species suffer a decline in vigour and the dunes become liable to blow-outs and other forms of erosion. This raises a management problem for coastal conservation of whether or not to encourage the spread of woody species across dune systems (e.g. sea buckthorn Hippophae rhamnoides; Fig. 7.46). It was specifically on this point that his Scottish Majesty King James VI (King James I of England) forbade 'for hereafter' in 1695 the removal from dunes of 'brent, broom, or juniper', all woody species (Gimingham, 1964). Where grazing from domestic livestock or rabbits is limited, dunes are readily colonized by pine and birch, with willow and alder in the wetter intervening slacks. This may increase stability by reducing wind erosion; however, it may also reduce species diversity and although the site may be preserved physically, species diversity is not necessarily similarly conserved.

Fig. 7.46 Sea buckthorn (Hippophae rhamnoides) spreading vegetatively by runners across a dry slack at Buddon Links, Angus, Scotland.
Coastal Habitat Destruction
Fig. 7.47 Goats supposedly put into the service of conservation by Scottish Natural Heritage in an attempt to reduce birch colonization of dried-out slacks at the Tentsmuir National Nature Reserve.

The answer to whether trees have a role in sand dune conservation lies in our long-term concept of what constitutes a viable dune system. Sadly, we are too used to considering sand dunes as a fringe of grassy hillocks bordering the sea. Less than 0.2% of the land surface of the British Isles is occupied by sand dunes and there remain few natural dune systems where development is not truncated to landward by roads, car parks or golf courses. It is to the shores of the Baltic, in Estonia and Latvia, that we have to turn to see this vegetation at its best, largely due to the draconian methods used in Soviet times to prevent illicit human migration to Finland. Some degree of disturbance, however, is necessary, either from grazing or recreational use, to keep the forest cover sufficiently open and encourage the plants of the undergrowth. Management has to be carefully balanced. There have been instances where misguided conservation measures such as introducing goats to the dune slack system to restrict the spread of woody plants have not only destroyed the biodiversity of the slacks but also devastated the lichen flora of the grey dunes (Fig. 7.47).

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