Where does most of the mass of a growing tree actually come from?

A tree's mass comes primarily from the air through photosynthesis: trees absorb carbon dioxide through tiny leaf pores and, using sunlight's energy, convert that carbon into sugars and eventually into the solid cellulose and lignin making up wood and bark. Only a small portion of its bulk comes from water and soil minerals, which mainly support the chemical processes rather than becoming structural material.

Minerals from soil are essential nutrients but contribute only a tiny fraction of a tree's weight, and water, while vital for growth, is mostly used and released rather than becoming permanent structure.

This means a mature tree is built largely out of thin air, and when wood burns or decomposes, much of that stored carbon is released back into the atmosphere, completing a cycle spanning the tree's lifetime.