Fig. 69. Red Cedar Avenue. Seeds dropped by birds which perched on the fences. Indiana. U. S. Forest Service.
The fact that conifers, as the longleaf pine, Fig. 46, [p. 200], and spruce, Fig. 55, [p. 211], are more apt to grow in pure stands than broad-leaved trees, is largely accounted for by their winged seeds; whereas the broad-leaved trees grow mostly in mixed stands because their heavy seeds are not plentifully and widely scattered. This is a rule not without exceptions, for beech sometimes covers a whole mountain side, as Slide Mountain in the Catskills, and aspens come in over a wide area after a fire; but later other trees creep in until at length it becomes a mixed forest.
The essential facts of the relation of trees to each other in the forest has been clearly stated by Gifford Pinchot thus:[1]
The history of the life of a forest is a story of the help and harm which trees receive from one another. On one side every tree is engaged in a relentless struggle against its neighbors for light, water and food, the three things trees need most. On the other side each tree is constantly working with all its neighbors, even those which stand at some distance, to bring about the best condition of the soil and air for the growth and fighting power of every other tree.
The trees in a forest help each other by enriching the soil in which they stand with their fallen leaves and twigs, which are not quickly blown or washed away as are those under a tree in the open. This collection of "duff" or "the forest floor" retains the moisture about their roots, and this moist mass tends to keep the temperature of the forest warmer in winter and cooler in summer. The forest cover, Fig. 55, [p. 211], consisting largely of foliage, has the same effect, and in addition protects the bark, the roots, and the seedlings of the trees from the direct and continuous hot rays of the sun. Without the shade of the leaves, many trees, as white pine, would quickly die, as may readily be seen by transplanting them to the open. The mass of standing trees tempers the force of the wind, which might overthrow some of them, and hinders the drying up of the duff.
Fig. 70. Shallow Roots of Hemlock. Bronx Park, New York, N. Y.
But trees hinder as well as help each other. There is a constant struggle between them for nourishment and light. To get food and water, some trees, as spruces and hemlocks, Fig. 70, spread their roots out flat; others, as oak and pine, send down a deep tap root. Those succeed in any environment that find the nourishment they need. Still more evident is the struggle for light and air. However well a tree is nourished thru its roots, unless its leaves have an abundance of light and air it will not thrive and make wood.
