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Hazard tree evaluation or tree risk assessment is a subject that all tree care professionals are familiar with to one degree or another, even though they may not be aware of their familiarity and knowledge. Many organizations offer classes, seminars and even certifications in the subject. But, at its most basic level, tree risk assessment is simply determining the safety and stability of a given tree.

Work-related tree risk assessment

By Michael Tain



Evidence of a severe lightning strike causing serious internal and external structural trunk damage.
Photo by Matt Logan

Hazard tree evaluation or tree risk assessment is a subject that all tree care professionals are familiar with to one degree or another, even though they may not be aware of their familiarity and knowledge. Many organizations offer classes, seminars and even certifications in the subject. But, at its most basic level, tree risk assessment is simply determining the safety and stability of a given tree.

Traditionally, the focus has been on the safety and stability of a tree, or trees, as it relates to a threat to life and property, typically of the homeowner in a residential setting, a business in the commercial setting, or the public at large in a municipal/governmental setting. This type of evaluation is obviously of great importance; yet the missing component in this traditional definition is the threat the tree may or may not represent to the tree care personnel who are assigned to remove or abate the hazard it represents to life and property. The changing dynamics and different forces the tree experiences during removal or pruning may present a much graver danger to the tree crew working on it, than it ever represented to property or the public. However, this can be rectified by the addition of a few simple steps in the pre-work planning process, and some knowledge of what the trees are trying to tell arborists about their own safety and security.


Visual assessments

Ideally, work-related tree risk assessment should start with the first person who looks at the tree, whether that is the salesperson, consultant or business owner. This will allow that individual to pass on vitally important information to the crew carrying out the work, while also enabling them to ensure that the needed gear and equipment is on hand (thus increasing both safety and efficiency). For example, a salesperson who does not perform some measure of work-related tree risk assessment may send a climbing crew out to remove a tree that is patently unsafe to climb, leaving the crew to make the difficult choice of going back for an aerial lift or “pushing on through” with the job by climbing. One option severely inhibits efficiency, while the other risks the crew’s wellbeing, obviously neither is acceptable. The first step of any tree risk assessment is a visual assessment; and should include both the inner and outer perimeter of the tree. Typically the outer perimeter will be evaluated first by proceeding completely around the tree beyond its drip line, visually inspecting it for defects, hazards and red-flag indicators that may or may not be evident when under the canopy. Additionally, this is also an excellent time to evaluate the ground and soil around the tree for signs of recent disturbances, excavation or construction that may have threatened its stability by impacting the roots. A survey of the inner perimeter of the tree is the next step, done within the drip line of the tree beneath the canopy, examining it for structural defects, red-flag indicators, and hazards that may not have been visible during the outer perimeter inspection. Additional examination of the ground and soil for disturbances or excavation should also be part of this step.


Red-flag indicators

The presence of red-flag indicators during the visual assessment of the tree may or may not indicate that the tree is unsafe on which to work. Rather, they give the arborist clues that the tree needs to be examined more closely, and that steps may need to be taken or the work plan changed to deal with the threat they represent. There are a wide variety of possible red flag indicators, but those that require further evaluation and investigation include the following:

The presence of fruiting bodies/mushrooms either in the root zone, pedestal, trunk or canopy.
The existence of either horizontal or vertical cracks.
Evidence of structural defects such as co-dominant leaders or included bark.
The possibility or signs of lightning damage.
The signs or symptoms of a particular insect infestation that may or may not affect the strength of the tree’s wood.
Hollows, cavities or other evidence of decay in the tree’s structure.
The presence of cavity-nesting animals or birds.


As stated previously, the simple existence of any or all of these red-flag indicators does not preclude proceeding with work on the tree. However, all should be further examined and evaluated for their severity, and the work plan possibly modified to account for their presence.


Pull test


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

A simple pull test is an excellent way to further evaluate the safety and security of a tree with red-flag indicators. The tree is loaded in a fairly horizontal vector or fashion through the use of a throwline placed in the upper canopy. Although a rope can certainly be used for pull testing, care should be taken to not “over” stress the tree, causing it to fail from one of the red flag indicators during testing. Gentle pulls are applied to the throwline in all four quadrants (north, south, east and west), and the tree observed for its reactions to this movement. Often this simple testing will reveal additional problem areas such as vertical or horizontal cracks. It can also present the evaluator with evidence of internal problems through the presence of excessive movement, or the lack of smooth movement down a particular section of the trunk, leader or branch. A lack of smooth movement beyond a particular point could be evidence of an internal defect or decay that is not allowing the force to continue past that point. While pull testing, it is important that another crew member evaluates the root zone area of the tree, and, in particular, the static relevant zone.


Roots and static relevant zone

The presence of a red-flag indicator, such as the fruiting bodies of fungal organisms, in the root zone of the tree is obviously a cause for concern and must be evaluated fully prior to work proceeding, as root decay or damage may critically inhibit the stability of the tree during work activities. But the tree’s reactions during pull testing in the static relevant zone are also of utmost importance. The static relevant zone is an area around the root flare of the tree equal to 150 percent of the tree’s diameter. For example, a tree with a dbh of 24 inches would have a static relevant zone of 36 inches. This area should be watched closely during pull testing for signs of cracking, heaving or other disturbances, which would all be indicators of a lack of stability.


Tree sections for evaluation

The form Integrated Risk Assessment Guide, available by clicking here, was developed and designed by Dwayne Neustaeter of Arboriculture Canada Training and Education. It 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 is as 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 shown on the form, should be evaluated visually, with additional inspection as needed, as well as examination during pull testing, if required. 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.


Work risk

Examination of the tree for work risk is the area in which a work-related tree risk assessment differs from a traditional hazard evaluation. Different tree care activities expose the tree to different forces and dynamics. These differing forces need to be taken into account during the work-related tree risk assessment to make sure that the tree is safe to work on; and, if not, allow the tree crew to refine their work plan. As shown on the form, different activities have varying effects on the tree, resulting in lesser or greater risk. A simple example is that pruning of smaller branches in a tree is going to generate much less force, and thus risk, than the rigging/removal of the whole tree.


This introduction to work-related tree risk assessment, though merely scratching the surface of a complex and multi-layered subject, should provide a starting point for climbing arborists to begin the vitally important process of not only evaluating trees for their hazards to property and others, but also the hazards the tree might represent to tree care professionals themselves.


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.comHe is currently located in Lancaster, Ky., and can be reached via e-mail at house@houseoftain.com

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