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How does older sapwood turn into heartwood?
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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.
As a tree grows and its diameter increases, the older, inner layers of sapwood cease to participate in the transport and storage of water and nutrients. During this transition, these inner portions are infiltrated by various chemical compounds, including oils, gums, resins, and tannins.12
As these cells die, the sapwood is officially converted into heartwood. This process often results in the heartwood becoming darker in color than the surrounding sapwood, although this color change does not occur in all species. Once formed, heartwood serves as a mechanically strong, decay-resistant core that typically persists for the life of the tree, providing structural support and potentially acting as a water reservoir for the remaining sapwood.231