Sapwood n. : The alburnum, or part of the wood of any exogenous tree next to the bark, being that portion of the tree through which the sap flows most freely; -- distinguished from heartwood.

Example:
What can cause a tree to start decaying and having holes in the Sapwood inside it in its eigth year of life?

Taxonomy
wood, tree, latewood, knots, heartwood, sapwood, rings, growth rings, earlywood, vessels, species, softwood, hardwoods, annual rings, timber, oak, hickory, proportion.

Defination from the web:
sapwood, while in others the change comes slowly. Thin sapwood is characteristic of such species as chestnut, black locust, mulberry, osage-orange, and sassafras, while in maple, ash, hickory, ...