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Why does sapwood contain more water than heartwood?

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temperate softwoods and hardwoods selected to show variations — wood Temperate softwoods (left column) and hardwoods (right column), selected to highlight natural variations in colour and figure: (A) Douglas fir, (B) sugar pine, (C) redwood, (D) white oak, (E) American sycamore, and (F) black cherry. Each image shows (from left to right) transverse, radial, and tangential surfaces. Click on an individual image for an enlarged view.

Sapwood contains more water than heartwood because it consists of the tree's living, outer layers, which are actively engaged in the transport of water and minerals to the crown. These cells remain functional for conduction, whereas heartwood is composed of older, dead cells that have ceased to participate in these life processes.123

As a tree ages, the inner layers of sapwood are converted into heartwood. During this transition, the cells are infiltrated by various chemical substances—such as oils, gums, resins, and tannins—which fill the space and contribute to the heartwood's darker color and increased resistance to decay. Because these deposits occupy the space where water would otherwise flow, and because the cells themselves are no longer alive or functional for transport, heartwood contains significantly less water than the active, conducting sapwood.134