Geography : Tropical Rainforests

The tropical rainforest is a biome with a constant temperature and a high rainfall. The level of humidity and density of the vegetation give the ecosystem a unique water and nutrient cycle. Rainforests around the world are threatened by human expansion.

Characteristics and climate

See where tropical rainforests are found on this biomes map.

A tropical rainforest biome is found in hot, humid environments in equatorial climates. They contain the most diverse range and highest volume of plant and animal life found anywhere on earth.

In general, tropical rainforests have hot and humid climates where it rains virtually everyday. The level of rainfall depends on the time of year. Temperatures vary through the year - but much less than the rainfall.

The graph shows average rainfall and temperature in Manaus, Brazil, in the Amazon rainforest. The rainy season is from December to May. Notice how much the rainfall varies over the year - the highest monthly rainfall is in March with over 300mm, while the lowest is in August with less than 50mm. Over the year, the temperature only varies by 2°C.

Monthly average of rainfall/temp in rainforest area of Manaus, Brazil

Compare these patterns with the one for savannah grassland and coniferous forest. Make sure you know the differences between them!

Rainforest water and nutrient cycles

Rainforest ecosystems are characterised by heavy convectional rainfall, high humidity, lushness of vegetation and nutrient-rich but shallow soil. These factors give rise to a unique water and nutrient cycle.

Rainforest nutrient and water cycle

Rainforest water cycle

The roots of plants take up water from the ground and the rain is intercepted as it falls - much of it at the canopy level. As the rainforest heats up, the water evaporates into the atmosphere and forms clouds to make the next day's rain. This is convectional rainfall.

Rainforest nutrient cycle

The rainforest nutrient cycling is rapid. The hot, damp conditions on the forest floor allow for the rapid decomposition of dead plant material. This provides plentiful nutrients that are easily absorbed by plant roots. However, as these nutrients are in high demand from the rainforest's many fast-growing plants, they do not remain in the soil for long and stay close to the surface of the soil. If vegetation is removed, the soils quickly become infertile and vulnerable to erosion.

If the rainforest is cleared for agriculture it will not make very good farmland, as the soil will not be rich in nutrients.

Rainforest soils

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Images and sounds of a tropical rainforest biome.

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Soils are red due to the high iron and aluminium content. There is a thick layer of leaf litter and decomposing organic matter on the surface.

Rainforest vegetation levels

Tropical rainforests have dense vegetation. From ground level up these levels of vegetation are:

• The shrub layer. It is dark and gloomy with very little vegetation between the trees. During heavy rainfalls this area can flood.
• The under canopy is the second level up. There is limited sunlight. Saplings wait here for larger plants and trees to die, leaving a gap in the canopy which they can grow into. Woody climbers called lianas avoid having to wait for gaps by rooting in the ground and climbing up trees to get to the sunlight.
• The canopy is where the upper parts of most of the trees are found. The canopy is typically about 65 to 130 feet (20 to 40 metres) tall. This leafy environment is home to insects, arachnids, birds and some mammals.
• emergents are the tops of the tallest trees in the rainforest. These are much higher, and so are able to get more light than the average trees in the forest canopy.

The graphic shows the levels of rainforest vegetation and the relative amount of sunlight that each one receives.

Vegetation levels in tropical rainforest

Adapting to rainforest life

The vegetation in the rainforest has evolved characteristics which help it to survive in this unique environment.

A fan palm

Buttress roots

Lianas

Strangler fig

Each has adapted to rainforest conditions in a different way.

• Fan palms have large, fan-shaped leaves that are good for catching sunshine and water. The leaves are segmented, so excess water can drain away.
• Rainforests have a shallow layer of fertile soil, so trees only need shallow roots to reach the nutrients. However, shallow roots can't support huge rainforest trees, so many tropical trees have developed huge buttress roots. These stretch from the ground to two metres or more up the trunk and help to anchor the tree to the ground.
• Lianas are woody vines that start at ground level, and use trees to climb up to the canopy where they spread from tree to tree to get as much light as possible.
• Strangler figs start at the top of a tree and work down. The seed is dropped in a nook at the top of a tree and starts to grow, using the debris collected there as nourishment. Gradually the fig sends aerial roots down the trunk of the host, until they reach the ground and take root. As it matures, the fig will gradually surround the host, criss-cross its roots around the trunk and start to strangle. The figs branches will grow taller to catch the sunlight and invasive roots rob the host of nutrients. Eventually the host will die and decompose leaving the hollow but sturdy trunk of the strangler fig.

For further information log on website :
http://www.bbc.co.uk/schools/gcsebitesize/geography/ecosystems/tropical_rainforests_rev4.shtml