From what I've heard from experienced carpenters and wood millers, growing conditions are a huge factor, as you want slow growing trees that have compact tree-rings. Also if you can get a wood piece with the core wood running through the entire length, you'll have a stronger work piece.
The strength of slow-growth vs fast-growth wood depends on the anatomy of the tree in question. Ring-porous woods (oak, ash) have (as the name suggests) growth rings that have a very distinct ring of pores running the length of the wood.
That means that fast-grown oak or ash is actually stronger than slow-grown. As you look at a cross section of the wood, slow-grown wood has a much higher amount of void space than fast grown.
As I understand it, the southern longleaf pines are the opposite. Because of how their growth rings are formed, they have alternate layers of more and less dense wood. Slow-grown has a higher percentage of the dense, stronger wood. On this point I'm speaking from reading, not firsthand knowledge. The only SYP we get up here in the northeast is pressure treated. PT works totally differently because it's so damned wet and I don't use much of it either way.
