Teacher sheet · Ages 15–16 · about 45–50 minutes
Trees as carbon stores and climate records
Lesson page: echoesof.science/school/trees/ages-15-16/ · Pupil worksheet: echoesof.science/school/trees/ages-15-16/print/
Learning aims
- Use the photosynthesis equation to explain how trees store carbon.
- Describe the functions of the trunk layers.
- Estimate stored carbon and CO₂ from girth, with unit conversions.
- Evaluate uncertainty in carbon estimates and in ring-based ages.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: where does the mass of a tree come from? Quick vote: soil, water or air? |
| 10 min | Text and worked example. |
| 20 min | Measure three trees outside; calculate in pairs. |
| 5 min | Inside the trunk game. |
| 5 min | Quiz; set worksheet. |
Materials for the experiment
- 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.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| A tree's mass comes mostly from the soil. | The carbon in wood comes from CO₂ in the air via photosynthesis; soil supplies water and minerals. |
| Carbon and CO₂ figures are the same thing. | Multiply the mass of carbon by 3.67 to get the mass of CO₂ it came from. |
| Ring counts give an exact age. | Missing or hard-to-see rings make counts estimates; dendrochronologists cross-date to check. |
Worksheet answers
- 6CO₂ + 6H₂O → C₆H₁₂O₆ + 6O₂. Carbon atoms from atmospheric CO₂ end up in sugar and then in wood, so the tree's dry mass is built largely from carbon taken out of the air.
- Carbon ≈ 3,500 ÷ 2 = 1,750 kg ≈ 1.8 t; CO₂ ≈ 1,750 × 3.67 ≈ 6,400 kg ≈ 6.4 t.
- For example: the table gives average values for a typical tree; 'half is carbon' is approximate; measuring error in circumference; the table rows are coarse; species and shape differ.
- The oak transpires about 151,000 kg of water in one year, about 14 times the mass of CO₂ the worked-example oak absorbed in its whole life: trees move far more water than carbon.
Quiz answers
- A tree's dry mass is estimated at 2,000 kg. About how much CO₂ has it absorbed? About 3,670 kg. Carbon ≈ 2,000 ÷ 2 = 1,000 kg; CO₂ ≈ 1,000 × 3.67 = 3,670 kg.[10]
- Where does photosynthesis take place? In chloroplasts. Chloroplasts contain the chlorophyll that absorbs light.[3]
- Which layer makes the trunk thicker? Cambium. The cambium is the thin growing layer.[1]
- Why is a ring count only an estimate of age? Some harsh years produce no ring. Prometheus had 4,862 rings but was estimated at about 4,900 years old.[8]
- Where water limits growth, a narrow ring most likely shows… a dry or harsh year. Trees make wider rings in wet, cool years.[4]
- At what height is girth measured in the Forestry England method? 1.3 m. Circumference is measured 1.3 m above the ground.[10]
Going further
Repeat the girth measurements every year in the same place and calculate how much CO₂ each tree has added. Compare with the class's own estimate of a school's yearly CO₂ emissions.
Sources used in this lesson
- [1] Parts of a tree (Minnesota Department of Natural Resources (Forest Treasures)): reliability High
- [3] Biology 2e, 8.1 Overview of photosynthesis (OpenStax (Rice University)): reliability High
- [4] Tree rings and climate (UCAR Center for Science Education): reliability High
- [5] Evapotranspiration and the water cycle (U.S. Geological Survey, Water Science School): reliability High
- [7] About the trees (Redwood National and State Parks) (U.S. National Park Service): reliability High
- [8] The Prometheus story (U.S. National Park Service, Great Basin National Park): reliability High
- [10] Calculating carbon (teacher resource) (Forestry England): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/trees/sources/
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