Where does a giant tree's wood actually come from?
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After you watchWhere does a giant tree's wood actually come from?
The short answer
The carbon in wood comes from atmospheric carbon dioxide. Dry wood also contains much oxygen and hydrogen; water supplies building material, while soil supplies essential minerals. Wood is not almost entirely carbon.
Try this next
- What changes if carbon dioxide is removed after the first comparison? Switch CO2 off and replay the model season. Compare both masses again; water and light remain available.
Now you — bend it
- What if In the leaf practice, lower only the light slider while carbon dioxide and water remain high.Compare the sugar-making readout. These practice sliders do not change the later season’s fixed mass illustration.
- What if After the first season, switch carbon dioxide off and replay.Compare both plant and soil masses. Switching the setting starts a new season; it does not change a season halfway through.
Can you prove it?Plant gain need not equal soil loss. — The model plant grows 1.0→22.0 units while soil falls 50.0→49.4. CO2 and water provide material from outside the soil; these are illustrative units.
Design your own test:Predict which input limits the model sugar-making rate, then move only that slider and check the readout.
Explain it to a 6-year-old: A tree uses carbon dioxide from air and water to build wood. Soil supplies small but important minerals.
The whole story
How it works
The carbon in wood comes from atmospheric carbon dioxide. Dry wood also contains much oxygen and hydrogen; water supplies building material, while soil supplies essential minerals. Wood is not almost entirely carbon.
What people get wrong
Many people think a tree's wood is made of soil it sucks up through its roots, so a growing tree must slowly eat away the dirt around it. In fact the soil barely changes weight as a tree grows, because most of the new wood is carbon caught from the air. The roots supply water and a few minerals, not the material the trunk is actually built from.
The catch
Building wood from the air's carbon is incredibly powerful, but it only works while there is light, because sunlight is the energy that snaps carbon and water into sugar, so in the dark the building stops. The roots and soil are still essential, but their job is mostly delivering water and a tiny amount of minerals, not providing the bulk of the tree's mass.
Questions kids ask
If wood comes from the air, why does a tree still need soil and roots?
The carbon in wood comes from atmospheric carbon dioxide. Dry wood also contains much oxygen and hydrogen; water supplies building material, while soil supplies essential minerals. Wood is not almost entirely carbon.
How can material from a gas become solid wood?
Carbon dioxide has mass even though we cannot see it. Plants use light energy to build carbon-containing molecules from carbon dioxide and water, then make materials such as cellulose.
Does the soil supply the whole mass of a growing tree?
No. Roots take up water and minerals, but much of the material in new wood comes from carbon dioxide and water. The experiment compares plant gain with soil loss rather than treating soil as the whole source.
Where does the oxygen we breathe fit in?
When a leaf snaps carbon dioxide and water into sugar using sunlight, it has a leftover: oxygen. The tree breathes that oxygen out into the air. So the same process that builds wood from carbon also makes the oxygen that animals and people breathe.
Talk about it
- Guess first: if we weighed the dirt in a flowerpot before and after a plant grew big in it, would the dirt be a lot lighter, a little lighter, or about the same?
- Where do you think most of a tree's heavy wood comes from — the ground, the water, or the air? What makes you say that?
- If a tree could talk, how would it explain making tons of wood out of something you can't even see?
For grown-ups
Photosynthesis fixes atmospheric carbon dioxide: 6CO₂ + 6H₂O + light energy → C₆H₁₂O₆ + 6O₂. The carbon skeleton of cellulose and lignin, which make up the bulk of dry wood, comes from that fixed atmospheric carbon, not from soil. Roots supply water and trace mineral nutrients such as nitrogen, phosphorus and potassium, which are essential but a tiny fraction of dry mass. Van Helmont's willow experiment in the 1640s first hinted at this when a tree gained about 74 kg over five years while the soil lost only a few grams.
Keep going
What else makes you wonder?
- If a tree breathes in carbon all its life, what happens to all that carbon when the tree dies or burns?
- Plants in the dark can't build wood — so how does a sprout in a dark drawer keep growing at first?
- If trees pull carbon out of the air, could planting more of them change how much carbon gas the whole sky holds?
