What is the dense inner core of a tree trunk called?

Heartwood is the dense, strong inner core of a tree, composed of dead or “retired” sapwood that no longer plays a role in the tree’s nutrient transport system. Unlike sapwood, heartwood is highly resistant to decay due to its reduced moisture content and the presence of natural compounds that make it less appealing to fungi and insects. As long as the tree’s outer layers—such as the bark and sapwood—remain intact, the heartwood continues to retain its strength and durability. In some tree species, heartwood contains naturally occurring chemicals, like tannins or oils, that enhance its durability, providing a natural resistance to decay and insect attack. However, the degree of this natural durability varies significantly between species; for example, cedar and teak have heartwood that is more resistant to decay, while other species may have less durable heartwood.

Source: Charltons Gates