Today forests occupy approximately one-third of Earth’s land area, account for over two-thirds of the leaf area of land plants, and contain about 70% of carbon present in living things. They have been held in reverence in folklore and worshipped in ancient religions. However forests are becoming major casualties of civilization as human populations have increased over the past several thousand years, bringing deforestation, pollution, and industrial usage problems to this important biome. 

Tropical forests
are characterized by the greatest diversity of species. They occur near the equator, within the area bounded by latitudes 23.5 degrees N and 23.5 degrees S. One of the major characteristics of tropical forests is their distinct seasonality: winter is absent, and only two seasons are present (rainy and dry). The length of daylight is 12 hours and varies little. Temperature is on average 20-25°C and varies little throughout the year: the average temperatures of the three warmest and three coldest months do not differ by more than 5 degrees.
Precipitation is evenly distributed throughout the year with annual rainfall exceeding 2000 mm. Soil is nutrient-poor and acidic. Decomposition is rapid and soils are subject to heavy leaching.
Canopy in tropical forests is multilayered and continuous allowing little light penetration. Flora is highly diverse: one square kilometre may contain as many as 100 different tree species. Trees are 25-35 metres tall, with buttressed trunks and shallow roots, mostly evergreen with large dark green leaves. Plants such as orchids, bromeliads, vines (lianas), ferns, mosses and palms are present in tropical forests.
Fauna include numerous birds, bats, small mammals and insects.
Further subdivisions of this group are determined by seasonal distribution of rainfall: 
Evergreen rainforest; no dry season.
Seasonal rainforest; short dry period in a very wet tropical region (the forest exhibits definite seasonal changes as trees undergo developmental changes simultaneously, but the general character of vegetation remains the same as in evergreen rainforests).
Semi-evergreen forest; longer dry season (the upper tree story consists of deciduous trees, while the lower story is still evergreen).
Moist/dry deciduous forest (monsoon); the length of the dry season increases further as rainfall decreases (all trees are deciduous). 
More than ½ of tropical forests have already been destroyed. 

Temperate forests
occur in eastern North America, northeastern Asia, and western and central Europe. Well-defined seasons with a distinct winter characterize this forest biome. Moderate climate and a growing season of 140-200 days during 4-6 frost-free months distinguish temperate forests. Temperature varies from -30°C to 30°C. Precipitation (75-150 centimetres) is distributed evenly throughout the year. Soil is fertile, enriched with decaying litter. Canopy is moderately dense and allows light to penetrate, resulting in well-developed and richly diversified understorey vegetation and stratification of animals. Flora is characterized by 3-4 tree species per square kilometre. Trees are distinguished by broad leaves that are lost annually and include such species as oak, hickory, beech, hemlock, maple, basswood, cottonwood, elm, willow, and spring-flowering herbs. Fauna is represented by squirrels, rabbits, skunks, birds, deer, mountain lion, bobcat, timber wolf, fox, and black bear. Further subdivisions of this group are determined by seasonal distribution of rainfall: 
Moist conifer and evergreen broad-leaved forests; wet winters and dry summers (rainfall is concentrated in the winter months and winters are relatively mild).
Dry conifer forests; dominate higher elevation zones; low precipitation.
Mediterranean forests: precipitation is concentrated in winter, less than 1000 mm per year.
Temperate coniferous; mild winters, high annual precipitation (greater than 2000 mm).
Temperate broad-leaved rainforests; mild, frost-free winters, high precipitation (more than 1500 mm) evenly distributed throughout the year. Only scattered remnants of original temperate forests remain.

Boreal forests, or taiga, represent the largest terrestrial biome. Occurring between 50 and 60 degrees north latitudes, boreal forests can be found in the broad belt of Eurasia and North America: two-thirds in Siberia with the rest in Scandinavia, Alaska, and Canada. Seasons are divided into short, moist, and moderately warm summers and long, cold, and dry winters. The length of the growing season in boreal forests is 130 days. Temperatures are very low. Precipitation is primarily in the form of snow, 40-100 cm annually. Soil is thin, nutrient-poor, and acidic.
Canopy permits low light penetration, and as a result, understorey is limited. Flora consist mostly of cold-tolerant evergreen conifers with needle-like leaves, such as pine, fir, and spruce. Fauna include woodpeckers, hawks, moose, bear, weasel, lynx, fox, wolf, deer, hares, chipmunks, shrews, and bats.