Why Do Trees Form Spiral Grain?
Spiral grain is the helical form taken by xylem tissues in their growth along a tree trunk or limb. Spiral grain is often conspicuous in snags that have lost their bark, as shown in the photo at left, and people love to speculate about it. You will hear, for instance, that it is caused by the Coriolis force or that trees always spiral one direction in the northern hemisphere and the other direction in the southern hemisphere.
The phenomenon and causes of spiral grain have received considerable study, perhaps because it affects the commercial value of wood. Kubler (1991) provides an extensive (though somewhat dated) bibliography. He noted that spirals are commonly observed in both directions (left-handed and right-handed), and that the direction of spiral can reverse several times during a tree's life. I have seen that attested in a decaying alpine log of Engelmann spruce. Kubler also noted that trees can spiral for many different reasons. For instance in the case of a tree growing out of the rock, the portion of the root system that has access to water and nutrients may be on one side of the tree, while the most productive part of the crown is elsewhere, and the tracheids follow a spiral path in order to convey the nutrients and water to the part of the crown where they are needed. Lateral translocation between tracheids being greater in Pinaceae than in Cupressaceae, this would suggest that spiraling is more pronounced in the latter family, and this I believe to be the case, seen for instance in side-by-side comparisons of pinyon and juniper (e.g., Pinus monophylla and Juniperus occidentalis). Spiraling can also occur (and this is probably more common) in response to stress: there is a helical stress imposed on any tree that is exposed to prevailing wind and has an asymmetrical crown, which is common in trees growing on exposed sites. Gravity can also impose a helical stress on a leaning tree. It has also been noted that spiral grain may make the tree stronger and better able to withstand stresses caused by wind, particularly if the direction of the spiral is periodically reversed. This concept was developed in some detail by Skatter and Kucera (1998) who studied wind effects on trees with asymmetrical crowns and showed that "spiral grain is an optimized growth feature when the trees are exposed to combined bending and torsion." They also assert that most conifers spiral at the same rate (called the grain angle) and show a change from left-handed to right-handed spiraling as they age. They also note that this pattern is observed in the Pinaceae, but is reversed in the Araucariaceae (only reported in two species).
Skatter and Kucera (1998) theorize that trees in general have asymmetrical crowns, with the side facing the sun (the south side in the northern hemisphere) having a larger crown than the side facing away from the sun. They show some data, collected in Norway, to support this, but note that they found no evidence that anyone else had studied systematic patterns of crown asymmetry. They then assert that crown asymmetry combined with prevailing westerly winds produces spiral grain with a predominant right-handed spiral in the northern hemisphere, and a predominant left-handed spiral in the southern.
Personally, I am skeptical. I would be encouraged if someone could show that the direction was reversed in areas with prevailing easterly winds, but there are no data. I note that Kubler found that spiral direction can reverse over time, sometimes several times during a tree's life, which shows that at best there might be a statistical preponderance of one spiral direction or the other. I note that there are virtually no data on spiral grain in southern hemisphere trees, and those are only for the Araucariaceae, which has significant wood anatomy differences from the Pinaceae studied by Skatter and Kucera (see their citations). There are additional problems, and enough knowledge gaps to fuel endless speculation.
Finally, there doesn't seem to be much known about how all this happens: what physiological stresses trigger which growth hormones, for instance, or what causes a reversal in the direction of the spiral. On balance, I still have a sense that the field is data-poor, and it's possible to generate lots of plausible hypotheses.
Kubler, Hans. 1991. Function of spiral grain in trees. Trees 5:125-135.
Skatter, Sondre and Bohumil Kucera. 1998. The cause of the prevalent directions of the spiral grain patterns in conifers. Trees 12:265-273.
Last Modified 2013-09-07