Why do they twist?
By Andrew Smith

There is nothing more enchanting to me than a twisted old juniper tree, it's silvered wood weathered with age. Such a tree evokes the windscarred mountain peaks, with horizons on blue horizons stretching away into the pale arc of sky. And such a tree evokes a hundred thousand starry nights, where coyotes gibbered their insanely lonesome songs and no one heard except the trees.

Sometimes I am lucky enough to collect one of these twisted patriarches of the mountains, and put it into a pot. They make a fine bonsai, and always remind one of some wild, lonely spot, lovely beyond all remembering.

When people see my old trees, with their corkscrew stems and beautiful spirals of dead wood running up the trunk, they almost always end up asking what made the trees grow so twisted. Was it the rotation of the Earth? The wind? The gravitational effect of the moon? An electromagnetic field, similar to what makes a compass needle point north? Why do they grow that way?

I first started thinking about this question several years ago, when a friend happened to remark that every dead snag we saw in the woods was twisted in the same direction; that is, the grain spiralled up the trunk to the right, no matter what angle you viewed the tree from. He suspected that this was due to the rotation of the Earth, the same way that water, because of the Earth's rotation, always spirals down a drain the same direction.

For years afterwards, I could not look at a dead tree without involuntarily checking to see which way the grain was spiralled. Since I make my living doing timber surveys, this meant that I had the opportunity to check countless thousands of dead trees. The results were always the same; the trees spiralled to the right. Actually, I have found a handful of trees, three or four at most, that spiralled to the left. But more on that later.

I asked silviculturalists and researchers why trees spiralled, and particularly why they spiralled in one direction only, but to no avail. No one knew the answer, and no one knew where to get the answer.
Then, a couple years ago, I was lucky enough to run across a research paper by Hans Kubler from the U. of Wisconsin entitled: Function of spiral grain in trees. The rest of this article will give some of the results of Kubler's research.

To get to the point right away, Kubler found that spiral growth conferred two main survival advantages for trees on harsh sites. First, spiral growth allowed water from each individual root to reach around to nearly every branch on the tree. And second, branches with spiral growth bent more easily, which allowed them to be more effective at dumping heavy snowfalls, and tended to prevent breakage in high winds. Both of these traits would be important for trees growing in harsh mountaintop conditions.

No cause for the direction of spiralling was given, but Kubler did note that almost all conifers start life with a left spiral, that is, the grain spirals upwards and to the left as you look at the tree. Then, after they become 10-15 years of age, almost all conifers switch to a right spiral for the remainder of their lives. As yet, this trait remains unexplained. And some trees switch back and forth throughout their lives.

As for survival advantage, Kubler found that spiral grain was definitely adaptive for dry, rocky sites. This has proved true in my experience; the rockier the site, the more twisted the trees are likely to be. And very seldom have I ever seen a tree with a noticeable spiral grain growing in good bottom land. In theory, the branches on a perfectly straight grained tree are fed only by the roots that are directly below them. Water from the root system follows the grain of the stem wood up the tree with minimal lateral movement. So, in a straight grained tree, if all the roots on the south side of the tree were cut, or did not receive moisture, the branches on that side of the tree would eventually die.

On a tree with spiral grain however, each root feeds nearly the whole tree, so if all the roots on one side of the tree die the foliage should survive unharmed. The reason for this is because the xylem, the stem wood that carries moisture from the roots to the crown, will spiral less far around the stem as the tree grows and stem diameter increases. Each new year of growth will be slightly offset from previous years growth, with the end result that the flow from one root will be distributed nearly completely around the tree bole, rather than just in a narrow band spiralling around the stem. This has been proven by injecting conifers with dye at the base. As conditions get harsher, the grain will tend to spiral at a more extreme angle around the stem.

And the system works in reverse too. Tree nutrients descend from the foliage in a spiral path to feed the whole root system, rather than just a single root. This return system is not quite as efficient as the root-to-foliage system is, since nutrients are transported only in a very thin layer of living cells called the phloem. Since the phloem is never more than one or two years growth thick, it lacks the depth to distribute its flow as widely as the xylem does. However, this is not a serious problem since tree roots can live for months or longer without food, while the foliage can generally only live a few days without water.

Old trees with spiral grain frequently have a beautiful corkscrew pattern of dead wood running up the stem. In extreme cases the majority of the stem is dead weathered wood, and only a thin strip of bark spiralling around the trunk is keeping the tree alive. I am not sure what causes this, but I would tend to believe that it probably originates from some stress in the foliage, with the resultant death of a narrow band of phloem cells down the stem, rather than from some stress in the roots.

Kubler also found that spiral grain actually made trees structurally weaker, but at the same time allowed them to bend more under wind and snow, and thus avoid breakage. So while a tree with a pronounced spiral grain will not make nearly as good a grade of lumber as its straight grained counterpart, it will have a definite survival advantage when it comes to shedding heavy loads of snow, or surviving a mountain windstorm.

Kubler found that genetics, age, and exposure to wind and dry conditions were the main determinants of spiral grain. Some trees seem genetically predetermined to show spiral grain no matter where they grow. In most however, spiral grain is a sign of harsh conditions; of fierce winds, unpredictable precipitation and great age.

So now, when you see that twisted old pine or juniper in a pot, you will know that this tree is a long term survivor of all nature has to offer. I have found that since spiral grain generally indicates decades or even centuries of poor growing conditions and very slow growth, it is one of the most accurate indicators of very old age in a tree, at least in the species I am familiar with. As such, it lends all the charm and charisma of bona fide antiquity to the trees it graces.