The forest is much more than an a.s.semblage of different trees, it is an organism; that is, the trees that compose it have a vital relation to each other. It may almost be said to have a life of its own, since it has a soil and a climate, largely of its own making.
Without these conditions, and without the help and hindrance which forest trees give to each other, these trees would not have their present characteristics, either in shape, habits of growth or nature of wood grain. Indeed, some of them could not live at all.
Since by far the greater number of timber trees grow in the forest, in order to understand the facts about trees and woods, it is necessary to know something about the conditions of forest life.
A tree is made up of three distinct parts: (1) the roots which anchor it in the ground, and draw its nourishment from the moist soil; (2) the trunk, or bole, or stem, which carries the weight of the branches and leaves, and conveys the nourishment to and from the leaves; (3) the crown, composed of the leaves, the branches on which they hang, and the buds at the ends of the branches. As trees stand together in the forest, their united crowns make a sort of canopy or cover, Fig.
55, which, more than anything, determines the factors affecting forest life, viz., the soil, the temperature, the moisture, and most important of all, the light.
[Ill.u.s.tration: Fig. 55. The Forest Cover. Spruce Forest, Bavaria, Germany. _U. S. Forest Service._]
On the other hand, every species of tree has its own requirements in respect to these very factors of temperature,--moisture, soil and light. These are called its _silvical characteristics_.
SOIL.
Some trees, as black walnut, flourish on good soil, supplanting others because they are better able to make use of the richness of the soil; while some trees occupy poor soil because they alone are able to live there at all. Spruce, Fig. 56, will grow in the north woods on such poor soil that it has no compet.i.tors, and birches, too, will grow anywhere in the north woods. In general, it is true that mixed forests, Fig. 57, _i.e._, those having a variety of species, grow on good loamy soil. The great central, deciduous Atlantic Forest grew on such soil until it was removed to make room for farms. On the other hand, pure stands--_i.e._, forests made up of single varieties--of pine occupy poor sandy soil. Within a distance of a few yards in the midst of a pure stand of pine in the south, a change in the soil will produce a dense mixed growth of broad-leaves and conifers.
[Ill.u.s.tration: Fig. 56. Virgin Stand of Red Spruce. White Mountains, New Hampshire. _U. S. Forest Service._]
[Ill.u.s.tration: Fig. 57. Typical Mixed Forest,--Red Spruce, Hemlock, White Ash, Yellow Birch, Balsam Fir, and Red Maple. Raquette Lake, New York. _U. S. Forest Service._]
The soil in the forest is largely determined by the forest itself. In addition to the earth, it is composed of the fallen and decayed leaves and twigs and tree trunks, altogether called the _forest floor_. It is spongy and hence has the ability to retain moisture, a fact of great importance to the forest.
MOISTURE.
Some trees, as black ash and cypress, Fig. 58, and cotton gum, Fig.
59, grow naturally only in moist places; some, as the pion and mesquite, a kind of locust, grow only in dry places; while others, as the juniper and Douglas fir, adapt themselves to either. Both excessively wet and dry soils tend to diminish the number of kinds of trees. In many instances the demand for water controls the distribution altogether. In the Puget Sound region, where there is a heavy rain-fall, the densest forests in the world are found, whereas on the eastern slopes of the same mountains, altho the soil is not essentially different, there are very few trees, because of the constant drouth.
[Ill.u.s.tration: Fig. 58. Cypress and Cypress "Knees." Jasper Co., Texas. _U. S. Forest Service._]
[Ill.u.s.tration: Fig. 59. Cotton Gums, Showing b.u.t.tresses. St. Francis River, Arkansas. _U. S. Forest Service._]
TEMPERATURE.
The fact that some trees, as paper birch and white spruce, grow only in cold regions, and some, as rubber trees and cypress, only in the tropics, is commonplace; but a fact not so well known is that it is not the average temperature, but the extremes which largely determine the habitat of trees of different kinds. Trees which would not live at all where there is frost, might flourish well in a region where the average temperature was considerably lower. On the other hand, provided the growing season is long enough for the species, there is no place on earth too cold for trees to live. Fig. 60.
[Ill.u.s.tration: Fig. 60. Northern Forest,--Young Spruce Growing Under Yellow Birch. Santa Clara, New York. _U. S. Forest Service._]
In general, cold affects the forest just as poor soil and drought do, simplifying its composition and stunting its growth. In Canada there are only a few kinds of trees, of which the hardwoods are stunted; south of the Great Lakes, there is a great variety of large trees; farther south in the southern Appalachian region, there is a still greater variety, and the trees are just as large; and still farther south in tropical Florida, there is the greatest variety of all. The slopes of a high mountain furnish an ill.u.s.tration of the effect of temperature. In ascending it, one may pa.s.s from a tropical forest at the base, thru a belt of evergreen, broad-leaved trees, then thru a belt of deciduous broad-leaved trees, then thru a belt of conifers and up to the timber line where tree life ceases. Figs. 61, and 62.
[Ill.u.s.tration: Fig. 61. Mixed Hardwoods on Lower Levels. Spruce and Balsam Dominate on Higher Elevations. Mt. McIntyre, Adirondack Mountains, New York. _U. S. Forest Service._]
[Ill.u.s.tration: Fig. 62. Scrub Growth on Mountain Top. Mt. Webster, New Hampshire. _U. S. Forest Service._]
LIGHT.
More than by any other factor, the growth of trees in a forest is determined by the effect of light. All trees need light sooner or later, but some trees have much more ability than others to grow in the shade when young. Such trees, of which maple and spruce are examples, are called _tolerant_, while others, for instance, larch, which will endure only a comparatively thin cover or none at all, are called _intolerant_. The leaves of tolerant trees endure shade well, so that their inner and lower leaves flourish under the shadow of their upper and outer leaves, with the result that the whole tree, as beech and maple, makes a dense shadow; whereas the leaves of intolerant trees are either spa.r.s.e, as in the larch, or are so hung that the light sifts thru them, as in poplar and oak. The spruces and balsam fir have the remarkable power of growing slowly under heavy shade for many years, and then of growing vigorously when the light is let in by the fall of their overshadowing neighbors. This can plainly be seen in the cross-section of balsam fir, Fig. 63, where the narrow annual rings of the early growth, are followed by the wider ones of later growth. A common sight in the dense woods is the maple sending up a long, spindly stem thru the trees about it and having at its top a little tuft of leaves, Fig. 64. By so doing it survives. The fact that a tree can grow without shade often determines its possession of a burnt-over tract. The order in the North Woods after a fire is commonly, first, a growth of fire weed, then raspberries or blackberries, then aspen, a very intolerant tree whose light shade in turn permits under it the growth of the spruce, to which it is a "nurse," Fig. 65. In general it may be said that all seedling conifers require some shade the first two years, while hardwoods in temperate climates, as a rule, do not.
[Ill.u.s.tration: Fig. 63. Cross section of Balsam Fir, Showing Fast Growth After Years of Suppression. Notice the width of the annual rings in later age compared with early. _U. S. Forest Service._]
[Ill.u.s.tration: Fig. 64. Tolerant Maple. The trees are too slender to stand alone. _U. S. Forest Service._]
[Ill.u.s.tration: Fig. 65. Intolerant Aspen, a "nurse" of Tolerant Spruce._ U. S. Forest Service._]
This matter of tolerance has also much to do with the branching of trees. The leaves on the lower branches of an intolerant tree will not thrive, with the result that those branches die and later drop off.
This is called "cleaning," or natural pruning. Intolerant trees, like aspen and tulip, Fig. 66, clean themselves well and hence grow with long, straight boles, while tolerant trees, like spruce and fir, retain their branches longer.
[Ill.u.s.tration: Fig. 66. Intolerant Tulip. Notice the long, straight boles. _U. S. Forest Service._]
The distribution of a species may also be determined by geographical barriers, like mountain ranges and oceans. This is why the western forests differ radically from the eastern forests and why the forest of Australasia is sharply distinct from any other forest in the world.
Any one or several of these factors, soil, moisture, heat, and light, may be the determining factor in the make-up of a forest, or it may be that a particular tree may survive, because of a faster rate of growth, thus enabling it to overtop its fellows and cut off their light. The struggle for survival is constant, and that tree survives which can take the best advantage of the existent conditions.
Besides these topographical and climatic factors which help determine the distribution of trees, a very important factor is the historical one. For example, the only reason by which the location of the few isolated groves of big trees in California can be accounted for is the rise and fall of glacial sheets, which left them, as it were, islands stranded in a sea of ice. As the glaciers retreated, the region gradually became re-forested, those trees coming up first which were best able to take advantage of the conditions, whether due to the character of their seeds, their tolerance, their endurance of moisture or whatever. This process is still going on and hardwoods are probably gaining ground.
Besides these external factors which determine the composition and organic life of the forest, the trees themselves furnish an important factor in their methods of reproduction. These, in general, are two, (1) by sprouts, and (2) by seeds.
(1) Most conifers have no power of sprouting. The chief exceptions are pitch pine and, to a remarkable degree, the redwood, Fig. 67. This power, however, is common in broad-leaved trees, as may be seen after a fire has swept thru second growth, hardwood timber. Altho all the young trees are killed down to the ground, the young sprouts spring up from the still living roots. This may happen repeatedly. Coppice woods, as of chestnut and oak, which sprout with great freedom, are the result of this ability. The wood is poor so that it is chiefly used for fuel.
[Ill.u.s.tration: Fig. 67. Sprouting Redwood Stumps. Glen Blair, Calif.
_U. S. Forest Service._]
(2) Most trees, however, are reproduced by seeds. Trees yield these in great abundance, to provide for waste,--nature"s method. Many seeds never ripen, many perish, many are eaten by animals, many fall on barren ground or rocks, and many sprout, only to die. The weight of seeds has much to do with their distribution. Heavy seeds like acorns, chestnuts, hickory and other nuts, grow where they fall, unless carried down hill by gravity or by water, or scattered by birds and squirrels.
Trees with winged seeds, however, Fig. 68, as ba.s.s, maple and pine, or with light seeds, as poplar, often have their seeds carried by the wind to great distances.
[Ill.u.s.tration: Fig. 68. Winged Seeds. 1, Ba.s.swood; 2, Box-elder; 3, Elm; 4, Fir; 5, 6, 7, 8, Pines. _U. S. Forest Service._]
Again some trees, as spruce, are very fertile, while others, like beech, have only occasional seed-bearing seasons, once in three or four years. Willow seeds lose their power of germination in a few days, and hence, unless they soon reach ground where there is plenty of moisture, they die. This is why they grow mostly along water courses. On the other hand, black locust pods and the cones of some pines keep their seeds perfect for many years, often until a fire bursts them open, and so they live at the expense of their compet.i.tors.
It is such facts as these that help to account for some of the acts of forest composition,--why in one place at one time there is a growth of aspens, at another time pines, at still another oaks; and why beeches spring up one year and not another. That red cedars grow in avenues along fences, is explained by the fact that the seeds are dropped there by birds, Fig. 69.
[Ill.u.s.tration: Fig. 69. Red Cedar Avenue. Seeds dropped by birds which perched on the fences. Indiana. _U. S. Forest Service._]
The fact that conifers, as the longleaf pine, Fig. 46, p. 200, and spruce, Fig. 55, p. 212, are more apt to grow in pure stands than broad-leaved trees, is largely accounted for by their winged seeds; whereas the broad-leaved trees grow mostly in mixed stands because their heavy seeds are not plentifully and widely scattered. This is a rule not without exceptions, for beech sometimes covers a whole mountain side, as Slide Mountain in the Catskills, and aspens come in over a wide area after a fire; but later other trees creep in until at length it becomes a mixed forest.
The essential facts of the relation of trees to each other in the forest has been clearly stated by Gifford Pinchot thus:[1]
The history of the life of a forest is a story of the help and harm which trees receive from one another. On one side every tree is engaged in a relentless struggle against its neighbors for light, water and food, the three things trees need most.
On the other side each tree is constantly working with all its neighbors, even those which stand at some distance, to bring about the best condition of the soil and air for the growth and fighting power of every other tree.
The trees in a forest help each other by enriching the soil in which they stand with their fallen leaves and twigs, which are not quickly blown or washed away as are those under a tree in the open. This collection of "duff" or "the forest floor" retains the moisture about their roots, and this moist ma.s.s tends to keep the temperature of the forest warmer in winter and cooler in summer. The forest cover, Fig.
55, p. 212, consisting largely of foliage, has the same effect, and in addition protects the bark, the roots, and the seedlings of the trees from the direct and continuous hot rays of the sun. Without the shade of the leaves, many trees, as white pine, would quickly die, as may readily be seen by transplanting them to the open. The ma.s.s of standing trees tempers the force of the wind, which might overthrow some of them, and hinders the drying up of the duff.
[Ill.u.s.tration: Fig. 70. Shallow Roots of Hemlock. Bronx Park, New York, N. Y.]