Wing M.R., Knowles A., Melbostad S. and Jones A.K. (2014) “Spiral grain in bristlecone pines (Pinus longaeva) exhibits no correlation with environmental factors.” Trees – Structure and Function. Vol. 28, No. 2, pp. 487-491. http://link.springer.com/article/10.1007/s00468-013-0965-y
Bristlecone pines are the oldest trees in the world. Some approach 5000 years old! California’s Ancient Bristlecone Pine Forest is just a few miles away from the University of California’s Barcroft Research Station, where Drake High has its cold frame project. We always pay several visits to the bristlecone pines whenever we go there.
While walking among the bristlecones, you are sure to notice that some trees have twisted trunks. They add to the beauty and other-worldliness of the scene. Many trees have straight trunks, though. Of the trees that twist, many are left-handed, but a few grow the other way: right-handed, like a corkscrew.
Nobody seems to understand spiral grain in bristlecone pines, or in any other tree species for that matter. (Spiral grain is not limited to bristlecones.) There are a number of hypotheses that get repeated:
- It’s “just genetics” – spiral grain confers no particular advantage to the tree, and is not caused by environmental factors. Sort of like handedness in humans. Then spiral grain should not correlate with environmental factors.
- Spiral grain makes the trunk stronger and more resistant to breakage. If that were so, we would expect to find proportionallly more dead and/or broken tree trunks that are straight, and more living/unbroken trees that are twisted.
- Just as sunflowers track the sun’s daily progress across the sky, so do trees. This causes spiral grain. The twisting should be left-handed. Trees with views of the western and eastern horizons blocked by ridges should exhibit less spiral grain. Southern hemispehre trees should spiral the opposite way.
- Torque from prevailing winds: Since there is more foliage on the sunny (south) side of the tree, the prevailing (westerly) winds push on that foliage and twist the tree. The spiral grain should be right handed. Sheltered trees should exhibit less spiral grain.
- Spiral grain helps distribute water from the roots evenly around the crown of the tree. Then maybe trees in assymetrical environments (on steep slopes or near streams) should exhibit more spiral grain?
- Coriolis effect: We do not believe that the coriolis effect could cause spiral grain in tree trunks, but if it did the twisting would be left handed, and southern hemispehere trees would be the opposite.
- Spiral grain relieves growth stresses during cell division. Then wouldn’t all the of the trees exhibit spiral grain? Why are most of them straight?
Surprisingly, all of these hypotheses can be tested without doing genetics. We just needed to survey a large sample of trees and use statistics. In the spring of 2010 Drake Students began a spiral grain survey of bristlecone pines. By the end of 2011 we had done over 600 trees.
Grove (Patriarch vs. Schulman)
Name of trail within grove
Tree species (bristlecone vs. limber)
alive vs. dead
trunk diameter at chest height
trunk twisting: right handed, left handed, neither?
Angle of horizon to east
Angle of horizon to west
handheld GPS receiver
clipboard and data tables
clinometer to measure angle of eastern & western horizon
corkscrew (to help discriminate between right-handed and left-handed spiral grain – – it’s hard to think straight at 11,300 feet!)