Sapwood is the living, outer portion of a woody stem or branch, while heartwood is the dead, inner wood. All wood starts as sapwood, then turns to heartwood as a tree ages.
Sapwood is formed just under the bark by a thin layer of living cells, the cambium. The cambium produces bark cells and wood cells. As trees grow, the width of their trunks increase because new layers of cells are added in the rings of sapwood.
Sapwood is crucial for transporting water and minerals from the roots to all parts of a tree. But as trees age and their trunks grow larger, the innermost part is no longer needed to conduct sap. The aging process causes the cells closest to the trunk's center to die, turning them into heartwood. Although heartwood does not transport water or minerals, it still serves a vital purpose in providing structural support for the tree.