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.]