Flowers of the Coast

SALT-MARSHES

Salt-marshes and their growth are described in detail in Chapter 5. They show better than any other feature the intimate relation between ecology and physiography in coastal evolution. They are built up of the mud, silt, and fine sand carried by the tidal (and other) currents, and deposited in quiet places. Their nature will depend much on the material of which they are composed; there is a great difference (see here) between the firm mud marshes of Norfolk and the sandy wastes of Morecambe Bay. Marsh growth begins in certain favoured places on the sea floor, especially on that part of it which is bared at low tide. Ideal conditions obtain on the coast of North Norfolk where formations like Blakeney Point, Wells Headland, Scolt Head Island, and the small high-tide island at Thornham provide shelter. Marsh growth takes place outwards from the original coast and inwards from the protecting ridge. Moreover, where structures like Blakeney Point throw off a number of laterals, or recurved ends, the best possible conditions for marsh formation often exist between neighbouring laterals, especially if they are so built as to make a narrow entrance to a relatively broad expanse inside (e.g. the Marram marshes at Blakeney Point). Into these quiet backwaters tidal water pours, and stands quietly for a period before the ebb (Pl. XIa). Sedimentation takes place readily, and since the water has to drain out through the narrow mouths, material deposited round the margins is undisturbed. Hence, a considerable quantity of mud soon gathers. The same thing happens in less enclosed places, but the speed of accumulation is generally less.1 Between Wells and Blakeney there are extensive marshes, only here and there fringed on their seaward side by a ridge. The larger waves break well out, so that in the quiet inshore water sedimentation can proceed.

The same quiet conditions often prevail in parts of estuaries and other embayments. There is considerable marsh growth in Hamford Water, around Canvey Island and Sheppey in the Thames Estuary, in Chichester Harbour, in Southampton Water and Poole Harbour, on the upper parts of Plymouth Sound and Milford Haven, and in the Bristol Channel. Many other places round our coasts show similar growth. The principles of accumulation are similar everywhere and need not be analysed in further detail.

Reference has been made to beach-drifting, to the movement of dunes, and to tidal and other currents. Let us look generally at Great Britain in relation to wind-systems. The prevalent winds (i.e. those blowing most frequently) in any part of this country are from a westerly direction, usually somewhat south-westerly in England and Wales. On our western shores the dominant winds (i.e. those having greatest power or effect) also blow from the same general direction. On the east coast, however, the dominant winds come from the quarter between north and east. How any wind will affect a particular stretch of beach must depend greatly on local conditions. To take an example: south-westerly winds will have great effect in Mount’s Bay, but not just east of the Lizard Peninsula. Allowing, however, for detail of this kind, the westerlies are responsible in the main for eastward directed beach-drift along the Channel, for that up the Bristol Channel, and for that along the coasts of Cardigan Bay. Another factor is also important—the relation of wind-direction to the amount of open water off a particular coast. On the Cumberland coast, the direction of beach-drift is north and south from approximately St. Bee’s Head. This is in general conformity with the amount of open water off these two parts of the coast. However, the relationship is better seen on the east side of England. Along the Norfolk coast, excluding minor exceptions, the travel of beach material is on the whole westwards from Sheringham along the north coast and south-east and south from that same place along the east coast of the county. The dominant winds and waves approaching the Cromer-Sheringham coast are from the quarter between north and east: these, working in with the extent of open water offshore and with the general trend of the coast, are mainly responsible for the outward drift from that locality. The southward drift of beach material continues, apart from a few minor interruptions, as far as the Thames.

On the whole (except in the inner parts of the Firths of Tay and Forth) beach material travels southwards from north-eastern Aberdeenshire all down the east coast of Great Britain. It is, however, along the more open coast south of Flamborough Head that this is most noticeable. Along the south shore of the Moray Firth and the coast as far as Banff and even Rosehearty the general movement of beach material is to the west, and southwards from Wick it is also directed towards Dornoch Firth and Inverness.

On an indented coast of hard rocks it is difficult to generalise. Each separate bay usually has its own beach, and whatever solid stuff travels round the enclosing headlands does so below water level and cannot easily be traced. The individual coves of Cornwall, Devon, Pembrokeshire, the north coast of Scotland, and elsewhere may have their beaches temporarily removed by storms, but they will gather again in normal times. It is probably true to say that each bay has its own shingle and sand economy. On relatively deep water coasts, such as that of the west of Scotland, it is impossible to generalise about the travel of sand and silt.

The main contrasts we have made between the different parts of the coasts of Great Britain may perhaps be related to an even more general factor. Apart from the Lancashire coast, and excluding local occurrences of boulder clay, it is approximately true to say that a line joining the mouth of the Exe to that of the Tees separates a region of softer rocks and simpler structure to the south and east from a more complicated region of harder rocks to the west and north. The former is associated with long lines of open beach and sweeping curves along which beach and long-shore drifting are well exemplified. The latter is often a coast broken by inlets and hard and rocky lines of cliff, along which lateral movement is irregular.

CHAPTER 3 SOME ECOLOGICAL CONSIDERATIONS

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.