GeoTopics @ GeoNet

Tropical Rainforest

Where are tropical rainforests found?
What is the tropical rainforest?
What is the structure of the tropical rainforest?
How did the tropical rainforest get like this?
What is the impact of humans on the tropical rainforest?
What is the future for the tropical rainforest? - Sustainable Development

Where are tropical rainforests found?

Tropical rainforests are located around the equator where temperatures stay near 80 degrees year round. Rainforests receive 160 to 400 inches (400-1000 cm) of rain each year. The largest rainforests are in Brazil (South America), Zaire (Africa) and Indonesia (South East Asia). Other tropical rainforest places are in Hawaii and the islands of the Pacific & Caribbean.

What is the tropical rainforest?

The Tropical Rainforest is a forest occurring in tropical areas of heavy rainfall. It is abundant with many species of wildlife and vegetation. Rainforests cover less than two percent of the Earth's surface. They are home to some 50 to 70 percent of all life forms on our planet. Rainforests are the most productive and most complex ecosystems on Earth.

What is the structure of vegetation in the rainforest?

[cross section in the rainforest]

The image above shows a typical cross section in the rainforest.

Emergents are the tallest trees and are usually over 50 metres tall. The Kapok tree is an example of an emergent.

The sea of leaves blocking out the sun from the lower layers is called the canopy. The canopy contains over 50% of the rainforest wildlife. This includes birds, snakes and monkeys. Lianas (vines) climb to the canopy to reach this sun light.

The under canopy mainly contains bare tree trunks and lianas.

The shrub layer has the densest plant growth. It contains shrubs and ferns and other plants needing less light. Saplings of emergents and canopy trees can also be found here.

The forest floor is usually dark and damp. It contains a layer of rotting leaves and dead animals called litter. This decomposes rapidly (within 6 weeks) to form a thin humus, rich in nutrients.

How did the tropical rainforest get like this?

The high rainfall and year-round high temperatures are ideal conditions for vegetation growth. The wide range of plants encourage a huge variety of insects, birds and animals.

What is the impact of humans on the tropical rainforest?

Deforestation (cutting down trees) is a major problem caused by humans in the tropical rainforest. Global Rates of Deforestation:

2.47 acres (1 hectare) per second: equivalent to two U.S. football fields
150 acres (60 hectares) per minute
214,000 acres (86,000 hectares) per day: an area larger than New York City
78 million acres (31 million hectares) per year: an area larger than Poland

The image below shows some of the causes and effects of deforestation in the Amazon Rainforest. Place you mouse over the image to disover these causes and effects. Click to view infomation on each factor.

Slash and burn
Most clearances are still by the local people and tribes needing land on which to grow crops. They clear the forest by ‘slash and burn’. Vegetation is cut down and then burned. The ash acts like a fertiliser adder nutrients to the soil. When the soil begins to turn infertile (usually after 3-5 years) the people move on. This is called shifting cultivation. It is a sustainable method of farming in the rainforest. It ensures the forest will recover.

Road Building
Transamazon.

Logging
Commercial logging is the major cause of primary rainforest destruction in South East Asia and Africa. World wide, it is responsible for the destruction of 5 million ha. per year. Logging roads enable landless people to enter the forest. In Africa, 75% of land being cleared by peasant farmers is land that has been previously logged.

Cattle Ranching
Ranching is a major cause of deforestation, particularly in Central and South America. In Central America, two-thirds of lowland tropical forests have been turned into pasture since 1950.

Hydroelectric Power
An unlimited supply of water and ideal river conditions have led to the development of hydro electric power stations (HEP Stations).

Farming
There are nearly 3 million landless people in Brazil alone. The government has cleared large areas of the Amazon Rainforest and encouraged people to move there. The scheme has not been successful. Farmers stay on the same land and attempt to farm the land year after year. Nutrients in the soil are quickly exhausted as there is no longer a humus layer to provide nutrients. The soil becomes infertile and nothing will grow.

Mining
The mining of iron ore, bauxite , gold, oil and other minerals have benefited many LEDCs. However, it has also devastated large areas of rainforest e.g. The Amazon.

Deforestation is causing many problems at a range of scales:

Local:

Ecosystem

About 80% of the rainforests nutrients comes from trees and plants. That leaves 20% of the nutrients in the soil. The nutrients from the leaves that fall are instantly recycled back up into the plants and trees. When a rainforest is clear-cut, conditions change very quickly. The soil dries up in the sun. When it rains, it washes the soil away. The rainforest never fully recovers. Wildlife and plant life is reduced.
Elimination of Indian groups and their way of life
Estimates suggest that 80% of forest Indians have died since the arrival of Europeans in the sixteenth century. Most have died from western diseases such as malaria to which they have no immunity. Those remaining have been forced away by the construction of roads, ranches, mines and reservoirs

Soil Erosion

When vegetation is removed soil is left exposed to the heavy equatorial rainfall. It is rapidly eroded. The removal of top soil means little vegetation will grow. Also, soil erosion leads to flooding as soil is deposited on river beds.

National:

Deforestation can consume a country's only natural resource. If deforestation is not managed in a sustainable manner a country's only natural resource could be lost forever.

Global:

Global Warming
Rainforest canopies absorb carbon dioxide which is a gas in the atmosphere. When the rainforests are burned and cleared, the carbon is released. Also, when trees are cut down they can no longer absorb carbon dioxide. This means more carbon dioxide is in the atmosphere. Carbon dioxide allows heat through the atmosphere (suns rays). However, it will not allow reflected energy to escape from the atmosphere. This is called the greenhouse effect and causes global warming.

What is the future for the tropical rainforest? - Sustainable Development

If development in the rainforest is to be sustainable (e.g. although the resources are used to aid development, it/they will still exist for future generations to use) a number of measures must be taken. These include:

Afforestation - Trees should be replanted in areas of deforestation.

Shifting Cultivation - Farmers should move on after 2-3 years to allow the rainforest to recover.

Rubber tapping - More sustainable methods of exploiting the rainforest should be pursued e.g. rubber tapping

Measuring trees - Trees should only be cut down when they reach a certain size. This will ensure younger trees survive longer and will encourage careful management of the rainforest.