Part 36 (1/2)

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 thras.h.i.+ng 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.

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