Tree Growth and Wood Formation

very severe drought in early sum- mer stop.s growth for a time, after late frosts, and in similar ca.ses. HEABTWOOD 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, A younger layers. nail driven into a tree 6 feet from the ground will .still be at the same height after it is buried under 20 or 60 or JOG layers of atuuial growth. But in most trees, like the oaks and pines, the wood becomes darker in color and harder after it has been in the 14 ti'ee for some j'ears. 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 hcartwood, which is dead, because it has nothing to do with growth (fig. 12). 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. When the tree is cut down, the sapwotxl rots more easil}' than the heartwood, because it takes up water readily and contains plant food, which dccaj's very fast. Not all trees have heartwood, and in many the difference in color between it and tlie sapwood is very sliglit. Since water from the roots rises only in the sapwood, it is eas' to Icill trees with heartwood by girdling them, provided all tiie sapwood is cut through. But in tho.se which have no heartwood the tuhes (if the older layers of wood can still (Dnvey water to the crown, and when such trees are girdled it is often several years before they die. A great many theoi'ies have been proposed to account for the rise of water into the tops of tall trees, s )me of which, as in the big trees of California, may be over 300 feet from the ground. But none of these theories is quite satisfactory, and it must ))e admitted that we do not yet know how the trees supply their lofty crowns with the water which keeps them alive. TREES IN THE FOREST. The nature of a tree, as shown by the white sap� bark. Milfonl. !') ,',,i 1 niter its behavior in the forest, is called jj^ silvicultural character. It is made up of all those qualities upon which the species as a whole, and ever^y individual tree, depend in their struggle for existence. The regions in which a tree will live, and the places where it will flourish best; the trees it will grow with, and those which it kills or is killed by; its abundance or scarcity; its size and -- rate of growth all these things are decided l)y the inborn (jualities, or silvicultural character, of each particular kind of tree. 15 THE VARIOUS BEaXJIREMENTS OP TREES. Dirt'ereiit sjx'cirs of trees, like ditl'eieiit. races of men, have special re(|iiireiiieiits for the things upon which their life depends. Some races, like the Eskimos, live only in cold regions. Others, like the South Sea Islanders, must have a very warm climate to be comforta- ble, and are short-lived in any other (tig. 13). So it is with trees, except that their difl'erent needs are even more varied and distinct. Some of them, like the willows, birches, and spruces of northern Canada, stand on the boundary of tree growth within the Arctic^ Circle. Other species grow only in tropical lands, and can not ri'sist even the lightest frost. It is alwaj^s the highest and lowest temperature, rather than the average, which decides where a tree will or will not grow. Thus the average temperature of an island where it never freezes may be onlj^ 60'^, while another place, with an average of 70�, may have occasional frosts. Trees which could not live at all in the latter on account of the frost might flourish in the lower average warmth of the former. In this way �the influence of heat and cold on trees has a great deal to do with their distribution over the surface of the whole earth. Their distril)ution within shorter distances also often depends largely upon it. In the United States, for example, the Live Oak does not 16 grow in Maine nor the Canoe Birch in Florida. Even the opposite sides of the same hill inaj' be covered with two difl'erent species, because one of them resists the late and early frosts and the fierce middaj' heat of summer, while the other requii'es the coolness and moisture of the northern slope (fig. 14). On the eastern slopes, where the sun strikes early in the day, frosts in the spring and fall arc far more apt to kill the young trees or the blossoms and twigs of older ones th;in on those which face to the west and north, where growth begins later in the spring, and where rapid thawing, which does more harm than the freezing itself, is less likely to take place. REaUIIXEMENTS OF TREES FOR HEAT AND MOISTURE. Heat and moisture a<'t together upon trees in such a way that it i.s A sometimes hard to distinguish their effects. diy country or a dr}' slope is apt to l)e hot as well, while a cool northern slope is almost always moister than one turned toward the south. Still the results of 17 the demand of trees for water-ciui usually l)c distinguished from the results of their need of warmth, and it is found that moisture has almost as great an influence on the distribution of trees over the earth us heat itself. Indeed, within any given region it is apt to be much more conspicuous, and the smaller the region the more noticeable often is its effect, because the contrast is more striking. Thus it is frequently easy to see the difference between the trees in a swamp and those on a dry hillside nearl)v, when it would be far less easy to distinguish the general character of the forest which includes both swamp and hillside from that of another forest at a distance. In many instances the demand for water controls distribution altogether. For this reason the forests on the opposite sides of mountain ranges are often composed of entirely different trees. On the west slope of the Sierra Nevada of California, for example, where there is plenty of moisture, there is also one of the most beautiful of all forests. The east slope, on the contrary, has compavativeh' few trees, Ijecause its rainfall is very slight, and those which do grow there are small and stunted in comparison with the giants on the west. Again, certain trees, like the Bald Cypress and the River_ Birch, are commonly found only in ver' moist land; others, like the mescpiites and the Pinyon or Nut Pine, only on the driest soils; while still others, like the Red Cedar and the Red Fir, seem to adapt themselves to almost any degree of moisture, and are found on both very wet and verj^ dry soils. In this way the different demands for moisture often separate the kinds of trees which gi'ow in the bottom of a valley from those along its slopes, or even those in the gullies of hillsides from those on the rolling land between. A mound not more than a foot above the level of a swamp is often covered with trees entirely different from those of the wetter lower land about it. Such matters as these have far more to do with the places in which different trees grow than the chemical composition of the soil. But -- its mechanical nature that is. whether it is stiff or loose, fine or coarse -- in grain, deep or shallow is very important, because it is dii'ectly connected with heat and moisture and the life of the roots in the soil. REQUIREMENTS OF TREES FOR LIGHT. The relations of trees to heat and moisture are thus largely respon.sible for their distribution upon the great divisions of the earth's surface, such as continents and mountain ranges, as well as over the smaller rises and depressions of every region where trees grow. But �while heat and moisture decide where the different kinds of trees can grow, their influence has comparatively little to do with the struggles of individuals or species against each othev for the actual possession 26900--No. 173--03 2 18 of the gTouncl. The outcome of thesse struggles depends less on heat and moisture than on the possession of certain ijualities, among which is the ability to bear shade. With regard to this power trees are roughly divided into two classes, often called shade-bearing and lightdemanding, following the German, but better named tolerant and intolerant of shade. Tolerant trees are those which flourish under more or less heavy shade in earlj^ youth; intolerant trees are those which demand a comparatively slight cover, or even imrestricted light. Later in life all trees require much more light than at first, and usually those of both classes can live to old age only when they are altogether unshaded from above. But there is always this difference between them: The leaves of tolerant trees will bear more shade. Consequently the leaves on the lower and inner parts of the crown arc more vigorous, plentiful, and persistent than is the case with intolerant trees. Thus the crown of a tolerant tree in the forest is usually denser and longer than that of one which bears less shade. It is usually true that the seedlings of trees with dense crowns are able to floui'ish under cover, while those of light-crowned trees are intolerant. This rough general rule is often of use in the study of forests in a new country, or of trees whose silvicultural character is not known. TOLERANCE AND INTOLERANCE. The tolerance or intolerance of trees is one of their most important silvicultural characters. Frequently it is the first thing a forester seeks to learn about them, because what he can safely luidertake in the woods depends so largely upon it. Thus tolerant trees will often grow vigorously under the shade of light-ci'owned trees (fig. 15) above them, while if the positions were reversed the latter would speedily die. The proportion of ditt'erent kinds of trees in a forest often depends on their tolerance. Thus hemlock sometimes replaces White Pine in Pennsylvania, because it can grow beneath the pine, and so be ready to fill the opening whenever a pine dies. But the pine can not grow under the hemlock, and can only take possession of the ground when a fire or a windfall makes an opening where it can have plenty of light. Home trees after being overshaded can never recover their vigor when at last they are set free. Others do recover and grow vigorously even after manj' years of starving under heavy shade. The Red Spruce in the Adirondacks has a wonderful power of this kind, and makes a line tree after spending the first fifty or even one m hundred years of its life reaching a diameter of two inches. The relation of a tree to light changes not onl}? with its age, but also with the place where it is growing, and with its health. An intolerant tree will stand more cover where the light is intense than in a cloud3' northern region, and more if H has plenty of water than J73 19 with ii scauty .supply- Vigorous seedlings will get along with less light than sickly ones. Seedlings of the same species will prosper under heavier shade if they have always grown under cover than if they have had plenty of light at first and have been deprived of it after^^ard.--. Ea tern N ith Car lina THE RATE OF GROWTH. The rate of growtli of different trees often decides which one will survive in the forest. For example, if two intolerant kinds of trees should start together on a burned area or an old field, that one which grows faster in height will overtop the other and destroy it in the end by cutting oif the light. Some trees, like the Black Walnut, grow rapidU- from their earliest youth. Others grow very slowly for the first few 3'ears. The stem of the Longleaf Pine, at 4 years old, is usuallj" not more than 5 inches in length. During this time the roots have been growing instead of the stem. The period of its rapid growth in height comes later. The place where a tree stands has a great influence on its rate of growth. Thus the trees on a hillside are often much smaller than those of equal age in the rich hollow below, and those on the upper 20 slopes of a high iiiountiiin arc ((minioiily starved and stunted in comparison with the vigorous forest lower down. The Western Chinquapin, which reaches a height of 150 feet in the coast valleys of nortTiern California, is a mere shrut) at high elevations in the Sierra Nevada. The same thing is often observed in passing from the more temperate regions to the far north. Thus the Canoe Birch, at its northern limit, rises only a few inches above the ground, while farther south it becomes a tree sometimes 120 feet in height. THE REPRODUCTIVE POWER OF TREES. Another matter which is of tin fleopest interest to the forester is the rcprodu<-tivc power of his trees H;xce])t in the case of sprouts and other growth fed by old roots, this depends first of all on the quantity of the seed which each tree bears; ])ut so many other considerations afi'cct the result that a tree which bears seed al)undantly may not re- produce itself very wel i A part of the seed is always unsound, and sometimes nuich the larger part, as in tlie case of the Tulip Tree. Hut even a great abundanceof sound seed does not always insure good reproduction. Theseeds may not find the right surroundings for successful germination, or the infant trees may pi'rish for want of water, light, or suitable soil. Where there is a thick layer of dry leaves or needles on the ground, seedlings often perish in great numbers because their delicate rootlets can not reach the fertile soil beneath. The same thing happens when there is no hunnis at all and the surface is hard and dry. The weight of the seed also has a powerful influence on the character of reproduction. Trees with iieavy seeds, like oaks, hickories, and chestnuts, can sow them only in their own neighborhood, except when they stand on steep hillsides or on the banks of streams, or when birds and squirrels carry the nuts and acorns to a distance. Trees with light, winged seeds (tig. 16), likethe poplars, birches, and pines, have a great advantage over the others, 21 l)eoause they can drop their seeds a long way oflf. The wuid is the means by whicrh this is l)ronght al)out, and the adaptation of the seeds themselves is often very curious and interesting. The wing of a pine seed, for example, is so placed that the seed whirls when it falls, in such a way that it falls very slowly. Thus the wind has time to carry it away before it can reach the ground. In heavy winds pine and other -- winged seeds are blown long distances sometimes as much as several miles. This explains how' certain kinds of trees, like the (iray Birch and the White Pine, grow up in the middle of open pastures, and how others, such as the Lodgepole Pine, cover great areas, far from the parent trees, with young growth of even age. Such facts help to explain why, in certain places, it happens that when pines are cut down oaks succeed them, or when oaks are removed pines occupy the ground. It is very often true that young trees of one kind are already growing unnoticed beneath old trees of another, and so are ready to replace them whenever the old trees are cut away. PURE AND MIXED FOREST. The nature of the seed has much to do with the ilistri))utioii of trees in pure or mixed forest. Some kinds of trees usually grow in bodies of some extent containing only a single kind; in other words, in pure forest. The Longleaf Pine of the South Atlantic and Gulf States is of this kind, and so is the Lodgepole Pine of the West. Conifers are more apt to grow in pure forest than broadieaf trees, because it is more common for them to have winged seeds. The gi'eater part of the heavy-seeded trees in the United States are deciduous, and most of the deciduous trees grow in mixed forest, although there are some conspicuous exceptions. But even in mixed forests small groups of trees with heavy seeds are common, because the young trees naturally A start up beneath and around the old ones. 'heavy seed, dropping from the top of a tall tree, often strikes the lower branches in its fall and bounds far outside the circle of the crown. Trees which are found only, or most often, in pure forest are the social or gregarious kinds; those which grow in mixture with other trees are called scattered kinds. Most of the hardwood forests in the United States are mixed; and many mixed forests, like that in the Adirondacks, contain l)oth broadieaf trees and conifers. The line between grega- rious and scattered species is not always well marked, because it often happens that a ti'ee may be gregarious