Trees as carbon stores and climate records
Trees turn carbon dioxide into wood and keep a year-by-year diary in their rings. Use the photosynthesis equation, estimate the carbon locked in a school tree from its girth, and judge how certain such estimates are.
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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.[3][5]
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.[1]
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.[5][7]
Forestry England's classroom method estimates stored carbon from girth: measure the circumference 1.3 m above the ground, read the dry mass from a table, take about half of it as carbon, and multiply by 3.67 to find the CO₂ absorbed. For an oak with a dry mass of about 5,771 kg: carbon ≈ 2,886 kg and CO₂ ≈ 10,600 kg, or about 10.6 tonnes.[10]
The factor 3.67 is the ratio of the mass of a CO₂ molecule to the mass of its carbon atom (44 : 12), so every kilogram of carbon in wood stands for about 3.67 kg of CO₂ taken from the air.[10][3]
Tree rings are a climate archive. Each year adds light spring wood and darker late wood; where water limits growth, wide rings mark wet, cool years and narrow rings dry or harsh ones. By cross-matching living and dead wood, dendrochronology reaches back more than 10,000 years, using cores only about 4 mm wide.[4]
Ring counts carry uncertainty. The bristlecone pine 'Prometheus' had 4,862 rings, but because harsh years can produce no ring its age was estimated at about 4,900 years.[8]
Try it: How much carbon is in our tree?
You need
- A flexible tape measure (at least 3 m)
- A metre stick to find 1.3 m
- Forestry England's circumference-to-dry-mass table (link on the sources page)
- Calculator, clipboard
- Optional: a log slice or photo of tree rings
Safety
Work in pairs with an adult nearby. Do not climb trees or lean on dead branches. Watch for roots and uneven ground. Do not damage bark with pins or tape.
What to do
- Pick three trees of different sizes in the school grounds.
- Use the metre stick to mark 1.3 m above the ground on the uphill side of each trunk.
- Measure the circumference at that height to the nearest cm. Two people measure independently; record both.
- Use the table to find the nearest dry mass for each circumference.
- Calculate carbon = dry mass ÷ 2 and CO₂ = carbon × 3.67.
- 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?
What happens, and why?
Wood is mostly made from carbon taken from CO₂ in photosynthesis, and about half of its dry mass is carbon. A bigger girth means much more wood, so the stored carbon rises steeply with circumference. The table and the 'half' rule are averages, so the result is an estimate, not a measurement.[10][3]
Play: Inside the trunk
Match each description to the right layer of the trunk.
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Quiz
Other ages: Ages 6–7 · Ages 10–11