UPDATE: I was recently contacted by a professor of mechanical engineering who has studied this phenomenon (from Schulgasser and Witztum, 2007). I wish to thank the authors for their comments and expertise regarding spiral grain mechanisms.
“Spiral grain in trees is formed during the process of cell division and maturation within the vascular cambium (sensu lato). Thus it is concluded that neither the slant of pseudotransverse divisions nor other ‘‘isolated events’’ (imperfect periclinal division, biased intrusive growth) are causative, but that they rather result from the fact that there is a radial gradient of the inclination angle (in the tangential plane) of fusiform cells, i.e. from the general tendency of a maturing cell to take on a preferred inclination with respect to the cell which immediately preceded it in its file. It is shown that the development of these stresses and the formation of spiral grain are just two aspects of the same process occurring throughout the cambial zone during cell maturation.”
From: Schulgasser, K. and A. Witztum. 2007. The mechanisms of spiral grain formation in trees. Wood Sci Technol 41:133-156.
Previous Response
Greetings Walter:
Well, spiral grain/bark patterns are somewhat of a mystery. I've seen these patterns in both hardwoods and conifers. From a traditional wood products standpoint, I'm well aware that this condition leads to poor quality lumber.
Now, I've tried to do a little research on the subject and I was able to come across one article that caught my attention. One hypothesis is that prevailing winds and asymmetrical crown positions lead to unusual torque. To compensate for the torque, a tree will sometimes develop this spiral grain pattern to endure these mechanical torque loads imposed upon it by prevailing winds.
There are a lot of other hypotheses out there and I've yet to come across "the" definitive answer to this long-standing question. Just one of those mysteries that we may have to live with for the time being.
