Forests

Many countries have developed national schemes for an ecological classification of their forests. Typically, these schemes are based on biophysical information and reflect the natural, large-scale patterns of species composition, soil type, topography, and climate. However, these classifications may vary greatly among countries, even for similar forest types.

An international system of ecosystem classification has been proposed by a scientific working group under the auspices of the United Nations Educational, Scientific and Cultural Organization (UNESCO). This scheme lists 24 forest types, divided into two broad classes: (i) closed-canopy forests with a canopy at least 16.5 ft (5 m) high and with interlocking tree crowns, and (ii) open woodlands with a relatively sparse, shorter canopy. A selection of forest types within these two broad classes is described below:

1. Tropical and Subtropical Forests.
   1. Tropical rain forest. This is a species-rich forest of angiosperm tree species (sometimes known as "tropical hardwoods") occurring under conditions of high rainfall and constant, warm temperatures. Consequently, the species in this ecosystem are tolerant of neither drought or frost, and the forest itself is commonly in an old-growth condition. Most of Earth's terrestrial biodiversity occurs in this type of forest ecosystem.
   2. Tropical and subtropical evergreen forest. This is also a rather species-rich forest, but occurring in regions in which there is seasonally sparse rain. Individual trees may shed their leaves, usually in reaction to relatively dry conditions. However, the various species do not all do this at the same time, so the canopy is always substantially foliated.
   3. Tropical and subtropical drought-deciduous forest. This is a relatively open angiosperm forest, in which tree foliage is shed just before the dry season, which usually occurs in winter.
   4. Mangrove forest. This is a relatively species-poor forest, occurring in muddy intertidal habitat in the tropics and subtropics. Mangrove forest is dominated by evergreen angiosperm trees that are tolerant of flooded soil and exposure to salt. Some genera of mangrove trees are widespread. Examples from south Florida are red mangrove (Rhizophora mangle) with its characteristic stilt roots, and black mangrove (Avicennia nitida) with its pneumatophores, which poke out of the oxygen-poor sediment and into the atmosphere.
2. Temperate and Subpolar Forests.
   1. Temperate deciduous forest. This is a deciduous forest dominated by various species of angiosperm trees growing under seasonal climatic conditions, including moderately cold winters. In eastern North American forests of this type, the common trees include species of maple, birch, hickory, ash, walnut, tulip-tree, oak, and basswood, among others (Acer, Betula, Carya, Fraxinus, Juglans, Liriodendron, Quercus, and Tilia, respectively).
   2. Temperate and subarctic, evergreen conifer forest. This is a northern coniferous forest (sometimes called boreal forest), growing in regions with highly seasonal conditions, including severe winters. The dominant genera of conifer trees are fir, spruce, pine, cedar, and hemlock, among others (Abies, Picea, Pinus, Thuja, and Tsuga, respectively).
   3. Temperate and subpolar, evergreen rain forest. This forest occurs in wet, frost-free, oceanic environments of the Southern Hemisphere, and is dominated by evergreen, angiosperm species such as southern beech (Nothofagus spp.) and southern pine (Podocarpus spp.).
   4. Temperate, winter-rain, evergreen broadleaf forest. This is an evergreen angiosperm forest, growing in regions with a pronounced wet season, but with summer drought. In North America, this forest type occurs in coastal parts of southern California, and is dominated by evergreen species of oaks (Quercus spp.).
   5. Cold-deciduous forest. This is a northern deciduous forest growing in a strongly seasonal climate with very cold winters. This forest type is typically dominated by angiosperm trees such as aspen and birch (Populus spp. and Betula spp.) or the deciduous conifer, larch (Larix spp.).