Tropical Rainforests: Extent, Type, Soil, Ecosystem and Biodiversity

The tropical rainforest is earth’s most complex biome in terms of both structure and species diversity. It occurs under optimal growing conditions: abundant precipitation and year round warmth. The World Wildlife Fund’s biome classification puts the tropical rainforests under Tropical Moist Broadleaf Forest.

Distribution of Tropical Rainforests

The Tropical rain forests is roughly located within 28° north or south of the equator , spread in Asia, Australia, Africa, South America, Central America, Mexico and on many of the Pacific Islands. They roughly cover 6-7% of earth’s area and are home to half of its biodiversity. The largest rainforests are in Brazil (South America), Democratic Republic of Congo (Africa), and Indonesia. Other tropical rainforests lie in Southeast Asia, Hawaii, and the Caribbean Islands.

The Amazon rainforest in South America is the world’s largest, covering an area about two-thirds the size of the continental United States.

Etymology: Rainforests

Rainforests are called so because they are wet due to round the year rains. There are apparently no seasons in Tropical rain forests near the equator, yet the tropical rainforests which are away from equator have only wet and dry seasons. Tropical rainforests receive 175 to 300 inches precipitation annually. Tropical rain forests are found in regions where temperatures and precipitation are high year-round. Mean monthly temperatures exceed 18 °C during all months of the year, due to location near to equator. Please note that there is no annual rhythm to the forest; rather each species has evolved its own flowering and fruiting seasons. Sunlight is a major limiting factor.

Layers of Trees in Rainforests

A tropical rainforest consists of four layers: the emergent trees, canopy, the understory, and the forest floor.

  • The emergent and canopy layers make up the very top of the rainforest, where a few trees, called emergent, poke out above the green growth to reach the sun. Most of the plant growth in rainforests is here, close to the sun.
  • Most rainforest animals, including monkeys, birds, and tree frogs, live in the canopy.
  • Below the canopy are the young trees and shrubs that make up the understory. The plants in this layer cannot grow to large sizes because the canopy blocks most of the sunlight.
  • The forest floor is almost bare because very little sunlight can get through the canopy and understory to reach the ground. This is where fallen leaves and branches rot quickly to release nutrients for other plants to grow.
  • Large mammals such as South American tapirs and Asian elephants who are too heavy to climb up into the canopy layer live in the dim light of the understory and forest floor.

Complex Ecosystem of Rainforests

In Rainforests, the plants and animals depend on each other for survival. For example, some insects can only survive in one type of tree, while some birds only eat one type of insect. If this tree is destroyed, the insects will have no home. If the insects die, the birds who rely on them for food will starve to death. Because of this interdependence, if one type of plant or animal becomes extinct, several others could be in danger of extinction as well.

Rainforest Soils

It would appear to us that tropical soils are very fertile in order to support this high productivity. But, it is incorrect to say so.  If we closely look at the system, we find that soils of Tropical Rain Forests are very thin and the rock below them highly weathered. An analysis of soils of tropical regions shows them to be virtually devoid of soluble minerals. Rocks weather rapidly due to high temperatures and abundant moisture, and millennia of rapid weathering and torrential rains to wash away nutrients from the soils have left the soils very low in nutrient stocks.

It has also been supported by the analysis of stream water draining tropical regions, which likewise reveals a scarcity of dissolved nutrients. Most tropical soils are clays with little soluble mineral content, and moderate to strong acidity which interferes with the ability of roots to take up nutrients. Only about 20% of the humid tropics has soils that can support agriculture, and most of this area is already in use.  In soils of the Tropical Rain Forests, the nutrients are found mainly in living plant biomass and in the layer of decomposing litter; there is little nutrient content of the deeper soil, as there is in temperate-zone ecosystems. This suggests that plants are intercepting and taking up nutrients the moment they are released by decomposition. Many organisms play role in decomposition process: termites, bacteria, fungi, various invertebrates.

Recycling of Nutrients

Due to the above mentioned reasons, the rainforest reuses almost everything that falls to the ground and decays.

When leaves fall from the trees, when flowers wilt and die, and when any animal dies on the forest floor, it decays and all of the nutrients in the decayed species are recycled back into the roots of the trees and plants. Only the top few inches of rainforest soil have any nutrients. Most of the nutrients are in the biomass, the bulk of animal and plant life above the ground. The roots of rainforest trees are not very deep; that way they can collect all of the nutrients in the top few inches of the soil. Rainforests even recycle their own rain. As water evaporates in the forest it forms clouds above the canopy that later fall as rain.

Biodiversity in Rainforests

Rainforests are home to half of all the living animal and plant species on the planet. High biodiversity appears related to high ecological specialization of species.  The rainforests are home to more worldwide species than all other biomes added together.

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