Sapwood

Sapwood, also called alburnum, is the living, outermost wood of a tree trunk and branches. It is the active pipeline of the tree's vascular system, transporting water and dissolved minerals upward from the roots to the leaves through specialized cells called vessels (in hardwoods) or tracheids (in softwoods). It also stores starches and other reserves in its living ray cells.

Sapwood is generally lighter in color than heartwood and more susceptible to decay because it lacks the preservative extractives found in the heartwood. The thickness of the sapwood layer varies by species: some trees like maples may have wide bands of sapwood, while others like black locust convert to heartwood quickly and have narrow sapwood bands.

Protecting sapwood is critical to tree survival because it performs all of the tree's water transport. Girdling, where the bark and sapwood are removed in a complete ring around the trunk, is fatal to a tree because it severs all upward water flow. This is why damage from string trimmers, rodents, or bark-stripping insects that removes bark in a ring around the base can kill an otherwise healthy tree.