The trees in a forest are all helped by mutually protecting each other against high winds, and by producing a richer and moister soil than would be possible if the trees stood singly and apart. They compete among themselves by their roots for moisture in the soil, and for light and space by the growth of their crowns in height and breadth. Perhaps the strongest weapon which trees have against each other is growth in height. In certain species intolerant of shade, the tree which is overtopped has lost the race for good. The number of young trees which destroy each other in this fierce struggle for existence is prodigious, so that often a few score per acre are all that survive to middle or old age out of many tens of thousands of seedlings which entered the race of life on approximately even terms.
Not only has a forest a character of its own, which arises from the fact that it is a community of trees, but each species of tree has peculiar characteristics and habits also. Just as in New York City, for example, the French, the Germans, the Italians, the Hungarians, and the Chinese each have quarters of their own, and in those quarters live in accordance with habits which distinguish each race from all the others, so the different species of pines and hemlocks, oaks and maples prefer and are found in certain definite types of locality, and live in accordance with definite racial habits which are as general and unfailing as the racial characteristics which distinguish, for example, the Italians from the Germans, or the Swedes from the Chinese.
The most important of these characteristics of race or species are those which are concerned with the relation of each to light, heat, and moisture. Thus, a river birch will die if it has only as much water as will suffice to keep a post oak in the best condition, and the warm climate in which the balsam fir would perish is just suited to the requirements of a long leaf pine or a magnolia.
The tolerance of a tree for shade may vary greatly at different times of its life, but a white pine always requires more light than a hemlock, and a beech throughout its life will flourish with less sunshine or reflected light than, for example, an oak or a tulip tree.
Trees are limited in their distribution also by their adaptability, in which they vary greatly. Thus a bald cypress will grow both in wetter and in dryer land than an oak; a red cedar will flourish from Florida to the Canadian line, while other species, like the Eastern larch, the Western mountain hemlock, or the big trees of California, are confined in their native localities within extremely narrow limits.
THE FORESTER'S KNOWLEDGE
The trained Forester must know the forest as a doctor knows the human machine. First of all, he must be able to distinguish the different trees of which the forest is composed, for that is like learning to read. He must know the way they are made and the way they grow; but far more important than all else, he must base his knowledge upon that part of forestry which is called Silvics, the knowledge of the relation of trees to light, heat, and moisture, to the soil, and to each other.
The well-trained Forester must also know the forest shrubs and at least the more important smaller forest plants, something of the insect and animal life of his domain, and the birds and fish. He must have a good working knowledge of rocks, soils, and streams, and of the methods of making roads, trails, and bridges. He should be an expert in woodcraft, able to travel the forest safely and surely by day or by night. It is essential that he should have a knowledge of the theory and the practice of lumbering, and he should know something about lumber markets and the value of lumber, about surveying and map making, and many other matters which are considered more at length in the Chapter on Training. There are as yet in America comparatively few men who have acquired even fairly well the more important knowledge which should be included in the training of a Forester.