Wood and Forest Part 36

But trees hinder as well as help each other. There is a constant struggle between them for nourishment and light. To get food and water, some trees, as spruces and hemlocks, Fig. 70, spread their roots out flat; others, as oak and pine, send down a deep tap root.

Those succeed in any environment that find the nourishment they need.

Still more evident is the struggle for light and air. However well a tree is nourished thru its roots, unless its leaves have an abundance of light and air it will not thrive and make wood.

[Ill.u.s.tration: Fig. 71. Long-bodied White Oak of the Forest. _U. S.

Forest Service._]

Even the trees most tolerant of shade in youth, like spruce, must have light later or perish, and hence in a forest there is the constant upward reach. This produces the characteristic "long-bodied" trunk of the forest tree, Fig. 71, in contrast to the "short-bodied" tree of the open, where the branches reach out in all directions, Fig. 72.

In this constant struggle for existence is involved the persistent attempt of scattered seeds to sprout whenever there is an opening. The result is that a typical forest is one in which all sizes and ages of trees grow together. Scattered among these are bushes and scrubby trees, called "forest weeds," such as mountain maple and dogwood, Fig.

80, p. 234, which do not produce timber.

[Ill.u.s.tration: Fig. 72. Short-bodied White Oak of the Open. Fort Lee, N. J.]

By foresters the trees themselves are cla.s.sified according to their size into:

Seedlings, less than 3' high, Saplings, Small, 3'-10' high.

Large, 4" in diameter, at breast height (4' 6").

Poles, Small, 4"-8" in diameter, at breast height.

Large, 8"-12" in diameter, at breast height.

Standards, 1'-2' in diameter, at breast height.

Veterans, over 2' in diameter, at breast height.

Every age has its own dangers. Many seeds never germinate, many seedlings perish because they do not reach soil, or are killed by too much or too little moisture, or by heat or cold, or shade. At the sapling age, the side branches begin to interfere with those of other saplings. Buds are bruised and lower branches broken by thrashing in the wind, and their leaves have less light. Only the upper branches have room and light, and they flourish at the expense of lower ones, which gradually die and are thus pruned off. Some trees naturally grow faster than others, and they attain additional light and room to spread laterally, thus overtopping others which are suppressed and finally killed, beaten in the race for life.

If the growth should remain about even so that the trees grew densely packed together, the whole group would be likely to be of a poorer quality, but ordinarily the few outgrow the many and they are called dominant trees. Even then, they still have to struggle against their neighbors, and at this, the large sapling stage, many perish, and of those that survive there are great differences in size. Trees make their most rapid growth in height, and lay on the widest yearly "rings," at the large sapling and small pole age, Fig. 114, p. 263. It is at this stage, too, if the growth is at all dense, that the young trees (poles) clean themselves most thoroly of their branches. The growth in diameter continues to the end of the tree's life, long after the height growth has ceased.

When trees become "standards," and reach the limit of height growth, thru their inability to raise water to their tops, their branches must perforce grow sidewise, or not at all. The struggle for life thus takes a new form.

How trees are able to raise water as high as they do is still unexplained, but we know that the chief reason why some trees grow taller than others, is due to their ability to raise water. The most remarkable in this respect are the California redwoods, the big trees, and certain eucalypts in Australia. This inability of trees to grow above a certain height results in a flattening of the crown, Fig. 73, and at this stage, the trees struggle against each other by crowding at the side.

[Ill.u.s.tration: Fig. 73. Flattened Crown of Red Pine. _U.S. Forest Service._]

Inasmuch as trees grow more sensitive to shade with advancing age, the taller trees have the advantage. Each survivor is one of a thousand, and has outlived the others because it is best fitted for the place.

This fact has its effect upon the next generation, because it is these dominant surviving trees which bear seed most abundantly. After the tree has finished growing in height and diameter most vigorously--the pole stage--and proved to be fitted for the place, its energy is largely spent in raising seed. As this process goes on generation after generation, only the best coming to maturity in each, the poorer sorts are sifted out, and each region and continent has those species best fitted to meet the conditions of life there.

This is the reason why exotics are very likely to be sensitive and perhaps succ.u.mb to influences to which native trees are immune.

Standards and veterans are the survivors of all the lower stages, each of which has had its especial dangers. If left alone, the tree gradually dies and at last falls and decays, adding somewhat to the fertility of the forest soil. From the point of view of human use, it would far better have been cut when ripe and turned into lumber. It is a mistake to suppose that the natural virgin forest is the best possible forest, and that it should therefore be left alone. In the National Forests the ripe lumber is sold and a considerable revenue is thus available. But nature's way with the dead tree is to use it to produce more life. How she does so will be explained in the next chapter, on the enemies of the forest.

[Footnote 1: Gifford Pinchot, _Primer of Forestry_, p. 44.]

THE FOREST ORGANISM.

REFERENCES:[A]

Pinchot, _For. Bull._ No. 24, I, pp. 25-66.

Bruncken, pp. 13-31.

_For. Circ._ No. 36, p. 8.

Fernow, _Economics_, pp. 140-164.

[Footnote A: For general bibliography, see p. 4]

CHAPTER VI.

NATURAL ENEMIES OF THE FOREST.

The natural enemies of the forest--as distinct from its human enemies--fall into three groups: (1) Meteorological, (2) Vegetable, (3) Animal.

METEOROLOGICAL FORCES.

[Ill.u.s.tration: Fig. 74. Effect of Wind, July, 1902, Ca.s.s County, Minnesota. _U. S. Forest Service._]

_Wind._ "Windfalls" are not an uncommon sight in any forest.

Frequently only small areas are blown down, one large tree upsetting a few others, or again a vast region is destroyed by great storms, Fig.

74. An area of many square miles in Florida covered with long-leaf pine was thus destroyed several years ago. The "slash" thus formed, when well dried, is particularly liable to catch fire and burn furiously. Windfalls are especially common among shallow-rooted trees, as hemlock, ba.s.swood and spruce, on sandy soil and on shallow soil underlaid with solid stone, especially where open s.p.a.ces give the wind free sweep. It follows that an unbroken forest is a great protection to itself. The only precautions against wind therefore, that can be taken by the forester, are to keep the forest unbroken by selecting only the larger trees for felling or to cut down a given tract by beginning at the side opposite the direction of prevailing storms and working toward them.

In sandy regions, the wind does immense harm by blowing the sand to and fro in constantly shifting dunes, Figs. 75 and 76. These dunes occupy long stretches of the Atlantic coast and the sh.o.r.e of Lake Michigan. Such dunes have been estimated to cover 20,000 square miles of Europe. Along the Bay of Biscay in France, the sand dunes formerly drifted in ridges along the sh.o.r.e, damming up the streams and converting what was once a forest into a pestilential marsh. This region has been reclaimed at great expense by building fences along the sh.o.r.e to break the wind and thus keep the moving sand within limits. In this way a million acres of productive forest have been obtained.

[Ill.u.s.tration: Fig. 75. Sand-dunes, Cape May, New Jersey. _U.S. Forest Service._]

[Ill.u.s.tration: Fig. 76. Sand-dune. Oregon. _U.S. Forest Service._]

On the other hand winds are beneficial to the forest in scattering seeds, weeding out weak trees, and developing strength in tree trunks.

_Drouth_ both injures the foliage of trees and causes defects in the grain of wood, the latter appearing as "false rings." These arise from the effort of the tree to resume growth when the water supply is restored. See p. 19.

_Water._ Certain trees have become accustomed to living in much water, as cedar and cypress have in swamps, and certain trees have become accustomed to periodical floods, but other trees are killed by much water. So when lumbermen make a pond which overflows forest land, the trees soon die, Fig. 77.

[Ill.u.s.tration: Fig. 77. Effect of Flooding. First Connecticut Lake, New Hampshire. _U. S. Forest Service._]

_Lightning_ frequently blasts single trees, and in dry seasons may set fire to forests. This is a much more important factor in the west than in the east,--in the Rockies, for instance, where there are electrical storms without rain.

_Fires_ will be considered later under man's relation to the forest.

[Ill.u.s.tration: Fig. 78. Slim Trees Bent Over by Snow; Stouter Trees Unharmed. Zurich, Switzerland. _U. S. Forest Service._]

_Snow and ice_ often bring serious harm to saplings by permanently bending them over, Fig. 78, or by breaking off tops and branches.

_Frost_ kills young plants; and sudden changes in temperature seriously affect grown timber, producing "frost checks" and "wind shakes." When there is a sudden fall in temperature, the outside layers of the tree, which are full of sap, contract more rapidly than the inner portions, with the result that the tree splits with a sudden pistol-like report, the check running radially up and down the tree.

This is called a "frost check" or "star shake," Fig. 41._a_, p. 47, and such wounds rarely heal, Fig. 79.

On the other hand when the temperature rapidly rises, the outside layers of the tree expand so much more rapidly than the inside, that they separate with a dull m.u.f.fled chug, the check extending in a circular direction following the annual rings. Such checks are often called "wind shakes" and "cup shakes," Fig. 41._c_, p. 47. These injuries are found in regions where sudden changes of temperature occur, rather than in the tropics or in very cold climates.