School › Rainforests › Ages 10–11
Rainforests: layers, soil and water
Why is a rainforest so rich in life but its soil so poor? Explore the four layers, find out how trees move water into the air, and run a fair test on what makes leaves release more water.
Read
Tropical rainforests grow between the Tropic of Cancer and the Tropic of Capricorn: mainly in South and Central America, West and Central Africa, Indonesia and South-East Asia. They have from about 2,000 mm to as much as 10,000 mm of rain a year and stay warm all year.[1][2]
Rainforests cover only about 2–6% of Earth's surface, but more than half of the world's plant and animal species live in them. The Amazon alone has around 40,000 plant species and nearly 1,300 bird species.[3]
There are four layers. Emergent trees stick out above the rest and face strong sun and wind; they have small, waxy leaves that lose less water. The canopy, 30–50 m up, cuts off most of the light and wind. The shady understory has small trees, vines and ferns with large leaves to catch the little light. The forest floor is the darkest layer.[4][2][3]
Surprisingly, most rainforest soils are not very fertile. In the warm, wet conditions fallen leaves rot very quickly, and the nutrients are taken up straight away by shallow tree roots and fungi, so few nutrients stay in the soil.[2][1][3]
Plants take up water through their roots and give it off as water vapour through tiny holes in their leaves called stomata. This is called transpiration. A large oak tree can give off about 151,000 litres in a year. It happens faster when it is warmer or windier and slower when the air is already humid.[6]
In the Amazon, water from the trees helps make the rain. Satellites showed that at the end of the dry season the first rain clouds form from water rising from the forest itself. If too many trees are cleared, the dry season could get longer.[5]
Try it: Do leaves give off more water in the sun? A fair test
You need
- Four clear plastic sandwich bags of the same size
- Twist ties or string
- Two similar leafy plants in pots (or two similar branches of the same bush)
- A sunny place and a shady place
- Kitchen scale or a small measuring spoon
- Paper towels
Safety
An adult chooses safe, non-poisonous plants. Do not pick or pull branches. Keep plastic bags away from faces. Wash hands after handling plants and soil.
What to do
- Choose two plants (or branches) of about the same size. Put one in the sun and one in the shade.
- Weigh each empty bag (or write down that it is dry).
- Put a bag over the same number of leaves on each plant and close it gently around the stem.
- As a control, close an empty bag in the sun and one in the shade.
- The next day, carefully remove the bags and compare the water inside. Weigh each bag again, or tip the water into a measuring spoon.
- Which plant transpired more? Did the empty bags collect any water?
What happens, and why?
Leaves release water vapour through their stomata; inside the bag the air becomes saturated and the vapour condenses into drops. Warmth increases transpiration, so the plant in the sun usually loses more water. The empty bags show that the water came from the leaves, not from the air. To be fair, use similar plants, the same number of leaves and the same bags, and only change sun or shade.[7][6]
Play: Which layer?
Sort each clue into the right rainforest layer.
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 15–16