A PRIMER OF FORESTRY PART I—THE FOREST

Fig. 1.—Roots, stem, and crown of a young Shellbark Hickory. Milford, Pa.

THE LIFE OF A TREE.

The object of forestry is to discover and apply the principles according to which forests are best managed. It is distinct from arboriculture, which deals with individual trees. Forestry has to do with single trees only as they stand together on some large area whose principal crop is trees, and which therefore forms part of a forest. (See frontispiece.) The forest is the most highly organized portion of the vegetable world. It takes its importance less from the individual trees which help to form it than from the qualities which belong to it as a whole. Although it is composed of trees, the forest is far more than a collection of trees standing in one place. It has a population of animals and plants peculiar to itself, a soil largely of its own making, and a climate different in many ways from that of the open country. Its influence upon the streams alone makes farming possible in many regions, and everywhere it tends to prevent floods and drought. It supplies fuel, one of the first necessaries of life, and lumber, the raw material, without which cities, railroads, and all the great achievements of material progress would have been either long delayed or wholly impossible. (See Pl. I.) The forest is as beautiful as it is useful. The old fairy tales which spoke of it as a terrible place are wrong. No one can really know the forest without feeling the gentle influence of one of the kindliest and strongest parts of nature. From every point of view it is one of the most helpful friends of man. Perhaps no other natural agent has done so much for the human race and has been so recklessly used and so little understood.

Fig. 4.—Trunks of two Red Firs. The figure of a man between them gives an idea of their great size, which, however, is not unusual. Olympic Forest Reserve, Washington.

THE PARTS OF A TREE.

In order rightly to understand the forest, something must first be known about the units of which it is made up. A tree, then, is a woody plant growing up from the ground usually with a single stem. (See fig. 1.) It consists of three parts: First, the roots, which extend into the ground to a depth of 3 or 4 feet, or still farther when the soil is not too hard and they do not find moisture enough near the surface. (See figs. 2, 3, and Pls. II, III.) They hold the tree in place, and take up from the soil water and certain mineral substances which the tree needs in its growth. Second, the trunk, stem, or bole, which supports the crown and supplies it with mineral food and water from the roots. (See fig. 4.) Third, the crown itself, with its net ork of branches, buds, and leaves, in which the food taken up by the tree from the soil and air is worked over and made ready to assist in the growth of the whole plant. (See figs. 5-7 and Pl. IV.) The crown has more to do with the life of the tree than its other parts, for the most important processes in the reproduction of the tree and the digestion of its food take place in the crown. For this reason, and because we can control its shape and size more easily and directly than that of the roots or trunk, the crown is of special interest to the forester. It is almost exclusively with the crowns that he has to deal in tending a crop of trees and preparing the way for the succeeding generation. As they stand together in the forest, the crowns of the trees form a broken shelter, which is usually spoken of as the leaf canopy, but which may better be called the cover. (See fig. 8.)

Plate I. A FOREST IN WESTERN WASHINGTON ALONG A WAGON ROAD.

Fig. 2.—Roots of the Western Hemlock. This young tree started on a fallen Red Fir; its roots spread under the moss and litter, and when fire came they were exposed. Olympic Forest Reserve, Washington.

Fig. 3.—Upturned skeleton roots of a Red Fir. The small roots have been burned away and the others cleared of soil by the fire. Olympic Peninsula, Washington.

Plate II. UPTURNED ROOTS OF WESTERN HEMLOCK, SHOWING HOW THOROUGHLY THE SMALL ROOTS PENETRATE THE SOIL. OLYMPIC PENINSULA, WASHINGTON.

Fig. 5.—Crown and stem of a young Western Larch. Priest River Forest Reserve, Idaho.

THE FOOD OF A TREE.

The materials upon which a tree feeds are derived from the soil and the air. The minute root hairs which spring from the rootlets take up water from the ground, and with it various substances which it holds in solution. These are the earthy constituents of the tree, which reappear in the form of ashes when any part of it is burned. The water which contains these materials goes straight from the roots to the leaves, in which a most important process in the feeding of the tree takes place. This process is the assimilation or taking up and breaking up, by the leaves, of carbonic acid gas from the air. It goes on only in the presence of light and heat, and through the action of chlorophyll, a substance from which the leaves and the young bark get their green color.

Plants containing chlorophyll are the chief means by which mineral materials are changed into food, so that nearly all plant and animal life depends upon them. Plant cells which contain chlorophyll break up the carbonic acid gas with which they come in contact, retain the carbon, one of its elements, and send back the other, oxygen, into the air. Then, still under the influence of the sunlight, they combine the carbon with the oxygen and hydrogen of the water from the roots into new chemical compounds, in which nitrogen and the earthy constituents mentioned above are also present; that is to say, the food materials which reach the tree through the roots and leaves are first digested in the leaves somewhat as food is digested in the human body, and are then sent to all living parts of the roots, stem, and crown, where they pass through another process of digestion, and are then either used at once in growth or stored away until the proper moment arrives. This is the general rule, but it is believed that in some cases food taken up by the roots can be used without first being digested in the leaves.

Fig. 6.—Crowns of the Black Hemlock (to the left) and Western Cedar. Washington Forest Reserve.	Fig. 7.—Stem and crown of a Longleaf Pine, the latter covered with moss swaying in the wind.

Plate III. ROOTS OF WESTERN HEMLOCK. OLYMPIC PENINSULA, WASHINGTON. A portion of the deep vegetable mold in which this tree stands has been burned away and the roots are exposed.

Fig. 8.—The forest cover. Spruce in Bavaria, Germany.

Fig. 9.—Yearly growth of a branch of Horse Chestnut. The hands of wrinkles mark the divisions between the growths of four successive years. The distance between these bands would never have been greater than it was when the branch was cut.

THE COMPOSITION OF WOOD.

Wood is made up chiefly of carbon, oxygen, and hydrogen. When perfectly dry, about half its weight is carbon, and half oxygen and hydrogen, in almost the same proportion as in water. It contains also about 1 part in 100, by weight, of earthy constituents, and nitrogen to the same amount. When wood is burned, all these materials disappear into the air except the earthy constituents. Now the nitrogen and water taken up by the roots were originally in the air before they reached the ground. It is true, therefore, that when wood is burned those parts of it which came from the air go back into it in the form of gas, while those which came from the soil remain behind in the form of ashes.

Fig. 10.—Bark of the Western Hemlock. Washington Forest Reserve.	Fig. 11.—Wood and bark of the Western Yellow Pine. The cut is perpendicular and the specimen stands as it did in the tree. The picture shows the division of the bark into scales by the successive layers of cork cambium. The true cambium is between wood and bark.

Plate IV. STEM AND CROWN OF A FOREST TREE IN BRITISH INDIA. The stem is about three feet in diameter.

HOW THE TREE BREATHES.

Besides giving out oxygen in assimilation, trees also take in oxygen from the air through their leaves, and through the minute openings in the bark called lenticels, such as the oblong raised spots or marks on the young branches of Birch and Cherry and many other trees. All plants, like all animals, breathe; and plants, like animals, breathe in oxygen and breathe out carbonic acid gas. This process of respiration or the breathing of the tree goes on both day and night, but it is far less active than assimilation, which takes place only in the light. Consequently more carbonic acid gas is taken into the tree than is given out, and the surplus carbon remains to be used in growing.

Fig. 12—Wood and bark of the Western Yellow Pine. The cut is a cross section and would have been horizontal as the specimen stood in the tree. Besides the division of the bark into scales this picture shows two of the deep-cracks in the bark, at the bottom of which lenticels are placed.	Fig. 13—Bark of the Western Yellow Pine. Outer surface, showing the scales made by the successive layers of cork cambium.

TRANSPIRATION.

The leaves give out not only the oxygen derived from the decomposition of carbonic acid gas taken from the air and carbonic acid gas produced in breathing, but also great quantities of water vapor. The amount of water taken up by the roots is very much larger than is required to be combined with carbon and the earthy constituents in the leaves. In order that fresh supplies of earthy constituents in solution may reach the leaves rapidly, the water already in them must be got out of the way. This is effected by transpiration, which is the evaporation of water from all parts of the tree above ground, but principally from the leaves. Even where the bark is very thick, as on the trunks of old Oaks and Chestnuts, transpiration goes on through the lenticels in the bottoms of the deep cracks. It sometimes happens, especially in spring before the leaves come out, that transpiration can not get rid of the water from the roots as fast as it rises, and that it falls in drops from the buds, or later on even from the leaves themselves.

Fig. 14.—By comparing this diagram with Pls. VII—IX and fig. 16, the place of each cut in the tree will be made clear.	Fig. 15.—Top of a common cork, slightly moistened to bring out the lines of annual growth, which are rather unusually plain in this specimen.

Plate V. BARK OF ONE OF THE BIG TREES OF CALIFORNIA. GENERAL GRANT NATIONAL PARK.

Fig. 16.—Wood of the Eastern Sycamore or Button-ball tree.

Fig. 17.—Cross section through a Black Oak. Milford, Pa. The silver grain, the rings of annual growth, and the dark heartwood and lighter sapwood are visible, and the line between the rough corky outer bark and mile thinner and lighter-colored inner bark may be seen.

THE GROWTH OF A TREE.

Fig. 18.—Cross section of a fallen Black Oak. Milford, Pa. The slabs shown in figs. 19 and 22 were sawed lengthwise from this tree, beginning where the black lines are seen on the cross section.

The addition of new material in the way described in the preceding pages is the foundation of growth. Except in the buds, leaves, fruit, and the twigs less than a year old, this material is deposited in a thin coat over the whole tree between the wood and the bark. The new twigs grow in length by a kind of stretching, but only during the first year. Thus it is only by means of these youngest twigs that a tree increases in height and in spread of branches. After the first year their length is fixed, younger twigs stretch out from the buds, and the older ones grow henceforth only in thickness. (See fig. 9.) The fresh coat of new material mentioned above covers them year by year. There are two layers in this coat, separated by a third one of tender forming tissues called the cambium, in which the actual making of the new substance goes on. The inner side of the cambium layer forms new wood, the outer side new bark. Besides the true cambium, which forms both wood and bark, there is another cambium which makes the corky outer bark, and nothing else. This cork cambium may encircle the whole tree, like the true cambium, as in the Red Cedar, or it may form little separate films in the bark, but in either case it dies from time to time, and is re-formed nearer the wood. (See figs. 10-13 and Pls. V and VI.)

Plate VI. BARK OF MATURE RED FIR. OREGON.

THE STRUCTURE OF WOOD.

Wood is chiefly made up of very small tubes or cells of various kinds, which have special uses in the life of the tree. Some conduct water from the roots to the crown, some store away digested food, and others merely strengthen the structure of the wood and hold it together. The wood of cone-bearing or coniferous trees (like the Pines and Spruces) has but few kinds of cells, while that of the broadleaf trees (such as Oaks and Maples) is much less simple. (See figs. 14, 16, 20, and Pls. VII-IX.) But in each case some of the cells have thick walls and small openings, an others wide openings and very thin walls. In climates which have regularly one season of growth and one of rest, like our own, the cells of the layer of new wood formed each year at the inner surface of the cambium are arranged in a definite way. When growth begins in the spring, and the fresh twigs and leaves put out, there is a great demand for water in the crown to supply these moist green new parts of the tree. Water rises in most trees through the newer layers of the wood, and especially through the last ring. Consequently, at first the tree makes thin-walled cells with wide openings, through which water can rise rapidly to the ends of the branches. Later on, when the demand for water is not so great, and there is plenty of digested food to supply building material, the cells formed are narrow and thick-walled. (See fig. 20.) Thus the summer wood in each year's growth is heavier, stronger, and darker in color than the spring wood. In the wood of many broadleaf trees, such as Oak and Chestnut, the spring wood is also marked by a band of open tubes of larger size called ducts. In others, such as Maple and Beech, these ducts are scattered through the whole season's growth, and in all conifers, as for example the Pines and Cedars, they are entirely wanting. But the differences in hardness and color between the growth of spring and summer are still present. It is sometimes possible to see the line which separates the growth of two seasons in the bark, as in the case of common cork, which is the outer bark of the Cork Oak, a native of southern Europe. (See fig. 15.)

Fig. 19.—Slab sawed lengthwise from a Black Oak. Milford, Pa. The saw passed about midway between the center of the tree and the bark. The line between the heartwood and the sap is plainly shown.

Plate VII. WOOD OF THE WHITE PINE.

Fig. 20.—Wood of a Spruce, greatly magnified. (From Hartig, Anatomie und Physiologie der Pflanzen, Berlin, 1891.)	Fig. 21.—A section of the common Staghorn Sumach, allowing the darkened heartwood, the white sapwood, and the inner and outer bark. Dark coloring matter is often deposited in the heartwood, as in the case here shown. Milford, Pa.

If the trunk or branch of an Oak tree is cut smoothly across, thin whitish lines may be seen running from within outward. Some of these lines begin in the center of the tree, and others in each one of the annual rings. These are the medullary rays, which make the silver grain in quartered Oak and other woods. (See figs. 17, 19, 22, and Pls. VII-IX.) They exist in all kinds of trees, but in many, as, for example, in the Chestnut and in most conifers, they are so fine as hardly to be seen with the naked eye. Seasoning cracks which run across the rings of growth always follow the lines of these rays, while others most often follow along some annual ring.

Fig. 22.—Slab sawed lengthwise from a Black Oak. Milford, Pa. The saw passed almost through the center of the tree, but not quite. The lines of annual growth are cut through obliquely, and the silver grain appears quite plainly, both in the middle and at the sides.

ANNUAL RINGS.

It is correct to speak of these rings of growth as "annual rings," for as long as the tree is growing healthily a ring is formed each year. (See figs. 17, 22, and Pls. VII-X.) It is true that two false rings may appear in one year, but they are generally so much thinner than the rings on each side that it is not hard to detect them. Very often they do not extend entirely around the tree, as a true ring always does if the tree is sound. Whenever the growth of the tree is interrupted and begins again during the same season, such a false ring is formed. This happens when the foliage is destroyed by caterpillars and grows again in the same season, or when a very severe drought in early summer stops growth for a time, after late frosts, and in similar cases.

Plate VIII. WOOD OF THE RED OAK.

Plate IX. WOOD OF THE SUGAR MAPLE.

HEARTWOOD AND SAPWOOD.

An annual layer once formed does not change in size or place during the healthy life of the tree, except that it is covered in time by other younger layers. A nail driven into a tree 6 feet from the ground will still be at the same height after it is buried under 20 or 50 or 100 layers of annual growth. But in most trees, like the Oaks and Pines, the wood becomes darker in color and harder after it has been in the tree for some years. The openings of its cells become choked so that the sap can no longer run through them. From living sapwood, in which growth is going on, it becomes heartwood, which is dead; because it has nothing to do with growth. (See figs. 19, 21.) It is simply a strong framework which helps to support the living parts of the tree. This is why hollow trees may flourish and bear fruit. Sapwood rots more easily than heartwood, because it takes up water readily and contains plant food, which decays very fast. Not all trees have heartwood, and in many the difference in color between it and the sapwood is very slight. Since water from the roots rises only in the sapwood, it is easy to kill trees with heartwood by girdling them, provided all the sapwood is cut through. But in those which have no heartwood the tubes of the older layers of wood can still convey water to the crown, and when such trees are girdled it is often several years before they die.

Plate X. CROSS SECTION OF A RED FIR, SHOWING SEASONING CRACKS AND ANNUAL RINGS. WESTERN OREGON. The rings are much winder near the heart than near the bark, as usually happens with larger trees. This tree grew very rapidly.

A great many theories have been proposed to account for the rise of water into the tops of tall trees, some of which, as in the big trees of California, may be over 300 feet from the ground. But none of these theories are quite satisfactory, and it must be admitted that we do not yet know how the trees supply their lofty crowns with the water which keeps them alive.