The Tropical Rainforest Ecosystem » Rainforest Layers and Characteristics

A forest built in layers

You already know where rainforests are and that the equatorial climate is hot and wet all year. This lesson is about what a rainforest looks like inside. The tropical rainforest ecosystem is built in layers, a bit like the floors of a very tall block of flats. Each layer has its own height, its own amount of light and its own animals and plants.

Because the climate is warm and wet all year, plants grow all year, and they compete hard for one thing above all: sunlight. The tallest plants win the light, and everything else grows in their shade.

The five layers

From the top down, the layers are:

Layer	Height (about)	Light	Life found here
Emergent layer	above 40 m, some trees reach 50 m or more	Full, bright sunlight; hot and windy	Birds of prey such as eagles, butterflies, bats and a few tall trees
Canopy	25 to 40 m	Most sunlight is caught here	The most life of any layer: monkeys, toucans, parrots, frogs, insects and flowers
Understory (under canopy)	5 to 25 m	Shady and humid	Young trees, tree frogs, snakes, lizards and many insects
Shrub layer	1 to 5 m	Very dim	Ferns, small shrubs, seedlings, insects and small mammals
Forest floor	ground level	Only about 2% of sunlight reaches it	Fungi, ants and termites, tree roots and larger animals such as tapirs

Heights are only a guide. Different books give slightly different numbers, so always say "about" and remember that the pattern matters more than the exact figure.

Key terms:

• Emergent layer: the tallest trees, which stick out above the rest of the forest.
• Canopy: the thick, continuous roof of leaves and branches below the emergents.
• Understory: the layer below the canopy, shaded by the leaves above.
• Shrub layer: low plants and young trees growing close to the ground.
• Forest floor: the ground at the bottom of the forest, covered in fallen leaves.

Light and life in each layer

Key characteristics of the rainforest

Very high biodiversity. Remember, biodiversity is the variety of living things in a place. The hot, wet climate lets plants grow all year, which gives food and homes for a huge number of insects, birds and mammals. A single patch of rainforest can hold more species than a much larger area of forest in a cooler climate. Many species are found nowhere else.

Dense vegetation. There are many trees, climbers and plants growing on other plants, all packed together. In places it is hard to walk through, especially near rivers and gaps in the canopy where light reaches the ground.

Tall trees with special features. Many rainforest trees are very tall, with straight trunks and few branches until the top. Two features are often named in the exam:

• Buttress roots: huge, wide roots that spread out above the ground around the base of a tall tree.
• Drip-tip leaves: leaves with a long, narrow, pointed end.

You will learn how these help plants survive in the next lesson, Rainforest Life: Nutrient Cycle and Adaptations. For now, just be able to name them and describe what they look like.

Evergreen trees. Most trees keep their leaves all year. So the forest always looks green.

Rainforest soils: latosols and leaf litter

You might expect such a lush forest to have rich soil. It does not. Rainforest soils are latosols: red or orange-coloured soils that are thin and low in nutrients. Heavy rain washes many nutrients out of the soil.

On top of the soil lies a thin layer of leaf litter, made of fallen leaves, twigs and dead material. In the heat and damp it rots away quickly, so the layer stays thin. The tiny amount of nutrients it releases is soon taken up again by roots. That is why the forest looks rich but the soil beneath it is poor.

Key terms:

• Buttress roots: wide roots spreading out above the ground at the base of a tall tree.
• Drip-tip leaves: leaves with a long, pointed tip.
• Latosol: a thin, red or orange, nutrient-poor rainforest soil.
• Leaf litter: the layer of fallen leaves and dead plant material on the forest floor.

How is a rainforest different from other forests?

Compare the rainforest with a forest in a cooler climate, such as northern Europe.

Feature	Tropical rainforest	Forest in a cooler climate
Climate	Hot and wet all year, no seasons	Distinct seasons, with a cold winter
Growing season	All year	Spring and summer only
Leaves	Mostly evergreen	Many trees lose all their leaves in autumn
Biodiversity	Very high	Much lower
Layers	Five clear layers, many very tall trees	Fewer layers, shorter trees
Vegetation	Very dense, very dark below	Lighter, more open

Exam-style question

Fig. 1.1 shows made-up data for five layers of a tropical rainforest.

Layer	Height (m)	Sunlight reaching layer (%)
P	Above 40	100
Q	25 to 40	40
R	5 to 25	10
S	1 to 5	5
T	0	2

Fig. 1.1

(a) Identify the layer that is the canopy. [1 mark]

(b) Describe the pattern shown in Fig. 1.1. [2 marks]

(c) Give two characteristics of rainforest soils. [2 marks]

(d) Explain why the tropical rainforest has very high biodiversity. [4 marks]