Why is the heartwood of the tree is harder?

Heartwood is harder than sapwood primarily because it contains a higher concentration of lignin, which provides structural support and rigidity. Additionally, heartwood consists of older, dead cells that have been filled with chemical compounds, such as tannins, which contribute to its density and durability. This makes heartwood more resistant to decay and pests, enhancing its overall strength and longevity compared to the softer, younger sapwood.