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,
