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Why can heartwood support a living tree even though its cells are dead?

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Tall giant sequoia trees with reddish bark and thick trunks rise closely together in a dense forest at Sequoia National Park, California. — Tree A stand of giant sequoia trees (*Sequoiadendron giganteum*) in Sequoia National Park, California.

Heartwood is able to support a living tree because it is composed of dead, mechanically strong tissue that does not require metabolic energy to maintain.12

As a tree grows, its inner layers of xylem eventually cease to transport water and nutrients, transitioning from sapwood into heartwood. During this process, the cells are infiltrated by various chemical compounds—such as oils, gums, resins, and tannins—which enhance the wood's resistance to decay.341

The structural integrity of this tissue is primarily provided by lignin, a complex organic polymer that reinforces plant cell walls. By providing a rigid, low-maintenance framework, heartwood allows a tree to grow large and withstand the compressive forces of gravity without the metabolic cost of keeping those inner cells alive. Because this woody tissue is essentially non-living, it persists for the life of the tree, serving as a stable structural core.524