THE PRINCIPLES OF PLANT GEOGRAPHY AND THE CHIEF PLANT FORMATIONS OF EUROPE AND NORTH AMERICA
We have now taken a general survey of the earth’s surface, have noted its mountain heights and its ocean depths, watched the formation of hills and valleys which is due to the joint action of atmospheric agents, running water and ice, and considered briefly some of the points of interest about climate. We next pass to that most characteristic feature of the surface, its clothing of plants. Except where the surface of the ground has been artificially sterilised by man, or is rendered unproductive by ice, by lava, by a total lack of water, or by the existence of poisonous salts, it is clothed with vegetation, and it is the presence of this vegetation which is its most obvious character.
Here, however, as in other regions of thought, the geographical standpoint has only been reached slowly. Man’s habit of analysis made him study grasses and trees for long generations before he got back to the forest and to the grassland as they occur in nature. Plants as individuals are the province of the botanist, but those plant groups which are the expression of the interaction of climatic factors, soil, and surface relief, are the concern of the geographer.
When we take a general survey of the face of the earth from the point of view of plant geography, we note three main conditions. In certain regions, alike in the tropics and in temperate zones, we find that plants reach their maximum size, combined with great differentiation of structure, and the formation of woody stems which offer great resistance to varying conditions of climate and weather. Such highly-organised plants form forests, which still dominate over a large part of the earth’s surface.
Man’s nearest allies, the anthropoid apes and the monkeys, are for the most part forest animals, and the lowest races of men are still forest dwellers. Where man is a forest dweller he seems not to reach his full size, as we see in the case of the pigmies of the Congo forest, and the negritos of the Philippines, and he suffers from a chronic insufficiency of food, which acts as a check both to his mental and physical development. There has, therefore, always been war between evolving man and the giants of the plant world, a war which has swept the forests away from many of the more civilised parts of the globe, and which still continues, though man’s victory is now so complete that he can afford to be generous, and give protection to the remnants of his former foe.
But over parts of the globe the climate, and especially the amount or distribution of the rainfall, makes it difficult or impossible for forests to grow naturally. Here other types of plants, lower in stature, and evading rather than facing the problems of winter cold or summer drought, flourish and form what we call the grasslands. The grasslands favour man in several respects. They feed the animals upon which he depends for food, for clothing, and for the conveyance of his person or property, and they offer much less resistance than the forest to his agricultural operations. Even the large herbivorous mammals which in their wild state haunt the forests, usually leave these at night to feed upon the grasslands, so that it is the grasslands which have largely fed man at every stage of civilisation. The atmospheric conditions within the woodlands also, the deficient sunlight, the humidity, and so forth, seem unfavourable to human development.
Where the conditions are especially unfavourable to plant life, we find that even the grassland plants are unable to keep up the struggle, and diminish in number, losing their power of forming a complete covering for the soil, and thus the grassland passes into desert, whether the hot waterless desert of low latitudes, or the cold frozen desert of northern ones.
In the most general sense, then, we may say that these three formations, woodland, grassland and desert, divide the surface of the land among them, and between them there is constant conflict. The grasslands are for ever attempting to encroach upon the woodlands, and in this attempt they have been assisted, sometimes to too great an extent, by the operations of man. Similarly the desert is always striving to encroach upon the grassland, and in this endeavour it has been sometimes involuntarily aided by man, who has also done much voluntarily to reclaim the desert land for the grasses.
Let us note next the particular conditions which favour woodland, grassland and desert respectively. The distribution of plants over the surface of the earth at large is determined by a number of factors, by the amount of heat, by the amount and distribution of precipitation, by the nature and strength of the winds, by the characters of the soil, and so on. But forests occur under the equator and also far to the north; we have cold deserts as well as hot ones; there are extensive grasslands in the Sudan as well as in the Canadian Far West. This proves that the varying amounts of heat may be neglected in considering the cause of the distribution of the three great plant formations.
Again, the soil is of minor importance, for different types of forest and of grassland occur on different types of soils. We are thus led to the conclusion that it is the precipitation and the wind which determine the distribution. To understand the reason for this we must consider the needs of different types of plants in the matter of water.
Plants can only take in the mineral constituents of their food in the form of a solution, and this solution must be weak, or it has a poisonous effect. For example, sulphate of ammonia is a valuable manure, but if a considerable amount be dissolved in water and applied to the roots of a growing plant, death may very likely take place.
It is a necessary consequence of the fact that plants can only absorb weak solutions of their food salts, that their roots take in more water than is actually needed by the plant. One of the functions of the leaves is therefore to get rid of surplus water, the process being called transpiration. Transpiration takes place faster in a tall plant like a tree, which grows up into dry layers of the air, than in a low plant like a grass. It takes place faster in windy weather than in calm. Other things being equal it takes place faster in warm weather than in cold, and the larger the plant and the more numerous its leaves the more water is given off, that is, the more water is returned to the air from the soil.
The result of all this is that forest trees require far more water than grassland. It has been calculated that a beech wood aged 50 to 60 years gives off during the growing season 354 tons of water per acre, which illustrates the drying effect of the presence of the wood. Similarly, the effect of tree-planting in the marshy regions of France and Italy, where the soil as a consequence has dried and the marshes disappeared, shows how great a demand upon ground water trees make, as compared with grasses and low growing herbs.
On the other hand, although trees take an enormous amount of water from the soil, they can draw their supplies from a large area. It is the extremities of the fine branches of the roots which take in the water, and these pass deep down into the soil, and spread out over a vast area. In other words, trees avail themselves of the water in the deeper layers of the soil, and can tolerate relatively long periods of drought, if the surface drying of the soil does not extend to the deeper layers. In hot summer weather grasslands brown and wither long before the trees show any signs of water-famine.
In consequence, we may say that as long as the total rainfall of a region is sufficient to ensure a constant supply of moisture in the subsoil during the growing season, trees can thrive, even if little or no rain falls during this season. On the other hand, drying winds are very hurtful to trees, especially if they occur at a period when the tree is unable, either because of the coldness of the subsoil, or because of its dryness, to take in fresh water to replace that which is lost. The hurtfulness of late frosts is largely due to the cold suddenly checking root absorption at a time when the growing parts, acted upon by the spring winds, are giving out water freely.
Grasses transpire less freely than trees, but their root system is much shallower and less well-developed. They depend upon the water in the upper layers of soil, and must have frequent, even if gentle, showers during their growing season, while they are quite indifferent to drought and even to cutting winds during their resting period.
A little reflection will show that it results from these facts that woodland, grassland and desert do not form a continuous series. It may quite well be that woodland passes through scrub into desert without the intervention of grassland. Right across Europe there is (or was) a broad belt of forest. Southward towards the Mediterranean this thins out into a characteristic form of scrub, called maquis, whose characters we shall describe later, and this scrub passes in all directions into desert land. Here no belt of grassland intervenes, for the rainless Mediterranean summer makes the growth of grass virtually impossible, except where special conditions, e. g. hills, introduce modifications. Contrasted with this we have the conditions in North America where, e. g. in Canada, the western coast is densely forest-clad, as is also the eastern region. In journeying eastward after crossing the Rocky Mountains the forest dies away into grassland, and the same thing happens, though more slowly, in a westward journey. The reason is that in this case there is a steady diminution of precipitation on passing to the interior, but what precipitation remains is, as we have seen, largely, though not wholly, summer rain, and is, therefore, sufficient to determine the growth of grass, though not of trees.
Again, in North Africa the forests of the Atlas Mountains pass directly, without intervening grassland, into the Sahara desert, but to the south of the desert the grassy and park-like Sudan separates the desert from the luxuriant tropical forest. In the latter case, however, it is possible that man’s influence has counted for something.
On mountains, in whatever latitude, the conditions are much more uniform, partly because it is wind, assisted by temperature variations, which is the dominating factor. Moisture is usually abundant, but high up what is called physiological drought occurs; that is, the temperature is too low for the plants to be able to absorb the abundant water.
In ascending any mountain, the following are the chief changes which occur. The lower slopes will probably be cultivated. As we ascend the precipitation increases, and forests appear. First we have probably a belt of deciduous trees, passing above into the more resistant conifers. This belt usually ascends higher on the south than on the north side, and higher on mountains which occur in a group than on isolated peaks. As the wind is more and more felt, and increases the dangerous transpiration of winter the trees become more and more dwarfed to escape its force. There may be a belt of prostrate mountain pines above, marking the tree limit; in any case the trees are gradually replaced by dwarfed shrubs. Then comes the zone of Alpine plants, the grasses making a complete sward, but being accompanied by many other plants. Gradually, as the soil becomes scantier, and the surface more rocky and exposed, the continuous sward disappears, and the conditions of a cold desert appear. A few scattered plants occur, ceasing near the snow-line, the highest being usually plants of simple structure like mosses and lichens.
As we have already indicated, in the case of the mountains of Europe there are often glacial shelves at considerable elevations, whose covering of fine débris determines the growth of peculiarly fine grass. The economic value of this grassland has in many cases in the Alps induced man to destroy the forest in order to increase pasture land. The result has often been disastrous, for once the trees are cut down the forest soil is rapidly destroyed by weathering, especially on slopes, the courses of streams are altered by the more rapid run-off, and widespread flooding and destruction of pastures have sometimes resulted. In North America, similarly, man’s attempt to increase pasture land or arable land at the expense of woodland has often led to disastrous consequences.
We have already spoken of the special features of the Mediterranean climate, and indicated that its peculiarities are reflected in its vegetation; we must now consider this vegetation in a little more detail. The fact that the region is chiefly visited by the inhabitants of more northern climates in spring gives rise to a somewhat erroneous impression in regard to the plants. In spring the Mediterranean vegetation is at its best. The mild winters permit the plants which further north die down or cease to grow, to go on blooming. The rains so moisten the soil that the first warm days cause very rapid growth in those plants which finish their activities before the hot, dry summer begins. They must flower and seed in spring, and die down till the rains of autumn awaken them again.
In our own country we have a few plants which hurry through their activities in this way. The lesser celandine, the wood anemone and a few others strive to flower and fruit before the forest trees are thickly clad with leaves. The snowdrop, even the wild hyacinth, though it is much later, similarly limit their active life to a short period in spring. This phenomenon, only suggested in our climate, is very marked in the Mediterranean area.
That region is especially characterised by its richness in bulbous and tuberous plants. These, as all who have grown hyacinths or narcissuses know, demand relatively large amounts of water during their short growing period. In spring, therefore, the shores of the Mediterranean are bright with many kinds of anemones, with narcissus, asphodel, bell hyacinth, Allium, tulips, and so on, all awakened by the spring warmth and the spring rains. Accompanying them are many bright-coloured annuals, also in a hurry to race through their life-history before the terrible drought of summer. Now also the grass grows, and the autumn-sown corn becomes tall. As the weather grows hotter and drier, the plants with bulbous and tuberous roots die down to the ground, the annuals die altogether, leaving their seeds to wait till the autumn rains before they sprout. The grasses turn brown, and the peculiar parched appearance of the Mediterranean summer spreads over the land.
To a northern visitor at this season it is not luxuriance but desolation which is the prevailing note. Except on the hill slopes there are no masses of broad-leafed foliage trees—there is not the deep bright green characteristic of the summer woods further north. The trees do not reach a great size; the leaves are usually small, and the fact that they strive to avoid the sun by arranging themselves with the edge upwards instead of the flat surface, makes them appear smaller than they are. They are often needle-shaped, sometimes shining and coated with resin, sometimes silvery owing to a coating of hairs on the under surface. Many plants have spines or thorns, and succulent plants like agave, aloe and prickly pear are common. The absence of a complete covering of vegetation causes the surface soil to dry completely, and so form clouds of dust which adds to the generally desolate appearance. Indeed, the brown powdery appearance of the soil is one of the points which especially strikes the stranger, accustomed to the darker, moister soil of the north, always covered with vegetation, except where man has interfered.
Here and there, however, are indications that even this parched brown earth holds wealth for man. The vines, if dusty and far less luxuriant than one expects, are loaded with ripening fruit. The gorgeous scarlet flowers of the double pomegranate gleam amid the dark foliage; the gnarled and twisted olives show on close inspection masses of small green fruits; the oleander bushes are covered with pink flowers; there are great round balls on the orange and lemon trees, and many other fruit trees are loaded with produce.
Let us sum up first what man gains from the plants of the Mediterranean, and then look at some points in regard to the wild plants. In the first place, we see that man takes advantage of the rapid growth of annuals in the early part of the year. The annuals most useful to him, here as elsewhere, are, of course, the cereals, especially wheat, which, if sown in autumn, is nourished by the winter rains, and grows rapidly with the warmth of spring to ripen in May, June or July, according to the locality.
In the second place, certain trees or shrubs, by reason of their resistance to drought, and their elaborate root system, which enables them to gather water from the deeper layers of the soil, will produce succulent fruits without needing artificial supplies of water. The most important of these, throughout the whole Mediterranean area, are the vine and the olive. The olive supplies the oil which is all the more necessary in that the absence of grass makes pastoral industries, and therefore the production of cheese and butter difficult or impossible except in the high grounds, while the vine supplies the wine which with bread and oil form the essential parts of the diet of Mediterranean man.
The olive tree, which is indigenous, may be regarded as one of the most characteristic trees of the area, and it is interesting to note that the novice not infrequently confuses it with another tree, almost as characteristic the evergreen or holm oak. The two are not nearly related, the olive belonging to the same family as the lilac and privet, while the evergreen oak is a true oak. Both trees, however, show similar adaptations to summer drought, and their resemblance to one another is a good example of convergence due to a similar environment. Both have small evergreen leaves; small that they may not lose too much water in summer, evergreen that they may assimilate even during the winter. Both have their leaves silvery beneath, which again prevents loss of water; both have gnarled trunks, branching low down, in order that the leaves may avoid the dry upper layers of the air. Adaptations of this kind are present to a greater or less degree in all the trees which are tolerant of Mediterranean conditions, and many of these trees yield useful fruits.
In addition to the cultivated plants mentioned, a great number of others are grown within the area, as we shall see later, but the point of interest is that the plants which have been of importance in the history of the region have been either annuals which ripened early, or fruit-bearing trees with special adaptations to resist drought.
Apart from the annuals and the bulbous and tuberous plants already described, the wild plants are chiefly shrubs or stunted trees with similar drought-resisting characters. During the long ages he has inhabited the Mediterranean, man has doubtless contributed largely to the destruction of the forests which are now, as we have seen, represented by the stunted scrub or maquis. But on climatic grounds we cannot suppose that the Mediterranean forests had ever the luxuriance of those further north, or of the tropical forests of the south.
Where there is sufficient rain chestnut woods occur, but this is only on the hill slopes. Above the chestnut, beech may occur, as in Sicily. The maritime pine and the Corsican pine form open woods in the damper places, and the picturesque stone pine, with its rounded head, is very characteristic. We have already mentioned the evergreen or holm oak as common, and the cork oak occurs abundantly in some places. These trees, with the cypress, must have formed the primitive forests, and they still constitute the most important forest trees of the area. The occurrence of a native palm (Chamærops) is interesting as suggesting the warmth of the climate, and even on the European shores the date palm is extensively planted, though its true home is the margin of the African and Arabian deserts.
Of the characteristic shrubs the most striking are perhaps the many species of Cistus, with large almost rose-like flowers, and leaves which attempt to adapt themselves to the climate by many different devices. Sometimes they are stiff and leathery, sometimes resinous, sometimes hairy. Many plants in the area have a coating of resin on their leaves. This, no doubt, preserves them against loss of water, but also probably protects against grazing animals. Goats thrive in the Mediterranean partly because of the catholicity of their taste in vegetation, and in consequence the plants have had to protect themselves against their appetite as well as against drought. Only those with some disagreeable quality, hairs, spines, resin, strong flavour, etc., could hope to protect themselves in the dry season, when grass is virtually absent. It is in consequence common to find aromatic or strongly-flavoured plants with glandular leaves; lavender, rosemary, myrtle, etc., are examples.
Other shrubby plants associated with the Mediterranean are oleander, the noble laurel, the tree heath, arbutus, many kinds of broom, and generally evergreen shrubs specially adapted to resist drought.
Let us turn from this picture to the appearance presented by Central and Northern Europe. As we have seen, the forest which once covered most of the area, except the steppe region of southern Russia, has largely disappeared, but enough remains to enable us to reconstruct the picture of the original forest.
As contrasted with the (chiefly) evergreen woodland of the Mediterranean, the forests of the low grounds are here deciduous. In summer clothed in magnificent foliage, well adapted to give off enormous quantities of water, in winter the trees stand tall and bare, exposing nothing but their branches to the winter blasts. While the buds of Mediterranean plants have no special means of protection, the typical forest trees of Central Europe have their buds carefully sheathed in scales, clothed in hairs, or coated with resin, to keep out alike the cold and the damp of the northern winter. While the leaves of Mediterranean plants are usually small, often coated with hairs beneath, often resinous, and so on, the forest trees further north have large leaves of delicate texture, with no special protection against drought.
Again, while the luxuriant forest of the tropics includes many different species of trees, the deciduous forests of cool temperate regions contain few species, and are often pure woods, that is, consist of one dominant species, forming beech woods or oak woods, and so on. The dense shade of the beech makes undergrowth difficult or impossible, but the other woods have a complicated undergrowth of many different kinds of plants, especially pronounced in spring before the leaves appear on the trees. But this undergrowth never reaches the luxuriance that it does in the tropical forest, and creepers and climbing plants are few.
As we ascend from the low ground to the higher, or as we travel northwards to high latitudes, the broad-leafed deciduous forests are replaced by coniferous ones. European conifers, with the exception of the larch, are evergreen, and all are more tolerant of cold and wind than deciduous trees. Pines, spruce, fir, larch, and silver fir are the most important kinds. Both at high altitudes and in high latitudes these conifers are often accompanied by birch, which is not a cone-bearing tree, but is very tolerant of cold and wind.
To the north there comes sooner or later a limit beyond which the cold and winds make further tree growth impossible. Here we come to a tundra region, where the place of trees is taken by low-growing shrubs, with small leaves and other adaptations to ensure against excessive loss of water. It is, as it were, the reappearance of the Mediterranean type, but here the cause is, not the absence of water, but the fact that the cold makes it impossible for the roots to absorb it. A condition of physiological drought results, and only plants well adapted to prevent undue loss of water can resist such conditions of life.
A somewhat similar type of vegetation occurs over vast areas in the more northern parts of Europe, forming the moors and heaths of much of Scotland, of parts of England and Ireland, of parts of Germany, and so on. Here the presence of peat produces conditions very unfavourable to plant life, except to certain shrubby plants such as heather and other plants of the heather family, juniper, bog myrtle, and so on, and some grasses and sedges, etc., all of which have special adaptations to life in a peaty soil. Over the large areas, therefore, covered by these heaths, trees are absent, or few, and this stunted shrubby vegetation takes their place.
Large areas of natural grassland, except for the tracts of pasture land already described in the mountain regions, are infrequent in Europe. They occur in Southern Russia and in the Hungarian plain, and form part of that great series of steppes and plains which stretches into Asia, and passes into a region of deserts.
The conditions favourable to the growth of grass here, instead of trees, seem to be purely climatic. Very important is the prevalence of strong cold winds during winter, which is a period of drought. The scanty rains come in early summer, which suits grasses admirably, while the total precipitation is too slight for trees. The summers are hot, and the rains cease early and give place to a period of drought, very injurious to trees, while it injures the grasses little, owing to the fact that they have had time to make their growth.
The abundant natural growth of grass makes these steppe regions well suited to the pastoral industries, which tend, as civilisation progresses, to give place to agriculture.
To sum up, we have seen that looking at Europe as a whole three great plant formations are represented. We have, first, the cool temperate forest, which once extended over the greater part of the continent, wherever the conditions were suitable. This has now largely given place to arable land. Next, we find round the Mediterranean sea, and in those great peninsulas and islands which are bathed by it, a zone of modified woodland passing into scrub, remarkable for the rapid growth of annuals in the early part of the year, and for the abundance of trees bearing useful fruits. Finally, linking Europe to temperate Asia, we have belts of steppe land, characterised by a luxuriant growth of grass in the early summer, and fitted by nature for pastoral industries, which do not thrive near the Mediterranean. Another way of putting the same facts would be to say that Europe proper is a region of temperate forest, linked to Africa by scrub land passing into desert, and to Asia by steppe land passing into desert.
The flora of North America, owing to the size of the continent, offers more resemblance to that of Asia than to Europe.
Bearing in mind what has been already said about the structure of North America—with its western mountain range and eastern uplands enclosing between them a region of moderate relief—and also what has been said in regard to its climates and to the influence of climate upon vegetation, it is relatively easy to deduce the main points in regard to the flora.
To the far north there is a treeless tundra region, quite comparable to that which occurs over vast areas in North Asia, and on a reduced scale in the northern part of the continent of Europe. Next we have a wide band of predominantly coniferous forest, which, although its species are different, yet in broad outline is entirely homologous with the coniferous forest found in northern Asia, south of the tundra region. In Canada this forest consists of spruces and larches, the species being peculiar to the continent. Mingled with the conifers are smaller numbers of the hardier deciduous trees, such as birches, poplars, and willows.
What we have already said as to the climatic differences between the eastern and western sides of continents will at once suggest that this band of forest is not likely to run directly across the continent from east to west. In point of fact it stretches from Labrador in a north-westerly direction to Alaska, leaving almost the whole of the western seaboard to be occupied by another type. This type is the extraordinarily luxuriant and beautiful western forest, consisting for the most part of conifers. It is largely these conifers which have enriched European parks and gardens within recent years, and although it is perhaps the great Sequoia (Wellingtonia) gigantea which has most impressed popular imagination, it must be remembered that size and luxuriance are characteristic of many species. This western forest stretches down the western seaboard to the State of California, and, indeed, persists until increasing aridity makes forest growth impossible. Its great luxuriance, compared with the scantier forests of the Mediterranean region in Europe, is partly to be ascribed to a greater rainfall, and doubtless partly to man’s interference, for the original forests of the Mediterranean must have been largely destroyed, as the western American forests are in process of being. One must remember also that the proximity of mountain ranges to the seaboard in western North America gives a heavy rainfall, and suitable places for forest growth. The fact that the trees are predominantly coniferous gives them great resistance to the summer drought. In front of the mountain ranges the coastal plain is occupied by an evergreen scrub vegetation comparable to that of the lowlands of the Mediterranean basin.
In British Columbia, where the Cascade Range lies at no great distance from the Rocky Mountains, the western coniferous forest practically clothes the whole area from the coast to the main range, but further south, where the Cascade Range and its continuation the Sierra Nevada are widely separated from the main range, a dry and semi-desert region occurs, between the two, which bears a desert type of vegetation, including especially a plant related to our wormwood, called sagebrush, with cactuses in the warmer parts. Another area which is too arid to carry trees, except where local conditions raise the rainfall, extends from Texas northwards to about the latitude of Edmonton or Battleford, and lies in the “rain shadow” of the Rocky Mountains. This is the region of the Great Plains, mostly too arid to carry anything but herds of cattle, and mostly forming natural pasture, being thus analogous to the steppes of Asia.
Eastward the rainfall increases, and we pass from the area of unreclaimed pasture to the prairies, now largely laid down to wheat and other food plants. Southward the Great Plains pass into the deserts of Mexico, but northwards they are separated from the northern coniferous forest by a belt of aspen, and it is in this region that the Canadians are steadily pushing the cultivation of wheat into the plains, wherever the local rainfall makes this possible.
So far we have left south-eastern Canada and the whole of the eastern and south-eastern States out of consideration. Speaking very broadly, we may say that all this area is clothed by a forest of mixed coniferous and broad-leaved trees which is comparable to the forest which covers the greater part of temperate Europe. But it is not to be expected that a forest which extends from the northern shores of the Gulf of St. Lawrence to the extremity of the peninsula of Florida, that is, through about 25 degrees of latitude, should be uniform throughout. In point of fact, botanists distinguish three separate zones. In south-eastern Canada and the New England states the Weymouth pine (Pinus strobus) predominates, being accompanied by limes, ashes, maples, oaks, elms, chestnuts, and so forth. Further south, and especially further west, extending to the Mississippi plains, there is a deciduous forest extraordinarily rich in species. Practically all our common genera of forest trees are represented, sometimes by very fine species, but in addition there are many genera with no European representatives. Very striking is the abundance of magnolias (whence the name of magnolia forest sometimes given to this type), and species of the laurel family, as well as of liquidambar. The magnolias and liquidambar are especially interesting, because they once occurred in Europe, their disappearance there being probably caused by the glacial period as explained on [p. 78].
We have emphasised above ([p. 137]) the luxuriance of the forests of the west coast of the States, but it should be noticed that luxuriant as its conifers are, there is a remarkable poverty in broad-leaved forms, as compared with these eastern forests, and this even in the warmer parts of the west coast. The reason is probably the same as in the Mediterranean region in Europe. The existence of a belt of desert to the south of the present “Mediterranean” region of western America made it difficult for the trees to migrate southwards at the onset of cold conditions in the glacial period, and thus many forms, which are known to have existed in California in Tertiary times, have now completely disappeared from the region, while they persist in the eastern forests to this day.
The third type of forest which occurs in the eastern half of North America is the “rain forest” of Florida and parts of the adjacent states. Here the rainfall is abundant all the year round, with a summer maximum, and the temperature is high. There is thus no need to economise water, and where the soil permits there is a luxuriant type of forest, which recalls that of the tropics, although it is poorer. Where soil conditions are unfavourable we have pine woods, conifers throughout the eastern United States always taking advantage of conditions relatively unfavourable to the broad-leafed trees.
Thus if we follow the eastern seaboard of the United States from Labrador to Florida we pass through the following floral regions:—(1) Coniferous forest, with relatively few species, (2) mixed coniferous and deciduous forest with chiefly the harder types of deciduous trees, (3) predominantly deciduous forest with many of the larger-leafed and more delicate forms, and finally (4) forest of the sub-tropical rainy type, intermixed with coniferous woods on the barren sandy soil and in the swamps.
The western coast shows more uniformity, the western type of coniferous forest stretching from Alaska to California, though it is richer, and more luxuriant in the warmer regions when moisture is still obtainable. As the moisture diminishes the forest dies away and desert or semi-desert conditions supervene.