Perhaps the prime reason why human planted and planned forests for timber is so popular?
I try to plan for it by making my cuts close, but not to final size, and then remeasuring and doing the final cut, but it is a pain. For me, the tension is always about how much extra to leave. I think that it also depends on the species of wood, in addition to how the tree grew and how it was milled.
I had no idea of the depth and complexity of milling logs when I first bought a sawmill. Today, I am aware the complexity exists but still have huge gaps in knowledge.
Trees growing on a slope typically will have tension induced from the extra strength needed on one side of the tree. So the pith will probably be off center.
If you mill up some 1x12's or whatever, how you cut it from the cant is going to affect which way its going to move or warp as it dries. Plain saw and get cupping. Plain saw through the pith and watch the board crack right down the center when drying.
Wood shrinks in all dimensions as it dries. So you need to mill a larger board than you need to account for shrinkage. But cut it using quartersaw method and you will alter how much it shrinks in thickness compared to width. And it goes on and on and on.
And then you get to heartwood vs sapwood. If the board you cut have some sapwood along the edges, its going to cause more problems when drying. Sapwood dries faster than heartwood, which induces tension that causes cracking.
You can air dry or kiln dry some species rapidly-- like pine. Try the same speed of drying with oak and watch it crack and check on you. If you exceed the maximum moisture removal rate when drying it will cause problems.
I haven't even touched on the bug calamity that has ruined probably 50+% of wood I milled over the last few years. This is a prime reason to keep a very clean area around the sawmill. Bits and pieces of wood, which are so common as a byproduct of handling the logs, are excellent breeding grounds for bugs. If you don't stay on top of keeping things clean you'll pay the price.
When I first got my sawmill, I expected I'd be the lumber king in short order. Throw the logs on, saw them, and take nice lumber off. But it is so, so much deeper than that. Instead, it has taken years to learn just how much I do not know.
