Sometimes trees grown in the open may become of considerable size, a foot or more in diameter, before any heartwood begins to form, for example, in second-growth hickory, or field-grown white and loblolly pines.
As a tree increases in age and diameter an inner portion of the sapwood becomes inactive and finally ceases to function.

This inert or dead portion is called heartwood, deriving its name solely from its position and not from any vital importance to the tree, as is shown by the fact that a tree can thrive with its heart completely decayed.

Some, species begin to form heartwood very early in life, while in others the change comes slowly.

Thin sapwood is characteristic of such trees as chestnut, black locust, mulberry, Osage orange, and sassafras, while in maple, ash, gum, hickory, hackberry, beech, and loblolly pine, thick sapwood is the rule.
There is no definite relation between the annual rings of growth and the amount of sapwood.

Within the same species the cross-sectional area of the sapwood is roughly proportional to the size of the crown of the tree.