Tropical rain forests are home to more species than all other land biomes combined. The leafy tops of tall trees- extending up to 70 meters above the forest floor form a dense covering called a canopy. In the shade below the canopy a second layer of shorter trees and vines forms an understory. Organic matter that falls to the forest floor quickly decomposes and the nutrients are recycled. An average of 50 to 260 inches (125 to 660 cm.) of rain falls yearly.
Rain forests belong to the tropical wet climate group. The temperature in a rain forest rarely gets higher than 93 °F (34 °C) or drops below 68 °F (20 °C); average humidity is between 77 and 88%; rainfall is often more than 100 inches a year. There is usually a brief season of less rain. In monsoonal areas, there is a real dry season. Almost all rain forests lie near the equator.
Rainforests now cover less than 6% of Earth's land surface. Scientists estimate that more than half of all the world's plant and animal species live in tropical rain forests. Tropical rainforests produce 40% of Earth's oxygen.
A tropical rain forest has more kinds of trees than any other area in the world. Scientists have counted about 100 to 300 species in one 2 1/2-acre (1-hectare) area in South America. Seventy percent of the plants in the rainforest are trees. About 1/4 of all the medicines we use come from rainforest plants.
Geographically located between the latitudes 10°N and 10°S of the equator, lowland tropical rain forest ecosystems share similar physical structure but vary in geology, species composition, and anthropogenic threats across the forests of Southeast Asia, Australia, Africa, and Central and South America. Approximately 50% of tropical rain forests are found in the Neotropics, primarily in the Amazon and Orinoco basin with patches in Central America, the Caribbean, and along the Atlantic coast of Brazil. African rain forests are mainly located in the Congo basin extending to the west coast and remnant forests remain in Madagascar. The Australian tropical realm (Oceania) includes Australia, New Guinea, and the Pacific Islands. During his travels, Alfred Russel Wallace noted distinct faunal, though not necessarily floral, differences between Australia and Southeast Asia and the "Wallace line" denotes this boundary. The severely fragmented areas of South and Southeast Asian rain forests account for less than 30% of rain forests worldwide and are found in India, Sri Lanka, mainland Southeast Asia, the Malay Peninsula, and Indonesia.
The climate of lowland tropical rain forests is warm, humid, and relatively stable. Tropical rain forests are characterized by mean annual temperatures ranging from 23 °C to 28 °C, with mean monthly temperatures no less than 18 °C and rarely exceeding 35 °C. Diurnal temperature fluctuations typically exceed mean monthly ranges, with annual temperature ranges of less than 5°C. Tropical biomes do not generally experience frost, even at high elevations, and tropical plants and animals do not tolerate freezing. Local variation in rainfall is much higher than temperature variation. Mean monthly precipitation exceeds 60 mm, and annual precipitation can exceed 10 m in a season, evergreen rain forests such as the northwestern region of Colombia known as the Chocó. Peak rainfall typically correlates with the intertropical convergence, which lies over the equators during the two equinoxes. In semi evergreen forests with seasonal variation in precipitation resulting in distinct rainy and dry seasons that drive plant phonological responses, mean annual rainfall is lower, with dry season months characterized by greater evaporative potential than precipitation. Average humidity in the forest understory is approximately 80 % with higher diurnal variation in the canopy.
These ecosystems comprise four distinct forest types. The first, lowland equatorial evergreen rainforests endures continuous and high rainfalls. The second, moist deciduous and semi evergreen seasonal forests, receives high overall rainfall, but a warm wet season alternates with a cooler dry season during which trees with null and void foliage lose their leaves. The third type, montane rainforests, is found in cooler-climate mountain areas, at altitudes ranging from 1,500 up to 3,300 meters. The fourth, flooded forests, is composed of swamps and floodplains and are permanently or seasonally subdued by flooding. Between 40 and 75 percent of all biotic species are indigenous to the tropical rainforests, which also harbor half of all the living animal and plant species on the globe and two-thirds of all flowering plants. Several million rainforest plants, invertebrates, and microorganism species are yet to be described.
Not all tropical forests are rainforests. Much of the tropics is arid and only 40% is forested. About half the tropical forest is rainforest; the remainder comprises seasonal forests, savannah woodland and other forms of open forest. Tropical moist forest is the collective term for rainforest, seasonal or monsoon forest, and mangroves.
Although, the tropical moist forest has the limelight, tropical woodlands are also of great importance. These forests may be under the greatest threat of overuse, through the gathering of fuel wood and fodder, grazing, and from the resulting soil erosion. This is not just an ecological disaster, but a human tragedy. Many people suffer malnutrition not only because of food shortages, but also because of a shortage of fuel needed for cooking. Fuel wood is a major forest product. Of the total tropical wood supply, 83% is consumed as fuel wood, 13% is consumed locally as timber, and only 4% is exported to the international timber trade. Sustainable production of industrial timbers is only part of the solution; efficient production and consumption of fuel wood are also essential.