Forest Zones, Subtropical

natural zones in which forest vegetation prevails, situated in the subtropical regions of the northern and southern hemispheres. They are characterized by mild winters that permit the plants to grow continuously. The westerly winds that prevail in both hemispheres throughout the year are responsible for the significant differences in external appearance of the landscapes found on slopes with different exposures. The subtropical forest zones are divided into subzones of semitropical forests, mixed monsoon forests, and Mediterranean arid-summer forests and scrub.

Semitropical forests are widespread in regions that are excessively wet throughout the year (the southeastern USA, the southern Brazilian Plateau, southeastern Africa, and New Zealand’s North Island). These regions have a total annual solar radiation of 500–630 kilojoules (kJ) per cm² (120–150 kilocalo-ries [kcal] per cm²), an annual radiation balance of 385–500 kJ/cm² (90–120 kcal/cm²), and an annual precipitation of more than 1,000 mm. Evergreens prevail. Summer-green broad-leaved forests including evergreen species are found on red-brown soil, yellow earth, and red earth. Bogs with specific hydrophytic vegetation are widespread.

Mixed monsoon forests develop in a warm climate where dry winters alternate with wet summers. They are found chiefly on the eastern boundaries of Asia, in North and South America, and in Australia, where the total annual solar radiation is 460–585 kJ/cm² (110–140 kcal/cm²), and the annual radiation balance is 210–250 kJ/cm² (50–60 kcal/cm²). The seasonal variations in warmth are comparatively minor, which accounts for the presence and almost year-round growth of evergreens. Characteristic of mixed monsoon forests is great species diversity (in eastern China, for example, about 20,000 species). Moisture-loving, multistory forests with lianas and dense underbrush are well developed in the southern subtropics. Near the boundary of the temperate regions the evergreen species become less numerous. The number of deciduous species increases in the mountains of the northern hemisphere. The mixed monsoon forests are made up of various species of evergreen oaks, related genera of the beech, camellia, magnolia, pine, fir, cryptomeria, and bamboo families, and representatives of the laurel and palm families. The prevailing soil groups—red earth and yellow earth—are low in humus, acid, frequently podzolized, and unstable in structure. Usually deficient in nutrients, they require fertilizers. The abundant rainfall, mountainous terrain, and cutting of forests have contributed to erosion in many places.

Mediterranean arid-summer forests and scrub are widespread primarily on the shores of the Mediterranean Sea. They occupy smaller areas in North America (California), western South America (central Chile), southern and western Australia, South Africa, and northern New Zealand. The climate of most of these areas is characterized by dry, warm summers and cool, wet winters. The average temperature of the warmest month varies from 18° to 28°C, and that of the coldest month, from 4° to 12°C. Precipitation ranges from 500 to 1,000 mm a year (about 2,000 mm a year in the mountains). The radiation balance is 210–250 kJ/cm² (50–60 kcal/cm²) a year.

Plant formations of coarse-leaved forests and scrub—xerophi-lous evergreens (maquis, garigues), small-leaved summer-greens (shibliaks), and communities of xerophilous semishrubs and shrubs (friganas [xerophytic shrub and semishrub vegetation])—have adapted to the Mediterranean climate. Evergreen oaks and various conifers (cedars and pines, including Italian stone pine) are common. Cinnamonic and brown forest soils are the prevailing types of soils. In some places yellow earth is well developed. Terra rossa is found on limestone.

Much of the land occupied by subtropical forest zones is used for farming. Rice, soybeans, tea, peanuts, cotton, and citrus fruits are grown in the semitropical and mixed monsoon forest regions. Olives, citrus fruits, figs, grapes, and grains are among the typical crops in the Mediterranean area, where animal husbandry is also important. Artificial irrigation is common in arid regions of the subtropical forest zones.