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.
CHAPTER 4 FORM AND HABIT OF COASTAL PLANTS
THE MAJORITY of the plants we find growing round the coast have to contend with unusually harsh conditions, and many of them are specially adapted to enable them to survive in their inhospitable habitats. In this chapter we shall consider some of the characteristic growth-forms they adopt.
Undoubtedly the main problem for most of these plants is to obtain adequate supplies of water, particularly in the early stages of their growth. This applies both to those growing in such obviously dry habitats as sand-dunes, shingle beaches or rocky cliffs, and to those growing in saline ground, such as salt-marshes or brackish swamps, although the reason for the difficulty is quite different in the two cases. The whole question of water-supply is sufficiently fundamental to merit discussion in some detail.
To deal first with the dry habitats; the whole trouble here is that they do not retain sufficient quantities of water in their surface layers, since the “soil” they provide is largely made up of coarse particles. The water-holding power of a soil depends in the first instance on the size of its particles. If these are large, water can percolate easily through them, and will also evaporate more quickly because of the large air-spaces between them. Thus the greater the number of small particles, the longer the soil will take to become dry after rain. Furthermore, it is a well-known fact that water tends to stick on to the outside of all relatively small particles on account of the force known as “surface-tension,” and since the total surface-area of a given weight of small particles is clearly greater than that of the same weight of coarse particles, the finer the soil the greater its powers of retaining water. But in addition to a lack of small particles, there is usually a shortage of humus in all the habitats in question. This important material, consisting of dead plant-remains in the process of decay, has already been briefly referred to (see here). Without discussing the varied forms in which this organic matter can occur, the amount present in a soil can be roughly guessed from its colour. Thus dark-coloured “peaty” soils contain the greatest amount and sands the least. All farmers are familiar with the fact that adequate quantities of this material are necessary in all “light” (i.e. coarse) soils, if they are not to suffer from frequent drought conditions. Humus possesses great powers of absorbing water, chiefly because much of it is usually in the form of very small particles of what are called “colloidal” size (i.e. they are so small that they easily pass through
