Tree Trunks Tell A Story
By CSU, Golden Plains Horticulture Associate, Linda Langelo
Do you notice the texture on the trunks of your trees? Most of the time we may not even notice the trunk's texture. If the texture has changed direction, this might indicate a stress! There are four basic stresses for a tree:
- Compression: is a squeezing action.
- Tension: is stretching or pulling action.
- Shear: occurs when components slide relative to one another.
- Torsion: is a type of shear stress caused by a twisting force.
According to the International Society of Arborists (ISA) an article titled, "Tree Risk Assessment: Loads and Growth Response," by Smiley, Matheny and Lilly: "When gravity acting on a branch pulls or bends it downward, the bending moment creates tension in the top of the branch (fibers are stretched), while the bottom of the branch is under compression (fibers are pushed together). In the middle of the branch, the "neutral plane" experience shear stress, where the fibers in tension and compression meet and try to slide in opposite directions. Bending, therefore, involves at least three stresses: compression, tension, and shear, and may also have a torsional component."
The picture of a tree trunk below quickly captured my attention:
Catalpa Tree- Photo Credit-L. Langelo
Interesting trunk, right? As you walk around the tree, the trunk twists in a spiral motion. This is an example of torsion of a tree's trunk. At one point in the history of this catalpa tree, a violent storm or high winds had enough force to twist the fibers of the tree. Therefore, it looks like there are lines that curve up and around the tree. Wind, particularly high winds are dynamic forces on trees. Now that this torsion has occurred this tree has become a hazard. This is a structural defect and it is a matter of time before the next environmental occurrence will take the tree down.
There is science behind how a tree responds to environmental forces. For example, think about the individual branches and then factor in the leaves. In other words, aerodynamic drag -- leaves in the wind. The ISA has formulas designed to measure these forces accurately. Data is collected to measure these forces to predict the tree's response. There is a great deal of math involved with tree sway motion. Trees can be pushed to lean because of wind speed and aerodynamic drag.
In this example, which shows a leaning tree, we could say that this tree is a hazard. The structural roots may have a harder time keeping this tree in the ground due to some of the science we know that has created this situation.
However, there is more science:
- Statis load, which is a load that has no movement. As trees grow larger, they become static.
- Dynamic load such as wind.
- Mass of trunk, branches and leaves.
- Spring -- wood Young's Modulus-the measure of stiffness of an elastic material.
- Damping has three components:
- Aerodynamic drag -leaves in the wind
- Viscoelastic damping -stem/root/earth
- Mass damping -limb sway interaction
This is the simplest way of describing how hazardous a tree can be. There is more to a leaning tree or a simple crack in the trunk. There is a history behind the crack, it may record the time of a violent storm or tornado or dramatic change in temperatures. Here is a picture of what is called a shear force when tree fibers are sliding relative/opposite directions to one another:
There are times when Mother Nature is not responsible for tree hazards. Here is an example below:
This is another type of compression from the staking material used to hold the tree in place against violent winds. This has constricted the tree's fibers severely. This threatens the health of the tree and further weakens the strength of the trunk.
In regards to staking, when doing so for long periods such as several seasons, this weakens the strength of the trunk. I had a client who staked Kentucky Coffee Trees with a two inch caliber trunk for over 3 years. By the time I was called to visit with the client, he became aware of his misunderstanding. These trees were being grown for sale. When the stakes were removed, the trees were more weeping than upright. A few were able to rebound, but most still had some weeping/bending in the trunk.
On a personal note, I do not stake trees. At the knee of the root ball I place a huge rock(s) around the root ball to allow the tree to sway until the roots are established. So far I have not had a problem. But again, that is my personal choice and experience. If you want more information on how to properly plant and stake trees, please click on this link:
But I digress......
Every trunk tells a story. There is much to the story of each tree. Every tree has to stand up to a lot in its lifetime. Let's give them the best chance we can.