Occasionally one comes across a living tree with a portion of its trunk, or its entire trunk, hollow. How is it possible for a tree to thrive even when its center, or heart, has completely decayed? It comes down to the different kinds of wood that are produced by a tree: sapwood and heartwood.
Sapwood (often light-colored) is the younger, living, outermost portion of a woody branch or tree trunk (just beneath the bark), while heartwood (often dark-colored) is the dead, inner wood. All wood in a tree is first formed as sapwood. Sapwood’s principal functions are to conduct water from the roots to the leaves (via xylem tissue) and to disperse nutrients made by the leaves to the rest of the tree (via phloem tissue). Heartwood (so called because of its central position, not because it is essential to the health of the tree) is basically non-functioning xylem tissue that has become blocked with resins, tannins, and oils. Although the dead heartwood can lend stability to a tree, it is no longer part of the transport system, and therefore, not vital to the tree.
Cavities and hollows typically result from an injury to a tree (usually caused by fire, storms, lightning, insects or birds) that exposes the heartwood. Bacteria and fungi lose no time moving in and beginning the decaying process, which can result in a hollow tree. Because the sapwood, and therefore the transport system, is still intact, the tree lives, despite the loss of its inner heartwood.
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