Tropical Rainforests » Tropical Rainforest Characteristics

The physical characteristics of a tropical rainforest

You met the tropical rainforest as one of the world's biomes in the last lesson. Now we look closely at what makes it special. Tropical rainforests grow close to the equator, in a belt roughly between the Tropic of Cancer and the Tropic of Capricorn. This lesson covers the physical features of the forest: its climate, layers and soils. Then we see how all the parts fit together.

Key terms:

• Tropical rainforest: a dense, tall forest found near the equator, with a hot, wet climate all year round.
• Canopy: the thick layer of leaves and branches formed by the tops of most of the trees.
• Emergent layer: the very tallest trees that stick out above the canopy.
• Leaching: when heavy rain washes nutrients down through the soil, out of the reach of roots.
• Nutrient store: a place where nutrients are kept, such as the soil, the leaf litter or the plants.

The climate

Because the rainforest is near the equator, the Sun is high in the sky all year. The weather is hot and wet, with almost no change between seasons.

Hot and wet conditions are perfect for plant growth. This is why the rainforest is so green and so dense. When you describe a climate in the exam, always give figures, such as the temperature range and the rainfall total.

The layers of the forest

A rainforest is built in layers. Each layer gets a different amount of light, so each one has different conditions.

• Emergent layer: the tallest trees, up to about 50 metres or more. They stand above the rest and get the most sunlight, wind and heat.
• Canopy: a thick, leafy roof about 30 to 45 metres up. It catches most of the sunlight and rain and is full of flowers, fruit and wildlife.
• Understorey: the shady layer below the canopy, with young trees and plants that cope with little light.
• Forest floor: dark, damp and covered in fallen leaves. Very little sunlight reaches it, so few plants grow there.

The canopy is the key to the whole forest. It is so thick that it shades everything below it and slows the rain before it hits the ground.

The soils

You might expect such a green forest to have rich soil. In fact, rainforest soils are usually poor in nutrients. The heavy rain causes leaching, washing nutrients down and away. The nutrients are mostly held in the plants and in a thin layer of fallen leaves on the surface, not deep in the soil.

The nutrient cycle in the rainforest

Remember, nutrient cycling means nutrients are passed around the ecosystem and reused. In the rainforest this happens very fast. There are three main stores:

• Biomass (the plants and animals): the biggest store, because the forest grows so quickly.
• Litter (dead leaves and branches on the surface): a small store.
• Soil: a small store, because of leaching.

Dead leaves fall all year round. The heat and damp let decomposers break them down very fast. Nutrients are released and shallow tree roots take them up again almost straight away. Rain adds a little, and leaching removes some.

Interdependence

Everything in the rainforest depends on everything else. This is called interdependence. Change one part and the others are affected.

• Climate and plants: the heat and rain let the plants grow thick and tall all year.
• Plants and water: trees take up water through their roots and release it from their leaves. This helps to make clouds and more rain.
• Plants and soils: falling leaves feed the thin soil. Roots and the canopy also help to protect the soil from heavy rain.
• Plants and animals: animals eat the plants and use the trees for shelter. Many spread seeds, which grows new plants.
• Forest and people: some people live in the forest and get food, shelter and medicines from it. If the forest is damaged, their way of life is damaged too.

Here is one chain to show how it all links. Heavy rain feeds the trees. The trees feed the animals. The animals spread seeds. The seeds grow into trees that hold the soil and protect it from the rain.

Exam-style question

(a) Describe the climate of a tropical rainforest. [2 marks]

(b) Explain why the soils in a tropical rainforest are poor in nutrients. [3 marks]