Factors & Forces: Identification of Structural Defects in Trees for Implementing Safe Work Plans
Climbing arborists and tree workers rely on the structural strength and stability of a tree’s branches, trunk and roots to support their weight and the forces associated with rigging operations. Failure of tree parts can result in uncontrolled falling of wood, resulting in property damage or injury to people. Much worse, failure of tree parts can result in uncontrolled falling of climbing arborists and tree workers. Therefore, detection of structurally weak or unstable parts of a tree is important before the climber ascends and throughout work operations. (Photo 1)

Photo 1: Detection of structurally weak or unstable parts of a tree is important before and throughout work operations. A Davey Tree residential and commercial crew work a crane removal in Chicago, Illinois. All photos courtesy of Davey Tree.
The Davey Tree Expert Company has developed a tree inspection method to be used by tree climbers, ground personnel, sales arborists and utility arborists. The objective of the program is for tree-industry professionals to systematically inspect trees and identify structural weakness before a climber ascends the tree and throughout work operations, and to use that information to implement safe work plans.
How?
How do we implement a system that can improve safe working practices without being overcomplicated and time consuming? By being practical and systematic. An effective tree-inspection protocol must be simple enough to commit to memory and able to be implemented within a reasonable amount of time. At the same time, it must be systematic, so each arborist follows the same step-by-step process of evaluating tree risk and everyone on the team contributes to developing a safe work plan.

Photo 2: When wind and decay converge.
Tree failure is most often caused by a structural defect and/or loss of wood strength. These defects can be present in a tree for years, or even decades, and yet the tree continues to survive. It is not uncommon to find trees with extensive internal decay that exists with the weakening wood for long periods. But what actually causes the tree to fail at any given time? The actual event of tree failure is frequently triggered by an external force that exceeds the strength of the existing defect. A load is applied, such as wind, ice or weight – including that from a climber or rigging – that triggers the failure, frequently at the point of the structural defect. The tree shown in Photo 2 failed when the force of excessive wind and the factor leading to loss of wood strength, namely decay, came together.
Developing a safe work plan
The process of developing a safe work plan begins with identifying factors that lead to strength loss in a tree. Once these defects are discovered, a work plan is developed that avoids or minimizes the forces applied to the defects during work operations.
Factors that lead to strength loss in trees are grouped into five categories:
(1) Branch and root attachment – What features of the branch unions and root collar are apparent that signal relative strength or weakness?
- Branch collar and branch bark ridge.
- Included bark.
- Epicormic branches.
- Branch-to-stem ratio and branch angle.
- Stem-girdling roots.
(2) Decay – Is decay present, and to what extent?
- Decay-detection tools (sounding mallet, etc.).
- Location of decay.
- Types of decay.
- Tree species’ response to decay.
(3) Damage – Is there evidence of damage that affects vigor and/or stability?
- Weather related.
- Mechanical.
- Soil compaction/excavation.
- Chemical.
(4) Response – Is there evidence the tree has responded to forces, damage or decay?
- Flexure wood.
- Compression/tension wood.
- Wound wood.
(5) Site – What site variables have contributed to tree vigor and/or stability?
- Soil/hydrology.
- Wind exposure.
- Construction/excavation.
- Recent change.
A systematic approach
Members of tree crews, as well as sales arborists and utility pre-planners, all are encouraged to inspect each tree with a systematic focus on each of the five categories of factors that lead to strength loss. For tree crews, this process doesn’t end when the work begins. Rather, it is an ongoing process that continues as climbers ascend and forces begin to be applied to the tree. By being vigilant, necessary adjustments to the work plan can be made to account for issues not initially discoverable from the ground.
Similarly, forces that are frequently triggers for tree failure can be grouped into five categories:
- Tree work – Managing the forces associated with climbing, rigging and other aspects of tree work is our primary focus.
- Gravity – The constant pull of gravity influences how trees build themselves. Do we see evidence that the tree has responded with appropriate wood?
- Weather – Wind and precipitation are the most common forces that trigger tree failure. Is there evidence of previous damage?
- Leverage – Force from any source, whether it’s wind, rigging operations or the weight of a climber, is magnified by leverage. How can we minimize leverage?
- Electricity – Direct and indirect contact with energized lines is a dangerous force. In addition, recognizing how previous line-clearance operations have affected the vigor and stability of a tree is important.
It is critical to realize that the weight of the climber and the weight of the wood lowered during rigging operations can be significant and forceful. By recognizing that forces are the trigger of most tree failures, work plans that control and minimize forces are devised that account for the structural weaknesses of a tree.

Photo 3: This is one way to have the list of factors to look for and the forces to manage right at your fingertips.
A list at your fingertips
How can we commit the process to memory? Have you ever written a brief shopping list on your hand so that you don’t forget the items you need? That’s where the idea came from to have the list of factors to look for and the forces to manage right at your fingertips. Conveniently, we have two hands, each with five fingers – one hand to remind us of the factors that lead to strength loss in the wood, and the other to remind us of forces that increase the likelihood of tree failure. We’ve created work gloves that list the five factors and five forces on the fingers. By having the reminders literally at our fingertips, we reinforce the practice of systematically inspecting trees and using the information to manage work-related forces. (Photo 3)
The Factors and Forces gloves are awarded to employees who complete the two-day, on-site training course or complete the online course offered through Davey’s learning-management system. During the training course, employees are introduced to several tools designed to help tree crews develop safe work plans and also to call an “All Stop” when the necessary resources (equipment, personnel, time) are not available. One of those training tools is the Complex-O-Meter. It is a graphic that is designed like an automobile speedometer, but rather than measuring velocity, it is a gauge for the complexity associated with tree work for both the personnel and the tree to be worked on.

Image 1: The Complex-O-Meter guides the arborist through a series of questions that consider the complexity and potential risk of an assignment.
The Complex-O-Meter guides the arborist through a series of questions that consider the complexity and potential risk of an assignment. (Image 1) Such questions include:
(1) How must the work plan take advantage of the strengths of the tree while avoiding the weaknesses?
- Can the work be accomplished from the ground?
- If not, is there functional access for an aerial lift?
- If not, are there appropriate tie-in points?
- If not, is there an adjacent tree with appropriate tie-in points?
- Can tree parts be allowed to free fall?
- If not, are the branches, trunk and roots strong enough to support rigging?
- If not, are there adjacent trees that can support rigging?
- If not, is there functional access for a crane?
(2) Do we have the appropriate resources to accomplish the work plan?
- Do we have the right people for the job?
- Do we have the right tools and equipment for the job?
- Do we have adequate time to safely complete the job?
Tree workers should understand that if, after conducting a tree-risk assessment and devising a safe work plan, it becomes apparent that the appropriate resources are not available to safely accomplish the work plan, they have a right to refuse to do the work until appropriate resources are available. The Complex-O-Meter heightens awareness of the crew as their work plan increases in complexity. This awareness reminds the crews that a greater level of skill, more experience, additional tools and perhaps more time are required as the level of work-plan complexity increases.
Another tool employed during the training course is the Branch Union Game (otherwise known as BUG). It is a deck of playing cards organized in a series of sections, including:
- Tree species.
- Trunk diameter.
- Tree condition.
- Site.
- Site condition.
- Job objective.
- Resources.
Teams of participants randomly select a card from each section, consider all of the variables and develop a work plan to safely achieve the objective. For example, a hand of cards may be a 36-inch cottonwood tree with basal decay that must be removed, located in a schoolyard with no functional access for a bucket truck. Some scenarios lend themselves to a work plan requiring low complexity. Others include variables that collectively require resources that are not available and lead the crew to a conclusion of an “All Stop.”
The use of games makes the learning engaging, and, with potentially thousands of combinations of variables in the game, it approximates the conditions that tree crews face on a daily basis. The objective is to encourage a systematic evaluation of the tree, the assignment, the resources that are available and the goal of developing a safe work plan, if possible. If a safe work plan is not possible, the objective is to reinforce with employees the right and responsibility of calling an “All Stop.” (Images 2-7)

Images 2-7: The Branch Union Game (BUG) has employees consider factors and variables that can lead to an “All Stop.”
Once all of the variables have been considered in developing a safe work plan, the final step in the Branch Union Game is to draw a Wild Card that inserts an unexpected wrench into the works. Wild Card variables include “Bucket truck out of service,” “Climber out sick,” “Ice on pavement,” “Windy” and other conditions that complicate the completion of the task. This is one more way the card game replicates situations faced in real-life conditions.
The objective
The objective of any safety-training program is to reduce the frequency and severity of incidents that may cause injury, and to empower all employees to STOP if they don’t feel comfortable completing an assigned task.
The Factors and Forces training course assists arborists in conducting a systematic tree-risk assessment, and knowing how to use that information to develop work plans that reduce forces applied to trees during work operations. In the Factors and Forces two-day training course, the tree-risk assessment information is supplemented with instructional tools. Each participant receives the special work gloves, a sounding mallet and chaining pin for detecting decay and the course manual, including the Complex-O-Meter.
The Branch Union Game cards are made available to participants to take back to their operations for use as a training tool for all employees involved in tree work.
Tree care professionals will work more safely as a result of recognizing the factors that lead to strength loss in trees and managing the forces associated with tree work. (Photo 4)

Photo 4: Training employees to recognize the factors that lead to strength loss in trees and managing the forces associated with tree work is aimed at everyone going home safe at the end of the day.
Robert “R.J.” Laverne, Ph.D., is manager of education and training with The Davey Tree Expert Company, an accredited, 52-year TCIA member company based in Kent, Ohio.
Alex Julius (she/her) is the employee-development and safety-training specialist at The Davey Tree Expert Company. She is a Board Certified Master Arborist (BCMA) and Certified Tree Worker Climber Specialist, and is Tree Risk Assessment Qualified. She is the co-author of “Tree Climbers’ Guide (4th edition),” and resides in Urbana, Illinois.
Tim Bushnell, CTSP and ISA Certified Arborist, is an arborist-skills specialist at The Davey Tree Expert Company. He chaired the ANSI A300 Committee and participated as a voting member in current and previous ANSI Z133 revisions (2006 and 2012). Tim is a past head technician for ISA’s ITCC. Most recently, he received the TCIA Pat Felix Volunteer of the Year Award and ISA’s Millard F. Blair Award for Exceptional Contribution to Practical Arboriculture.
This article is based on a presentation they made on the same topic at TCI EXPO ’23 in St. Louis, Missouri. To view a prerecorded video made for that presentation, go to tcimag.tcia.org and, under the Resources tab, click Videos.