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Rainforests: water, nutrients and climate feedback
Turn rainfall into litres, measure transpiration rates, compare conflicting figures from different sources, and explore how the Amazon helps trigger its own wet season.
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Tropical rainforests lie between the tropics, with average daily temperatures of about 20–25 °C and 2,000–10,000 mm of rain a year. Sources disagree in detail: National Geographic gives 1,800–2,500 mm and 18–30 °C. Comparing ranges like these, and asking how each was measured, is part of judging evidence.[1][3]
Rainfall in millimetres is a depth: 1 mm on 1 m² is 1 litre. Manaus in Brazil averages about 2,323 mm a year, so every square metre receives about 2,323 litres, and a hectare (10,000 m²) about 23 million litres. Its wettest months have about 310 mm and its driest (August) 68 mm.[1]
Rainforests cover only about 2–6% of Earth's surface yet hold more than half of all plant and animal species; the Amazon alone has an estimated 400 billion trees and 2.5 million insect species. The vertical structure (emergent layer, canopy at 30–50 m, understory, forest floor) creates many different habitats with very different light, wind and humidity.[3][2][4]
Nutrient cycling is fast: in the hot, humid conditions organic matter decomposes quickly and nutrients are absorbed rapidly by roots and fungi, so most soils are not very fertile.[2][1]
Transpiration moves water from the soil, through the plant and out of the stomata as vapour. Its rate rises with temperature and wind and falls as humidity rises. A large oak tree can transpire about 151,000 litres a year, which is roughly 400 litres a day on average.[6]
Using satellite measurements of deuterium in water vapour, scientists found that at the end of the dry season the clouds over the southern Amazon form from water transpired by the forest, which helps start the wet season in mid-October. This is a feedback: clearing and burning forest could weaken the rain pump; in some observations the wet season has begun up to a month later since the 1970s, and continued drying could turn parts of the forest into savanna.[5]
Try it: Measuring transpiration rate by mass loss
You need
- Two or three similar potted plants
- Clear plastic bags or cling film to cover the pots and soil (not the leaves)
- Electronic balance (0.1 g)
- A small desk fan
- Thermometer (and a hygrometer if available)
- Squared paper (1 cm²) for leaf area
- Spreadsheet
Safety
Keep the fan's cable and plug away from water and wet hands; an adult plugs it in. Use non-poisonous plants and wash hands after handling soil. Mop up spills so no one slips.
What to do
- Water all plants equally an hour before. Cover each pot and the soil surface with plastic so water can only leave through the plant.
- Weigh each plant and record the mass, temperature and time.
- Place one plant in still air and one in front of the fan on low (same light and temperature). A third plant with all leaves removed can act as a control.
- Weigh each plant again after 24 hours (and 48 hours if possible).
- Estimate the total leaf area by tracing a few leaves on squared paper and multiplying by the number of leaves.
- Calculate the rate of water loss in g per hour and in g per hour per 100 cm² of leaf. Compare still and moving air, and estimate the uncertainty from the balance and the leaf area estimate.
What happens, and why?
Almost all the mass lost comes from water leaving the leaves as vapour (transpiration), because the pot and soil are sealed. Moving air carries away the humid air next to the leaves, so transpiration is faster with the fan; warmer air also speeds it up and humid air slows it down. The leafless control shows how much water is lost in other ways.[6][7]
Play: Faster or slower?
Would this make a plant transpire faster or slower? Sort each one.
Your best score is kept only on this device. Nothing is sent anywhere.
Quiz
Other ages: Ages 6–7 · Ages 10–11