The life and nature of a forest:  When we think of a forest we are apt to
    think of a large number of individual trees having no special
    relationship to each other.  Closer observation, however, will reveal
    that the forest consists of a distinct group of trees, sufficiently
    dense to form an unbroken canopy of tops, and that, where trees grow
    so closely together, they become very interdependent.  It is this
    interdependence that makes the forest different from a mere group of
    trees in a park or on a lawn.  In this composite character, the
    forest enriches its own soil from year to year, changes the climate
    within its own bounds, controls the streams along its borders and
    supports a multitude of animals and plants peculiar to itself.  This
    communal relationship in the life history of the forest furnishes a
    most interesting story of struggle and mutual aid.  Different trees
    have different requirements with regard to water, food and light. 
    Some need more water and food than others, some will not endure much
    shade, and others will grow in the deepest shade.  In the open, a
    tree, if once established, can meet its needs quite readily and,
    though it has to ward off a number of enemies, insects, disease and
    windstorm—­its struggle for existence is comparatively easy.  In the
    forest, the conditions are different.  Here, the tree-enemies have to
    be battled with, just as in the open, and in addition, instead of
    there being only a few trees on a plot of ground, there are
    thousands growing on the same area, all demanding the same things
    out of a limited supply.  The struggle for existence, therefore,
    becomes keen, many falling behind and but few surviving.

[Illustration:  FIG. 124.—­Measuring the Diameter of a Tree and Counting its Annual Rings.]

This struggle begins with the seed.  At first there are thousands of seeds cast upon a given area by the neighboring trees or by the birds and the winds.  Of these, only a few germinate; animals feed on some of them, frost nips some and excessive moisture and unfavorable soil conditions prevent others from starting.  The few successful ones soon sprout into a number of young trees that grow thriftily until their crowns begin to meet.  When the trees have thus met, the struggle is at its height.  The side branches encroach upon each other (Fig. 123), shut out the light without which the branches cannot live, and finally kill each other off.  The upper branches vie with one another for light, grow unusually fast, and the trees increase in height with special rapidity.  This is nature’s method of producing clear, straight trunks which are so desirable for poles and large timber.  In this struggle for dominance, some survive and tower above the others, but many become stunted and fail to grow, while the majority