The North American Forest
( Originally Published Early 1900's )
THERE are in the northern temperate zone three great forest regions : The eastern Asian, including principally the eastern portion of Siberia, Manchuria, and the Japanese archipelago; the European; and the American forests, including great portions of the United States and Canada. The American forest may be subdivided into three groups : The great eastern forest, which originally covered nearly all the territory on the Atlantic side of the Mississippi, and in several places extends considerably beyond that river; the forests of the Rocky Mountain regions, including the minor mountain ranges of the great basin; and the Pacific coast forest. The immense areas lying between these subdivisions are occupied by the grass plains of the eastern slope and the alkali and sage-brush deserts of the interior, both of them distinguished by the almost total absence of tree growth. On the northern edge these three forest zones converge, so that there is a subarctic forest belt stretching from ocean to ocean. We have before us the task of describing the relations existing between the life of the American people as a social organism, and the forest with its manifold productions. It seems natural to begin by considering the characteristics of the forests themselves, such as they were before the -hand of man wrought the vastly changed conditions under which they now exist. In doing so we cannot think of delineating in detail the botanical or physiographic phenomena of this great form of plant life. He who would learn of the different species of trees composing it, their dendrological character, their manner of life and growth, will have to look elsewhere for such information. Nor will we have space to wax eloquent about the beauty of our sylvan inheritance. Al-though the esthetic point of view has its very important place in our relations to surrounding nature, we cannot in this volume concern ourselves with that aspect. What we in this chapter seek to impress firmly upon the minds of such as may feel inclined to follow the author through these pages is simply this : the manner in which the various trees are associated in the wilderness is not the result of accident, but determined by complex, but very definite laws. If one had a perfect knowledge of those laws he could predict with absolute certainty the number, species, and character of the trees to be found in any given locality. And further : the forest is not a thing that was made once and remained the same ever after. It has grown and developed as a whole, just as each individual tree grows from infancy to old age. Again, this growth and constant change takes place according to very complex and unalterable laws. Each tree does not constitute an independent entity, but is affected in every moment of its life by every other tree and minor plant of the entire forest, and in turn itself influences every other tree. The forest therefore constitutes an organism, having a united life different from, but de-pendent on, the life of its individual members. - Within this organism, a never-ceasing struggle is going on, tree fighting against tree, species against species; while the entire organism carries on a warfare with other plant associations, such as the prairie and the bog, in which it is sometimes vanquished, sometimes victorious.
To the ingenuity of man, with the help of such knowledge as he may acquire of the laws regulating the life of the forest organism, it is possible to make use of the various phases of this warfare for his own purposes. By creating conditions favoring some particular species of tree he is able to help it to spread and flourish at the expense of its competitors; and similarly he may create conditions which help the forest as a whole to maintain itself against the aggression of other plant associations. No small part of silviculture—or the art of caring for woodlands—consists in just such interference in the natural processes of forest development, rather than planting and sowing. In order to understand the relations of forestry to our national life, the reader should have in his mind an outline at least of what American forests are like, and also of how they came to be what they are.
Each of the three main subdivisions of the North American forest has peculiarities distinguishing them from the others. Let us first look at the great Atlantic region. This immense territory was, when white men first came to our shores, almost uninterruptedly covered with forest. For thousands of square miles, in many portions of it, there was not enough open space to establish a forty-acre farm on. There was an occasional strip of sedge-covered marsh along the streams ; an open bog had here and there taken the place of a former lake. The few small clearings made by the Indians were hardly worth counting. Thus the interminable woods extended from the salt meadows of the tide-water line to the Appalachian Mountain chain, swept up its ridges and peaks, leaving bare but a few of the highest tops, filled the broad longitudinal valleys, and descended into the great rolling plain of the Mississippi country. But here its character changed by degrees. More and more frequently the vast continuity of it was interrupted by prairies, grass-covered and flower-studded, many of them of vast extent. Towards the north, to be sure, in what are now the northern portions of Michigan and Wisconsin, North-eastern Minnesota, and especially the immense territory between the Great Lakes and Hudson Bay, there are no prairies. The dense unbroken forest, the "heavy timber" as it is called by the people of the locality, is there bounded quite abruptly by the treeless expanses of the Great Plains, where the miserable inhabitants talk of a forest when they see a few willow shrubs and poplars in a ravine by the river. But farther to the south, there was a large region where forest and prairie struggled for mastery, with the result that, generally speaking, the prairies covered the undulating uplands, sedge marshes the wet, broad depressions, and forests the river valleys, as well as the few hilly places. There were also light groves, called openings, in many places in the uplands.
The species constituting this great area of primeval forest were far from being the same in all parts of the territory. Far to the north, on the bleak shores of Hudson Bay, hardly anything was found except spruce (Picea mariana and P. canadensis), together with the balsam poplar (Populus balsamifera). As you go south, the first additional trees you meet with will be the little jack pine (Pinus divaricata) and the aspen (Populus tremuloides). Soon you enter the domain of the king of lumber trees, the stately white pine (Pines strobus), with her cousin, the misnamed Norway (P. resinosa), and the dignified, slow-growing hemlock (Tsuga canadensis). By this time you are fairly within the realm of the broad-leaved trees, the oaks, maples, beeches, chestnuts, the walnuts and hickories, the tulip tree and sassafras, the gigantic sycamore, or buttonwood as they call it in the Eastern States. The farther south you go, the greater becomes the number of species, until when you reach about the latitude of Kentucky or Southern Illinois the list becomes so long that it would fill several pages of this volume. Here and in the region of North Carolina the eastern forest reaches its greatest development so far as diversity of trees is concerned. Farther south, the coniferous trees, especially the different kinds of pine, again become more prominent, as they were towards the border of British America. The southernmost end of the region, the extreme southern part of Florida, with the adjoining islands or "keys," takes on a different character, many varieties properly belonging with the typical West Indian species. The most distinguishing feature of the whole region, excepting the arctic edge, is the prevalence of broad-leaved trees. Over very wide areas not a coniferous tree can be seen, unless it be the arbor-vita (Thuja occidentalis) and tamarack (Larix laricina) of the swamps. Elsewhere broad-leaved and coniferous trees grow mingled together, with broad-leaved ones holding the decided majority; while, to be sure, there are also large areas where the pines and their congeners exclude their broad-leaved rivals almost altogether.
But in the other two forest zones, the Rocky Mountain and the Pacific, the coniferous or evergreen trees have almost a monopoly. Not as if there were no broad-leaved species in those sections. There are many, oaks, maples, poplars, and others. But they form little scattered groves here and there, or crouch in the ravines of the mountain streams, without impressing themselves upon the character of the landscape. There is another feature of the western forests, or the larger portion of them, which makes their aspect quite different from eastern woodlands. That is the fact that the trees, in many cases, stand far apart, so that their crowns do not always touch. While in the east the trees, both broad-leaved and ever-green, stand so close together that the branches inter-mingle and form a dense canopy, through which but few scattered rays of sunshine ever reach the ground, such is not the case in the forests of the Rocky Mountains or the Sierra Nevada. One of the consequences of this manner of growth is that the soil in the western forests becomes much drier than it ever does in the east. This fact will probably have an important bearing upon silviculture when that is begun in earnest in the western country. Another feature distinguishing the forests of the Rocky Mountain region from those of the Atlantic side is that they nowhere cover such immense areas in unbroken compactness. They are distributed along the mountain ranges in belts, leaving bare the highest portions where reigns the eternal snow, and rarely occupying the broader valleys and plains.
Not only have the pines and other coniferous trees the overwhelming majority of numbers in those western forests; they also display a far greater variety of species, compared with eastern woods. Some of the western species, like the western white pine (Pinus flexilis) or the bull pine (P. ponderosa), spread over large areas, while others, like the Monterey pine (Pinus radiata) or the redwood (Sequoia sempervirens), are restricted to a few valleys or mountain ranges. Of the 109 coniferous trees native to the United States and enumerated in Sudworth's list, eighty belong to the country west of the great plains, twenty-eight to the eastern forest, while only one, the common juniper (Juniperus communis), in-habits portions of both regions.
What has been said about the peculiarly open character of much of the western forest does not apply to those portions found in the western halves of Oregon and Washington, and thence stretching along the coast into Alaska and far towards the Arctic regions. On the contrary, these are among the densest woods in the whole world, where under the vaults of the immense crowns, that are swung from the column-like trees at a height of 100 or 150 feet, eternal twilight covers the ground. Of all the forests of the world these have the most gigantic trees, barring only the sequoias of California, of which we will speak anon. Compared to the spruces, firs, and pines of the Puget Sound region, the mightiest of eastern pines and even the giant sycamores of Illinois and Indiana river bottoms are but smallish. No one who has entered those dense forests composed of trees 250 feet tall and having six and more feet in diameter has failed to be impressed with their grandeur, and literature is filled with attempts to describe their majesty. But they are surpassed by the redwood (Sequoia sempervirens), that magnificent, cypress-like giant which forms a belt, from ten to twenty miles wide, along the California coast from the Oregon boundary to a point in Monterey County, a little north of San Francisco. And even the redwood is not equal in size to the famous big tree (Sequoia gigantea), the pride of the Sierra. This tree is known to tourists principally by the comparatively few specimens growing in the protected groves of Calaveras and elsewhere. But it reaches its grandest development farther south, in the southern part of the Sierra, where it forms, not small groves, but extensive forests.
How did the forests and the species composing them come to be distributed over the North American continent in just the peculiar manner in which we find them Before very much was known about plant geography, people used to be content with saying that each tree found itself in that region the natural conditions of which were most adapted to its nature. But such vague answers no longer content us to-day. The science of palaeobotany, or the knowledge of the plants which existed on the globe in former geological periods, has helped us on the track of this secret of nature. To be sure even now we know these things fragmentarily only, and an almost limitless field is here still open to investigation. But this we can now affirm : The distribution of trees is due to two sets of factors,—one topographical and climatic, based upon the differences of soil, elevation, temperature, humidity, and so forth. The other factor is historical, arising out of the order in which the seeds of different species were deposited in each particular locality, or failed to be so deposited.
The palæobotanists, drawing their conclusions from the remnants of wood, impressions of leaves, flowers, and fruit, and other small relics of extinct vegetation which are found imbedded in rocks and beds of coal or peat, have established the fact that during what are known as tertiary times, vast forests, composed of trees not very different from those now growing in the United States, existed in far northern regions, nearly up to the pole, where now everything is decked with ice and snow. But the warm climate , of the tertiary ages was succeeded by the secular winter, which is known as the glacial period of the quaternary epoch, and of which most of my readers have heard. Farther and farther south crept the great glaciers, joined by those flowing down from the high mountains of the west, until the whole northern part of the continent, as far south as the latitude of Cincinnati, and even beyond, was covered with a sheet of ice of immense thickness, leaving but here and there an island uncovered, like the celebrated driftless area of South-western Wisconsin. Before the advance of the ice and the cooling of the climate that was both cause and con-sequence of the glaciation, the forests succumbed, and the species composing them were either extinguished or be-came restricted to more southern latitudes. But after thousands of years the climate gradually became milder, the edge of the continental ice sheet slowly melted, forming immense rivers and lakes. As the land was laid bare, vegetation recovered the lost territory step by step. At first, the character of the land recently left by the ice was undoubtedly very much like what we find to-day in the Barren Grounds, the solitary regions in the northern part of the British dominions. There we find vast areas covered principally with mosses belonging to the genus Sphagnum, interspersed with a few sedges and numerous species of the heath family. It is a vegetation very similar to that of the peat bogs found occasionally in the Northern States. These moss prairies or tundras were gradually supplanted by the advance-guard of the forest ; spruces and poplars, that came slowly marching up from the south and are, in all probability, continuing their advance to this day. These most arctic of American trees were followed by the pines, and these again by the more southern species of hardwood, maples, oak, beeches and the like. While thus the forest as a whole was advancing northward, the various species fought among themselves for each locality. This fight continues to the present day, and is changing the distribution of species from century to century. It is very probable, for in-stance, that both north and south the hardwoods are gaining ground at the expense of the pines and spruces. The hemlock does not reproduce itself in a portion of Wisconsin, and therefore will die out there when the present generation of trees has run its course, unless man interferes. The red and black oaks are gradually supplanting the white oaks in many parts of the country.
We have repeatedly used the terms warfare and fighting for the competition of trees among themselves, and it is time to give the reader an idea of how such warfare is going on. Trees have no claws and teeth with which they can attack each other as animals do, but they can fight, nevertheless, and the numbers of the dead and crippled in these battles are tremendous.
There are two things which every tree needs, or it must die : moisture for its roots, to carry water containing in solution various mineral salts to all parts of its body, and light to enable the leaves to assimilate the material so furnished, and build up the various vegetable tissues making up the tree. Each kind of tree, by reason of its specific characteristics, requires these two things in varying proportions. For each species there is a mini-mum of light, and the accompanying warmth, and also a maximum; exceeding these limits, suffering begins. The same holds true of moisture, as well as some other requisites. For instance, most trees will die if their roots are immersed in water for a large part of the growing season. But a few, like the black ash or bald cypress, will grow lustily on swamps wet the year through. They have a very high moisture maximum. On the other hand, these species would not flourish on a dry, rocky ridge. The conditions there would be below their moisture minimum; while a black spruce, for instance, will grow in a Northern Wisconsin swamp no less than on a dry rock in the Adirondacks, because it is adapted to a very large range of moisture conditions.
As to light, dendrologists have divided trees into two classes, light-loving and shade-enduring species. The two classes, however, connect by imperceptible transitions. Generally speaking, the needs of a tree as regards light can be told by the character of the shade its own crown makes. The oaks, for instance, need much light, and an oak sapling that stands in a dark, shady place will never grow to be a good-sized, healthy tree. Now, everybody must be struck with the light character of an oak grove, where the sun's rays everywhere penetrate to the ground and paint fantastic figures on the vigorous growth of grass and herbage. A maple grove is much darker, and if you enter a wood composed of beeches or hemlock, you find yourself in almost nocturnal twilight, where no ray of the sun succeeds in reaching the ground. The need of light for the light-loving trees is particularly great in their youth; while on the other hand, the seedlings of the shade-enduring trees often die when they are exposed to too bright sunshine. If the seed of a light-loving tree should fall under the crowns of a group of beeches, it would have very little prospect of growth, while the young beeches would grow lustily. Here is one of the ways in which species of trees carry on their war-fare. Suppose that on a tract of land covered with oak, birches, or other light-lovers, the seed of beech, maple, or other shade-endurers should be deposited. The light coming through the crowns of the established species would be sufficient to start the young invaders into vigorous growth; the seedlings would gradually develop into trees, each forming its usual dense crown and casting a deep shade on the ground under it. By and by they will distribute their seeds, some of which will grow into trees, and it will make no difference to them whether they are rooted under their parent species or under the neigh-boring oaks. In either case, the light conditions are favorable. But with the seeds cast by the oaks things are different. If they sprout under one of their parent species they will grow. But those that come to lie under the beeches do not find light enough, and either do not sprout at all, or soon languish and die. By and by some of the old oaks will perish, from accident or age. In the new growth the beeches already have the majority, and the percentage in their favor is constantly increasing. After some centuries the oaks will have disappeared, and in place of the sunlit oak grove there now stands a cool, shady beech wood. The war has resulted in victory for the invader. Of course, the light conditions are not the only factors to decide the struggle, else the light-loving trees would long ago have become extinct. It might hap pen, for instance, that when the seeds of the shade-enduring species reached the place, they found the light conditions favorable all over the locality, but in one half of it the soil or the moisture relations were such that the newcomers could not endure it. Then the result of the war would be, that on one half of the tract the beeches have superseded the oaks, while on the other the oaks remain in undiminished vigor. The conditions affecting the outcome are rarely so simple as we have here assumed for the sake of clearness. Ordinarily, they are exceedingly complex, so that it becomes very difficult to trace them. But a knowledge of these processes is necessary for the skilful pursuit of silviculture. One important practical rule we may mention here, which is based on this observation that the species of tree growing in any given place is not always directly regulated by the natural circumstances of the locality, but influenced by the competition of other species. It is this: The fact that in any region a species is never found except in places of some special character, as in swamps, or on sandy soil, does not prove that it will not flourish elsewhere. It may have been driven into these retreats by its competitors, and would really much prefer the better places from which it has been excluded by them. This may often be of importance in silviculture, when it is desired to grow a tree outside of its apparent favorite habitat.
Just as each species competes with every other species for the most favorable places, so every individual tree competes with every other, whether of its own kind or a different species. Again, the main objects of the struggle are light and moisture. To gain these necessaries, each tree adapts its manner of growth, the shape of its trunk, branches, roots, and leaves in a most marvellous manner. Everybody must have noticed that no tree is the exact counterpart of another of the same species. Aside from differences in age and size, each tree has a different way of disposing its branches, twigs, and leaves.
This difference is invariably exactly of the kind which is most favorable to the growth of the tree under the particular local circumstances among which it must develop. As a tree cannot run away, it has to make the best it can out of the situation in which it finds itself as a seedling. Sometimes the devices the tree hits upon in difficulties are absolutely startling. Here is an illustration : At Devil's Lake, Wisconsin, a pine tree is standing on the side of the almost vertical quartzite rocks of the locality. It had originally sprouted in a cleft where there is hardly a shovelful of soil. The tree is now about six inches in diameter. From its little cleft, it sends out a single root, as thick as the trunk, along a narrow ledge, on which there is practically no soil at all. On the surface of this ledge, lying on the exceedingly hard rock, this root runs along, almost horizontally, for twenty-six feet, where it finds an accumulation of soil and enters the ground.
While there are infinite variations of form growing out of this struggle for moisture and light, there are a few general rules of practical importance to the forester. A tree standing in the open, where the light strikes its crown from all sides, forms a round, symmetrical top, with the lowest branches not very far above the ground. Or if it is of the kind that has a pyramidal growth, like the spruces and most other coniferous trees, it may be clothed with living branches to the very earth. If the same kind of tree should grow in a place where it gets full sunlight from some directions, while other sides are shaded, the branches will all, or nearly all, grow towards the light, thus forming asymmetrical crowns. Now sup-pose that a tree of the same species should find itself standing in a dense clump of trees, where the light can-not reach it from any side. Then its only salvation is to reach the light which comes from above. Consequently it sends out few and small lateral branches, but puts all its energy into height growth, until it has grown above the shade cast by the surrounding trees. This done, it begins to spread its leafy branches in all directions, to absorb as much of the loved sunlight as possible. Where a number of trees grow closely together, so as to mutually hinder the light from reaching their leaves, a race for the sun ensues between them, in which those are victorious which by reason of their more energetic height growth first show their tops above the others. Then these begin to spread their side limbs, thereby throwing their rivals into ever denser shade, and the latter cease to grow vigorously, and in course of time may even die. If they are of the shade-enduring kind, they have, of course, a better chance to survive than if they were light-lovers. In such a case they may even succeed, after a while, in their turn, in overtopping the first victor, be-cause that ceased to grow much in height when the spreading of the branches began. But generally the tree which has once been overtopped by its neighbor never grows into a very large and vigorous specimen, but joins the ranks of the suppressed, which the skilful forester cuts out sooner or later to make room for their betters.
Not only do whole trees languish and die when they are overshadowed by others, but every branch shares the same fate if at any period of its life it is deprived of the free access of light, either by other branches of its own tree, or by neighboring trees. Where the trees stand close together, this happens to all the lower branches, which usually die and fall off during the first few years of their lives, so that not only are few side branches produced, but what few there are soon disappear again, Trees so grown consequently show tall trunks with only a few branches towards the top.
Now it happens that the quality of lumber cut from trees with tall stems is very much better than that produced by trees where the branches are many and reach far down the trunk, for every branch means a knot in the lumber. Consequently the forester who desires good lumber aims to make his trees tall and with as few branches as possible on the lower part of their trunks.
When the tree has succeeded in growing above the heads of competitors and begins to spread its crown, it changes its economy in various ways. For one thing, as it is now enabled to provide itself with more leaves, it has a chance to produce a greater quantity of wood : for each leaf is a laboratory where the material is distilled out of which wood and other vegetable tissue are formed. This increased formation of wood results in an increase of the diameter of the trunk, while the height growth is no longer as rapid as before. At the same time, the character of the wood changes, especially in those trees which have two kinds of wood, an inner core of heart-wood, and a surrounding layer of sap-wood. This is the case with most of our lumber-producing trees. The greater the diameter of such a tree trunk, the smaller the proportion of sap-wood, while a tall tree of very small diameter is nearly all sap-wood. As heart-wood is much more valuable for timber purposes, it follows that to make his trees most valuable the forester allows them to follow up the period of rapid height growth by a period of prevalent diameter increase. In other words, he now cuts away the weaker, half-suppressed trees, so that the remaining ones get the benefit of an open stand. Where this happens naturally in the wilderness, by one cause or the other, the result is, of course, the same as where the new condition is produced artificially. The forester's art in silviculture never amounts to anything more than giving special direction to the processes initiated by nature.
One of the principal weapons which trees have in the propagation of their species is the production of immense quantities of seed, which are spread broadcast, trusting to accident that some will find a favorable spot to sprout and grow into a new tree. Evidently, the more seeds are sown, the greater is the probability that some of them will find such a spot. Therefore trees that are very fertile have an advantage over trees which produce a less quantity. But no matter whether few or many seeds are produced, a very small percentage ever succeed in becoming trees. In fact, it happens not rarely that of all the seeds scattered over the ground in any given year not a single one ever reaches the state of a seedling tree. Those who have never observed these relations are apt to assume that in a given tract of woodland, growing healthily under undisturbed natural conditions, one will be able to find trees of all ages, from the patriarch of several centuries down to the little seedling just showing the tip of its stem above the litter on the forest floor. But such conditions are rather rare, and the reason for that is not very hard to find. In the first place, the trees do not bear seed every year. Varying according to species and perhaps to habitat and other conditions, what is known as a seed year occurs but once in three, four, or five years, as the case may be. In such a year, every tree of the species, old enough to bear fruit at all, is full of them, while in other years only here and there a few are ripening. Consequently it is only in seed years that there is much hope for any seeds to find a favorable sprouting place. But even then it may happen that not one of them has such luck. The condition of the ground has much to do with this. It may be that by one cause or other, as, for instance, too much moisture or too great dryness, it has become unfavorable to the seedlings, either by reason of its chemical or mechanical condition. The older trees are little affected by the change, for they send their roots deeply into the subsoil and the character of the surface layer is of relatively little importance to them. But the seedling depends for its life upon the condition it finds in this top-soil. More often the ground is shaded too much either by the crowns of the old trees themselves or by the undergrowth, which in turn may consist of former generations of young trees of the same species or of shrubs belonging to entirely different kinds of plants. In still other cases the ground may have been invaded by grasses or herbs, forming a matted tangle of roots and stems which make it difficult for the tree seeds to sprout. Under any of these and similar conditions long periods may elapse during which no reproduction of trees takes place, and the supply of many seed years may go to waste. But sooner or later an opportunity will come; and the trees are always on the lookout to take advantage of accidents. One of the commonest of such accidents is the death and fall of one of the giants of the forest. The prostrate trunk for a number of years encumbers the ground, but it has torn a wide breach into the leafy canopy on top, through which the bright sun-light enters the shady depth of the wood. Gradually the fallen tree decays, helped in this process by manifold fungi and other cryptogamic plants. After a while all that remains of what was once a tree is a heap of rich brown vegetable mould. As yet this is no place for a tree. Only mosses, ferns, and a few flowering plants which like to feed on organic matter and are known to science as saprophytes, or decay plants, find a congenial home here. But gradually, h y various processes, among which the burrowing of animals plays no small part, the vegetable mould is mixed with the underlying earth, and true soil formed. Now is the time for the tree seeds, but if they do not hasten to occupy the spot, a host of other plants, herbs, grasses, and shrubs are lying in wait to get themselves established and preempt the ground. Of course, during the whole time while only fungi, mosses, ferns, and other specially adapted plants could live in the decaying mass, seeds of other species continued to arrive on the spot, but found it impossible to germinate. But now this has become possible for them, and intense rivalry between them follows. In this, if luck is with it, a young tree may come off victorious and in course of time develop into another giant like the fallen one on whose grave it grows.
This is but one instance where an accident afforded opportunity for reproduction of trees in a wood other-wise unfavorable to young growth. Of course the variety of circumstances making such opportunities is infinite. Often it happens in the primeval wilderness that whole bodies of trees are overthrown by violent winds, and then the conditions brought about by the fall of a single tree are repeated on a larger scale. This matter of windfalls is perhaps not quite understood by the average layman. Generally speaking, trees adapt their manner of growth so as to withstand the violence of all winds to which they are likely to be exposed. The means by which they in-crease their power of resistance to storms are various. One of these is the elasticity of their fibres, even of the trunk, by virtue of which they bend before the wind, but immediately resume the upright position when the blast ceases. Another means of protection is a root system going very deeply into the ground, combined with great strength of the trunk. Unless the latter quality were added, a strong wind might not be able to uproot the tree as it would a shallow-rooted one, but the very strength and unyielding quality of the root would in-crease the danger of the trunk being broken off. Still another useful device is the very common thickening of the lower part of the bole just at the place where the greatest strain is suffered when the tree is bent. Now, within the limits set by the characteristics of each species, each individual tree develops these means of defence to a greater or less degree according to the measure of its exposure; that is, a tree growing in a place where strong winds are constantly blowing, as on the crest of a high mountain, develops as deep and stout a root, and strength-ens its other wind defences just as much as its specific nature will permit. On the other hand, a tree growing in a protected ravine does not waste energy on such useless objects, but puts it into other forms of life activity. The trees in the midst of a compact forest protect each other, and consequently develop relatively shallow root systems. Only those at the edge of the wood, where the wind can reach them better, send their roots down deeply. Now suppose that, either by the hand of man or natural causes, an opening is made in the forest, so that individuals which heretofore stood surrounded by trees are now exposed to the wind on one or more sides. The trees now forming the edge of the wood will at once proceed to strengthen their root systems and thicken their boles, until they are as well prepared to resist the violence of the wind as if they had grown in an exposed situation from the beginning. But this process takes a number of years, and in the meantime they are in constant danger of being uprooted or broken off. Undoubtedly the majority of disastrous windfalls are in situations like the one described, where the trees had not yet be-come adapted to new conditions. But it may happen, of course, that a storm of unusual violence overthrows trees which had bravely withstood all ordinary tempests. The trees sometimes left standing by settlers on their clearings nearly always succumb to the wind sooner or later. Some species, which have shallow roots under the best conditions, are more liable to windfalls than others. Such are, for instance, the basswood (Tilia americana) and the hemlock (Tsuga canadensis). Others, like the various walnuts and hickories, develop deep and stout tap-roots even in the most sheltered situations, and consequently suffer little from this particular danger. Windfalls are a great detriment to the American forests. In addition to the direct damage, the tangle of drying branches and twigs affords one of the best starting-points for forest fires. The overturned trees are at once attacked by a host of insects and fungi which sometimes spread upon the adjacent sound timber and injure it.
To return to the opportunities which tree seeds find for sprouting, such as are small and perhaps provided with wings, or other devices enabling them to float for awhile in the wind, have evidently a better chance than heavy seeds which cannot fall far from the parent stem. To the former class belong, among others, the seeds of poplars and birches, while conspicuous in the latter are the oaks and hickories. It is impossible to describe here even a few of the wonderful devices by which many seeds ac-quire this useful power to travel. How much advantage a tree derives from such power of its seeds can be seen conspicuously in the case of the large areas in the Great Lake region which have been deprived of their former pine growth by the lumberman's axe and the fire. On these "slashings" the first trees which appear to pro-vide a new forest growth are almost invariably poplars, especially the kind known as trembling aspen, and the white birches. These have especially effective apparatuses which enable their seeds to travel long distances. The pines, of which there are usually quite a number left on these "slashings," being trees that were too small for the lumberman, or of defective timber, have seeds which can be carried by the wind but a few rods at best. Consequently they cannot at once cover the whole area, the way the aspens do. But this instance also points the moral that the race is not always to the swift. Though the pines do not travel far, their little seedlings come up in numbers within a few rods about each seed tree. The growth of aspen, as well as bracken, grass, brambles, and other vegetation invading these areas, unless it gets to be too dense a tangle, is of advantage rather than otherwise to them, for it keeps off some of the scorching sunshine to which pine seedlings are rather sensitive. The aspen grows rapidly into saplings six and more feet high. The pines, for the first few years, grow but a few inches. Then they begin to shoot upwards, and by the time they are about fifteen years old, their tops begin to show above those of the aspen, that are now ten to fifteen feet high. Five years more, and the pines are throwing the aspen into shade and hindering their growth ; another decade, and most of the aspens have died out because, being light-lovers, they could not thrive in the shade of the pine, which has now recovered the ground it lost thirty years ago. An exactly similar alternation of trees can be observed in New York and New England, with the exception that there spruces usually play the part taken by pines in the Lake region. Undoubtedly other sections of the country might furnish parallel cases where trees have an advantage at the start which they lose later on in the rivalry with other species.
Attentive readers must have observed that the dangers threatening a tree are by no means over when the seed has found a favorable locality and developed into a seed-ling. Just as very few seeds ever sprout at all, so very few infant trees ever reach old age. A very large old tree takes up a hundred times as much room as a young sap-ling. This room must be provided by killing off the weaker individuals competing for it. A wood composed mainly of very old trees will show far fewer individuals to the acre than one stocked with young ones. But the crown canopy may be just as dense, and the quantity of timber contained in it is apt to be far higher.
It would require a volume by itself to describe in detail the manifold conditions under which the warfare of the forest is carried on. We have, almost at random, picked out a few of the phases which influence its progress. These illustrations were designed to impress upon the reader the fact that a forest, left to the undisturbed action of natural forces, does not remain unchanged from century to century, but is different to-day from what it was yesterday, and will be still different to-morrow. As the individual tree lives through various life stages, from infancy to old age, so the forest as a whole matures and grows old. But while the individual, when its limit of age is reached, must die, the forest has the power of constantly regenerating itself, so that its continuity may remain unbroken for countless ages. There are, to be sure, certain slow secular changes which may in the long run destroy a forest altogether. Thus the forests growing in the northern half of our continent in tertiary ages were destroyed by the long glacial winter. But that is a matter of many thousands of years. Humanly speaking, there is no reason why a forest, taking its vast extent as a whole, should not live forever.
Another important principle we have tried to impress by our cursory observations on the inner life of a forest: Multifarious and bewildering as the variety of its life phases is, the forest and the changes constantly going on in it are not the disorderly results of accident. In their astonishing complexity they are yet dependent on a few simple laws of nature. To the degree in which we understand these and their workings, to that degree we will be able to control their results. As we proceed in the consideration of the subject-matter of this volume, we will have frequent occasion to treat of the forest as subject, not to natural forces, but to control by the will of man, who may destroy, maintain, or regenerate it as suits his purposes. To understand clearly how such control is possible, we must bear in mind that it is done, not by suspending or reversing the action of the processes of nature, but by guiding and giving special directions to them.
Such guidance and control are possible only to men who have a knowledge of those natural processes. Not as if anybody now possessed or was ever likely to possess such knowledge perfectly. But even an imperfect knowledge gives us a means of exercising some influence. It is only within a relatively short time that a partial under-standing of the life processes of a forest has been accomplished anywhere, and in America we are still far from knowing as much of our forests as the Europeans know of theirs. During the greater part of our history, we were very far from exercising an important influence on our forests. On the contrary, our history as a nation was far more intensely influenced and largely determined by the primeval woods.