In the carbon cycle, it's not just about the individual tree — the entire forest plays a role. Leaves take in carbon dioxide, converting it to sugar, which is carbon-based. Some of the sugar is used immediately for energy, converted back to CO2, and released into the atmosphere. The rest is stored in living wood or dead matter, such as fallen leaves and branches. Old-growth forests, in particular, store vast amounts of carbon while continuing to absorb CO2.

The illustration below, by Mieke Roth, is available as a full-size print-quality PDF file here.