Without these conditions, and without the help and hindrance which forest trees give to each other, these trees would not have their present characteristics, either in shape, habits of growth or nature of wood grain. Indeed, some of them could not live at all.
Since by far the greater number of timber trees grow in the forest, in order to understand the facts about trees and woods, it is necessary to know something about the conditions of forest life.
A tree is made up of three distinct parts: (1) the roots which anchor it in the ground, and draw its nourishment from the moist soil; (2) the trunk, or bole, or stem, which carries the weight of the branches and leaves, and conveys the nourishment to and from the leaves; (3) the crown, composed of the leaves, the branches on which they hang, and the buds at the ends of the branches. As trees stand together in the forest, their united crowns make a sort of canopy or cover, Fig. 55, which, more than anything, determines the factors affecting forest life, viz., the soil, the temperature, the moisture, and most important of all, the light.
Fig. 55. The Forest Cover. Spruce Forest, Bavaria, Germany. U. S. Forest Service.
On the other hand, every species of tree has its own requirements in respect to these very factors of temperature,—moisture, soil and light. These are called its silvical characteristics.
SOIL.
Some trees, as black walnut, flourish on good soil, supplanting others because they are better able to make use of the richness of the soil; while some trees occupy poor soil because they alone are able to live there at all. Spruce, Fig. 56, will grow in the north woods on such poor soil that it has no competitors, and birches, too, will grow anywhere in the north woods. In general, it is true that mixed forests, Fig. 57, i.e., those having a variety of species, grow on good loamy soil. The great central, deciduous Atlantic Forest grew on such soil until it was removed to make room for farms. On the other hand, pure stands—i.e., forests made up of single varieties—of pine occupy poor sandy soil. Within a distance of a few yards in the midst of a pure stand of pine in the south, a change in the soil will produce a dense mixed growth of broad-leaves and conifers.
