The New International Encyclopædia/Forest
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FOREST (OF. forest, Fr. forêt, It., ML. foresta, forest, from Lat. foras, foris, out of doors, fores, door, Gk. θύρα, thyra, OChurch Slav, dvīrī, Lith. dùrys, Goth, daúrōns, OHG. turi, Ger. Thür, AS. duru, Eng. door). A tract of land covered with a natural growth of trees. From the standpoint of vegetation the world may be roughly divided into forest, grassland, and desert, the area of each being determined by various climatic factors. Among these climatic factors atmospheric moisture takes a prominent place, as can readily be seen in comparing a vegetation chart of the world with a rainfall chart. Other things being equal, the greater the rainfall, the richer the forest. Forests seem to be in a measure independent of the seasonal distribution of rain, since they occur in regions of daily rain, of summer rain, or winter rain. Endurance through dry seasons is made possible by the great depth of tree-roots, and also by the thick and leathery leaf-texture in the case of evergreens, or by the shedding of leaves in deciduous trees. Because of the heat, more water is required by a tropical forest to meet the demands of transpiration than by a forest in the temperate zone. Another factor, perhaps of equal importance with moisture, is wind. Kihlman has shown that the presence or absence of trees in arctic regions is not a question of cold, nor even of a season's length, but of winter winds; trees grow only where they are protected from the great loss of water by transpiration induced by dry winter winds by being buried under the snow; the height of the trees thus marks the winter level of the snow. Since the winds of eastern Argentina are strong during the resting period, grassland is present, though the moisture is sufficient for a forest.
The forest formations of the world may be divided into eight types, based chiefly on the ecological characters of the leaves. (See Leaf.) (1) The evergreen forest of the tropical regions of diurnal rainfall. This forest is especially well developed in the regions of the trade winds in oceanic climates, as of Brazil and Malaysia. This type is often called the rainy forest, and may be taken as representing the climax of the world's vegetation. Here plants grow in vast profusion and great diversity of form, and lianas, or climbing plants, and epiphytes reach their greatest development. Simultaneous periodicity is largely wanting, so that the forest is always in active life. (2) The deciduous monsoon forest, especially characteristic of the monsoon district of India, differs from the forest first named chiefly in having simultaneous periodicity. The other characters of the rainy season are present, but in a less complete degree. (3) The evergreen forest of the temperate zone is essentially an extension of the tropical evergreen forest into the cooler regions, especially of the Southern Hemisphere. It is peculiar to pronounced oceanic climates with markedly uniform temperature and moisture. (4) The deciduous forest of the north temperate zone is the typical forest formation of the eastern United States. The forests of beech, maple, chestnut, oak, etc., are too familiar to need description. The radical difference between the forests of the same latitudes in the Northern and Southern hemispheres is doubtless associated with the continental climates of the one and the oceanic climates of the other. (5) The deciduous savanna forest of the tropical and warm temperate regions is transitional between forest and grassland (q.v.), having a park-like aspect, which is due to scattered trees in a district where grasses form the chief undergrowth. Such a forest commonly has a moderate rainfall. (6) The thorny or scrubby forest of tropical and warm temperate regions where the rainfall is slight is transitional between forest and thicket (q.v.). (7) The forest of temperate regions where the rainfall occurs in winter, is finely shown in the Mediterranean region; coarse and leathery but large evergreen leaves, like those of the holly, laurel, oleander, and the evergreen oaks, may be taken as typical of such regions. (8) The conifer forests, the pines and firs with their leathery, needle-shaped evergreen leaves, form great forests in the colder regions of the temperate zones, especially of the Northern Hemisphere.
The forests heretofore discussed are all climatic and widespread. Edaphic (q.v.) or local forests also occur. Indeed, in most of the regions where the above climatic types are found, there are localities in which other forest combinations are present. For example, in a swamp in the deciduous zone of the northern United States there may be found tamarack, spruce, and white cedar. Close observation in such a place for many years would doubtless show the gradual dying out of these trees and their replacement by the ordinary members of the deciduous forest. On a hill there may often be found a pine-plant society, but this is not a permanent condition. Pines are often likely to be followed in a natural sequence by oaks, and they in turn by maples and beeches. These changeable plant societies may be called edaphic, while the ultimate forest toward which all are tending may be called the climatic formation. Viewed in this light the eight great forest types outlined above are forest formations.