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.
If a tree grows all its life in the open and the conditions of soil and site remain unchanged, it will make its most rapid growth in youth, and gradually decline.

The annual rings of growth are for many years quite wide, but later they become narrower and narrower.