Teacher sheet · Ages 15–16 · about 45–50 minutes
Rainforests: water, nutrients and climate feedback
Lesson page: echoesof.science/school/rainforests/ages-15-16/ · Pupil worksheet: echoesof.science/school/rainforests/ages-15-16/print/
Learning aims
- Convert rainfall depths to volumes and compare monthly data.
- Explain rapid nutrient cycling and poor rainforest soils.
- Measure transpiration rate, control variables and estimate uncertainty.
- Explain the forest–rainfall feedback and evaluate differing figures from sources.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: how many litres of rain fall on your classroom floor area in a year in Manaus? |
| 10 min | Read the text; rainfall and transpiration calculations. |
| 20 min | Set up the mass-loss practical (weigh again after 24–48 h). |
| 5 min | Faster or slower? game. |
| 5 min | Quiz; discuss why sources give different numbers. |
Materials for the experiment
- 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.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| Rainforests have rich soils, so cleared land will make good farmland for a long time. | Most rainforest soils are not very fertile; nutrients from dead leaves are taken up quickly by roots and fungi instead of being stored in the soil. |
| All mass a potted plant loses is water used for growth. | Almost all of the short-term mass loss is water leaving the leaves by transpiration. |
| Different numbers in sources mean one source is wrong. | Ranges often differ because places differ and sources average in different ways. |
Worksheet answers
- Volume = 2.5 m × 10,000 m² = 25,000 m³ = 25,000,000 litres.
- Loss = 412.6 − 398.2 = 14.4 g; rate = 14.4 ÷ 24 = 0.60 g/h; per 100 cm² = 0.60 ÷ 3.6 ≈ 0.17 g/h per 100 cm².
- Any two: rainforests in different places really get different amounts; different years or periods are averaged; one source may describe typical forests and the other the full range including the wettest places; definitions of 'rainforest' differ.
- Trees transpire water vapour that forms the clouds that start the wet season. Clearing and burning removes trees, so less vapour is released and the wet season may start later; a longer dry season dries the forest further, and parts could turn into savanna.
Quiz answers
- How many litres of rain fall on 1 m² in a year with 2,000 mm of rain? 2,000 litres. 1 mm on 1 m² equals 1 litre.[1]
- Manaus has 310 mm of rain in its wettest month and 68 mm in August. About how many times wetter is the wettest month? About 4.6 times. 310 ÷ 68 ≈ 4.6.[1]
- Why are rainforest soils generally poor even though plant growth is huge? Nutrients are recycled rapidly into living plants. Decomposition and uptake are both fast.[2]
- Which change would decrease the transpiration rate? Higher humidity around the leaves. Water evaporates less easily into air that is already moist.[6]
- How did scientists show that Amazon trees start the wet season? Satellite measurements of deuterium in water vapour. Plant water vapour carries more deuterium than ocean vapour.[5]
- A large oak transpires about 151,000 litres a year. About how much is that per day? About 400 litres. 151,000 ÷ 365 ≈ 414 litres.[6]
Going further
Plot NASA's monthly rainfall for Manaus as a bar chart and mark the dry season. Then research one rainforest product or houseplant and trace where it comes from.
Sources used in this lesson
- [1] Mission: Biomes – Tropical rainforest (NASA Earth Observatory (Mission: Biomes)): reliability High
- [2] Rainforest (summary) (Encyclopaedia Britannica): reliability High
- [3] Rain forest (National Geographic Society (Education resource library)): reliability Medium-high
- [4] Rainforest layers (Smithsonian Tropical Research Institute): reliability High
- [5] The Amazon makes its own wet season (NASA Earth Observatory): reliability High
- [6] Evapotranspiration and the water cycle (U.S. Geological Survey (Water Science School)): reliability High
- [7] Transpiration bag (Water cycle for kids) (U.S. Geological Survey (Water Science School)): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/rainforests/sources/
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