ENTS, Larry, Don,

I have seen the same spiraling pattern in a wide variety of tree species.
None of the explanations presented in the article make much sense to me.
They have not even demonstrated that trees tend to spiral in one direction
more than another. There is no common point of reference to say which
direction the tree is spiraling. If a tree is spiraling clockwise as looking
down at it from above, then looking at it standing the grain would go upward
to the right and down to the left.

The first thing to do would be to note the species of the tree, the
location, and direction of spiral,  Once you had a decent data set, then
some analysis of the information could be made.

If I were to guess, and it is a guess, I would think the spiraling pattern
is a genetic trait and not one developed on the fly in response to average
wind direction ad the direction of the sun.  Certainly it is not related to
the Coriolis effect on something as small as a tree trunk.  My guess as to
why the tree grain spirals is that it is to provide additional flexibility
in response to wind stress,  I think the twisted grain would be stronger in
response to a wind than would a straight grained tree.  The tree tends to
twist in response to winds rather than snap.  There is no single direction
of weakness as is formed by the structure of the parallel grains, every direction
is equally strong with the grain spiraling around the trunk.  The spiraling
may not be in every specimen of a tree depending on its own genetic make-up
and could require some triggering stress to develop.

We need some field data and a structural engineer to look at the mechanics
of the spiral grain.

Ed Frank