What it shows
A tree stores carbon taken from the air in its wood. Measuring its girth and height gives a rough estimate of its wood volume, mass and the carbon dioxide it has removed from the air.
The science
Trees take in CO2 by photosynthesis and build wood from the carbon. About half of the dry mass of wood is carbon, and one carbon atom of mass 12 comes from a CO2 of mass 44, so the CO2 removed is 44/12 times the carbon mass. The trunk can be treated as a cone or a cylinder: volume V = form factor x pi x (D/2) squared x H, where D is the diameter at chest height of 1.3 m, from girth divided by pi, and the form factor is about 0.5 for a tapering trunk. Branches and roots add more, so this estimate is rough and may be out by a factor of 2.
Materials
Cost: free. Many items come from home or from waste; spare parts such as LEDs, motors and wire are in the store.
- A measuring tape or a long string and a ruler
- A clinometer or a protractor with a plumb line
- A friend to measure a distance on the ground
- A calculator
- A list of wood densities for common trees from a forestry website
- A notebook
- A tree to measure, with permission
Safety
For this project: Do not climb trees. Measure from the ground, keep away from dead branches and ant nests, and work with an adult.
Never use mains electricity: power models with cells, batteries or a low-voltage adapter. Sharp and hot tools, flames and lab chemicals need an adult or your teacher beside you. Keep magnets, button cells and small parts away from young children, and wash your hands after making.
All the safety rulesStep by step
8 steps. Do them in order, and test each part before you go on to the next. Write what you notice in a notebook as you go.
Measure the girth of the tree trunk at 1.3 m above ground with the tape and divide by pi to get the diameter D.
Find the tree height with a clinometer: stand a measured distance away, measure the angle to the top, and use H = distance x tan(angle) + your eye height.
Calculate the trunk volume as 0.5 x pi x (D/2) squared x H.
Look up the typical dry wood density for the species, for example about 600 kg per cubic metre for many hardwoods, and multiply to find the trunk’s dry mass.
Add about 30 per cent for branches and 20 per cent of that for roots if you wish, and write the assumptions.
Multiply the dry mass by 0.5 to get the carbon stored and by 44/12 to get the CO2 removed from the air.
Measure two smaller trees of different girth and plot the estimated carbon against girth to show how it grows quickly with size.
Compare with a published estimate that a mature tree may absorb about 20 kg of CO2 per year, and calculate the years needed to reach your tree’s total.
What you should see
A table of girth, height, volume, mass and carbon for each tree, with the large tree storing hundreds of kilograms of carbon, equal to well over a tonne of CO2, and a graph that rises steeply with girth.
Viva questions
A teacher or a judge will ask how it works. Try to answer each question before you open it, then compare.
1Why is carbon about half of the dry mass of wood?
Wood is mostly cellulose and lignin, which are made of carbon, hydrogen and oxygen with carbon around half.
2How do you convert carbon mass to CO2 mass?
Multiply by 44/12, the ratio of the masses of CO2 and carbon.
3Why is the estimate rough?
Tree shapes, wood density and branch and root mass differ from the assumed values.
4Why is planting trees not a quick solution to climate change?
Trees take many years to store large amounts of carbon.
Take it further
Estimate the total carbon in all the trees of the school campus with a tree census and a formula.
Change one thing, measure what happens, and add a table or chart of your readings: that is what turns a model into your own investigation.
Free ebooks for this project
Science projects
Your Science Exhibition Guide
Choose a project, build it, present it and answer the judges with confidence
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Investigatory Projects for Class 11 and 12
Topics, method and report structure for physics, chemistry and biology