Here's why:

* Heartwood is the older, inner wood that no longer transports water and nutrients. Its cells are filled with resins and other substances that make it strong and resistant to decay.

* Sapwood, the outer layer of wood, is the living part of the tree. This is where water and nutrients travel up from the roots to the leaves.

* As a tree ages, the sapwood gradually becomes heartwood. The cells in the heartwood die, but they still provide structural support and protect the living sapwood from disease and insects.

So, while the core of a tree is technically dead, it plays a vital role in the tree's overall health and longevity.