I've been told that it's the ratio between early and late wood that makes the difference.
In ring-porous woods (hickory, ash, locust, osage, oak, ...) early wood (Spring wood) is the early very porous wood the tree makes to let juices flow again rapidly from roots to leaves, allowing bud burst and leaf expansion. Buds are nothing but miniature leaves and shoots, ready to expand. It just requires pumping them up with sap to allow enlargement of the leave cells. This is what this spring wood is for. This also means that spring wood takes a nearly fixed or at least minimum proportion of wood (relative to the volume of the crown of the tree), but the downside is that it has low density (consisting mostly of water-bearing channels), and is therefore rather weak.
Latewood (summer wood) is wood being made after leaves have expanded and photosynthesis is at its peak. This wood is much denser and therefore stronger. The thicker this summer wood growth, the stronger the wood. The thickness is determined by how fast the tree can grow, which depends on availability of light, water (if there's shortage) and nutrients. So I suppose you get the strongest wood on rich, well-drained soils with plenty of light.
Still, if you have plenty of light on all sides, the wood will bud and grow on all sides, and you don't get knotless straight staves. So in order to have straight staves, you want competition for light on the sides (thereby avoiding lateral bud burst), or you have to prune side branches regularly in order to coerce the tree to grow tall, straight and without low bifurcations.
In conifers, however, early wood has a different function. Early wood is also less strong, but it is less clear what determines how much early wood is laid down compared to stronger late wood. There, the strongest wood comes from slow growing trees with very dense growth rings, which also yields denser wood.
In diffuse-porous hardwoods, the situation is rather unclear. There it seems medium growth rates (whatever that may be) yields the strongest woods.
But in the end, it's average density of a log that will dictate strength. I have seen some very fine-ringed black locust of higher density that broad-ringed specimens. So how to pimp density through growth conditions, I don't know...
Joachim