A search among hardwoods of high specific gravity and hardness to augment the waning supply of dogwood for shuttles was rewarded by finding that open-grown sugar maple trees contained in their short trunks wood of the required weight, hardness, and other essential attributes for shuttles, including the qualities of high shearing strength and smoothness of wear. The open-grown sugar maple trees had increased rapidly in diameter and averaged only 7 growth rings to the inch, compared to 22 for old-growth sugar maple in Pennsylvania. The wide-ringed open-grown trees produced wood averaging 0.77 in specific gravity. Other sugar maple stands averaged 0.68 in specific gravity. A comparable specific gravity value for dogwood is 0.78.

Sugar maple must be hard when it is used for flooring, furniture, and agricultural implements. But for bearing blocks, bowling pins, printers' type, cogs, shoe lasts, and butchers' blocks, it must be still harder. Purchasers of sugar maple lumber frequently specify sapwood because of its uniform light color. Wide sapwood also is expected to yield a wood superior in hardness. Trees in farm woodlands produce wide sapwood when given growing space enough to maintain rapid growth in diameter.

Here's what happens when a nonuniform growth of Southern yellow pine is cut into lumber: A is a cross section of the yellow pine log, showing where planks are cut from it; B shows the planks immediately after being sawed from the log; and C shows what happens because of high longitudinal shrinkage of the edge nearest the pith.

With other hardwoods, including the oaks and yellow poplar, the relationship between specific gravity and growth rate, such as that shown by ash, hickory, and sugar maple, has been found to follow the same general trends with respect to changes in width of growth rings during the development of a tree or stand. This led to a rule based on a record of the historical environment and development of a tree, as follows:

Wood of fairly uniform weight and strength is produced in broad-leaved trees (hardwoods) that have maintained or increased their growth rate from the center outward. A slowing of growth rate at any period in the life of the trees is accompanied by the production of wood of lower specific gravity. These facts make it possible from an examination of the growth rate throughout the life of a tree to determine whether the wood contained in a tree is all of high specific gravity, or whether with later years a declining growth rate produced wood of lower specific gravity and strength.

To improve the growth of cone-bearing trees, or softwoods, such as southern yellow pine and Douglas fir, some modifications of forestry practices are necessary from those required for growing wood of high specific gravity in hardwood species. This applies especially to the growth in diameter in young trees.

High specific gravity and strength in southern yellow pine was found in wood containing a high percentage of summerwood, which is the darker portion of the annual growth ring. The summerwood portion of the growth ring was found to be about twice as strong as the springwood.

Uneven distribution of young trees over an area was one of the prevailing causes of great variation in the specific gravity of wood in second-growth southern yellow pine stands. Young pine trees that had a large amount of growing space had larger, more wide-spreading crowns than trees that grew in closely stocked portions of a stand. Such trees grew in diameter at a rapid rate at first, but since a high proportion of the annual growth ring was springwood the resulting wood was of low specific gravity. Young trees with smaller crowns in more densely stocked portions of the same forest had a slower rate of growth and contained heavier and stronger wood with proportionately much less springwood.