This is the conclusion of a two-part article that takes a close look at the commonly held notion that removing deadwood from trees improves their health. Part 1, “On Removing Deadwood,” ran in the February issue of TCI (tcimag.tcia.org/magazine-archive).
In Part 1 of this article, we proposed that removing deadwood from a tree isn’t necessarily beneficial for the tree and in some instances can be detrimental to it. While there may be reasons to remove limbs or sections, such as the proximity of targets below, tree health is unlikely to be among them. And, while removing deadwood may be a revenue producer for a tree care company, it should not be sold by an arborist as something to improve the health of a tree.
In sections 1 and 2, we discussed “abscission” or “cladoptosis” as the natural shedding of branches and a means of self-preservation for the tree, and the differences between abscission and CODIT (compartmentalization of decay in trees). Now I want to address the common notion some arborists use in support of removing deadwood from trees to improve their health.
Arguments for removing deadwood
The common argument that deadwood removal benefits the tree is such: Removing deadwood benefits the tree because it allows the tree to close over wounds faster. This position was made frequently by those in the deadwood thread on TreeBuzz, mentioned in Part 1, and they mostly relied on anecdotal evidence.
The argument has an issue in its design, in that it does not acknowledge a difference in the ways branches die, ergo the mechanisms in place. It lumps all deadwood together. An abscised branch has no wound to close, where a non-abscised, damaged branch does. Generally, the only wounds that need closing are the ones made by making cuts on live tissue.
I found the image above in Gilman’s An Illustrated Guide to Pruning. Given the way the image is illustrated, removing the stub would benefit the overall tree. (Image 1)
For this image to be true, though, that dead branch would have likely either received a heading cut or broken off while living, both of which would have allowed air entry into the branch when it was alive. The tree pictured is either a poor compartmentalizer or has a low-energy state. This image does not describe an abscised branch, for abscised branches do not depend on wound closure to protect the main stem to which they’re attached, as described in Section 1. The branch’s abscission layer prevents decay ingress while the dead branch is attached to the tree and after the limb falls off.
In ISA’s Science of Arboriculture Podcast, episode “What Does Science Say about Pruning Mature Trees,” speaker Linda Chalker-Scott gives a very compelling lecture. In her discussion about pruning, Chalker-Scott talks about the avoidance of leaving stubs. She does not specify either live or dead stubs. The speaker also does not go into detail about why one should remove stubs. I think she’s talking specifically about when we prune live material on mature trees, not leaving living stubs.
Chalker-Scott is properly applying CODIT’s principles. She’s emphasizing that by leaving living stubs, we’re making the compartmentalization process take longer when cutting live tissue. I do not disagree with her on that. I think, however, that it could be specified she’s not talking about abscised dead branches.
It is also possible her reasoning could be categorized in what’s called the Sugar Stick Theory (Dujesiefken), the more common argument supporting removing deadwood.
The Sugar Stick Theory
The deadwood thread on Tree Buzz that I mentioned earlier is fascinating. The discussion began back in 2008 and is still ongoing. The basis of the debate is the same premise as this article: Does removing deadwood actually help a tree? Many arborists in the thread claim that retaining deadwood means retaining a food source for pathogens.
The decaying organisms that consume the already-dead wood are not inherently pathogenic. Pathogens target living wood. Not all fungi are pathogenic, and most are saprophytic (“Saprophytic Fungi” fungimap.org.au). Extremely virulent pathogens exist, but they’re few and far between.
A breach by opportunistic fungi is a failure on the tree’s part. What puts the fungi in a position to succeed is the tree’s low-energy state, not an abundance of “food.” The “food source” of a pathogenic fungus would be living tissue. Dead pieces are biologically separated, or in the process thereof, from the tree, remember?
We understand that opportunistic fungi can break through the defenses of stressed, low-energy plants. The
removal-of-food-source concept applied to non-abscised deadwood doesn’t remove the food source for fungi, because the branch has decayed into the branch attachment. It accelerates the compartmentalization process, but it doesn’t entirely remove the food source.
All of the outlined mechanisms trees utilize to prevent decay function optimally when the tree is not stressed, or in a high-energy state. Does removing the “food source” for fungi impact the tree’s energy state? No, it doesn’t.
To address that, it is more worthwhile to address the tree health in the soil, rather than pruning.
By removing deadwood of any kind, we’re addressing a symptom, not the cause. Wood dies in a number of ways: abscission, damage, retrenchment, insects, etc. In any case, removing the dead material does not address the cause. I don’t think removing deadwood directly improves the health of a tree.
Abscised branches do not depend on wound closure to protect the main stem they’re attached to. So to me, it does not stand to reason that removing that type of deadwood improves tree health. In very unhealthy trees, the removal of non-abscised dead branches in tandem with real health-improvement procedures may have a small and indirect benefit.
Should the tree be in a healthy energy state, it seems we’re comfortable relying on its defense mechanisms. We should be striving to improve a tree’s health then. There are many well-supported procedures we could perform on trees if health were our actual concern.
Contrary to Gilman’s image in the previous section, Dujesiefken states, “The only purpose of deadwood removal is to keep trees in a safe condition.” Notice the use of the explicative “only.” (Dujesiefken, Dirk, et al. Trees – a
Lifespan Approach: Contributions to Arboriculture from European Practitioners)
Don’t get me wrong; I love climbing and chasing deadwood out of tree crowns, though I dispute the claim that it makes any direct impact on tree health. If I could find a way to justify it as wholly beneficial, I would. I’m not arguing against removing hazardous deadwood or improving aesthetics. It must be said, too, that a vast majority of trees do not get their deadwood manually removed by an arborist. The mechanisms they have in place are generally effective.
It is particularly challenging with this subject to point at things concretely, because of the time it takes to study trees and because of the incredibly high number of variables involved in tree systems. One major question stands out: What metric would we even use for measuring to determine if removing deadwood makes a positive impact? Each thing I can think of is a combination of factors, such as growth rate or decline rate – how could we narrow down the influence of a single factor? I’m not entirely sure how we could quantifiably determine if removing deadwood directly benefits the tree.
Implications and questions
This section contains some questions that come to my mind after reading a lot about this subject.
I wonder, could a bad pruning cut initiate an abscission response? I’ve seen small branches killed by heading cuts. The tree detects the dysfunction (i.e., loss of leaves and disrupted auxin levels) and begins to abscise the branch.
In Pallardy’s textbook, The Physiology of Woody Plants, he and the other authors say abscised branches may or may not have an abscission zone. Clarity on this topic would be helpful.
I’d like to do more wood dissections on branch-attachment zones of non-abscised dead branches and abscised branches. Seeing inside will further develop my opinion on this subject. If anyone has any before-and-after images of dissections they’ve done, I’m interested in seeing those.
A comparison involving the use of either a Resistograph or tomograph would be insightful, to get a closer look at these defense mechanisms. I am interested in comparing tomographic images of unclosed wounds made on live tissue with tomographic images of similarly sized, abscised branches of the same species.
Two of the works cited here mention plant-part senescence. While trying to find a difference between abscission and senescence, I found very muddy explanations. Some resources I skimmed even lump the two together. In short, it is my understanding that senescence is a product of age that, in some cases, could be called natural crown retrenchment. In other cases, senescence was summed up in casual reading as “plant parts die when they get old.” I didn’t look very deeply into this because this phenomenon has fewer recognizable patterns than other common tree processes.
It is possible that, had I looked more deeply into senescence, it could have been relevant to this article. If you have any insight on this, let me know.
Certain subjects don’t get reassessed once the community has agreed on them, such as the nature of deadwood. Do people think removing deadwood is beneficial to a tree’s health? Yes, they do! Both arborists and homeowners seem to believe that. This article has laid out reasons why it would be disingenuous to claim that removing deadwood directly improves the health of a tree, though doing so may, in some cases, have a small and indirect benefit.
In my experience, the arboriculture industry here in the U.S. could be a more ethical and honest one when it comes to interacting with and selling to homeowners. The use of explicit language in any profession is what protects us and educates others. Pruning deadwood is for safety, and selling it as improving a tree’s health would be disingenuous.
The late Bob Wulkowicz, a profoundly thoughtful arborist, said in the deadwood thread on Tree Buzz:
Chain saws and bucket trucks changed this industry significantly, and we need to recognize (admit) that legacy. On one hand, we had greater access and increased efficiencies in the removal of limbs and wood. We also set the scene for increased profits and steady employment. Those are realities, but there is an inherent slippery slope of replacing conscious tree care with how much wood is put in the chipper.
Larger organizations in the arboriculture community inadvertently perpetuate the poor science by presenting information as finite rather than part of an evolving science. Smaller companies and arborists get their information from them, and then it gets sold to homeowners. The extensive and frivolous pruning the residential arboriculture industry sells in the name of tree health is absurd, even if it is genuine and innocent ignorance.
Wulkowicz also says in the forum:
We should also remember the deadwood fills our gas tanks and pays our mortgages, so there are powerful forces aligned to defend and encourage deadwood removal. The important thing is to try to maintain some sort of perspective that recognizes the truths of both biology and economics.
He’s exactly right; “Big Arb” is a large body that can’t pivot nimbly – no big industry can. It can’t very well say, “Removing deadwood is pretty benign,” because that is how a lot of us make our living. Arborists have a duty to not take advantage of the resilience of trees, and likewise, a duty to not take advantage of people’s ignorance. If small-company operators think deadwood removal benefits a tree’s health, then that makes it easier for them to sell the idea to homeowners.
I’m not saying people are intentionally misled (although sometimes that is the case); certain things just don’t get carefully examined, because some decision already has been made. My thoughts about this are best summed up by Wulkowicz again:
…all that one had to do to justify pruning was to waive a page with the printed reasons of “Why we prune.” Now a number of those pronouncements are simply embarrassing. Those dogmas and then reconsiderations are always likely to be a part of evolving science. It is also interesting that an awful lot of people think that those facts are at the end of finite knowledge and anything “further” is irrelevant or irritating.
Reassessing commonly held beliefs and standards is vital to all of us as students of arboriculture – it’s how we’re all going to improve. Removing deadwood may not be detrimental to the tree, but that doesn’t inherently mean it is beneficial. Do not forget, as arborists, we’re supposed to be helping trees.
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Chalker-Scott, Linda. “Science of Arboriculture Podcast.” What Does Science Say about Pruning Mature Trees? Oct. 20, 2017
“Deadwood.” The BuzzBoard,
Dujesiefken, Dirk, et al. Trees – a Lifespan Approach: Contributions to Arboriculture from European Practitioners. Fundacja EkoRozwoju, 2016.
Gilman, Edward F. An Illustrated Guide to Pruning. Delmar, 2012.
Hirons, Andrew D., and Peter Thomas. Applied Tree Biology. John Wiley & Sons, Inc., 2018.
Pallardy, Stephen G., and T. T. Kozlowski. Physiology of Woody Plants. Elsevier, 2007.
“Saprophytic Fungi.” Saprophytic Fungi, fungimap.org.au/about-fungi/saprophytic-fungi
Shigo, Alex L., et al. Compartmentalization of Decay in Trees. Forest Service, U.S. Dept. of Agriculture, 1977.
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Jeremiah Sandler, a Certified Arborist and ISA Tree Risk Assessor Qualified, is owner/operator of Tree First Arboriculture in Royal Oak, Michigan. This article first appeared on his website, www.treefirst.org.