INTRODUCTION TO FORESTRY
THE WORD "forest" has been variously defined. As men have become better acquainted with the forest and its uses, the definitions have changed. It is difficult to reconcile some of the earlier definitions of the forest as "a waste grounds belonging to the king" (Blackstone), with such modern definitions as "a complex association of trees, shrubs, and other plants in which each individual plays some part in the life of the community" (Graves and Guise). Some foresters recognize management as a factor in the definitions, but forests can and do exist without the slightest semblance of management.
To the forester, the forest embraces more than trees and shrubs. It is an association or community of trees, shrubs, soil and soil organisms, animals, birds, and insects, each of which exerts important influences on the ultimate character and value of the area. This association should extend over a considerable area. The farm wood lot of less than 5 acres would not be classified as a forest, although it is possible to practice forestry in it.
Trees in the forest usually differ from those grown in the open or in orchards. Their shape and the absence of lower limbs is a result of natural pruning by mutual shading. It is possible to have a number of "wolf" trees in a forest. These low-crowned, wide-spreading individuals do not have typical forest form, but they exist as forest trees.
Thus we see that "forest" is a term that cannot be sharply defined. A small tract, called by foresters a wood lot, may be a forest to a city dweller. The easterner may consider the extensive areas of chaparral and manzanita in the Southwest as brushy wastelands, although they are important watershed forests.
For the purpose of this book, let us define the forest as an association of trees (as in large plantations with no undergrowth), or shrubs (as in the watersheds of the Southwest), or both (as in the hardwood and mixed forests), growing on a considerable area, upon which it is possible to practice forestry.
What Is a Forest?
Forest grown trees.
Open grown trees.
Part of the forest heritage of the West.
"Forestry" like the term "forest" has been defined in many ways. In Europe the first definitions of forestry laid stress on the ability of the forest and the forester to produce and maintain game for the royal hunt. The old European forester was game keeper and policeman of the hunting preserve.
When a scarcity of timber supplies became evident in the Old World and its danger was forecast in America, forestry became the raising of continuous timber crops, and game production was relegated to minor importance. With further study of forests and forest influences, scientists discovered that protection of important watersheds, distribution of precipitation, erosion prevention, and partial control of drying winds were important forest functions. Many forest areas are managed largely with these ends in view, timber production and game management being of secondary consideration.
In recent years increased leisure time has created a demand for another forest product which in some areas exceeds all others in importance. This product, not measurable in dollars and cents or in cords or board feet, is recreation.
Some definitions of forestry imply that the profession is one to be practiced on nonagricultural land. Forestry may be, and is, practiced on land which, if cleared of trees, would be far better agricultural soil than that on which many farmers are struggling to grow crops of corn or potatoes. Much poor farm land now under cultivation would be better employed raising trees. An ideal plan might be to convert the submarginal land (that which can barely produce an income from farm crops) into forests, and such forested lands as might produce profitable agricultural crops into farms. That ideal situation has not been reached, so forestry is practiced on rich as well as on poor land.
Forestry is the production and maintenance of themany and varied products of the forest. It has been defined as ascience, but it is a combination of many sciences such as botany,biology, physics, and mathematics. It has been defined also as an art(the application of these sciences). The sciences are fundamentals uponwhich the art is based. Forestry includes the study of these sciencesand their application, which is the art. Hence forestry may be simplydefined as the science and art of managing forests so that they yieldcontinuously their maximum of wood products, values, andinfluences.
No wood is wasted in the Old World.
See Ch. II, p. 31.
Steep slopes make poor farms, but they can support good forests.
Science or Art?
A tree is a woody plant exceeding 10 feet in height,with a single stem unbranched for some distance above the ground. Treeslive on from year to year, increasing in height and diameter eachseason. They differ from shrubs in: (a) their manner ofbranchingshrubs branch at, or close to, the ground; (b) theultimate height attainedtrees commonly reach 180 to 200 feet andoccasionally more than 300 feet, shrubs rarely exceed 25 to 30 feet andare usually much smaller; and (c) the shape of their crownstreesdevelop crowns characteristic of the species, shrubs with indefinitestem arrangement rarely assume a characteristic form. Certain species,however, such as the sumacs and willows, which are properly classifiedas trees when growing in favorable locations, may become shrubby underadverse conditions.
The tree is composed of three main partsroots,stem, and crown. Roots extend deep into the soil and anchor thetree against strong winds. They search out the mineral elements andwater necessary for maintenance of tree growth. Large roots bear smallerrootlets, and these rootlets in turn bear fine, hairlike roots whichabsorb the dissolved nutrients from the soil and transfer them to therootlets, from whence they begin a journey through the roots to thestem.
The stem (shaft, trunk, or bole) is a mass ofelongated cells or tubes tightly bound together with other smaller cellsand shielded against mechanical injury and extreme temperatures by aprotective covering of bark. The vital or living mass of tissue in thestem is a thin sheath of active cells, called cambium, separating thebark and wood. Through the tubes of the new wood, inside the cambium,the dissolved food substance passes to the crown.
The crown includes the branches, twigs, leaves, andbuds. The wood structure of the branches is similar to that of thetrunk, the cambium layer and conducting tubes extending to the tips ofthe smallest twigs.
Many theories have been advanced to explain the flowof water and food from the roots to the crown. Early students of plantphysiology attributed the ascent of sap to capillary action. Capillarityis the tendency of liquid to rise in fine tubes. If a fine glass tube,open at both ends, is placed upright in a vessel of water, the waterlevel in the tube will be higher than that in the vessel. The smallerthe tube, the higher the water will rise. Oil rises in a lamp wick bycapillary action; and water will ascend a dry stick if one end isdipped into a vessel of it. The height to which liquid may rise bycapillarity is limited, however; it is impossible for sap to reach thecrowns of large trees like the redwood and sequoia by capillarityalone.
Another force, long thought to be the cause of sapascension, is atmospheric pressure. As leaves transpire water, accordingto this theory, a vacuum is created. Atmospheric pressure was thought toforce sap upward to fill the void. The action of atmospheric pressuremay be observed in the common mercury barometer. The greatest height towhich water can be raised by atmospheric pressure is about 32 feet, butsap in trees rises to many times this height.
Pressure exerted by the roots has also been creditedwith causing sap ascent. That root pressure is an active force may beseen when a tree has been cut, especially in the springthe stumpwill "bleed." This force also is limited and cannot transport sap togreat heights.
The most generally accepted explanation of sapascension is the "cohesion" theory. The column of water in a tree may belikened to a long string. A pull exerted by transpiration (release ofwater vapor by the leaves) on one end of this string causes movement allalong the line to the roots. Energy for this process is supplied by thesun. As the sun evaporates the transpired water, it draws the watercolumn upward to the leaves. Water under such circumstances has atendency to stay together, or cohere. Cohesion is greater in sap than inpure water. The transpiration pull on the sap is sufficient to move thecolumn to great heights, and the cohesive force of the sap is strongenough to hold the fine columns or "strings" together. Although no onevessel or group of vessels extends the entire length of the tree, theredoes exist an unbroken series of columns that zig-zag in many directionsfrom the leaves to the roots.
Plant physiologists are still seeking explanationsof the details involved and the movement of sap is still beinginvestigated. It is probable that all the forces mentioned in thisconnection contribute to sap ascension. Clearer and more definiteconclusions may be obtained by further research.
However this may be, by some force or series offorces the sap (water and dissolved nutrients) is carried to the leaves.The leaves are, in effect, small factories where the raw materials aremanufactured into food. Each leaf bears many small pores or mouths,called stomata, through which it takes in air. Air is composed ofnitrogen and oxygen, with small quantities of argon and carbon dioxide,varying amounts of water vapor, and minute quantities of other elements.Carbon dioxide is separated from the rest of the air by the leaves andis combined, in the presence of sunlight, with water and otherelements. Wood is composed chiefly of carbon, oxygen, and hydrogen. Thecarbon is derived from the air, and the oxygen and hydrogen from thesoil water that has ascended to the leaves. Mineral elements from thesoil comprise about 1 percent of wood. It is these earthy constituentsthat remain as ashes when wood is burned.
The tree's raw food must be dissolved in water sothat it can ascend to the height of the crown, and when the rawmaterials, including water, are combined with carbon dioxide muchunnecessary water remains. This is given off (transpired) through thepores of the leaves and is evaporated by the sun.
After manufacture, the elaborated food materialreturns downward through the cells and tubes of the inner bark, to thetwigs, branches, trunk, and roots. The channels of food transportationin a tree may be compared with the blood streams in a humanbeingblood being carried to the lungs to acquire oxygen, and thenflowing to the growing parts of the body.
A cross-section of the trunk reveals a series ofrings, one for each year of the tree's life. In a large tree theinnermost group of rings is dead, hard, brittle, and stiff. Thefunction of this core of heartwood is to maintain the tree in an uprightposition. Around the heartwood is a sheath of younger wood or sapwood.The sapwood, still alive, helps in the transportation of food and water.Surrounding the sapwood is a fine layer of small cells, called thecambium, which is really the growing portion of the stem.
Elaborated food from the leaves travels downwardthrough the inner bark and is diverted laterally into horizontallyarranged cell groups or rays. Cambium cells absorb this food substanceand grow.
Cells increase in number by cell division. A cellgrows and splits into two smaller cells; the two small cells grow, andeach splits into two more. By cell division and growth, the cambiumforms new wood on its inner side and new bark on the outside.
The wood growth early in the season (spring-wood) isformed when the trunk must transport vast amounts of water and food;therefore, its cells and tubes are large and thin-walled. The"summerwood" is composed of smaller, heavier-walled cells. This differencein cell structure produces the visible annual rings which can readily beseen on the stump of a tree. It is possible to tell the age of a tree bycounting the rings from the center or pith, to the cambium. The tree'shistory is shown in a cross-section of the trunka poor growingseason results in a narrow ring, a rich season in a wide one, and firesleave tell-tale scars. Removal of nearby competing trees also isindicated by the wider rings.
As the cambium manufactures new wood it increases insize, forcing the bark outward. The older bark, being dead, cannotstretch and expand. Consequently it cracks into the plates, ridges, orscales that are characteristic of some trees. The eucalyptus, or bluegum, sheds its bark annually. The bark of the redwood, on the otherhand, remains for years, becoming very thick and ridged beforedropping.
To perform the functions of growth, the tree musthave heat and light in addition to water, minerals, and air. Themanufacture of raw food by the leaves requires sunlight. In a denseforest the leaves soon disappear from the shaded understory and thetrees develop new ones in the canopy. Some trees are better able towithstand shade than are others. These are known as "tolerant" trees.Beech, hemlock, and balsam fir are examples of tolerant trees, whereasblack locust and the larches are intolerant. But no tree can live verylong without some sunlight. All growth processes of a tree requireenergy which is derived from light and heat. Tree growth is negligiblein winter. In the extreme North, trees are often stunted and smallbecause their growth is retarded by lack of sufficient heat.
According to their sizes, Gifford Pinchot hasclassified trees into seven groups:
1. Seedlings: Trees of seed origin, up to 3feet in height.
2. Small saplings: Trees 3 to 10 feethigh.
3. Large saplings: Trees more than 10 feet inheight, and up to 4 inches in diameter.
4. Small poles: Trees 4 to 8 inches indiameter.
5. Large poles: Trees 8 to 12 inches indiameter.
6. Standards: Trees 1 to 2 feet indiameter.
7. Veterans: Trees more than 2 feet indiameter.
See Wood Technology, p. 151.
Water rises higher in fine tubes.
Oil rising by capillarity.
Mercury rises by atmospheric pressure.
Resin flowing from stump by root pressure.
Cohesion of liquid.
Section of leaf.
Transportation channels in wood.
A ring for each year's growth.
Three stages in cell division.
Trees Must Have Light and Heat.
THE FOREST FLOOR
The term "forest floor" is often confused with"forest cover." The forest floor is that covering lying close to themineral soil under the forest. Forest cover includes trees, underbrush,and herbaceous growth.
The forest floor resembles a rug in its make-up. Itis compact in its lower layers and lighter in its upper layer. It ismade up of fallen leaves, twigs, pieces of bark, fruits and nuts,rotting logs, down trees, and other vegetative matter. It consists ofboth plant and animal life.
There are actually three layers of the forest floor.The first one, resting on and merged with the mineral soil, is a massof humus. It is damp, dark in color, and composed of thoroughly decayedlitter (fallen leaves, twigs, and bark). The middle layer is made up ofpartially decayed litter. The character of the leaves, twigs, and othercomponents may be seen in the compound, and in it animal life andchemical forces are at work reducing this mass to completelydisintegrated organic matter which gradually changes into soil. It ishumus in the making.
The third and topmost layer is exposed to the air. Itis made up of newly fallen leaves, twigs, and wood particles. This leafycovering is to the forest floor as the nap is to a rug. Raindrops fallingon this cover are checked and broken up, and the run-off enters thespongy soil. When air currents pass through the forest this mass oflitter keeps the soil from drying.
Bacteria and fungi in the forest litter makedecomposition possible. Humus is changed into soil more readily inlocations where the seasons are warm and long. On the other hand, wherethe seasons are short, the litter accumulates in greater quantities andprotects the soil and roots from killing freezes.
The Forest FloorFallen Leaves, Twigs, and Bark.
A section of the forest floor.
Like a Huge Sponge.
See pp. 20, 22.
Swarming Bacteria, Rapidly Growing Fungi.
THE TREE COMMUNITY
Trees in the forest are comparable to human beings ina social or economic community, except that the tree's inherent savagestruggle for existence is more openly ruthless than man's subtle,diplomatic schemes to get ahead. As there are classes of people in theeconomic and social scales, so there are classes of trees in the forestcommunity, as follows:
Dominant: Overtopping the rest of thestand.
Codominant: Beneath the dominant but receivingfull sunlight on top and sides of crown.
Intermediate: Beneath the codominant,receiving sunlight on top only; growth retarded by dominant andcodominant classes.
Suppressed: Beneath all other living classes;receiving little or no sunlight; little chance for recovery.
Dead: Trees which have succumbed in thestruggle.
Each tree succeeds in its life struggle only to theextent that the trees in the higher crown classes allow it to succeed.When the larger trees are removed, through death, lumbering, or silviculturalcuttings, the intermediate and suppressed ones have a chanceto recover. Some trees, however, cannot reestablish themselves whenreleased from long suppression.
The life and death struggle results in a survival ofthe fittest. On some sites and under certain conditions the fittest treemay be an important timber species. On others it may be a weed for whichthe lumberman can find no market. The forester attempts, through wiseuse of the axe (removal of certain trees) and underplanting, to convertthe stand to trees of economic value.
Competition for soil nutrients (foods) often resultsin well developed and widespreading root systems. Competition for lightusually develops tall, straight survivors, and that is one of thereasons trees are placed close together in plantations. Such spacingcauses the ultimate death of over half the seedlings, but the trees thatsurvive usually are well formed timber trees devoid of large lowerlimbs.
Practically all trees are tender when young, and aresusceptible to scorching by direct rays of the sun. Foresters protectsuch trees by planting them beneath "nurse" trees. The beech with itsthin canopy is an excellent nurse tree. It guards against the direct sunrays but permits enough light to filter through to sustain the youngtrees beneath it. Intolerant trees, that is, trees unable to withstandexcessive shade, planted under young beech soon harden and reach formore sunlight, finally overtopping the beech.
Some treesthe locust and Scotch pine forexamplesadd nitrogen to the soil through nodules formed on theirroots. Trees like spruce which ordinarily perish on poor, sandy soil maystill be grown on such sites if mixed with nitrogen-producingtrees.
Other trees have poisonous effects. It has been notedthat often no vegetation except grass will grow beneath the blackwalnut. Poisons given off by the roots of black walnut may be fatal toany woody vegetation with which they come in contact.
Let us follow the struggle for existence in a forestin its natural state, one in which forestry is not being practiced. Wewill assume that the forest consists of large white pines and hemlocks.The larger pines have overtopped the hemlocks and are dominant, butbeneath their shade the tolerant hemlocks are thriving.
As the older pines die and succumb to insects anddecay, openings are left in the canopy. Ordinarily a dying treeproduces an abundance of seed in its final years. These seeds and thoseof neighboring trees fall to the groundto the dense brush-coveredareas of laurel and rhododendron, to the open spaces in the shadows oftowering pines and hemlocks, and to the sun-lit patches of forest floorvacated by dead trees. Warm rains cause the seed to sprout. Those in thethickets are quickly choked by the brush, those in the shadows die fromlack of light, but those in the openings put forth tiny stems and roots.While they are tender and succulent, the seedlings are in constantdanger from all the herbivorous animals of the forest. Deer and otherbrowsing animals feed on the new leaves, and birds eat the tendersprouting seed.
After the first season, barely half the crop is heft,Where there is no protective leaf canopy, heavy rains gouge theseedlings from their beds, frost kills many more, the scorching rays ofthe sun burn up their unprotected buds and needles, and brush and weedscompete with them for possession of the openings and for the foodelements in the soil. A few of the trees, however, become established infavorable spots.
The following spring the older trees burgeon forth,with new shoots and leaves, to fill the holes in the canopy; the shadedeepens, and the seedlings begin a new struggle for light, water, andfood. As the snow melts, they become exposed once more to the deer andrabbits; and late frosts retard their growth.
The closing of the canopy of the older trees allowssufficient light for seedling growth and protects them from the directrays of the sun. But as the canopy closes more and more each year theshade becomes too intense. Being less tolerant than the hemlock, theyoung pines yellow and begin to die. If the canopy remains closed, allthe young pines will die and the tolerant hemlocks will have fullpossession of the area. But now a storm blows down a large overmaturepine. This permits more light to reach the understory. Responding tothe light, many of the young pines recover, and rapidly grow to reach aplace in the sun. Growth of the hemlock also is accelerated. All thistime numerous insects and diseases are attacking the trees, and anintense competition for soil, water, and food takes place. The weakenedtrees succumb to these attacks, and the stronger ones take possession ofthe root and crown space vacated by their dying neighbors.
Thus the trees pass through the sapling stagetheintolerant white pines gaining dominance because of their strivingfor additional light. They are now tall saplings with pointed crowns andstraight boles which excess shading has made limbless.
Overtopping the hemlocks, however, does not give thepines full possession of the site. The tolerant hemlocks thrive well inthe shade; and when the pines settle down to grow in diameter, thehemlocks continue to grow in height. Since pine demands more light thanhemlock, successive generations of competition for a site usually resultin the complete suppression of the pine. White pine seedlings cannotendure the dense shade of mature pines, but hemlock can establish itselfunder shade.
Although this is the story of but two species,similar struggles go on wherever two or more trees are contending forthe same limited area. Shrubby and herbaceous plants enter into thestruggle particularly in the earlier stages; insects and diseases areoften determining factors in the ultimate plant growth.
Classes of Trees are Like Classes of People.
Young trees are suppressed in dense forest.
Tolerant species may thrive in dense forest shade.
Nitrogen nodules on roots.
Tolerant hemlocks under pine.
Young pines start in openings.
Pines die in dense shade.
A New Struggle.
Pines Must Have Light.
Pines respond to light.
Pines reach for the sun.
Hemlocks reproduce in shade.
The forest is an association of trees, shrubs, and other forms of life.
The forest is much more than trees and shrubs. It isa complex association of many forms of life, struggling together andagainst each other to grow and reproduce. Competition for light and foodin the forest usually results in tall, straight trees free of largelower limbs. Often, however, this survival of the fittest does notproduce the best timber species or the species best adapted to man'suse.
Forestry, by applying the knowledge of tree growth,soils, spacing, planting, improvements, and protection to timberproduction, has directed the forces of nature to fulfill man'sneeds.
The tree is almost as complex as the forest itself.It is a living plant, comparable in many instances to man. Like man, itneeds air, light, heat, water, and food for its growth. Its rootspenetrate deep into the ground in search of food material; its trunkcarries the raw food to the crown where the leaves combine it withelements from the air to form nourishment for the growing cells.
Billions of minute cells and tubes make up thesubstance we know as wood, and each year many more cells are addedthrough the growth of the cambium layer. As the cambium grows it adds anew layer of wood on its inner side and a new layer of bark on its outerside. In the spring the cambium grows large cells with thin walls, butin the summer smaller thicker-walled cells are grown. The differencebetween the spring and summer wood may readily be seen on a cross-sectionof the tree trunk; and it is possible, by counting these annualrings, to determine the tree's age, and by examining the size and conditionof the rings, to study the tree's history and to predict futuregrowth.
The transportation of food material from the roots tothe crown is a subject which has long interested scientists. Manyexplanations have been attempted but the most generally accepted one isthe "cohesion theory" which likens the columns of sap to a series ofstrings pulled upward by the sun's power to evaporate moisturetranspired, or given off, by the leaves.
One of the important components of the forest is theforest floor (the soil, humus, and litter). It is from this source thatthe tree receives most of its food. In cross section the forest floorappears as a number of intermingling layers of leaves, twigs, and otherlitter in various stages of decomposition, resting upon and mergingwith the mineral soil. Bacteria and minute insects live in the soillayers and hasten the disintegration of litter into rich soil.
The forest floor acts as a huge sponge in absorbingrainfall and snow melt, retaining some of it for plant use butpermitting most of it to trickle slowly into springs and streams tomaintain constant flow.
Tree roots in the forest floor compete for water andfood, while their trunks and crowns struggle for light. Consequentlydifferent classes of trees appear. Those seed which have been fortunatein falling in open spaces or on good soil thrive and grow into healthytrees; others, less fortunate, fight a losing battle against naturalforces. Some trees are endowed with power to resist shade and will liveon year after year in the shadows of towering neighbors. Grazing animalseat or trample young seedlings, insects and diseases maintain a constantattack so that only a small percentage of the seeds which reach goodsoil finally become timber producers.
Refer to Forest Competition, p. 9.
The Tree, pp. 3, 7.
Wood Structure and Growth, pp. 6, 7.
Refer to How Sap Rises, pp. 4, 5.
Refer to The Complex Forest Floor, pp. 8, 9.
See Tree Classes, pp. 9, 10.
Refer to Tree Enemies, pp. 58, 59.