Where does most of a tree's mass come from?

The air. Almost everyone assumes a tree is made of soil, and it is very largely made of carbon that arrived as carbon dioxide gas through pores in the leaves - plus water. The soil supplies minerals in small quantities and somewhere to stand.

Jan Baptist van Helmont worked this out around 1640 with an experiment anyone could repeat. He planted a willow in a weighed quantity of dried soil, watered it for five years, and weighed both again: the tree had gained about 74 kilograms and the soil had lost a couple of hundred grams. He concluded, wrongly, that the mass came from water - photosynthesis was two centuries away - but he had already demonstrated that it did not come from the ground.

This is why forests matter to the carbon budget in the way they do. A growing tree is an accumulation of atmospheric carbon held in place, and a burning or rotting one puts it back. It is also why the phrase carbon sequestration is literal rather than metaphorical: the wood in a table is atmosphere that a tree took out and a carpenter has kept out for a while longer.