The forest is a tract of land covered with trees which grow close together or, as one American has expressed it, the forest is an overproduction of trees on a certain area. This over-production of trees is an important factor of forest growth, the factor of sociability. Trees growing far apart and trees growing close together differ in their height and diameter growth, size of crowns, form of stem, and quality of wood produced. The natural reproduction of isolated trees is exposed to adverse climatic influences, while the reproduction in a close stand develops under the protection of a canopy. Trees in the open live their normal age, whereas most of the trees in a dense forest arc eventually killed by their neighbours In a young forest there may be as many as 50 .ООО seedlings per acre, while in a forest stand one hundred years old there may be only 100 trees per acre. Through thise enormous rate of mortality, nature eliminates weak individuals incapable of competing with their more vigorous associates. If seedlings in artificial reforestation arc planted 160 far apart, a true forest is not created, but rather an orchard of short, brushy trees, worthless as timber. There is no struggle for life in such plantations, and unfit specimens have a chance to grow and reproduce. Thus tree planting which disregards the social aspects, tends to bring about a general degradation оf the present stand, as well as degeneration of the future forest which may originate from inferiorseed trees.
The intimate association of trees in forest stands entails not only struggle for life among both species and individuals, but also their cooperation in grown. The planting of white birch and other pioneer trees to encourage the growth of spruce seedlings is a commonly quoted illustration of “cooperation” between tree species. Of equal silvicultural importance is the combination of pine and a soil conserving un – derstory of oak, beech, or hornbeam. Yellow birch,
basswood and a number of other species that never occur in pure stands, appear to be dependent upon hard maple or oaks which form the bulk of the forest. Optimum development of white spruce takes place only in association with tolerant hardwood which provide shade and fertilizing litter.
The trees are the most outstanding but not the only constituents of the forest. Each forest stand is associated with shrubs herbaceous plants, mosses, lichens, fungi, bacteria, protozoa, worms, insects, birds, mammals, and other animals. The occurrence of all these organisms is not accidental; their existence depends upon the food furnished by their associates. For this reason the forest is referred to as association of mutually related plants and animals, called “Biocenose”, “Biome”, “community” or “life unit” Competition or antagonism, commensalism, symbiosis, saprophytism. and parasitism, are the basic forms of interrelationship that keep аll of the associated members in a state of “biological equilibrium”. The disturbance of this equilibrium by man’s activity may cause grave consequences. In localities where wolves, foxes, and other predators are exterminated, the number of rabbits or other rodents may increase to such an extent as to arrest the natural, as well as the artificial regeneration of the forest. The destruction of shrubs and ground vegetation by grazing may induce an emigration of birds from the area. The absence of birds may permit the local multiplication of destructive insects, like the gypsy moth and sawfly, and the subsequent devastation of extensive forest