occupants are of greater size and stronger growth, so that the earlier colonists are eventually swamped by them. Later on, these in turn may be choked out by other even stronger plants. Thus each successive community, by modifying the soil, tends to make the habitat more suitable for the growth of new species, but in so doing lays the way open for its own ultimate destruction. For example, many of the early colonists in the mobile sand of young sand-dunes are unable to exist in the thick sward of grasses and other plants which cover the surface of the older dunes, and the marram-grass itself is eventually stifled when the surface of the sand becomes completely fixed. The early colonists of sand-dunes, however, not only add humus to the sand but also modify the habitat by anchoring the surface of the sand. Only a limited number of pioneer plants can exist in the shifting sand between the clumps of marram-grass on the young dunes, but they all make their contribution towards fixing the surface of the sand. As a result of their efforts, it gradually becomes possible for a greater variety of plants to become established, and eventually the characteristic close sward of fixed dunes is produced.
In many cases a modification of the habitat may be produced by the combined efforts of plants and physical factors. The colonisation of the bare soft mud on the edge of a salt-marsh is a good example of this. The pioneer plants, such as glasswort or rice-grass, are instrumental in stabilising the mud and also add humus to it. In addition, they aid the natural physical process in which mud is deposited by causing a distinct slackening of the tide as it ebbs and flows over them, and in this way the level of the habitat is gradually raised and stabilised so that other plants can become established.
Generally speaking, all succession is directed towards developing the most complex vegetation which the climate will permit, no matter what the nature of the original habitat may have been. The ultimate vegetation produced in this way is called the climax formation or the climatic climax. The communities making up this formation will be more or less stable and will not be seriously threatened by new invaders. In most of England and the southern part of Scotland, if the vegetation were left completely undisturbed, oak or beech forest would eventually be developed. In the north of Scotland and most of the central portion also, if we exclude the tops of the higher mountains, the climatic climax would, however, be pine forest, an association of the Northern Coniferous Forest Formation. In comparatively recent times, most of the British Isles was forested in this way, but the large-scale felling of our woodlands during the Middle Ages and later has almost obliterated the natural forests. Nowadays, as a result of intensive agricultural operations, the climax formation is rarely reached in the course of natural succession. Where suitable areas exist, which are not cultivated or grazed, the absence of suitable seed-parents in the immediate neighbourhood precludes the development of natural woodland. Ecologists recognise, however, a number of relatively stable subclimaxes in the vegetation of this country, which are developed under the conditions which are normally present.
Any natural succession of communities which replace each other in a particular habitat is called a sere. Thus those which succeed each other in a salt-marsh all belong to the halosere, salt being the master-factor controlling each of them, and those developing on blown sand to the psammosere (Greek: psammos=sand). The sea-coast provides practically the only habitats in this country where one can see a more or less complete series of communities starting with bare ground and finishing with a type of vegetation which remains comparatively stable under the particular conditions. Elsewhere, succession can be most easily observed in an area which has previously carried some fairly stable type of vegetation, but which has subsequently been modified in some way or other. This is well illustrated when a wood is felled or a heath is burnt and is known as secondary succession. Good examples of this type of development can also be seen along the coast, as for instance when the surface vegetation on a sand-dune is broken through and the strong winds produce a “blow-out” (see here), which is then recolonised in much the same way as the fresh sand on the newest dunes (Pl. XXI). As another example, the seaward edge of a salt-marsh sometimes becomes eroded as a result of a sudden change of current or for some other reason. The original vegetation is thus destroyed, but in course of time the mud on which it originally grew may be colonised once more to form what is called “secondary marsh,” usually at a different level from the original one.
When we come to look into the reasons why particular plants grow where they do, we find that there are a large number of factors to take into account. Most of these are closely inter-related in the effects they produce, but it is worth while to discuss briefly some of those which are especially important in determining coastal vegetation.
The climate of the country is obviously of the greatest importance, for it controls such factors as the duration and intensity of the sunlight, the range of temperature, the rainfall, the humidity of the atmosphere and the strength of the winds. Climatic factors show their effect most clearly when vegetation is studied on a broad geographical basis, but even in a relatively small area like that of the British Isles the effects of small differences in climate are quite noticeable. Thus the average rainfall and the humidity of the air is much greater on the west coast than on the east, and this probably accounts for many small differences in the distribution of plants along the two coasts. It is certainly responsible for the much richer moss flora found on the western sand-dunes compared with those on the east coast, and may partly account for the occurrence of certain typical “Atlantic” species along our western and south-western coasts. In the same way, the mean temperature in the North is distinctly lower than that in the South, and this is one of the factors responsible for the absence or rarity of a number of plants in Scotland and northern England, which are comparatively common in the South, and also for the fact that certain characteristic north European plants are only found in the North.
Wind is obviously a very important factor in all coastal habitats. Its most pronounced effect is that it increases the loss of water vapour from the leaves of plants by constantly bringing dry air into contact with them. As a result, the growth of many seaside plants is considerably retarded and they are often found in a very stunted form. To combat this, many coastal plants adopt a mat or rosette habit for much of the year. Exposed parts of the coast are generally destitute of trees, and such few trees as do occur near the coast are usually found tucked away in sheltered valleys, or combes as they are called in the West Country. Trees and hedges in coastal areas often assume very distorted forms, which show clearly the direction of the prevailing winds (Pl. VIII). This is due to the fact that only the shoots on the leeward side can develop normally, those continually exposed to the prevailing winds being dried off and killed. In this way they appear to have been blown over by the strength of the wind, whereas actually their peculiar shapes are due to the unequal development of the shoots on their two sides. The effect of wind in retarding growth is most marked on the east and north-east coasts, which are exposed to the driest winds, although it is very noticeable on any of our coasts.
Another group of factors to be considered depend upon the general topography of the habitat and may be called physiographical factors. The angle at which the ground slopes, the aspect or direction of the slope and the height of the land above sea-level, are examples of these. The familiar coastal processes of erosion, silting and the blowing of sand, which are discussed in Chapter 2, also come into this class. In addition, the prevalence of strong winds along the coast, whose effects have just been described, is clearly due to a combination of climate and topography. It is hardly necessary to give illustrations of the result on the vegetation produced by all these factors; the relation of the highly specialised community of plants which are found on mobile sand with their habitat, for instance, is sufficiently obvious. Some of them, however, become particularly important when we consider cliff-vegetation. Thus the angle at which the cliffs slope largely controls the amount of soil available for supporting plants in the rock crevices, and will indeed determine the stability of the surface of the cliff itself, if it is composed of soft material. The height above the sea will also determine the amount of spray to which the habitat is exposed, and most cliffs show some zoning of the vegetation which can be correlated
