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.
Fig. 56. Virgin Stand of Red Spruce. White Mountains, New Hampshire. U. S. Forest Service.
Fig. 57. Typical Mixed Forest,—Red Spruce, Hemlock, White Ash, Yellow Birch, Balsam Fir, and Red Maple. Raquette Lake, New York. U. S. Forest Service.
The soil in the forest is largely determined by the forest itself. In addition to the earth, it is composed of the fallen and decayed leaves and twigs and tree trunks, altogether called the forest floor. It is spongy and hence has the ability to retain moisture, a fact of great importance to the forest.