Does removing dead wood from the canopy actually produce some net benefit for the tree? Does anyone out there actually know of any research that suggests trees respond positively when dead wood is removed?
I'm well aware of the benefits to people from removing heavy dead branches over "targets", however I'm looking for scientific literature that suggests trees benefit when dead wood is removed. Please point me to this literature.
I'm looking for actual published peer reviewed research papers that have focused on canopy dead wood. I've searched google scholar, Cambridge Abstracts, J-Stor and others, but haven't had any luck. I've read almost all that Shigo published and understand that CODIT will happen around senescing (sp) branches regardless of whether or not the dead branch is still attached. Shigo, as far as I know, never suggested CODIT would be enhanced by removing dead wood. As for Gilman, if he or any of his students have conducted research on this, I would love to read it. The link to the "gray" literature from Gilman suggests that it is his opinion that dead wood "could" pose a risk to tree health.
Tom, interesting research subject. Please keep us informed of what you find. I have no scientific documentation but strong anecdoctal evidence from field observations during our own work.
For many years I had told clients that removal of deadwood was primarily cosmetic (barring hazardous situations). But as the years went by it became increasingly apparent, at least to us, that the trees that were deadwooded were indeed healthier and more vibrant than neighboring trees that were not.
This is in mostly residential work but also stands of conifers in the forest interface. Whereas this does not produce an immediate response but the overall appearance after time (5 years or so) is noticeably different.
My hunch is that as the wounds finish "healing" and closing up, the tree can now put its energy elsewhere rather than keep investing it on the already dead branch. If you leave the dead branch on, the tree has to try to compartmentalize the whole branch, rather that just cover over the cut.
Looking at wild trees in woods habitat its clear that attached dead wood is the natural order of things. Excessive dead wood removal reduces wildlife nesting and feeding opportunities. Many tree species in woods settings successfully drop limbs and seal off the collar without human intervention. In a residential setting it makes sense to remove dead wood hazards over targets. It really comes down to what species of tree it is, where the dead wood is in the tree, what caused the branch to die and what the projected long term effect is of keeping a particular dead branch. As much as we like absolutes it's probably not possible to create a clear rule that dead wood should or should not be removed, good assessment is required per individual case weighing all of the factors.
I'm thinking that there are so many variables per species, dead wood location on the tree and habitat context that a study would be inconclusive in determining ultimate net gain or loss from human intervention. Look at Bald Cypress for example, it is very long-lived with a completely hollowed trunk.
We should also consider the role of that wood after it dies. In the forest, like moss said, dead wood is a home to creatures. But when it falls from the tree it becomes mulch and becomes part of a never ending cycle that keeps the soil rich.
If we deadwood a tree and haul the wood away, we break that cycle.
Here's an anecdotal story about this topic. Since the results would be impossible to replicate you have to decide how credible the outcome is.
A compan that I know had a client with a large American elm that was infested with a number of critters including fungus, bugs etc. that were weakening the tree. IN order to keep the nasties at bay a very involved and expensive chemical treatment and fertilizer program had been followed for several years. The tree had improved just a little but the client was concerned about the expenses. The arborist suggested a VERY thorough deadwooding down to pencil and matchstick diameter. This trim job wasn't cheap either.
Over the next couple of growing seasons the arborist tracked what the tree needed to keep the nasties at bay. What was found was that the costs for treatment plummeted and the tree improved. The variable was the thorough deadwooding so the conclusion was that the pathogens had deadwood to 'eat' and openings into the tree for exploitation. The tree does expend energy isolating deadwood and trying to grow over stubs. With that energy left and reallocated to growth the tree seemed to be in better shape.
It would be nice to read about peer reviewed research about this.
ICAN out of Evergreen College in Washington would likely be a resource. At one time I had a subscription to 'The dead wood Journal' which highlighted research into the topic as it related to forests.
Ancient Trees has something to say on this topic as well. For urban trees it is a matter of dealing with a single tree in an artificial environment. Forest trees work in a completely different dynamic and the interactions with the rest of the biosphere they exist in become the context by which the impact of removing deadwood is evaluated. The pathogens and bugs we often refer to as "nasties" in the first context are harmful and lead to hazard conditions. In the second the become part of the life cycle of the forest and are given a rightful place to exist in.
Thanks for all the insight. I've debated/discussed this topic many times, and all my searching for information has led to some papers about coarse woody debris (CWD) in forest ecosystems, but very little talk about canopy dead wood. This was surprising to me because our industry has been advocating it for years as a way of improving tree health and reducing the risk of fungal infection. Maybe it's because of historic problems like dutch elm disease, where infection can often initiate in small branches from feeding beetles. I could see how it might help an American elm if you pro-actively removed all the deadwood.
I also agree it depends on the species, and like Moss said all urban environments require unique consideration. Forest canopies have only been studied for the last 25 years or so, and urban canopies have received even less investigation (by scientist). we all know that science is never absolute. Because of this, I can only really tell a client that removing dead wood will make the tree safer, and it might look better, "cleaned out". But I can't really tell them if there's a positive "tree health" feedback.
It seems as an arborist any actions taken on the above ground portion of the tree are most often not in the best interest of the tree. Conversely, when we alter the below ground portion (mulching, changing grade etc.) it is possible to have the greatest impact in terms of 'tree health'.
Myth 25 Big Callus means strong healing. Wound closure and compartmentalization are two separate processes.
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Because of this, I can only really tell a client that removing dead wood will make the tree safer,
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Rather than use the term 'safe' you might consider talking in terms of risk reduction. Telling a client that a tree is 'safe' means that there are no risks. We all know that isn't possible unless the tree is removed and the stump ground out too.
Tom, this is a link to the ancient tree newsletter. I don't think you'll find a straightforward answer to your question. Look at these ancient trees where they have survived for centuries with all the defects and issues we would be resolving supposedly. Was deadwooding necessary to the success of the tree or did deadwood in the canopy create a problem for the tree?
You may find some insight into the value of deadwood in trees from the standpoint of habitat. Like any of the defects we see in trees the definition "hazard" applies only if a target exists. Discussing the removal of deadwood and the advantages/disadvantages requires a discussion on what the client's needs are with respect to the tree. Is it a front yard tree or specimen tree where it's aesthetic value is most important? What is their ideal as far as that is concerned? Can deadwood be left to enable the continuation of the natural ecosystem that is a tree?
You've opened up a whole different world of arboricultural practices that we are yet to wrap our heads around.
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...historic problems like dutch elm disease, where infection can often initiate in small branches from feeding beetles. I could see how it might help an American elm if you pro-actively removed all the deadwood.
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But those beetles bore into live wood at branch junctures (crotches). In order to remove the beetles, the branch would have to be removed using a very flush flush cut. Additionally, in this scenario (DED), removing a branch once it is dead would be too late, because the fungus would already have moved to other parts of the tree.
Sanitation pruning is typically performed by removing flagging (dying) branches and making the cut well below the ailing branch, cutting into and removing some healthy material as well as the dying branch.
I think your general question about the benefits of removing deadwood is very important and, as you are finding, not well researched or documented.