How tough trees withstand strong Wyoming wind? Simple: thigmomorphogenesis - Casper, WY Oil City News
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How tough trees withstand strong Wyoming wind? Simple: thigmomorphogenesis

Snow wilts trees and power lines on Tuesday, Sept. 8, 2020. (Dan Cepeda, Oil City)

CASPER, Wyo. — The Cowboy State is expected to see damaging winds on Wednesday, with gusts up to 80 mph expected on both I-25 and I-80 in southeastern Wyoming.

The winds may cause damage to trees and power lines and the National Weather Service has cautioned that many areas of the state could see widespread power outages.

While powerful winds pose a risk to trees, how is it that they don’t all topple over when a strong gusts come through? Simple: thigmomorphogenesis.

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“Thigma” is a Greek word meaning “to touch” and the term thigmomorphogenesis refers to the way in which plants are able to alter their growth patterns in response to mechanical sensations, according to Wikipedia.

Amanda Bunce with the University of Connecticut’s Stormwise provided the following definition:

“Thigmomorphogenesis is the mechanism by which plants develop the capacity to handle the force imposed on them by the wind. We call this developing wind-firmness.”

Trees gain strength over time from swaying in the wind, allowing them to better withstand directional forces, according to Purdue University Urban Forestry Specialist Lindsey Purcell’s “Trees and Storms” document that.

“If conditions are consistent, trees will initiate changes in their development to compensate for these loading patterns,” Purcell wrote. “They are biologically engineered to adjust to external loading, under normal conditions. This fiber strength can minimize the impact of external forces, unless they are excessive, such as during a storm.”

Bunce explained that the size and shape of a tree dictates the amplitude and frequency of how it sways.

“Swaying is a tree’s natural method for the dissipation of the energy exerted upon it by the wind, but there exists the dangerous possibility of achieving an amplitude of sway greater than the tree’s elastic capacity to return upright,” she wrote. “That’s when trees fall down. That’s when we have tree/utility interaction problems.”

“We might ask ourselves ‘why does this not happen more often?’ When a big windstorm hits, why don’t more trees succumb to the forces? As it turns out, all plants, trees included, adapt to the patterns of physical stimulation they experience as they grow.”

Other examples of physical stimulation which plants can respond to include things like water or touch from animals.

“As a plant experiences these physical agitations, it develops a capacity to withstand them,” Bunce adds. She noted that the term “thigmomorphogenesis” was coined by Dr. M.J. Jaffe in 1973 to refer to this ability of plants.

“Jaffe, in 1980, showed that rubbing the stems of bean plants produced the same responses (shorter, thicker stems) as exposing them to wind,” Bunce said in giving examples of how thigmomorphogenesis has been demonstrated scientifically.

While trees may be able to adapt to various forces including wind and weight from ice and snow, Purcell notes that “some storms can exert extreme forces, resulting in injuries of varying degrees.”

She describes common types of damage trees can experience from wind exposure:

  • wind throw: “The entire tree is ‘pushed over’ by high winds. The loading forces on the aerial tree portions are too great for the developed root system. Previous harm from activities such as construction damage, lack of maintenance and attacks from pests predisposes the tree to damage with poor anchorage or health.”
  • crown twist: When the crown of a tree has uneven geometry from the spread of leaves and branches, high winds can cause the tree to twist. “Uneven wind loading on the lopsided crown produces a damaging twist on major branches and the stem. This twisting causes torsional stress, much like twisting a rope backwards, resulting in splits and cracking. This is especially damaging around old wounds or other defects, which can lead to failure.”
  • stem failure: “Trees will fail at their weakest point. Typically, failure is caused by the combination of a defect and external forces. Old injury sites or wounds are common on tree trunks, and these damaged areas can lead to tree failure under excessive loads. Also, trees newly exposed to prevailing winds from development or removal can be at greater risk. Trunks can snap or buckle if the tree is unable to withstand the wind forces.”
  • root failure: “Stability and anchorage are important roles of the root system for keeping a tree upright. Both the structural, woody roots and the fine absorbing roots create an effective network of strength to withstand wind-loading forces. As the tree becomes larger, increased stress is put on the roots to maintain stability. Roots that are restricted, diseased or damaged can cause trees to lean and fall.”
  • branch failure: “Tree branches are easy victims of loading forces. Whether it is axial loading from snow or ice accretion or lateral, side loading from wind, damage is frequent. Because branches can sometimes be poorly attached to the main stems, they can easily experience injury. However, this structural arrangement allows the branches to be flexible and disposable, and easily shed in times of stress to help dampen the impact of wind, minimizing damage to the larger, structural branches. Poor architecture and development, such as codominant stems and included bark in branch junctions, predisposes branches to failure and often are the first to experience damage.”

In addition to describing damage which trees can experience, Purcell also provides measures people can take to help prevent damage or what can be done to help remediate such damage.

That information is available online.