What in the world caused this?
Early in my career, I followed an older colleague through a dark forest. He’d been telling me highly suspect stories of Sasquatch sightings in that very forest, when suddenly his voice quieted. “We’re almost there,” he whispered. We slipped into a light-filled gap. Looming above us was a white pine tree, long dead, its bark long gone. “Look at the trunk,” he whispered. I saw my first argyle tree.
Since then, I’ve seen plenty of trees with argyle patterns on their trunks—white pines and ponderosa pines, sometimes in person but usually in photos that get forwarded to me, all of which come with the question, “What in the world caused this?” along with some awkward suggestions. One such suggestion: “Sasquatch using nunchucks.”
I don’t think so.
As a professor of forest biology in Oregon, I’ve been able to ask a good number of other forest researchers and wood scientists, “What in the world?” We have a best guess, but (of course, waving our arms) caution that we’d know a lot more if we actually made some systematic observations, dissections, and mechanical tests. (I have a bunch of suggestions, if you’re interested!) But lacking those, here’s what we think: The cross-hatches help lower stresses when the trees twist in the wind.
What made us think that?
We came to that conclusion after making a lot of simplifying assumptions. If a tree were a column made of something like cheese that has the same strength in all direction, and you twisted that cheese column, the highest stresses would be at +45° and -45° to the direction of the column—exactly where we see these X’s. The extra cheese material in the X’s would provide more material in a local area to bear the stress, and so the stress at any one point in the cheese would be lower.
Even though there are many ways in which a tree trunk is not like cheese, there is evidence in other cases having to do with tree support that trees develop a shape that will make the stresses more-or-less even within the trunk it is “loaded” (under stress). For example, trees rock in the wind, causing a local stress concentration at the tree’s base—and in response, trees are wider at their bases. That butt swell (the actual technical term!) decreases the stress concentration by giving more wood for the stress to be spread over. People have used finite-element modeling in many situations to predict the stem shape that will confer uniform stresses throughout (such as where a trunk and branch meet, forked trunks, and trunks growing around fixed objects like rocks). The modeled shapes are usually remarkably close to what trees make.
What assumptions did we make that might be wrong?
Trees don’t only alter the amount of wood they put in different places as they grow, but they also alter the mechanical properties of that wood—so they could, for example, make stronger wood in some places and more flexible wood in other places. If so, then their growth could result in uniform stresses using a combination of material properties and shape. We made this best guess on shape alone.
And wood doesn’t have the same properties in every direction the way cheese probably does. Wood is much stronger along its main axis (up and down) than across it. In that way, a tree trunk is more similar to a bundle of uncooked spaghetti noodles than cheese. If you yank the noodles lengthwise, they don’t stretch appreciably. If you grab the two sides of the bundle and yank outward, the bundle falls apart. But even so, if you twist the column of noodles, you will cause stresses on the surface at +45° and -45°.
And wood fibers don’t necessarily run straight up and down the tree; they may run in spirals. In a tree with such “spiral grain,” the tree is strongest when pulled along the same angle as the fibers, not straight up and down. But argyle trees do not have spiral grain: their fibers run up and down, as can be seen from the vertical cracks in this dried tree.
And the wood fibers can vary significantly in their mechanical factors from place to place in the tree. The variation occurs through different amounts of cell wall (which alters wood density) or through changes in the orientation of the strength elements within the walls of the wood cells. These wood cells are about the size of an eyelash and hollow, and have several layers in their walls. Each layer has internal reinforcements (strands of super-strong cellulose) at a characteristic angle, like steel-belted radial tires. If the reinforcements in the thickest layer are at a flat angle, the wood is weaker when it’s pulled up and down than if the reinforcements run more vertically. But even so, these potential differences in material properties are unlikely to have much effect on where the primary stresses are on the trunk’s surface when it twists.
On top of all of the differing wood properties in different places, trees aren’t untapered columns and their cross-section isn’t necessarily circular. And they have branches.
But even in the face of these potential wrong assumptions, a column-like structure that is twisted will have its highest principle stresses on the outside of the trunk at + 45° and –45°: exactly where the X’s are situated.
Why are there X’s rather than just ribs?
Twisted stems untwist, too. When a tree rotates in one direction, it develops high principle stresses at +45°. When it untwists, the trunk will have high principle stresses at -45°.
These X’s are only on one side of the tree-trunk though, and in some trees, the X’s do trail off into ribs that are longer in one direction than the other. I presume the assymetrical lengths of the ribs in the X’s are related to higher average forces in one direction than the other.
Why would a tree twist?
A tree would be vulnerable to twisting if it grew in a windy site, and a) had larger branches on one side of the trunk than the other, or b) if its environment had more wind on one side of the canopy than the other. Either way, the wind’s force on branches on one side of the tree is greater than on the opposite side, and the trunk twists.
Why are the X’s in those particular spots on the trunk?
In trees that have fainter X’s, the X’s are centered where a branch used to be. The branches would have been the levers that the wind pushed on, and so the high stress concentrations—and the center of these X-s—would be at these branch junctions. I would guess that all the X’s originated at the base of branches.
Why do only a few trees in a stand have argyle stems?
Like people, individual trees have different genotypes. The trees that make argyles apparently have a propensity to react to the wind stress by developing argyles. Other individuals in the stand may have different reactions—perhaps active ones, such as having shorter branches or not growing so tall, and perhaps passive ones, such as breaking more often.
Are argyle trees associated with habitat?
Yes. Argyle trees seem to develop in places that are prone to high winds, such as in open stands on hillsides and near ridges. In Oregon, you can see them from Highway 20 near Hoodoo where the B&B Fire Complex burned 90,000 acres in 2003. The bark has fallen from these dead trees in the wind-exposed pass area, so there are a lot of trunks to look at. (Be sure to look at sites where ponderosa pine is coming back, not other species.)
How about tree size?
I’ve only seen the argyle pattern in fairly large-diameter trees (at least a couple feet in diameter near the base). I expect the development will not happen until the tree has been growing long enough for its stem of, say, 6” diameter to reach into the windier zone of the atmosphere.
So that’s our best guess, but only the trees know, deep down, what causes argyle stems. I’m arm-waving. We need branch-waving instead. Until we get that, I’d love to hear your ideas and observations!
Some work examining the uniform stress hypothesis:
- Morgan J, Cannell MGR. 1994. Shape of tree stems—a re-examination of the uniform stress hypothesis. Tree Physiology 14:49-62.
- Mattheck C. 1995. Biomechanical optimum in wood stems. In Plant Stems: Physiology and Functional Morphology (ed. B. L. Gartner). Springer, pages 75-90.
- Dean TJ, Roberst S, Gilmore D, Maguire DA, Long JN, O’Hara, KL, Seymour RS. 2002. An evaluation of the uniform stress hypothesis based on stem geometry in selected North American conifers. Trees 16:449-568.
Thanks for discussion—especially to Dean DeBell for showing me my first argyle tree, to Bob Leichti, who coined the term “argyle stem,” to John Nairn who was patient with all my ideas, and to Everett Hansen for tolerating slow trips when I kept jumping out of the car or off the trail to take pictures.
I went out and checked the trees around here and none of them are wearing argyle socks! That was an interesting article.
Thanks, Margaret! I guess you’d know argyle if you saw it. Maybe some Fair Isle at your place?
This phenomenon is new to me. Could it be that these are scars? Perhaps the tree is not merely adding reinforcing tissue, as it is It might brace a lateral branch, but thickening as a result of repairing damaged cells, as a broken bone thickens when mended. I’d think “X”s would only form when the wind blew from alternate directions, unless it blew continuously in the same direction for months, then ceased. This inquiry ties in with my wondering whether twisted trunks, which would lack stiffness, might, at some point in their lives, reverse their twist so that an inner helix would be ensconced in an outer helix running in the opposite direction. If that were possible, it would produce a structure that was super stiff.
HI, Chip. There are lots of things wrapped up (haha) in your comments. To start with the end, yes, most trees actually have “interlocked grain,” in which the prevalent grain angle differs among the different growth rings. It’s been studied systematically in one specis, and this does make a structure that’s super-stiff, as people know if they run across wood in the log pile where this interlocked grain is very prominent.
As for the Xs only forming when wind blows from opposite directions, trees in strong wind don’t simply conform to that push and stay there (with the exception of something quite limber like coconut palms). They thrash around. Folks have put strain gauges on them to ask things like whether the branches dampen or exacerbate teh forces (looking at resonant frequencies), although I don’t know of work looking at torsion. Te twisted trees will revert to vertical grain when the wind isn’t blowing, and will also pass through vertical grain as the thrash.
My brother Rog asked the same thing. As for scarring, this is really interesting about plants. Only about 5% of the sapwood (the outer inch or so) is living cells, but even they have rigid “shells” already. They are aligned in vertical and radial (spoke-like) stripes a few cells across (as epithelial cells to longititudinal and radial resin canals, and as rays), and probably those cells have the capacity to make new growth if there’s an injury, like if a branch breaks. But they can’t do intrusive growth, and so if the wood just cracks, they don’t produce new material (called callus cells). These argyle trees have a continuous sheath of growth on the surface, like what a candle gets when its dipped in wax; all the cells are made at the same time. You can see that from the anatomy at a macroscopic scale.
Trees do have cracks that form in stems, in many situations. Sometimes there are “shakes” in which there are breakages between growth rings, but those usually happen in the heartwood where nothing can grow. We’ve seen cracks along the rays in the sapwood, that become real problems if people turn the wood into veneer (which is why we looked at it). People didn’t end up figuring out why it was in the plantations it was in, although they had in common that they were in high nutrient environments, and there seemed to be a strange shape to the colored heartwood (it was often star-shaped, with uneven divisions between the heartwood and sapwood. But what’s interesting is that the cells broke along the rays, but the ray cells (alive) did not differentiate and make any new growth.
The lateral bracking is dicey to imagine. We’ve talked this through a lot on this end. Trees have no means of developing strength in the “tangential” (hoop direction, the direction a belt would go in if it were a person). The cambium, where wood cells are made, has progenitor cells that are axial, but none that go around the circumference. On top of that, we don’t know of a way that cells can strengthen the bonds with adjacent cells to their sides (they’re held together with an amorphous goo of high-molecular weight lignin). So this idea of these X’s being in the zones of the principle stresses seems liek the most likely idea.
We toyed with other hypothesis but they didn’t seem likely. (One was that the ridges act like pleats, and flatten when the tree twists, which woudl decrease the strain, so the wood would not reach the critical strain. No, for several reasons. Another is that the higher diameter of the ridges increases the 2nd moment of area, but its role in strengthing a column in torsion would be unclear.)
Trees often form ridges under branches (that look like wrinkles). They haven’t been studied biomechanically, I don’t think, but tree biologist are pretty certain they have a biomechanical role.
Thanks for your comments! It would be interesting to do some real testing, such as bring some trees with this patterning into the lab and do torsion tests, and compare them to trees without the patterning in torsion tests. (I don’t have funding, but we do have enormous Instrons in my buildings. One would have to build a chuck for the torsion tests though!) Etc…… 🙂 Again, thanks for your ideas!
Dear Barbara, Thank you so much for the detailed explanation of conjecture concerning argyling. After reading it, my best guess is that the cambium layer cracks in one direction as a gust torques it, then in the other direction when it snaps back. The healed cracks form small ridges. Ensuing growth amplifies these ridges. Twist a sapling in each direction until it’s bark cracks, then examine it after a couple of years.
On another topic, most pertinent to fruit trees, but possibly creating a new category of forest product: Sap should be the perfect wound dressing for trees, preventing the wood from drying and mold and insect intrusion. I would think it could be harvested somewhat as maple syrup is. One might suppose that sap from the same species would be preferable, but it may be that sap from other species would be easier to obtain and might have some qualities – antibacterial or structural – lacking in the native sap. Pine sap, for instance, might be more easily gathered. I gathered some resin balls from our apricot tree, dissolved them in water and applied the paste to a wound. When dry, it resembles varnish. When wet, it is a gel. It doesn’t trap gas and bubble like a commercial dressing. What do you think?
Could argyling trees be twisted by other trees toppling next to them, twisting them in one direction to snap back, or to be hit by neighbors falling on the other side and twisting them in the other direction?
That’s a good question. I don’t think so, because that would be a one-time stress. Usually trees grow swellings (like at the base of the tree, the top end of roots, and the undersides of branches) gradually, starting at some point when they tree is smaller, and increasing as they get older. An imperfect example is that one heavy snowfall won’t cause trees to grow a more conservative form to handle the weight of snow, but if they grow in a snow zone (and have a lot of snowfalls each year), they’ll grow in a way that accommodates (anticipates) the snow load. Thanks for that idea though.