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
[Illustration: FIG. 4.--WILD CARROT (DAUCUS CAROTA), GROWING UNDER GREAT
EXPOSURE. 1/2.]
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
[Illustration: FIG. 5.--SECTION ACROSS INROLLED LEAF OF CROWBERRY
(EMPETRUM NIGRUM), MUCH ENLARGED.]
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
[Illustration: FIG. 6.--DIAGRAM ILLUSTRATING SUCCESSION OF VEGETATION IN
LAKES.
_a_, Marsh zone; _b_, reed zone; _c_, zone of floating vegetation; _d_,
zone of submerged vegetation.]
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.”