Resources and environment, Year 10: tropical rainforests

More than 2,000 mm

HintLondon gets about 600 mm; a rainforest gets over three times that.

The whyRain falls in almost every month, often as a heavy downpour in the afternoon. It is also hot all year, with no winter. Heat and water together give plants a growing season that never stops.

Leaching

HintIt sounds like the name of a blood-sucking worm.

The whyRainforest rain is so heavy that it carries dissolved nutrients deep into the ground. This is one reason the soil is poor even though the forest above it is so rich.

In the living plants (the biomass)

HintIt is not under your feet. Look up.

The whyDead leaves rot within weeks in the heat and damp. Shallow roots take the nutrients straight back up before the rain can wash them away. So nutrients spend almost all their time inside the trees.

They give off water vapour, which forms clouds and falls again as rain

HintThink of transpiration from billions of leaves.

The whyRoots take up water and the leaves release it into the air. The air above the forest stays humid, clouds build during the day and the water comes down again. Remove the trees and the area becomes drier.

With the trees gone, the soil's few nutrients are used up and washed away

HintConsider what the forest canopy once dropped on the ground, and what downpours do to bare earth.

The whyThis is shifting cultivation. Because the plots are small and are left to recover for many years, the forest grows back. It shows people as one part of the system, depending on the same nutrient cycle as the plants.

emergents

HintThey "come out" above everything else.

The whyA rainforest has layers: the forest floor, the under canopy, the main canopy and, above it, these scattered giants. They get the most sunlight but also the most wind and the driest air.

Buttress roots

HintNamed after the stone props on the walls of a cathedral.

The whyRainforest trees root shallowly because the nutrients are near the surface. A shallow root cannot hold up a 40 m tree, so the base of the trunk spreads out into ridges that steady it. The wide base also gathers nutrients from a larger area.

Rain runs off the leaf quickly, so it stays light and free of mould

HintImagine the spout of a jug.

The whyLeaves in the rainforest are wet for much of every day. A film of water blocks light and lets algae and fungi grow on the surface. A pointed tip and a waxy surface channel the water away within minutes.

lianas

HintTarzan swings on them.

The whyVery little light reaches the forest floor. These climbers use the tree trunks as a ladder, so they reach the light without spending energy on a thick trunk of their own.

It can live and feed in the canopy without coming down to the ground

HintMost fruit and leaves are high up.

The whyThe canopy is where the sunlight is, so it is where most leaves, flowers and fruit grow. Animals that can climb, swing, glide or fly reach that food. The dark forest floor has far less to eat.

half

HintA simple fraction, and not a tiny one.

The whyConstant heat, water and light, and many layers to live in, allow a huge number of species to exist side by side. This is why the loss of rainforest matters far more for biodiversity than its area suggests.

Clearing even a small area can wipe out a whole species

HintCompare it with an animal found right across a continent.

The whyA species found in only one place is called endemic. In a UK wood most species also live in thousands of other woods. In a rainforest a single valley may hold plants and insects found nowhere else on Earth.