A whole bunch of scientific studies have been conducted on the effects of growth rate on wood strength. Some solid rules of thumb have emerged (most of these center around commercially important species, which excludes most classic bow woods). On ring porous woods (ash, elm and oak among common woods, plus osage, mulberry, black locust, laburnum etc.), ring thickness largely correlates with density. In other words, wood grown fast is denser and stronger than wood grown slowly. The early wood layer of a ring porous wood's yearly growth is weak mush: slow-grown ring porous wood has lots of this and little else, since the strong latewood part of the rings is thin. Very quickly grown wood, with very wide rings may be less dense (and even looks like that) than somewhat more slowly grown wood - there probably are species-specific optimal ring widths. And a high S.G. wood can of course objectively be dense and strong even when thin-ringed (lilac is the densest and strongest wood here, ring-porous, and always grows slowly).
In conifers, the situation is reversed: slow growth and thin rings equal dense, strong wood. Countless studies on dozens of conifer species confirm this. The thinner the rings, the more dense, strong latewood in comparison to earlywood a piece of coniferous wood contains. But ring-count is not the sole answer, since, as others have pointed out, the density increase is not a constant but subject to differences in genetics, growth conditions etc. In yew, for instance, a relatively coarse-ringed specimen from one location may have equal density to another from somewhere else with a higher ring count. And two pieces with equal ring count can have measurably different density, each following their own genetically(?) determined range. But I'd sure like to see J.D.'s 150rings/inch yew that's light and flimsy (unless disease or decay are at play)! Take 1000 yew samples of various growth rates and measure their density: a definite trend emerges. The few exceptions don't change the trend.
In diffuse-porous woods (maple, beech, birch etc.) ring thickness does not correlate with density: they can be dense with either wide or thin rings. Many trees that seem diffuse-porous are in fact semi-diffuse porous; walnut, cherries, rowans and many other trees in the Rose family, for instance. In these the difference between early and late wood is not as pronounced as with ring porous woods, but is still there if one looks closely. Specimens with little porous early growth and thick, dense late growth are dense and strong.
The thing is, trees are too complex beings to fit into simple equations. But the basic rules of strength-growth correlation still apply most of the time, and are a good starting point for evaluating most bowstaves.
Tuukka
