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In their simplest forms, tree dynamics is the way a tree moves and responds to forces both natural and artificial, and risk assessment is evaluating a given tree for the hazard it presents to civilians and tree care professionals.

Tree Dynamics and Risk Assessment

By Michael “House” Tain



An example of a structural failure in the scaffold section during an ice storm.
Photo by Michael “House” Tain

The subjects of tree dynamics and risk assessment are vitally important for every tree care professional. Industry professionals do not have to look very hard to find a multitude of classes, audio-visual options, and books explaining the ins and outs of tree risk assessment; nor do they have to look far to see the results of risks underestimated, tree dynamics misunderstood, or hazards even unrecognized as the bodies and injuries pile up monthly throughout the nation. In their simplest forms, tree dynamics is the way a tree moves and responds to forces both natural and artificial, and risk assessment is evaluating a given tree for the hazard it presents to civilians and tree care professionals. At times, both of these topics have been misunderstood or misinterpreted. In the case of risk assessment, the focus was often on civilians rather than the workers involved. Certainly the risk presented by a tree to the public needs to continue to be part and parcel of a risk assessment, but it is equally important that the folks who work upon it, and the forces they create, be considered as well. Although there is no substitute for hands on field education, the basic principles and knowledge introduced here will help tree crews better understand not only what to look for, but why it is so important.


Tree dynamics

As mentioned, tree dynamics is the way the tree moves and responds to those forces that affect it. This may be due to natural forces such as being exposed to prevailing winds and developing stronger wood on one side of the trunk or the other in response, supporting/stabilizing roots, or even a particular canopy shape. An observant tree care worker will notice that a healthy tree uses its entire structure to absorb and dissipate the force of the wind, with all of its parts moving regardless whether they are directly affected by the force. This same process holds true when artificial forces are at work on the tree, a large branch removal will be reflected by movement throughout the canopy. Conversely, an absence of movement in a branch or a particular part of the canopy can be evidence of a disconnect from the tree’s natural energy absorption system, such as decay, a hollow, or even a crack. The individual tree’s system or dynamic should be taken into account prior to starting work, as it will provide excellent indicators on possible problem areas, and even dictate how the work should be carried out.


Visual inspection

The most effective, safe and efficient assessments of a tree are those that start with the first tree professional who looks at the tree — the salesperson, bidder or company owner. This way, the work can be written up in such a way that the crew is not only prepared as far as needed gear and equipment, but also primed for possible problem areas. Regardless of whether the tree was assessed for risk prior to the job being bid, it most definitely should be prior to any work being done by the crew. All crew members should take part as much as possible, as different eyes will see different things. The visual inspection should go all the way around the tree and check both the inner and outer perimeter. The outer usually being out beyond the drip line and the inner nearer to the trunk. Both vantage points will allow different possible problems to be seen; and care should be taken to examine the tree carefully for any red flags both aloft and on the ground.


Red flags

Red flags don’t necessarily mean to cease work and start packing up the truck, but they do mean the crew has to look a little bit closer to make sure the flags don’t present a problem. Just as there are a lot of different trees, there are a lot of red flag indicators. Those that definitely need to be checked out more closely include:

Horizontal or vertical cracks in any part(s) of the major structure of the tree.
Mushrooms or fruiting bodies in the root zone or in the structure of the tree.
Signs of co-dominant leads or included bark at major branch attachments.
Signs of, or verbal evidence of, lightning strikes.
Insect infestation signs or symptoms, which could include both those harmful to the tree and to the climber (wasps, bees, etc.).
Hollows, cavities, or other evidence of decay in the tree’s structure.
The presence of cavity-nesting animals or birds.
Evidence of recent construction, disturbance, trenching, grade changes, etc., in the tree’s root zone.


Pull testing

Pulling on a tree is not only an excellent way to see how well it tolerates the movements of climbing and rigging, but also gives a quick and dirty example of tree dynamics. The testing is best done with a throwline located pretty high in the canopy to avoid the temptation to overload the as-yet-unknown strength of the tree. Pulling in horizontal and vertical directions allows for observance of the tree’s response; and doing it in all four directions around the tree will often reveal additional information. Gentle pulls that get the canopy swaying can often reveal areas where cracks open up, or even areas with no movement at all, indicating some sort of disconnect through decay or defect. In addition, a crew member keeping an eye on the root zone will be able to see any lifting or cracking that might take place in that area.


Roots and static relevant zone

Mushrooms and fruiting bodies were mentioned as red flags; and should certainly be examined more closely if they are found in the root zone of the tree as they may indicate a lack of stability. But the most important area for evaluation of fruiting bodies or during the pull testing is what is termed the static relevant zone. This area has been shown to be vital to tree stability with regard to roots; and is an area roughly equal to 150 percent of the tree’s diameter out and around the root flare. For example, a tree with a dbh of 24 inches would have a static relevant zone of 36 inches and this is the place where mushrooms or cracking during pull testing would be of most concern.


Tree sections for evaluation

The Integrated Risk Assessment Guide was developed and designed by Dwayne Neustaeter of Arboriculture Canada Training and Education, and is used by both that organization and North American Training Solutions, along with a variety of public and private tree care organizations/companies throughout the United States and Canada. The way the tree to be evaluated is divided into sections may differ in some ways from those with which some tree care professionals are familiar. A description of the sections follows:

The scaffolds are those leaders and branches that make up the structure of the tree’s canopy.
The trunk is the continuation of the trunk of the tree beyond its first branches.
The pedestal is the lowest portion of the tree’s trunk from the root zone extending up to its first branches.
The roots are the root zone along with the vitally important static relevant zone.

Each of these sections of the tree, as can be seen on the form, should be evaluated visually, with additional inspection as needed, as well as examination during pull testing, if required. An additional part of the form takes into account the type of work that will be done on the tree, thereby accounting for the forces that different tree care operations will exert on it. Tree care professionals should feel free to integrate this form, or whatever sections of it they feel applicable, into their existing tree risk assessment procedures, but should be aware that additional practice and training is necessary to fully understand and employ it effectively.


This is a brief introduction to the concepts of tree dynamics and risk assessment. But with the basic knowledge and principles discussed here, tree crews should have a better idea of not only what to look for, but how and why to look for it. The end result will hopefully be fewer surprises from unnoticed hazards or ignored tree dynamics, and safer, more efficient tree work for all.


Michael “House” Tain is a contract climber, splicer, educator and writer associated with North American Training Solutions www.northamericantrainingsolutions.com  and Arbor Canada Training and Education www.arborcanada.com He is currently located in Lancaster, Ky., and can be reached via e-mail at house@houseoftain.com

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