How do trees turn CO2 into wood?

In photosynthesis, leaves use sunlight to combine CO2 from the air with water, making sugars and releasing oxygen. The tree uses some sugar for energy and breathes part of that carbon back out (respiration). What remains builds new wood, bark, leaves and roots.

So the carbon a tree stores is its net growth: what it captures minus what it releases. A healthy, growing tree adds carbon every year. A stressed, sick or dying tree may add little or start losing it.

How much of a tree is carbon?

The 2006 IPCC Guidelines for National Greenhouse Gas Inventories (Volume 4, Chapter 4, Table 4.3) give a default carbon fraction of 0.47: about 47% of dry tree biomass is carbon. For tropical and subtropical forests the range is 0.44 to 0.49.

To turn carbon into CO2, multiply by 44/12 (about 3.67), the ratio of the molecular weights of CO2 and carbon.

Illustrative conversion (not a per-tree figure): 1 tonne of oven-dry wood × 0.47 = 0.47 tonne of carbon × 3.67 = about 1.72 tonnes of CO2 removed from the air to build it.