Runout

[John Hamlett]

Another observation;
I've seen trees with more wind near the ground and less further up. Your straighter splitting billets may be from higher in the tree. If that's the case, the ones with more twist should be bigger billets(?).

[Barry Daniels]

My forester friend talked to a friend of his who is a dendrochronologist (tree ring expert) about grain twist. He said they don't know why it happens but they usually only see it on the outer portion of older trees. They do not think it is related to wind. They see grain twist going either direction with no apparent reason. Sorry if this wasn't much help.

[Mario Proulx]

That does help, Barry. It means that we've all been collectively licking our finger and sticking in the air, guessing at the wind direction(yes, pun intended... <bg>)...!

[David King]

I think twist is just the tree's way of telling us it doesn't want to be part of an instrument.

[Chuck Tweedy]

A luthier, a dendrochronologist and the Pope walk into a bar ...

[David Malicky]

I'm no expert at all on this, but was curious so did some googling. It looks like a number of causes are supported by research, including Mario's and Alan's thoughts on strength and reacting to offset loads. And as Mario said, it appears to be another nature + nurture phenomenon, with some species predisposed to it, but environment important for how much it's expressed.

The traditional standard reference appears to be Harris (1988) "Spiral grain and wave phenomena in wood formation", who found a lot of genetic evidence, but also controversial evidence on environment. It's not on google books, though; I could only find a brief review (2nd paragraph has a snippet of good info): http://forestry.oxfordjournals.org/cont ... .2.extract

The Introduction to this article...
http://www.treeringsociety.org/TRBTRR/T ... _13-24.pdf
has a very nice summary of the research as of the mid 2000s. Some snippets:
"A common view today is that spiral grain growth is initiated by genetic factors..."
"Also, ethylene production in the wood itself is suggested to influence or even initiate spiralgrain growth"
"...spiral grain might strengthen the tree to protect it from breakage when external mechanical loads like bending and torsion are applied"
"Various environmental factors are believed to influence spiral grain growth, including gravity, wind, light and solar movement, aspect, slope, soil, altitude, water distribution, or injuries to the tree..." [Those are a list of hypotheses that have been examined -- the article briefly outlines whether there is evidence for them.]

Some paper abstracts on specific causes (not that these trump the review above)...
To distribute water all around the tree when roots are dry on one side: http://www.springerlink.com/content/t6588m24684238v3/
To resist external torque (theory): http://www.springerlink.com/content/3v2280780613591m/
To resist external torque/bending from wind (data): http://www.springerlink.com/content/a6khrex54dek18ud/
To resist asymmetric wind loading (data): http://www.springerlink.com/content/l74rbfgvr02m7xqt/

So, it sounds like genetics is needed as a starting point, but many things can pull the trigger.