Case Study: Codominant Tops with Bark Inclusion - Western Redcedar
A property owner is concerned because he noticed a crack in his otherwise healthy western redcedar (Thuja plicata).
The subject tree is an established, healthy western redcedar growing on the north face of a steep slope. The diameter at standard height is 35.5 inches, and it is 95 feet tall. Codominant tops originate at 48 feet and have included bark in the union which has started to crack down the main lower stem. The crack extends 16 feet below the union.
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Subject tree viewed from southwest side. (Taylor, 2019.)
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Subject tree viewed from east side. (Taylor, 2019.)
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Given the limited evidence of wound wood along the majority of the crack compared to the overall good health of the tree, and bright interior wood, it is likely that the lower portion of the crack has worsened recently and the defect at the union has a probable likelihood of failure over a one year time frame. Using the ISA Tree Risk Assessment process (Dunster, et al., 2017), a Level 2 Tree Risk Assessment gives an overall risk rating of “High” when target location, fall distance, and severity of impact are factored in.
Initial mitigation options include removal, reduction to a habitat snag, or installation of bracing rods and static cables (Dunster, et al., 2017, pg. 143) with biannual monitoring (Smiley and Lilly, 2013, pg. 45). Because the homeowner expressed interest in keeping the tree, the recommended action is to move forward with the support system.
Considerations include available budget for ongoing tree inspections and maintenance, and further damage to the tree if the change in range of motion affected loading (Smiley and Lilly, 2013, pg. 11). Because western redcedar is decay resistant (Sturrock, Braybrooks, and Reece, 2017) the large wound and multiple smaller wounds from installation of the support system are of less concern than it would be for most other species.
Considerations include available budget for ongoing tree inspections and maintenance, and further damage to the tree if the change in range of motion affected loading (Smiley and Lilly, 2013, pg. 11). Because western redcedar is decay resistant (Sturrock, Braybrooks, and Reece, 2017) the large wound and multiple smaller wounds from installation of the support system are of less concern than it would be for most other species.
Diagram from the Best Management Practices booklet for ANSI A300 Part 3 showing approximately where all cabling types should be installed. (Smiley and Lilly, 2013, pg. 12.)
In 2022, the European Arboricultural Council organized the publication of a European Arboricultural Standards, which includes a specification that “Static cabling must be located in the static (lower) part of the crown.” (pg. 8), and “Static systems should be installed in the lower ¼ of the crown (measured from the fork), preferably as close to the junction as possible.” (pg. 12). The use of “must” rather than “should” for static cable placement is stronger language and, while not enforceable in the US where a different standard exists, should be considered as a reasonable justification for deviation from the A300 recommendation due to its recent date of publication and more thorough and descriptive guidance for each distinct type of support system.
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The ANSI A300 Part 3 Standard Practices, which covers supplemental support systems currently in use in the US, were published by the ISA in 2013* and states that “Anchors should be installed at or near a point two-thirds (⅔) of the length/height of the branch or leader to be supported, measured from the junction to be supported.” (pg. 15). It also states that “Support cables should be taut following installation.” (pg. 14). The language in both cases (“should”) allows for interpretation as long as it can be justified, but many municipal codes state that tree work must comply with ANSI A300 standards (ex. Seattle Municipal Code 25.11.100 B-2), and arborists may feel pressure to work to the guidance put forth in the standards due to fear of regulatory action. Since that resource was published, several different opinions and manufacturer documentation suggest that perhaps this specification is not appropriate for both static and dynamic cabling systems, and the installation height for static support systems should be lower to allow more natural movement of the upper canopy (Cobra, 2021, pg. 18; Rinn, during a presentation about a separate topic). Guidance from manufacturers of dynamic support systems typically recommend some amount of slack built into the system (Cobra, 2021, pg. 18), which is also in conflict with the advisory recommendation in the A300, but, again, the language allows for interpretation when “defensible reasons for non-compliance exist.” (Tree Care Industry Association, 2013, pg 23). 
Diagram from the European Arboricultural Standards showing approximately where the static and dynamic zones are to inform cable installation depending on cable type and tree characteristics. (TeST, 2022, pg 12.)
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Ultimately, for the subject tree, determining the correct height for static cable installation to minimize the likelihood of further damage or failure requires a climbing inspection to assess the distance between the two primary codominant stems, inspect for decay or other damage not visible from the ground, and decide if the strength of the tops as they get narrower will be sufficient to maintain the support they are intended to provide to one another.
Preliminary mitigation prescription is to install four ¾ inch machine threaded rods through the low split offset from each other, and one two feet above the union. Additionally, install 1 or more ⅜ inch EHS (extra-high-strength) steel cables at approximately 8 to 12 feet above the defective union, pending climbing inspection to determine if any additional smaller upright stems originate in the area may also require support to account for the change in motion.
Preliminary mitigation prescription is to install four ¾ inch machine threaded rods through the low split offset from each other, and one two feet above the union. Additionally, install 1 or more ⅜ inch EHS (extra-high-strength) steel cables at approximately 8 to 12 feet above the defective union, pending climbing inspection to determine if any additional smaller upright stems originate in the area may also require support to account for the change in motion.
View from client's deck during an air show - they want to keep this tree even
though their view could have been improved by removing it.
though their view could have been improved by removing it.
*The newly updated 2023 version of the consolidated ANSI A300 is available for purchase, but is so recently published that my copy hasn’t arrived yet! I hope the new standards cover these questions about cabling, but at the time this assignment was written this information was unavailable.
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This project was built as an assignment for the Tree Risk Management module within the MSc Arboriculture and Urban Forestry course at Myerscough College. All photos and content are my own unless otherwise attributed or cited. All names, addresses, and dates are made up, but accurately reflect season and site conditions at the time of assessment.
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