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Photosynthesis builds the sugar that trees turn into wood: 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂, driven by light absorbed by chlorophyll in chloroplasts. CO₂ enters, and O₂ and water vapour leave, through the stomata.

The trunk is layered: outer bark protects; phloem carries sugars from the leaves; the thin cambium divides to make the trunk thicker; xylem (sapwood) carries water and minerals up; and heartwood in the centre no longer conducts water but gives support.

Transpiration moves very large volumes: a large oak can transpire about 151,000 litres a year. Coast redwoods lift water to heights of up to 112 m and take about 40% of their moisture from fog.

Experiment: How much carbon is in our tree?

1. Pick three trees of different sizes in the school grounds.
2. Use the metre stick to mark 1.3 m above the ground on the uphill side of each trunk.
3. Measure the circumference at that height to the nearest cm. Two people measure independently; record both.
4. Use the table to find the nearest dry mass for each circumference.
5. Calculate carbon = dry mass ÷ 2 and CO₂ = carbon × 3.67.
6. Estimate the uncertainty: how much does the result change if your circumference is 5 cm out, or you pick the next row of the table?

Tree	Circumference (cm)	Dry mass (kg)	Carbon (kg)	CO₂ (kg)
Tree 1	Estimate
Tree 2	Estimate
Tree 3	Estimate

Questions

1. Write the balanced equation for photosynthesis and explain how it links CO₂ in the air to the mass of a tree.

2. A tree has a dry mass of 3,500 kg. Calculate the carbon stored and the CO₂ absorbed. Give your answer in tonnes to 2 significant figures.

3. List two reasons why your carbon estimate for a school tree is uncertain.

4. A large oak transpires about 151,000 L a year. Assuming 1 L of water has a mass of 1 kg, compare this with the oak in the worked example (CO₂ ≈ 10,600 kg absorbed over its life).

Sources for every fact on this sheet: echoesof.science/school/trees/sources/

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