onal 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.

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. Since each succeeding ring is laid down
on the outside of the wood previously formed, it follows that
unless a tree materially increases its production of wood from
year to year, the rings must necessarily become thinner. As a
tree reaches maturity its crown becomes more open and the annual
wood production is lessened, thereby reducing still more the
width of the growth rings. In the case of forest-grown trees so
much depends upon the competition of the trees in their struggle
for li