=918. Forest Societies.=[52]—Each different climatic belt or region has its characteristic forest. For example, the forests of the Hudsonian zone in North America are different from those of the Canadian zone, and these in turn different from those in the transition zone (mainly in northern United States). The forests of the Rocky mountains and of the Pacific coast differ from those of the Alleghanian, Carolinian (mainly middle United States) or Austroriparian (southern United States) areas. Finally, tropical forests are strikingly different from those of other regions. Similar variations occur in the forests of other regions of the globe. The character of these forests depends largely on climatic factors. The character of the forest varies, however, even in the same climatic area, dependent on soil conditions, or success in seeding and ground-gaining of the different species in competition, etc. =919. General structure of the forest.=—Structurally the forest possesses three subdivisions: the floor, the canopy, and the interior. The floor is the surface soil, which holds the rootage of the trees, with its covering of leaf-mold and carpet of leaves, mosses, or other low, more or less compact vegetation. The canopy is formed by the spreading foliage of the tree crowns, which, in a forest of an even and regular stand, meet and form a continuous mass of foliage through which some light filters down into the interior. Where the stand is irregular, i.e., the trees of different heights, the canopy is said to be “compound” or “storied.” Where it is uneven, there are open places in the canopy which admit more light, in which case the undergrowth may be different. The interior of the forest lies between the canopy and the floor. It provides for aeration of the floor and interior occupants, and also room for the boles or tree trunks (called by foresters the wood mass of the forest) which support the canopy and provide the channels for communication and food exchange between the floor and canopy. The canopy manufactures the carbohydrate food and assimilates the mineral and proteid substances absorbed by the roots in the soil; and also gets rid of the surplus water needed for conveying food materials from the floor to the place where they are elaborated. It is the seat where energy is created for work, and also the place for seed production. [Illustration: Fig. 489. Mature forest of redwood (Sequoia sempervirens). (Bureau of Forestry, U. S. Dept. Agr., Bull. 38.)] =920. Longevity of the forest.=—The forest is capable of self-perpetuation, and, except in case of unusual disaster or the action of man, it should live indefinitely. As the old trees die they are gradually replaced by younger ones. So while trees may come and trees may go, the forest goes on forever.