Why Do Trees Twist?

This old question came up again on a recent drive through central Oregon. Western Junipers were everywhere. On one section of route 26 (a lonely one-lane highway) I thought, “OK, let’s settle this once and for all. Do conifers in the northern hemisphere all twist one way?”

I looked to the right. The junipers twisted to the right. I looked to the left. Those trees twisted to the left. I got goosebumps. Was I driving down a line of demarcation? The next mile disproved this. The ones on the right twisted left. The ones on the left twisted right. I breathed a sigh of relief. Chaos. So comforting.

Juniper twisting to the right

About twenty years ago I read several competing ideas about why trees twist. On coming home from my road trip I wanted to see if anything new had been added to the conversation. The following are a few tidbits on the current thinking about trees twisting.

One contributor from University of Alaska, Geophysics Institute writes:

“…the matter can be related to the Coriolis effect of the earth’s rotation. In the northern hemisphere, all moving objects are diverted ever so slightly to the right. Nielsen thinks that possibly when a tree is rocked by winds, the tip might tend to rotate in a counterclockwise circle when viewed from above. This would lead to a clockwise spiral twist.

“Foliage tends to be thicker on the south side of the tree because of better sunlight. Prevailing winds, in most of the tree-growing northern hemisphere, are from the west. Combine these factors, and the westerly wind pushing on the thicker south side of the tree, year after year, causes an asymmetrical wind loading which slowly twists the tree around in the observed direction.”

Rocky Mountain juniper with twists

I liked the comments of Christopher Earle from the Gymnosperm Database because of his admission that twisting is observed in both directions, and his general hesitation in making strong conclusions:

“Kubler [notes] 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. 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.

“Skatter and Kucera 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.”

Steve Varland and I collected this 2,520 degree (7 twists) Ponderosa Pine

For the semantically concerned there is yet one last qualifier: 

“Barry Donnellan, a Fairbanks attorney, observes that the term “spiral” is not correct in the sense that we’ve been using it here. The preferred use of spiral, he points out, is the description of a plane curve like a neatly coiled garden hose lying flat on the driveway. If you raise one end of the coiled hose, you would have the shape that we’re talking about, which is a helix. But, as he says, who ever speaks of a “helical” staircase?”

Ten years from now I’m sure to be driving down another juniper-riddled landscape and wonder the same niggling question, “What’s new in the spiraling—or as we now stand corrected, ‘helical’—tree debate?”