As readers of this blog and our work may know, codominant branches are unreliable predictors of tree failure during storms. At the same time, numerous studies have found that codominant branch unions are weaker than those with more favorable aspect ratios or attachment types. This disconnect may reflect the difference between asking what is stronger and what is strong enough. It could also stem from studying unions in isolation under static loading conditions, which may overlook the dynamic and complex interactions between tree crowns and wind during storms. Whatever the cause, this kind of inconsistency in the research is what really intrigues us.

Biomechanical puzzles that may or may not ever be fully resolved aside—why not aim to grow the strongest tree possible by implementing a structural pruning program early on? Research has shown that formative pruning goals can be achieved with minimal cost and cutting during this stage of a tree’s life. Given that, the approach appears to have few downsides and helps avoid more difficult decisions later in a tree’s maturity—when aggressive pruning may result in large wounds (which are a significant predictor of storm-related failure).

With that in mind, we’d like to highlight a recent study on structural pruning published in Arboriculture & Urban Forestry. In this week’s Rooted in Tree Research, we feature another Tree Research Journal Club post, discussing an article that was published about six months ago:

Loyd AL, Smiley ET, Fite KL. Effects of Different Amounts of Codominant Stem Subordination on the Growth and Aesthetics of Acer rubrum ‘October Glory’. Arboriculture & Urban Forestry (AUF). 2024 Nov 1;50(6):414-26.

In this article, the authors examined how trees with codominant stems respond to removal cuts and varying intensities of subordination cuts. The results of this well-written and straightforward study may be especially informative for arborists working with the same or similar species.

What was done?

To test how different levels of pruning affect codominant stems, the researchers performed a single reduction or removal cut on one stem of a pair of codominant branches on 56 medium-aged red maple cultivars (Acer rubrum ‘October Glory’). The cuts varied in intensity, removing approximately 25%, 50%, 75%, or 100% (i.e., full removal) of the stem’s diameter at the union. Eleven trees were left unpruned (0%) as controls.

Over the following three years, the authors monitored changes in aspect ratio, stem diameter growth, sprouting (i.e., sprout count and diameter), percent wound closure, and visual appearance. Visual ratings were provided by three ISA Certified Arborists based on their perception of client expectations. Ratings were assigned as follows: 1 – not acceptable, 2 – moderately acceptable, and 3 – acceptable.

What was discovered?

Pruning dosage reflected a balance between aesthetics and long-term structural improvement. The lower pruning levels (0%, 25%, and 50%) received the highest visual ratings (at or near 3 – Acceptable), but were the least effective at addressing the codominant stem issue.

Branches reduced by 50% to 75% showed a significant decrease in aspect ratio (i.e., were less codominant) compared to non-pruned control branches after one growing season. However, at the 25% reduction level, changes in aspect ratio were not significantly different from the controls during the same timeframe. From the first to the second growing season, only the more drastic 75% reduction cuts continued to show a significant effect.

Pruning also triggered sprout production. In general, sprout production was lower in the 25% reduction treatment. Average sprout caliper did not differ among treatments. Additionally, wound closure was greatest in the 25% reduction group.

What we like about this paper

The authors do an excellent job of producing an accessible article with practical findings. The writing is logical, concise, and includes guidelines for practice. They published in an open-access journal, ensuring their findings are readily available to those who need them most. It’s also the issue’s ISA Certified Arborist Continuing Education article, which we take as a strong signal that the Arboriculture & Urban Forestry’s editorial team sees it as both relevant and potentially valuable to the credentialed arborist community.

As researchers who do this sort of work, we also appreciated the visual rating system used in the study. Ratings were determined by consensus among three arborists with years of professional experience—individuals who have likely encountered situations where pruning both met and fell short of client expectations. This approach was a clever and practical way to assess aesthetic outcomes.

Minor grievances

As mentioned at the start of this post, the current literature contains some conflicting data regarding the strength loss and risk associated with codominant stems. However, you wouldn’t get that impression from this paper. While a fair number of studies have emerged in recent years on this topic (including a paper we recently covered) only one of the studies cited in this article is less than 10 years old.

In fact, of the 13 studies cited, the median and average publication ages were 19 and 21.3 years, respectively. For context, two decades ago there were no iPhones or Facebook—something some of our readers may have a hard time imagining in a world now shaped by smartphones and social media. It was also a time when Andrew (with approximately 95% fewer gray hairs) was studying urban forestry as an undergraduate at the University of Wisconsin–Stevens Point and Alyssa was a junior in high school, serving as president of both the journalism and drama clubs. Our personal understanding and the way we talk about codominant stems has shifted significantly since we first encountered these older works. It would have been encouraging to see more recent research represented, even if the authors referenced newer studies merely to disagree with their conclusions.

Conclusions

For the species assessed, reduction pruning cuts in which the cut was 50% to 75% of the branch diameter at its base provided the best control of codominant stems (up to two years) while minimizing visual impact.

About this blog

Rooted in Tree Research is a joint effort by Andrew Koeser and Alyssa Vinson. Andrew is a Research and Extension Professor at the University of Florida Gulf Coast Research and Education Center near Tampa, Florida. Alyssa Vinson is the Urban Forestry Extension Specialist for Hillsborough County, Florida.

The mission of this blog is to highlight new, exciting, and overlooked research findings (tagged Tree Research Journal Club) while also examining many arboricultural and horticultural “truths” that have never been empirically studied—until now (tagged Show Us the Data!).

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