A client was asking me the other day about a couple of ailing sugar maples in front of her house. The sidewalk next to them had been replaced last year, and this year the 14″ caliper trees were showing signs of stress — smaller leaves than usual, and fewer of them. I told her that it was likely the sidewalk work had damaged (or eliminated) significant root mass.
She pointed out that it was the leaves on the yard side of the trees that looked bad. How could that be? Well, many trees actually grow in a spiral fashion. Often, when you see crown problems on one side of a tree you’ll discover a girdling root gripping the trunk on the tree’s opposite side; conversely, if you find root problems on one side of a tree, you might find that foliage and limbs on the opposite side from the roots are showing signs of distress.
A couple of online resources discuss this phenomenon: in the journal Trees; Structure and Function, Hans Kubler writes about the spiral formation of tree grain as a way for the tree to distribute water and carbohydrates evenly through the tree, especially when one area in the root zone is dry.
Larry Gedney, writing in Alaska Science Forum, presents a couple of ideas from colleagues; one, that the Coriolis effect determines a tree’s spiral tendencies, and another, that prevailing wind loads combine with denser south-side foliage to twist a tree.
Hmm. We may never know — perhaps the spiral twist could be chalked up to some factor so far undiscovered — but Professor Kubler’s hypothesis seems to make sense. Looking at these two photos of the Catalpa tree outside the Lincoln, MA Public Library illustrates the plausibility of his concept: a sidewalk and road run right next to the tree, which would limit its rooting space and its ability to take up water on that side.
Catalpa next to road and sidewalk
How might this spiralling tendency be exaggerated for use in a design? I could see a grove — or even just a trio — of trees like this as a setting for a dance performance.
What a tree reaching behind itself looks like
Deborah Howe
Toby Wolf
I don’t know that Kubler’s hypothesis, that the twisting distributes water and carbohydrates evenly throughout the tree, squares with what you observed with your client’s trees. If Kubler is right, the disruption of the roots would result in a more or less even impact throughout the canopy of the tree, no?
Wouldn’t there be a structural advantage to the twisting? A brick wall is stronger if the courses are staggered to avoid vertical stacking; a cable is stronger if its strands are twisted together. I’d guess that a spiral structure would make a tree’s trunk harder to bend or break.
Aha! Many years ago, RISD students were required to take a course involving nature. And one recurring piece of this was nature’s spiras: conch shells, for instance. So … there’s nothing new under the sun, and this could pertain to trees as well All this spiralling may not necessarily be due to small boys or wind.
Well, actually, in the case of the client’s trees, my thinking is that the amputation of roots on one side of the tree would prevent the dispersal of nutrients and water to the other side of the tree — what growth is possible with the removal of those roots would be due to the the dispersal from the remaining roots, but with a spiral arrangement the vascular system would be disrupted at the tree’s base.
The spiral (or, as the second article points out, helical) structure would make a trunk stronger. I would think that a combination of factors goes into dictating how a specimen of any given genus grows: structural issues, chemical issues, the size of the cells, the density of the lignins, the tree’s situation, the climate it lives in, etc. etc. etc. I remember a conversation I had with an arborist about 10 years ago when a number of American elms on Boston Common were giving up the ghost to Dutch Elm Disease. Their decline was incredibly fast; within two or three weeks, five or six trees had died and been removed from my walk to work. The arborist pointed out that we’d had a wet spring, and elms have relatively large xylem and phloem cells, which grew even larger with all the water available. That size meant that the water flowing through those cells could and did also transport the fungus quite efficiently — moreso than might have happened in a dryer year.
I was driving down the VFW parkway, and one of the Red Oaks in the median caught my eye with a pronounced helical pattern. I kept my eye on the other median trees as I drove on, but didn’t see any others.
The helical tree looked to be the same size as the others, and there weren’t any pronounced differences between its growing environment and theirs. It may have been part of the original planting, or it may have been a replacement tree that went in while the others were still young. I wonder if it’s a genetic trait that varies from one individual to another.