A tree is built mostly from air

A large tree weighs tonnes, yet it doesn't leave a hole in the ground. Most of its mass came in through the leaves, as carbon dioxide.

Ask where a tree's wood comes from and most people point down, at the soil. It is a sensible guess. It is also mostly wrong.

One of the first people to test it was Jan Baptist van Helmont, a chemist and physician in what is now Belgium, in the first half of the 1600s. He grew a willow in a measured amount of soil, gave it nothing but water for five years, and then weighed everything again. The tree had gained about 74 kilograms. The soil had lost only about 57 grams.

Van Helmont concluded that the tree was made of water. He was partly right: water is one of the ingredients. But he had missed another one that he could not see or weigh in a pot.

That ingredient is carbon dioxide, and it took more than a century to find. Experiments in the 1700s showed that plants change the air around them, and in 1779 Jan Ingenhousz showed that they need light to do it. By 1796, Jean Senebier had found that green plants take in carbon dioxide and give out oxygen in the light. Soon after, Nicolas-Théodore de Saussure showed that a plant's gain in weight comes from both the carbon dioxide and the water it takes in.

What wood is made of

In photosynthesis, a plant uses the energy of sunlight to turn carbon dioxide and water into sugars, and then builds those sugars into cellulose, lignin and the rest of its body. By weight, wood is roughly 50 percent carbon, 42 percent oxygen and 6 percent hydrogen. Nitrogen and the other elements, such as calcium and potassium, add up to only about 2 percent.

Almost all of that carbon arrived as a gas, through tiny pores in the leaves, from air in which carbon dioxide is only a small fraction of a percent. A forest is, in a very real sense, air that has been gathered up and made solid.

The soil still matters. It holds the water and supplies minerals a tree cannot live without. But the bulk of the tree, the part you can saw, stack and burn, came from the sky. When wood burns, that carbon goes back into the air, and the small pile of ash left behind is mostly the minerals that came up from the ground.