Where Does the Wood in a Tree Come From?

A tree may weigh several tons, yet it does not acquire most of that material by consuming the soil beneath it. Much of its dry mass began as an invisible gas in the air.

Leaves take in carbon dioxide through microscopic openings called stomata. During photosynthesis, energy from sunlight helps combine carbon dioxide with water. The plant produces sugars and releases oxygen. It then rearranges those sugars into cellulose, lignin, and the many other carbon rich substances that form trunks, branches, roots, and leaves.

The carbon atoms in a wooden table therefore once floated through the atmosphere.

In the seventeenth century, Jan Baptist van Helmont conducted an early experiment with a willow tree. He weighed the soil in a pot, planted the tree, and added only water. After five years, the tree had gained about 74 kilograms while the soil had lost very little. Van Helmont concluded that the added mass came from water. He had demonstrated that it did not come mainly from soil, but he did not yet understand the role of carbon dioxide.

Roots remain essential. They supply water and minerals such as nitrogen, phosphorus, potassium, and magnesium. These nutrients allow growth and chemical activity, but they account for only a small portion of the tree’s dry substance.

A tree feels like the most solid possible product of the ground. Its body, however, is largely sunlight powered carbon assembled from the atmosphere.

Forests make air visible. Year after year, they take a gas too thin for us to see and turn it into rings of wood strong enough to support branches, animals, houses, and ships.