Rainforest Layers

A forest in four storeys

You already know from studying ecosystems that tropical rainforest grows near the equator because it is hot and wet there all year. This module starts where that finishes. A rainforest is not simply a lot of trees in one place. It is built in storeys, one above another, and each storey is a different world to live in. Here is the idea that explains the whole thing, and it is worth holding on to because almost every question on this topic is answered by it. Light arrives at the top. Everything below is competing for what is left, and there is not much left. The tallest trees push above everything else and get full sun; just below them the treetops form a near-continuous roof that catches most of the light that remains; below that it is dim, and at ground level it is dark for most of the day. So the structure of a rainforest is a record of that competition. And once you see it that way, the adaptations stop being a list to memorise. Buttress roots, drip tips, climbing plants, animals that live their whole lives off the ground: every one of them is an answer to a question about light, water or height.

Words for the layers

Seven terms to use precisely. Notice that each definition says something about light or height, because that is what the terms are really about.

From the top of the forest down

Put the layers of a tropical rainforest in order, starting at the top.

• Emergent layer: the tallest trees, standing clear in full sunlight
• Canopy: the near-continuous roof of treetops that catches most of the light
• Understorey: dim, with shrubs and young trees waiting for a gap
• Forest floor: dark, with very little growing

Why the floor is so dark

Very little grows on a rainforest floor. What is the main reason?

• The canopy above absorbs most of the sunlight, so very little reaches the ground
• It is too cold at ground level for plants to grow
• There is not enough water at ground level
• Animals eat every plant that tries to grow there

The roof and the floor

The same forest, at the top and at the bottom. Reading these two columns against each other is the quickest way to see that light drives everything else.

Match each layer to its conditions

Growing all year round

Now the parts that depend on each other, which is what your specification means by interdependence. The climate comes first: hot all year, wet all year, with no real dry season and little variation between one month and the next. Nothing stops growth, so growth never stops. That single fact drives the rest. Because it is hot and wet, dead leaves and animal remains reaching the forest floor are broken down very quickly rather than building up. The nutrients released are taken up again almost immediately by the dense growth above, and because the trees have shallow, spreading roots they can capture those nutrients near the surface. So the store of nutrients in a rainforest is mostly in the plants themselves, not in the ground. That is why the soil is surprisingly thin and poor beneath one of the richest ecosystems on Earth, which surprises students every year. The forest also feeds its own climate: trees take up water and release it from their leaves, and much of the rain that falls on a rainforest came from the forest itself. People are part of this too. Indigenous peoples have long lived in rainforests, taking what they need in small quantities and moving on to let the forest regrow. What happens when the trees are cleared, and how forests can be used sustainably, is studied in the next module on this topic.

Filling in the structure

A tropical rainforest grows in layers. The tallest trees form the _____ layer, standing clear above the treetop roof called the _____, which absorbs most of the sunlight. Below it the _____ is dim, and the forest _____ is dark, so little grows there. The climate is hot and _____ all year, so dead material is broken down very quickly and the nutrients are taken straight back up by the plants.

Two answers to heavy rain

Select the TWO leaf features that are adaptations to very heavy rainfall specifically.

• Drip tips, which let water run off the leaf quickly
• A waxy leaf surface, which helps water run off rather than soak in
• Buttress roots spreading out from the base of the trunk
• Growing as a climbing plant up a tree trunk

Explaining an adaptation properly

Adaptation questions are where marks are most often lost, and always in the same way: the student names the feature and stops. Naming is description; the question asks for explanation. A full answer has three parts, and here it is done twice. Take buttress roots. Feature: large roots that spread out from the base of the trunk. Condition: rainforest soils are thin, so tall trees root shallowly rather than deeply. Benefit: the buttresses spread the weight and give stability, so a very tall tree can stand in shallow soil without falling. Now take the epiphyte. Feature: a plant growing on the branches of another plant instead of in the ground. Condition: the forest floor is dark, because the canopy takes most of the light. Benefit: by growing high up it reaches the light directly, and it takes the water and nutrients it needs from the air and rain rather than the soil. Notice the shape of both. The middle sentence is the one that does the work, because it names the condition the adaptation is answering. If you can add the words "because the rainforest is..." to your answer, you are explaining. If you cannot, you are still only describing.

The answer that names but never explains

Four sentences from answers about rainforest adaptation. Select the ONE that names a feature without ever explaining what condition it answers.

• Leaves have drip tips so that heavy rain runs off quickly instead of sitting on the leaf.
• Many plants have drip tips and buttress roots, which are important rainforest adaptations.
• Lianas climb the trunks of trees because the light is at the top and this is a way of reaching it.
• Tree roots are shallow because the nutrients are concentrated near the surface of the soil.

From heat to thin soil

Assemble the chain that explains the rainforest's most surprising feature.

Identify it from its features

You are given descriptions of rainforest species you have never met. Work out where each one lives, and why.

• A plant has very large, broad leaves and grows slowly close to the ground. What does that suggest?
• An animal has a gripping tail, strong limbs and eats fruit. Where does it most likely live?
• A tree is unusually tall, has a thick trunk and wide buttress roots. What is it adapted for?
• You are asked why a rainforest contains so many different species. What is the best explanation?

Explain the structure and the adaptations

Explain how a tropical rainforest is structured, and how plants and animals are adapted to the conditions at different heights within it.

• Describe the climate in words: hot all year, wet all year, no real dry season
• Name the four layers in order from the top down
• Explain why the forest floor is dark, and what that means for what grows there
• Explain two plant adaptations using feature, condition and benefit
• Explain one animal adaptation and say which layer it suits
• Explain why so many species live in a rainforest
• Explain why the soil is thin even though the forest above it is dense
• Finish by linking the structure back to competition for light