Heartwood

Heartwood is the older, central wood in a tree trunk that has ceased conducting water and nutrients. As sapwood ages and is displaced outward by new growth, its living cells die and the wood often becomes infused with extractive compounds like tannins, resins, and oils that darken its color and increase its resistance to decay. This transformation from sapwood to heartwood is a natural aging process.

Despite being composed of dead cells, heartwood serves a crucial structural function, providing the majority of the trunk's mechanical strength and rigidity. It acts as the backbone of the tree, supporting the massive weight of the canopy and resisting wind forces. The accumulated years of wood growth in the heartwood core give large trees their remarkable strength-to-weight ratio.

When heartwood decays due to fungal infection entering through wounds, the tree can develop internal cavities. While a hollow tree has lost structural support, the remaining cylinder of sapwood and intact outer heartwood can still provide significant strength, much like a hollow pipe is stronger than a solid rod of the same weight. Arborists use tools like resistographs and sonic tomographs to assess the extent of internal decay without invasive investigation.