The relationship between width of growth rings and the mechanical properties of wood is discussed under Rate of Growth.

In this connection, however, it may be stated that as a general rule the wood laid on late in the life of a tree is softer, lighter, weaker, and more even-textured than that produced earlier.

It follows that in a large log the sapwood, because of the time in the life of the tree when it was grown, may be inferior in hardness, strength, and toughness to equally sound heartwood from the same log.
After exhaustive tests on a number of different woods the U.S.
Forest Service concludes as follows: "Sapwood, except that from old, overmature trees, is as strong as heartwood, other things being equal, and so far as the mechanical properties go should not be regarded as a defect."[22] Careful inspection of the individual tests made in the investigation fails to reveal any relation between the proportion of sapwood and the breaking strength of timber.
[Footnote 22: Bul.

108: Tests of structural timber, p.

35.] In the study of the hickories the conclusion was: "There is an unfounded prejudice against the heartwood.