Sapwood is comparatively new wood.  There is a time in the early history of every tree when its wood is all sapwood.  Its principal functions are to conduct water from the roots to the leaves and to store up and give back according to the season the food prepared in the leaves.  The more leaves a tree bears and the more thrifty its growth, the larger the volume of sapwood required, hence trees making rapid growth in the open have thicker sapwood for their size than trees of the same species growing in dense forests.  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.  If the rings are narrow, more of them are required than where they are wide.  As the tree gets larger, the sapwood must necessarily become thinner or increase materially in volume.  Sapwood is thicker in the upper portion of the trunk of a tree than near the base, because the age and the diameter of the upper sections are less.

When a tree is very young it is covered with limbs almost, if not entirely, to the ground, but as it grows older some or all of them will eventually die and be broken off.  Subsequent growth of wood may completely conceal the stubs which, however, will remain as knots.  No matter how smooth and clear a log is on the outside, it is more or less knotty near the middle.  Consequently the sapwood of an old tree, and particularly of a forest-grown tree, will be freer from knots than the heartwood.  Since in most uses of wood, knots are defects that weaken the timber and interfere with its ease of working and other properties, it follows that sapwood, because of its position in the tree, may have certain advantages over heartwood.

It is really remarkable that the inner heartwood of old trees remains as sound as it usually does, since in many cases it is hundreds of years, and in a few instances thousands of years, old.  Every broken limb or root, or deep wound from fire, insects, or falling timber, may afford an entrance for decay, which, once started, may penetrate to all parts of the trunk.  The larvae of many insects bore into the trees and their tunnels remain indefinitely as sources of weakness.  Whatever advantages, however, that sapwood may have in this connection are due solely to its relative age and position.