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IT IS HARDLY possible to understand how the vegetation is distributed round the coast-line without having some slight acquaintance with the principles of plant ecology. In this chapter we shall therefore consider quite briefly what ecology is about and also take the opportunity to explain some of the terms which are commonly used by ecologists. No attempt will be made to go more deeply into the subject than is necessary to follow the method used in the later chapters, which describe in detail the characteristic vegetation to be found in various typical habitats along the shore. For a fuller account of the subject the reader is referred to Professor A. G. Tansley’s fine book, The British Islands and their Vegetation. In the following short account the examples have been chosen as far as possible from seaside vegetation in the hope that the main characteristics of coastal habitats in general will become apparent.

Plant ecology is concerned with the study of plants in their natural habitats and their relations with their surroundings. It is thus primarily a field study and can be worked out only in the place where the plants are actually growing. The present popularity of both plant and animal ecology is to a certain extent a reaction from some of the more specialised lines of inquiry in biology, which have to be carried out indoors in laboratories.

One of the most fundamental differences between plants and animals is that the former are fixed in the soil, and cannot therefore move about when they are growing. They are thus, of necessity, gregarious and have to lead a communal life. Plants are, in fact, usually found in well-marked communities, whose composition depends on the nature of the habitat and a number of other factors, some of which are discussed later in this chapter. The word plant community is a general one which is used to describe any collection of plants growing together which can be said to possess a definite individuality. If there is much bare ground between the individual plants, which is available for colonisation by other species, the community is said to be open. The plants found growing on the front (seaward) range of sand-dunes in an area of blown sand form a typical open community (Pl. VII). Other obvious examples to be found amongst coastal vegetation are the communities inhabiting exposed sea-cliffs (Pl. II), and the mobile mud along the edges of salt-marshes (Pl. XIII). When the vegetation is more or less continuous, and competition for the available space becomes an important factor, the community is said to be closed. An open community generally represents an early stage in the colonisation of an area, but it may also be found in a habitat where the conditions are so harsh that plants have great difficulty in existing at all.

Although the individual members of an open community depend largely on the nature of the habitat, the amounts and nature of the species present will depend increasingly on their inter-relations in the available space. Usually one or more dominant species, which are mainly responsible for the general appearance of the community, can be recognised. They are frequently the tallest-growing plants present and may thus exercise a profound influence upon the other inhabitants of the community, particularly by competing successfully for the available space or by causing shade. As examples from coastal vegetation, we may mention rice-grass (Spartina townsendii), which is the main dominant species in the communities formed on the soft mud of salt-marshes along the south coast (Pl. XIV), and the sea-rush (Juncus maritimus), which frequently dominates a zone along the upper edges of salt-marshes elsewhere. The other species associated with these dominants are known as subordinate species. If these are found in nearly every example of a community, they are called constant species. Any other plants which turn up from time to time in the community, but are not really characteristic, are known as casuals.

Plant communities may be of very different sizes and importance, and it is customary to divide them into various classes. The largest unit of vegetation is called a plant formation and usually refers to a broad type of vegetation which remains roughly the same over a whole continent or even throughout the world. The character of a formation depends on the nature of the habitat and it reflects this in the distinctive life-forms of its principal species. Thus the Salt-marsh Formation contains a highly characteristic population of halophytes, whose specialised life-forms reflect the most important feature of the habitat, that of its periodical immersion by sea-water. Similarly the Sand-dune Formation contains another very characteristic population of plants, many of which are xerophytes and specially adapted to grow in the semi-arid conditions of blown sand. (Some ecologists restrict the use of this term to the ultimate climax vegetation which can be developed in a habitat under given climatic conditions, and would not therefore refer to either of these essentially transitional types as formations.)

The term plant association has in the past been used to refer to so many different units of vegetation that, to avoid confusion, it has not been employed in this book. It is now generally accepted that it should be used to describe a relatively large unit, usually a geographical sub-division of a formation which is characterised by a particular dominant species. As an example, we could say that the Oak-Beech Association is the typical form in the British Isles of the main European Deciduous Forest Formation. In the same way, the Marram-grass Association is typical of the Sand-dune Formation in this country, though associations with other plants as dominants may be found in similar habitats in other parts of the world.

From the point of view of our discussion of coastal vegetation, however, the most important unit to define is the plant consociation. This is a smaller affair than either of those so far mentioned, although it was frequently called an association in the old days. It consists of a community with (usually) a single dominant species. Salt-marshes generally show well-marked examples, since the vegetation often occurs in distinct zones. Thus the lowest strip is often dominated by annual glasswort (Salicornia stricta) (Pl. XIII), and other typical zones are dominated by such plants as sea-aster (Aster tripolium) sea manna-grass (Puccinellia maritima) (Pl. XIb), sea-lavender (Limonium vulgare) (Pl. 5), etc. Plant consociations are often named after the Latin name of their dominant species by adding the suffix etum to the stem of the Latin name of the genus. Thus the consociations referred to above are usually called the Salicornietum, Asteretum, Puccinellietum and Limonietum respectively. Should there be any possibility of confusion over the identity of the dominant species, the specific name is usually added in the genitive case. For example, consociations dominated by two separate rushes are found in salt-marshes in different areas, and the word Juncetum maritimae is therefore used for that dominated by the sea-rush, to distinguish it from that dominated by the mud-rush, which is called Juncetum gerardii.

The smallest unit with which we need concern ourselves is the plant society. This is a purely local community, which may sometimes be noticed within a consociation, dominated by a species which would be considered a subordinate one if the consociation were viewed as a whole. Societies generally owe their origin to some small local differences in the habitat. Thus the sea-purslane (Halimione (Obione) portulacoides) often forms a distinct society along the sides of the creeks which cut through the Puccinellietum or Asteretum in a salt-marsh, because the soil there is better drained (Pl. XV). Another type of society is a layer society, which can be observed when the vegetation is composed of plants of very different heights. This is most obvious in a forest, but an important society of mosses and lichens can often be seen below the main herbaceous layer on the older sand-dunes, and there is frequently a layer of shade-loving plants in the Juncetum maritimae in a salt-marsh.

In explaining the various units of vegetation which are recognised by ecologists we have tacitly assumed that they remain stable and possess a constant composition and structure. This is, however, by no means the case; nearly all vegetation is continually changing, although the rate at which this is proceeding varies greatly. Some communities appear to be remarkably stable, but others are mere passing phases, which soon give place to others. We ought therefore to look upon all these units as representing positions of relative equilibrium into which plants group themselves for a time. Generally speaking, the changes which are in progress all tend towards a position of greater stability. All progressive change of this kind is known as succession.

There are two main types of change which can bring about a succession of vegetation. To the first type belong all those which are caused by purely physical factors which alter the habitat in some way, making it less suitable for the first occupants and more suitable for others. A long-term example of this kind of change would be a gradual alteration in the climate; there is plenty of geological evidence of the effect of such climatic changes in past eras upon the vegetation of the British Isles. It is often possible, however, to see much more rapid changes in progress. For example, the sand on the sea-shore always contains enough salt to make it somewhat alkaline, but as soon as it has been raised above the level of the highest tides in the form of a sand-dune, the salt will rapidly be washed out by the rain. If there is only a small amount of calcium carbonate (another substance causing alkalinity) in the sand, this will also in time be washed out from the surface layers, and plants which prefer more acid conditions can then become established. In this way, the first colonists, which prefer neutral or slightly limy soils, will be gradually replaced by others and eventually “dune-heath,” with heather as the dominant species, may sometimes be produced. Another example is provided in some dune areas, where water tends to accumulate between the ranges of the older dunes, producing a totally different type of habitat within the main area of blown sand. Here a community consisting almost entirely of marsh plants frequently appears. Yet another example of the effect of a physical change can often be seen in salt-marshes, where the tide has been artificially excluded from the upper levels by the construction of some sort of barrier. Here the vegetation is rapidly changed by the appearance of numbers of non-halophytes as soon as the rain has washed out the residual salt from the soil.

The second type of change which may bring about a succession of vegetation is one produced by the plants themselves. When any bare ground is colonised, there is nearly always at first a fairly rapid series of changes in the composition of the plant communities. The first colonists or pioneers will almost invariably give way to others later, and these in turn may afterwards be replaced by still others until a relatively stable equilibrium is reached between the habitat and its vegetation. This type of development can probably be observed taking place along the coast better than anywhere else in the country. The usual way in which plants alter a habitat is by adding humus to it. Humus is the dark organic material produced by the partial decay of plant remains, and as the first colonists die off this material begins to accumulate in the surface layers. In course of time the physical properties of the soil are modified by the addition of this humus and, in particular, its water-holding power is steadily increased. As a result of this, it becomes possible for a wider selection of plants to gain a footing. As a rule, the new 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 with this factor. The direction towards which a cliff faces is also important in determining the amount it will be exposed to the prevailing winds and thus, indirectly, the amount of spray it is likely to receive, and will also control the duration of the periods of shade. There is often a marked difference in the vegetation of cliffs with different aspects, in particular those on the opposite sides of small islands.

Another group of factors, which in some ways show the most pronounced effects on the composition of the vegetation, are those related to the physical and chemical properties of the soil. These are called edaphic factors (Greek: edaphos=the ground). On the coast the commonest physical characteristic of most habitats is that of a poor water-supply. Sand-dunes, shingle beaches, and most cliffs are all subject to periodical drought conditions, which are aggravated by the drying winds. We shall see in the next chapter that the leaves of many seaside plants are equipped with devices of various kinds to check excessive loss of water, and that their root-systems are often very extensive. The amount of air contained in the soil is also related to its physical state, and it is noticeable that a number of plants, such as marram-grass on dunes, sea purslane in salt-marshes, and the shrubby seablite (Suaeda fruticosa) on shingle grow luxuriantly only when their roots are well aerated.

The chemical nature of the soil is also of great importance. Salt is obviously the master-factor in determining the highly specialised vegetation of salt-marshes, and the presence of halophytes in other coastal habitats, such as shingle beaches and exposed cliffs, shows that there also salt spray is deposited in sufficient amount to be an important factor. The ultimate vegetation developed on sand-dunes also varies greatly with the amount of calcium carbonate initially present in the sand. We have already seen that, if this is small, it will be washed out of the surface-layers in time, and that typical plants of acidic soils like heather and heath may eventually appear, as the supply of humus increases. Many west coast dunes, however, have been formed from sand which contains so much calcium carbonate in the form of broken shells that the relatively slow leaching action of the rain has produced little effect on it. As a result, the final vegetation on these dunes remains fundamentally calcicole (lime-loving), and is remarkably similar in composition to the grassland commonly found on chalk and limestone. In the same way, chalk and limestone cliffs may be expected to show some different plants from those which are found on acidic rocks.

Finally, we must say a word about the effects on the habitat caused by living organisms. These are called biotic factors (Greek: bios=life), and include the activities of man and his animals, the effects of rabbits, birds and insects and those produced by the plants themselves. The effects of previous generations of plants in altering the physical and chemical properties of the soil have already been briefly discussed. As far as man is concerned, his activities are less in evidence along the coast than in most parts of the country, since coastal areas do not lend themselves well to agricultural development. Nevertheless, in a thickly populated area like ours, there is no region where the hand of man has not played some part in modifying the vegetation. For instance, large areas of many salt-marshes are used for the grazing of cattle, which has the effect of restricting some plants but not others. Many old salt-marsh areas, too, have been completely transformed by drainage operations or the construction of sea-walls to exclude the tides, and the laying out of golf-courses has altered the vegetation in sand-dune areas in a number of places. Moreover, in certain districts marram-grass has actually been planted to stabilise shifting sand-dunes, and elsewhere rice-grass has been employed in a similar way for reclaiming salt-marshes, so that it is often impossible to distinguish between natural and partly artificial vegetation. Nor should it be forgotten that the large-scale felling of the native forests all over the country in the past has had the indirect effect of preventing the natural development of the climax vegetation in many suitably undisturbed areas along the coast.

Rabbits are frequently responsible for considerable modification of the vegetation, and are often extremely common in coastal areas. In particular, the grassland on the tops of cliffs is often infested with them, and the older sand-dunes provide a veritable rabbit’s paradise. In all probability, the somewhat stunted vegetation which is so characteristic of such areas results as much from its being continually nibbled by rabbits as from its exposure to strong winds. Some plants, however, are more attractive to rabbits than others, so the actual composition of the vegetation may be considerably altered. Even salt-marshes are not exempt from the attentions of rabbits; in some districts, for instance, it is unusual to see more than a quite small proportion of the sea-aster plants reaching the flowering stage. Birds also sometimes have a marked effect on the vegetation, particularly when large colonies gather on small islands for breeding purposes. Needless to say much excreta is deposited on the cliff-ledges and cliff-tops near their nesting sites, and the increase in the amount of nitrogen and phosphates in the soil produced in this way has the effect of altering the composition of the vegetation considerably.

The above brief summary can do no more than suggest the kind of factors which must be looked for if we are to make any attempt to understand why coastal vegetation is distributed as it is, and why particular species occur in some places and not in others. Our knowledge of these matters is still extremely incomplete, and it is well to realise that much valuable information can still be easily collected by amateur botanists who are prepared to make a fairly detailed survey of the vegetation in a particular habitat and to keep their eyes open for the factors which have been responsible for its composition.