But in this strife there is also mutual aid.  Each tree helps to protect its neighbors against the danger of being uprooted by the wind, and against the sun, which is liable to dry up the rich soil around the roots.  This soil is different from the soil on the open lawn.  It consists of an accumulation of decayed leaves mixed with inorganic matter, forming, together, a rich composition known as humus.  The trees also aid each other in forming a close canopy that prevents the rapid evaporation of water from the ground.

The intensity of these conditions will vary a great deal with the composition of the forest and the nature and habits of the individual trees.  By composition, or type of forest, is meant the proportion in which the various species of trees are grouped; i.e., whether a certain section of woodland is composed of one species or of a mixture of species.  By habit is meant the requirements of the trees for light, water and food.

[Illustration:  FIG. 125.—­Mountain Slopes in North Carolina Well Covered with Forests.]

    Some trees will grow in deep shade while others will demand the
    open.  In the matter of water and food, the individual requirements
    of different trees are equally marked.

The natural rapidity of growth of different species is also important, and one caring for a forest must know this rate of growth, not only as to the individual species, but also with respect to the forest as a whole.  If he knows how fast the trees in a forest grow, both in height and diameter, he will know how much wood, in cubic feet, the forest produces in a year, and he can then determine how much he may cut without decreasing the capital stock.  The rate of growth is determined in this way:  A tree is cut and the rings on the cross-section surface are counted and measured; see Fig. 124.  Each ring represents one year’s growth.  The total number of rings will show the age of the tree.  By a study of the rings of the various species of trees on a given plot, the rate of growth of each species in that location can be ascertained and, by knowing the approximate number of trees of each species on the forest area, the rate of growth of the whole forest for any given year can be determined.

[Illustration:  FIG. 126.—­Bottom Lands Buried in Waste from Deforested Mountains.  Wu-t’ai-shan, Shan-si Province, China.]

[Illustration:  FIG. 127.—­Eroded Slope in Western North Carolina.]

Forests prevent soil erosion and floods:  Forests help to regulate the
    flow of streams and prevent floods.  Most streams are bordered by
    vast tracts of forest growths.  The rain that falls on these forest
    areas is absorbed and held by the forest soil, which is permeated
    with decayed leaves, decayed wood and root fibers.  The forest floor