Tropical and subtropical moist broadleaf forests

Tropical and subtropical moist broadleaf forests, also known as tropical wet forests, are a tropical and subtropical forest biome. The biome includes several types of forests, including lowland tropical rain forests, which receive high rainfall year-round; moist deciduous and semi-evergreen forests, with high overall rainfall marked by strong seasonal variations; montane rain forests found in cooler-climate mountainous areas; and freshwater swamp forests and peat swamp forests.

Tropical and subtropical moist broadleaf forests are found in a belt around the equator and in the humid subtropics, and are characterized by warm, humid climates with high year-round rainfall. Tropical and subtropical forest regions with lower rainfall are home to Tropical and subtropical dry broadleaf forests and Tropical and subtropical coniferous forests. Temperate rain forests also occur in certain humid temperate coastal regions.

Tropical and subtropical moist broadleaf forests are common in several terrestrial ecozones, including parts of the Afrotropic (equatorial Africa), Indomalaya (parts of the Indian subcontinent and Southeast Asia), the Neotropic (northern South America and Central America), Australasia (eastern Indonesia, New Guinea, and northern Australia), and Oceania (the tropical islands of the Pacific Ocean). About half of the world's tropical rainforests are in the South American countries of Brazil and Peru. Rain forests 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.

rainfall : high, year round, sometimes with seasonal variation.
temperature : always warm
soil : generally poor and leached.
plants : high diversity
animals : high diversity

Characteristics

Rainforests are characterized by high rainfall, with definitions setting minimum normal annual rainfall between 2,000 mm (about 78 inches or 2 meters) and 1700 mm (about 67 inches). The soil can be poor because high rainfall tends to leach out soluble nutrients.

Rain forests are home to two-thirds of all the living animal and plant species on the planet. It has been estimated that many hundreds of millions of new species of plants, insects, and microorganisms are still undiscovered and as yet unnamed by science. Tropical rain forests are called the "jewel of the earth", the "Earth's lungs", and the "world's largest pharmacy" because of the large amount of natural medicines discovered there.

Despite the growth of flora in a rainforest, the actual quality of the soil is quite poor. Oxisols, infertile, and deeply weathered, have developed on the ancient Gondwanan shields. Rapid bacterial decay prevents the accumulation of humus. The concentration of iron and aluminium oxides by the laterization process gives the oxisols a bright red color and sometimes produces minable deposits (e.g. bauxite). On younger substrates, especially of volcanic origin, tropical soils may be quite fertile.

The undergrowth in a rainforest is restricted in many areas by the lack of sunlight at ground level. This makes it possible for people and other animals to walk through the forest. If the leaf canopy is destroyed or thinned for any reason, the ground beneath is soon colonised by a dense tangled growth of vines, shrubs and small trees called jungle.

Trees

There are several common characteristics of tropical trees. Tropical species frequently possess one or more of the following attributes not commonly seen in trees of higher latitudes.

Many species have broad, woody flanges (buttresses) at the base of the trunk. Originally believed to help support the tree, now it is believed that the buttresses channel stem flow and its dissolved nutrients to the roots. Large leaves are common among trees of the C layer. Young individuals of trees destined for the B and A layers may also have large leaves. When they reach the canopy new leaves will be smaller. The large leaf surface helps intercept light in the sun-dappled lower strata of the forest. Drip tips facilitate drainage of precipitation off the leaf to promote transpiration. They occur in the lower layers and among the saplings of species of the emergent layer (A layer).

Trees are often well connected in the canopy layer especially by the growth of woody climbers or lianas, plants with epiphytic adaptations, allowing them to grow on top of existing trees in the competition for sunlight.

Other characteristics that are more frequent in tropical tree species than in temperate forests include:

Exceptionally thin bark, often only 1-2 mm thick. It is usually very smooth, although sometimes covered with spines or thorns.
Cauliflory, the development of flowers (and hence fruits) directly from the trunk, rather than at the tips of branches.
Large fleshy fruits attract birds, mammals, and even fish as dispersal agents.

Rainforest layers

The rainforest is divided into four different parts, each with different plants and animals, adapted for life in that particular area.

Emergent layer

This layer contains the emergents—trees which tower above the canopy. The trees are usually evergreens, which are able to withstand the harsh temperatures and high winds. Eagles, butterflies, and bats inhabit this layer.

Canopy layer

Away from river banks, swamps and clearings where dense undergrowth is found, the forest floor is relatively clear of vegetation, as little sunlight penetrates to ground level. The densest areas of biodiversity are found in the forest canopy, a more or less continuous cover of foliage formed by adjacent treetops.

The canopy, by some estimates, is home to 40% of all plant species, suggesting that perhaps half of all life on Earth could be found there. A quarter of all insect species are believed to exist in the rainforest canopy.

Scientists have long suspected the richness of the canopy as a habitat, but have only recently developed practical methods of exploring it. As long ago as 1917, U.S. naturalist William Beebe declared that "another continent of life remains to be discovered, not upon the Earth, but one to two hundred feet above it, extending over thousands of square miles".

True exploration of this habitat only began in the 1980s, when scientists developed methods to reach the canopy, such as firing ropes into the trees using crossbows. Exploration of the canopy is still in its infancy, but other methods include the use of balloons and airships to float above the highest branches and the building of cranes and walkways planted on the forest floor.

Understory layer

There is a space between the canopy and the forest floor, which is known as the understorey (or understory). This is home to a number of birds, monkeys, snakes, and lizards. The leaves are much larger at this level. Insect life is also abundant.

Forest floor

This region receives only 2% of the rainforest's sunlight. Thus, only specially adapted plants can grow in this region. It also contains decaying plant and animal matter, which disappears quickly due to the lack of sunlight.

Fauna

Tropical rainforests support a diverse array of fauna including mammals, reptiles, birds and invertebrates. Mammals may include primates, felids and other families. Reptiles include snakes, turtle, chameleons and other families. Birds include such families as vangidae and Cuculidae. Dozens of families of invertebrates are found in tropical rainforests.

The rainforest as a source of drugs

Tropical rain forests are called the 'world's largest pharmacy' because of the large amount of natural medicines discovered there. Nearly half of the medicines that we use come from the rainforests. For example, rain forests are responsible for containing the "basic ingredients of birth control hormones, stimulants, and tranquilizing drugs" (Banks 36). Curare (a paralyzing drug) and quinine (a malaria cure) are also found there. Scientists believe that the cures for many more diseases will be discovered there in the future. Currently, 121 prescription drugs currently sold worldwide come from plant-derived sources. And while 25% of Western pharmaceuticals are derived from rainforest ingredients, less than 1% of these tropical trees and plants have been tested by scientists.

Threats

The most severe threat to the Tropical Rainforest is Human intervention. Clearing and degradation of tropical rain forests for timber, grazing land and agriculture, continues to lead towards severe soil erosion of already nutrient-poor soils. The rapid rate of clearing is destroying the homes and biodiversity in one of the most animal and plant prolific biomes on earth. Due to all of this, tropical rainforests now only cover 6% of the earth’s surface.