CHAPTER II

PLANT ASSOCIATIONS

“It is perhaps also proper to take into account the situation in which each plant naturally grows or does not grow. For this is an important distinction, and specially characteristic of plants, because they are united to the ground and not free from it like animals.”--THEOPHRASTUS: _Enquiry into Plants_, I. iv.

Before setting about discussing the various types of vegetation which our own country presents, it will be well to have a general idea of the extent to which the main types are developed, and of the amount to which agriculture has interfered with the native flora. We have seen that the natural vegetation of the greater part of the British Isles is woodland: yet so profoundly has human industry altered the face of the country that woodland, natural or planted, occupies only about one-twentieth of the surface of England, rather less of Scotland and Wales, and about one-seventieth of Ireland. Much of the former woodland is now represented by “arable land,” which covers over one-third of England, and about half that proportion of the other parts of the British Isles. Permanent grassland, partly natural, partly replacing ancient woodland, bulks large in England and Wales, occupying about two-fifths of the whole country; in Scotland and Ireland the proportion is much less, but in those countries a large area is under moor, heath, or natural grass, over which wander great herds of sheep and cattle. A. G. Tansley[3] thus contrasts (in percentages) the area of cultivated land (on which natural vegetation has been to all intents destroyed), with the area on which natural or semi-natural conditions still prevail:

England. Wales. Scotland. Ireland.

Cultivated land 75 59 25 ? 20-30 Land under natural or semi-natural vegetation 15-20 40 70-75 ? 70-80

It will be seen how little of the original vegetation of England is left to us for purposes of study--less than one-fifth, almost the whole of which has been influenced to some degree by human operations; while in Scotland and Ireland a much larger area is more or less in its primitive condition. The Scottish mountain-sides and Irish moorlands still to a great extent retain a natural flora, save that the greater number of grazing animals which they now support, as compared with the times when wolves and other enemies roamed unchecked, leaves its impress upon the vegetation.

Viewing the plant world as a whole, its primary divisions, from the point of view of ecology, are governed by the factor of rainfall. It is true that the plants of the Tropics differ profoundly from those of the Temperate regions, and those again from the plants of the Arctic. But this is a difference in the _species_ and families which constitute the vegetation, rather than a difference in the _types of vegetation_ or plant formations which occur. A certain area in Siberia may not have one species in common with a certain area in India, but in both we may find the three great vegetation types of forest, grassland, and desert. A rainfall gradient, on the other hand, will cause a progressive change in vegetation type, as may be seen in crossing North America from east to west, where the forests of the New England States give way as precipitation diminishes to the prairies of the middle States, and these again to the deserts which stretch far over the west. It is only in the extreme north that temperature, apart from precipitation, becomes the dominant influence in determining the presence or absence of vegetation, or its character.

Within any one climatic region--say within the British Islands--the _soil_ in which the plants grow is the controlling factor in determining the character of the plant population. And while a classification by _plant form_--such as woodland, grassland--is often convenient, when we come to analyze the various plant associations which colonize the ground, it will be found that similarity of form-type does not necessarily imply affinity as regards either physiological conditions or floristic constituents. Thus, a Beech wood on the Chalk has really no affinity with an Oak wood on the Coal-measures, save that they are both woods: they shelter plant groups of quite different composition, one a constituent association of the Limestone Formation, and the other of the Formation of Clays and Loams, according to modern English classification. Similarly, the Hazel copse which covers the screes of Farleton Fell has no close relation to the Hazel copses along the Westmorland becks, although the dominant plant--the Hazel--is the same in both cases: soil is the controlling factor, and the one is related to the limestone vegetation of the hill above, the other to the vegetation of the loams and peaty soils of the adjoining mountain-side. In the British Isles the leading plant formations are those of clays and loams, of sands and sandstones, of siliceous soils, of calcareous soils, of peat, of marsh, of lakes and rivers, of salt-marsh, sand dune, and shingle beach; also, governed by the climatic factor, alpine vegetation stands somewhat apart. While the vegetation of some of these, such as salt-marsh or peat, usually presents a uniform aspect, others, such as the clays, sands, and limy soils, display each a characteristic type of woodland and of grassland, as well as other variants, dependent on the composition, depth, and wetness of the soil, the degree of exposure, and so on: these form the _associations_ which together constitute the formation. Each association, if the plants composing it be examined, will be found to consist of an assemblage of species, large and small, brought together by their superior fitness for the particular conditions which prevail. There are mostly in each association one or more dominant species--such as the trees of an Oak wood, or the Heather of a moor--which by their abundance or vigorous growth control the association. The shelter which they give may protect some of the members of the community: the shade which they cast may keep out other plants which otherwise would invade the ground. The association will include some species specially adapted to the particular conditions which prevail, and perhaps not found elsewhere in the area; these are the indicator plants of the association, which give it its special character, and which will help us to identify the association should we encounter it again; there will be others--dependent species--which are attracted by the shade, or shelter, or other advantages which the growth of the dominant plants affords: and there will be others, again--probably many--of wide distribution, which are merely as much at home here as elsewhere. But all grow here because they are better fitted for the particular conditions prevailing than are the other plants of the surrounding area. On Farleton Fell, for instance, among the most abundant species which fill the crevices of the limestone plateau are two ferns--the Limestone Polypody (_Polypodium Robertianum_) and the Rigid Buckler Fern (_Lastrea rigida_). Though there is rocky ground of many kinds in the Lake District, these two plants are never found save on similar outcrops of the Carboniferous Limestone, and they are clearly specially fitted for life in the hollows of this rock. But the same rock crevices also harbour many species which are found equally on the soils derived from the slate rocks or sandstones. To take another instance: many of our most familiar spring flowers are woodland plants--the Primrose (_Primula acaulis_), Wood Anemone (_A. nemorosa_), Wild Hyacinth (_Endymion nonscriptum_). These rejoice in the humus soil which is formed from the dead leaves of preceding years; they flower before the trees are in full leaf, thus securing plenty of light and air for their period of growth; and they are accustomed to have their stems and roots protected from summer heat by the leafy canopy overhead. Transplanted into an adjoining sunny pasture they will soon die out. They are characteristic members of the woodland association of one or more formations. But with them we shall find other species, such as the Wild Strawberry (_Fragaria vesca_), which are equally at home on dry sunny banks or even on sand dunes.

If we ask _why_ the plants group themselves into the associations which we may study any day in the country, in many cases the answer is not obvious. It is clear that while many species accommodate themselves easily to different soils or different degrees of light or of moisture, others have small powers of accommodation, and are in consequence restricted in their range. By long usage many plants have acquired special characters enabling them to live under special conditions--some examples will be discussed a little later--and in some such cases it is easy to correlate the peculiar characters of the plant with those of the habitat. But in many other cases the relation is not obvious. For instance, we cannot tell, by examining a plant, whether it is partial to a limy or to a non-limy soil; yet many plants are poisoned by lime, while others, though generally capable of growing in a soil devoid of lime (if planted in a garden), are nevertheless absent from the non-calcareous areas adjoining their limestone habitat; in other words, they can hold their own on limestone, but are unable to do so elsewhere. The two ferns already mentioned (_Polypodium Robertianum_ and _Lastrea rigida_) are cases of the latter kind; while some of the most familiar of our hillside plants, such as Foxglove (_Digitalis purpurea_) and Broom (_Sarothamnus scoparius_), are instances of the former.

If, however, we consider some of the formations or associations which are the result of extreme conditions of environment, we get more light on the relations between the plants and the factors which control the vegetation. Take the case of the plants inhabiting desert regions such as were discussed in Chapter I. Here the outstanding feature is scarcity of water, and the plants display various remarkable adaptations which fit them for a thirsty life. There are three ways to meet scarcity of water--facilities for gathering it, arrangements for storing it, and economy in using it; and arrangements for all three are familiar features of desert plants. To effect the first, the root-system is extended, and is often enormously developed in proportion to the aerial parts. This adaptation may be studied in the flora of dry places in our own country, such as shingle beaches and sand dunes, which are characteristic semi-deserts. Take such plants as the Sea Holly (_Eryngium maritimum_), the Sea Convolvulus (_C. Soldanella_), or the Sea Sedge (_Carex arenaria_), and compare the extent of the root-system or underground stems with that of the aboveground portions. Fig. 4 represents the Wild Carrot (_Daucus Carota_) as found growing under extreme exposure on the west coast of Ireland. To meet the conditions the tall branched stem has been entirely dispensed with, and the terminal umbel is seated on the ground in the middle of a ring of leaves. In this way the plant prepares to resist both drought and wind. Water storage is often developed in different parts of xerophytes (drought-resisting plants)--in roots, or stems, or leaves, which become much enlarged, and at the same time covered with a

highly impervious skin, so that they act as veritable cisterns. In plants like the Cacti water storage in the stems is carried very far indeed; while in such genera as the Stonecrops (_Sedum_) the leaves are often so swollen and charged with water that they lose up to 98 per cent. of their weight if they are dried. Prevention of excessive loss of water by transpiration is effected in plants of dry places mainly by reduction in the size of the leaf and by protection of its surface. Leaf reduction is very marked in many dry countries. If we compare the flora of the Mediterranean region (a dry area) with that of Middle Europe or of England, we shall be struck with the prevalence in the former of small-leaved twiggy plants--Lavender (_Lavandula_) and Rosemary (_Rosmarinus officinalis_) will serve as examples. Often leaf-reduction is carried much farther, and we need not go beyond our own commons to find a good example, for in the Gorse (_Ulex_) flat leaves are entirely absent and the branches are shortened and converted into prickles, thus largely reducing the surface exposed to the sun and wind. The seedling Gorse has little trifoliate leaves, which remind us of its affinity to the Trefoils and Brooms, but they are discarded almost at once, to fit the plant better for life in the dry, breezy localities which it favours. Reverting to the Mediterranean flora, a characteristic of its plants is the prevalence of a grey hue in their stems and leaves, such as we see in the Pinks and Achilleas of our rock gardens. This is due to a coat of wax, as in the Pinks (_Dianthus_), or a felt of hairs, as in the Achilleas, designed to check excessive transpiration. The coatings of hairs are often of great beauty and complexity, and form an almost impenetrable covering to the leaf surface, protecting the upper side from the fierce rays of the sun, and on the underside sheltering the stomata, or minute openings through which the plant exhales the surplus water drawn up from the roots and inhales carbon dioxide. Another very beautiful device for protecting the underside of the leaf, and one which may be studied in many of our commonest plants, consists of the inrolling of the edges, often combined with a wrinkling or ridging of the underside, so that the stomata are set in deep hollows, communicating with

the open air only through narrow openings. The leaves of some of our common grasses show these characteristics to great advantage. And again the stomata are often sunk in little pits, by which device they obtain further protection. If we now examine the plants composing the sand-dune or shingle-beach associations in the light of these facts, we shall find them full of interest. The plants are well equipped to meet the adverse conditions of a very porous soil, drying winds, and scorching sun. Note the grey felt of hairs which protects the leaves of the Horned Poppy (_Glaucium flavum_), the tough, waxy skin which covers the Sea Holly (_Eryngium maritimum_), the extensive underground stem-systems of the fleshy-leaved Sea Convolvulus (_C. Soldanella_) and Sea Purslane (_Honkenya peploides_). Even the annual plants display similar characters. In the great desert regions the annuals are often quite normal in structure: that is because they appear during the brief rainy season, and pass away before the fierce heat of summer sets in. But on our shingle beaches the annuals grow throughout the summer, and need protection against drought: so the Sea Rocket (_Cakile maritima_), the Sea Whin (_Salsola kali_), and others are very fleshy plants; their leaves are small, with an impervious skin, their root-systems are better developed than in most annuals. The grasses and sedges of these places, such as the Bent (_Ammophila arenaria_), Sea Wheatgrass (_Triticum junceum_), Sea Sedge (_Carex arenaria_) have underground stems which burrow widely through the sand, with an extensive root-system, and tufts of inrolled leaves beautifully protected against over-transpiration and well worth microscopical examination.

If we turn from the shingle beach to the salt-marsh, where water is very abundant, we shall be struck by the peculiar fact that its vegetation displays characters quite similar to those we have just been studying. How can we reconcile this with the theory that the peculiar characters of the shingle-beach plants are correlated with lack of moisture? The explanation is to be found in the fact that plants have difficulty in absorbing water if it is highly charged with mineral substances in solution. In the salt-marsh the heavy muddy soil is impregnated with common salt (chloride of sodium): the plants absorb it with difficulty; and in consequence they are faced with the same main problem which confronts the Sea Holly and Sea Whin, and they meet it in the same way. Indeed, the salt-marsh plants appear to be more highly specialized, for very few intruders from outside can venture in, while on the beach we may meet with many plants which belong to other formations growing successfully, at least for a time. The salt-marsh flora is very exclusive, and contains but few species which we encounter in other situations. Some of them are also found on dry sea-rocks--the Sea Pink (_Statice Armeria_), Scurvy-grass (_Cochlearia officinalis_), Sea Aster (_A. Tripolium_), and so on; showing that soaking soil is in no way essential to their growth. (The first two reappear among alpine plants on some of our higher mountains, pointing again to an analogy of conditions not altogether understood.) But the salt-marsh formation as a whole is perhaps the most distinctive as regards its composition of any of the plant-groups of our country. It is dominated by such species as the grey leathery-leaved _Obione portulacoides_, the small-leaved, thick-stemmed Sea Pink, the Sea Wormwood (_Artemisia maritima_), which is all covered with a silky coat; the pools are fringed with _Scirpus Tabernæmontani_, a dwarf greyish copy of the Common Bulrush of our lakes, and filled with the narrow-leaved _Ruppia_ and _Zannichellia_; and in the muddiest places are little forests of Glasswort, leafless, very fleshy, the flowers reduced to mere essentials and buried in the fleshy stems (Fig. 2, p. 18).

Again, it is easy to trace the relationship existing between plant form and soil conditions in the bogland flora; and these relations, unexpectedly enough, turn out to be analogous to those obtaining in the case of the salt-marsh. The sodden peat, sour and badly aerated, and poor in mineral salts, is poor also in the bacteria which feed upon and destroy dead vegetable matter, with the consequence that acid humus compounds collect in the half-decayed vegetable mass; water charged with these substances is as unsuitable for plants as is the water of the salt-marsh. In spite of the wetness of the peat, water is in this case also a desideratum; and the moorland plants, like those of the sea fringe, possess special adaptations for economizing it. This usually takes prominently the form of a reduction of leaf-surface. The dominant plants, such as the Ling (_Calluna vulgaris_) and Purple Heather (_Erica cinerea_), have minute leaves with reflexed edges and special structure to protect the stomata. The grasses and sedges which abound have similar characteristics; the whole vegetation tends to be small-leaved and long-rooted. A few of the plants, such as the Eyebright (_Euphrasia_), eke out the scanty food-supply by a semi-parisitism, robbing their neighbours of portions of their hardly-won sustenance; one or two others, such as the Bladderwort (_Utricularia_), which floats in the bog-pools, and the Sundew (_Drosera_), which fringes their edges, entrap insects and digest their juices, helping out their scanty rations with an animal diet. On the moors the peculiar soil conditions determine definitely the type of vegetation, which, over large areas, is as uniform and monotonous as that of the salt-marsh.

We see, then, that the peculiar character of several of the most marked of native plant formations--those of shingle, of salt-marsh, and of moor--are due primarily to scarcity of water. They are drought formations, produced either by physical drought, as in the case of shingle, which fails to retain water, or by physiological drought, as in the salt-marsh or bog, where, though water is present in abundance, it is not in a condition in which plants can readily make use of it.

Let us now go to the opposite extreme, and consider the plant formation which characterizes lowland lakes and rivers, where water suitable for plant use is

superabundant. In such places we are faced with a vegetation exhibiting a great number of species and a marked variety of form, and by no means so easy to correlate with its environment as those which we have been considering. In a wide sense, the nature of the vegetation is largely dependent on the degree of aeration of the water and the amount of dissolved mineral salts which it contains, an increase of either (within limits) resulting in a richer flora. But in any one area it is clear that depth of water is the controlling factor: the plants are arranged in zones, one succeeding another as the bottom shelves. Two main zones are conspicuous: (1) A zone of tall reed-like plants near the margins, which farther out is succeeded by (2) a zone of lax floating plants which either have leaves resting on the surface or grow entirely submerged. Above the former a belt of marsh plants links the reed zone with the vegetation of the soils of normal moisture; below the latter, should the water increase in depth, we reach an aquatic desert region, where the reduction of light renders plant growth difficult, and eventually inhibits it. Let us consider the conditions prevailing in the reed zone. Here the plants are essentially aerial, and though they have their feet in water, the stems and leaves rise far above it. Water-level is variable in lakes and rivers; the plants are usually tall, so that even in case of flood the leaves and flowers will not be drowned. Wave action on lake-shores is somewhat violent, and in flooded rivers a strong current may sweep through the vegetation; we see the advantage of the slender elastic stems and narrow leaves that characterize the plants: compare Reed (_Phragmites_), Reed-mace (_Typha_), Flag (_Iris_), Bur-reed (_Spargarium_), Bulrush (_Scirpus_); and these characters also fit them for the windy nature of their habitat. The denuding effect of wave or current action is countered by the network of creeping stems and abundant roots which the plants possess, forming a tough felt which floats, and by its growth and decay helps materially to form fresh land. Another effect of the creeping and branching stem-systems is the production of extensive and dense groves of many of the species.

When we pass beyond the reed zone, a completely different type of vegetation prevails. Here the plants are essentially aquatic. They make no effort to raise their stems and leaves above the water surface; but almost all of them raise their flowers into the air, though the seed is often ripened below the surface by a downward curving of the stem. These plants, surrounded by water, use their roots chiefly as anchors, and absorb through their stems and leaves the water from which they obtain the necessary mineral salts. As regards the supply of oxygen and carbon dioxide which the air supplies to them, those with floating leaves absorb it from the atmosphere, while those whose leaves are submerged have to subsist on the small quantity of these gases which is dissolved in the water--no wonder that such plants are rare in stagnant waters where aeration is poor. To assist respiration and transpiration, abundant and often comparatively gigantic air-spaces are provided in roots or stems or leaves, giving them a cellular appearance, and making them singularly light and spongy in texture. The leaf system of those plants which possess floating leaves--such as Water Lily (_Castalia_ and _Nymphæa_) or Common Pondweed (_Potamogeton natans_), are well worth study. They are tough, to withstand battering by waves; the stomata are situated, not on the lower side of the leaf, as in land plants, but on the upper side, where they are in contact with the atmosphere; and the upper surface is waxy or oily, so that it is not wetted and the stomata are not blocked. Changes of water-level are met by means of long flexible stems, rising not vertically from the root, but at an angle, so that the leaves can rise with a rise of water-level. But not all the plants are anchored to the bottom. Some, which favour especially ditches and quiet waters, float freely with roots hanging down in the water--the Frog-bit (_Hydrocharis_) and Duckweeds (_Lemna_) are familiar examples. In the Duckweeds true leaves are absent, but the tiny stems are flattened and green and serve the same purpose, the minute flowers being borne on their edges. A few plants, such as the smallest of the Duckweeds (_Wolffia arrhiza_) and the Bladderworts (_Utricularia_), have gone farther still, and have dispensed with roots altogether. In _Wolffia_, indeed, the degeneracy of structure which results from the simplification of life problems in plants which live thus floating freely in water, is carried to its extreme limit. Leafless, rootless, and almost flowerless, it maintains itself by the budding of its tiny green fronds, a life-history as primitive as that of the lowly Algæ among which it lives. In the Bladderworts, the long flaccid stems, clothed with much-divided leaves converted in part into ingenious insect-traps (see p. 188), hang limply in the water, sending up boldly into the air their flowering shoots with yellow Snapdragon-like blossoms. In most of such free-floating plants, compact buds are formed at the tips of the shoots in autumn, and while the rest of the stem dies away these sink to the bottom and remain there safe from frost and storm until the spring, when they rise to the surface and produce a new crop of plants.

We have now glanced at the most distinctive of the plant formations which we meet with in our own country, and find that they accompany extreme conditions relating to water and soil: it remains to return to the consideration of the vegetation which develops under conditions of a more normal character--on ordinary soils, in fact, which are neither very wet nor very dry. Such conditions are precisely those which are required for agricultural purposes; and over the wide areas where they prevail, we find, as pointed out already, mere fragments of the native associations remaining in an undisturbed condition. This renders their study more difficult, and the difficulty is heightened by the fact that while the physical conditions show no contrasts so marked as those which we have been considering, the formations which can be distinguished are several, and each contains several associations--often a woodland, a scrub, and a grassland type. Thus, the formation which occupies calcareous soils exhibits characteristic woodlands--woods of Ash (_Fraxinus excelsior_), for instance, and on the downs peculiar woods or scrub of Box (_Buxus sempervirens_), Juniper (_Juniperus communis_), Yew (_Taxus baccata_), or Hazel, as on Farleton Fell. It also bears some very marked types of grassland, as on the chalk downs; and the limestone pavement of Farleton Fell is a special variant of this. Similarly, clays and loams, sands, and siliceous soils possess similar characteristic types of vegetation. But the consideration of these would occupy more space and lead us into more technical detail than the scope of this book warrants. For an account of these associations, written by botanists who have made a special study of them, the reader is referred to Tansley’s “Types of British Vegetation.”