ered
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
is, moreover, covered with a heavy undergrowth and thus behaves like
a sponge, absorbing the water that falls upon it and then permitting
it to ooze out gradually to the valleys and rivers below. A forest
soil will retain one-half of its own quantity of water; i.e., for
every foot in depth of soil there can be six inches of water and,
when thus saturated, the soil will act as a vast, underground
reservoir from which the springs and streams are supplied (Fig.
125). Cut the forest down and the land becomes such a desert as is
shown in Fig. 126. The soil, leaves, branches and fallen trees dry
to dust, a