Structure of wood

The core of a tree consists mostly of starch and it's function is storing nutrients for next yearly growth and branching. In the lower part of the trunk the core is dead.

Sapwood consists of 10-15 of the innermost yearly rings. It's cellural structure is distinct from younger rings. Sapwood always has knots and the wood cracks easily. This is the worst quality wood in the trunk.

Heartwood is dead wood that serves as a supporting "beam" for the living surface wood. In many tree species the heartwood is darker than surface wood and it's usually the most valuable part of the trunk. Heartwood formation occurs spontaneously.

Sapwood is living wood, in which water and nutrients move from the roots to the treetop. In light species of tree, such as spruce and birch the border between heartwood and sapwood is barely visible.

Bark consists of many different sections