Tropical and subtropical moist broadleaf forests

Tropical and subtropical moist broadleaf forests, also known as tropical wet forests and tropical rainforests, 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, northern and eastern 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.
Abiotic factors: Hot and wet year-round; thin, nutrient poor soil
Dominant plants: broad-leaved evergreen trees that are tall and form a canopy over the forest floor; ferns; large woody vines that form the understory under the canopy
Geographic locations: parts of South and Central America, South East Asia, parts of Africa, southern India and North East Australia

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", and the "world's largest pharmacy" because of the large amount of natural medicines discovered there. Tropical rain forests are also often called the "Earth's lungs", however there is no scientific basis for such a claim as tropical rainforests are known to be essentially oxygen neutral, with little or no net oxygen production.,^[1]

Despite the growth of flora in a rainforest, the actual quality of the soil is usually 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

Rainforest on Fatu-Hiva, Marquesas Islands

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

Many tree 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 and shrubs of the understory and forest floor layers. Young individuals of trees destined for the canopy and emergent 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 and are made possible because the lower layers are largely protected from winds which damage large leaves in the canopy. Canopy leaves are usually smaller than found in understory plants or are divided to reduce wind damage. The leaves of rainforest understorey trees also often have drip tips which facilitate drainage of precipitation off the leaf to promote transpiration and inhibit the growth of microbes and bryphytes which would damage or smother the leaf .

Trees are often well connected in the canopy layer especially by the growth of woody climbers known as lianas or by 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 rainforest tree species than in temperate forests or drier tropical regions 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, monkeys, 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 decomposing plant and animal matter, which disappears quickly due to the high humidity and temperature found in this region.

Human uses

Habitation

Most tropical rainforests have historically not supported dense human populations. Food resources within the forest are extremely dispersed due to the high biological diversity and what food does exist is largely restricted to the canopy and requires considerable energy to obtain. Rainforest soils are often thin and leached of many minerals, and the heavy rainfall can quickly leach nutrients from rainforest plots cleared for cultivation. Nonetheless humans have and do exploit rainforests for food and shelter in several parts of the world. Many agriculturalists, particularly within the Amazon and New Guinean rainforests, obtain their food primarily from farm plots cleared from the forest and hunt and forage within the forest to suplement this. Other peoples described as rainforest dwellers are hunter-gatherers who subsist in large part by trading high value forest products such as hides feathers and honey with agricultural people living outside the forest. Other groups of hunter-gatherers have exploited rainforest on a seasonal basis but dwell primarily in adjacent savanna and open forest environments where food is much more abundant. ^[2]Humans make their shelter from large leaves and branches and depend on the trees themselves to function.

Cultivated foods and spices

Coffee, chocolate, bananas, mangoes, papayas, avocados and sugar cane all originally came from tropical rainforests, and are still mostly grown on plantations in regions that were formerly primary forest. In the mid-80s and 90s, 40 million tons of bananas were consumed worldwide each year, along with 13 million tons of mangoes. Central American coffee exports were worth US$3 billion in 1970. Much of the genetic variation used in evading the damage caused by new pests is still derived from resistant wild stock. Tropical forests have supplied 250 cultivated kinds of fruit, compared to only 20 for temperate forests. Forests in New Guinea alone contain 251 tree species with edible fruits, of which only 43 had been established as cultivated crops by 1985.^[3]

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 were discovered in rainforests. For example, rainforests contain the "basic ingredients of birth control hormones, cocaine, 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.

Tourism

Currently one the largest economic values of tropical rainforests comes in the form of tourism. People travel both nationally and internationally to experience rainforests firsthand. The economic benefits of tourism is the most promising way in which rainforests may be preserved.

Timber and fuelwood

Rainforest are an important source of timber in most parts of the world, particularly of high value timbers such as cedar and ebony.

Animal products

Rainforests provide numerous animal products including honey, game meat and associated trophies such as hides and ivory.

Ecosystem services

In addition to extractive human uses rainforets also have non-extractive uses that are frequently summarised as ecosystem services. Rainforests play an important role in maintaining biological diversity, modulating precipitation infiltration and flooding, increasing scientific knowledge and in the spiritual wellbeing of humans. Such ecosystem services are of use to humans without the need for any modification or management of the forest itself.

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.