Wood density and strength vary within species to such an extent (even excluding abnormal growth like compression and tension wood), that any worthwhile study on wood properties would need a very large number of specimens per species. I wonder if that was done here? Also, many of the very densest and strongest woods grow only to small dimensions and are therefore rarely found in comparisons like this. Most of the woods I use for my selfbows because of their exceptional strength and elasticity are practically never found in lumberyards or wood dealers. I do prefer self-harvested wood for many other reasons, too (keep the jokes coming!)
