How Defects Increase Likelihood of Tree Failure

In the February 2023 issue of TCI Magazine, we reviewed some important mechanical concepts that arborists should know to better understand the likelihood of tree failure (“What Is Tree Biomechanics, and Why Should I Care?”). From that article, we learned that the likelihood of failure depends on two factors: the loads a tree experiences and the tree’s load-bearing capacity. We also learned that failure occurs when the loads exceed the load-bearing capacity. That definition applies not only to trees, but to pretty much everything – ropes, ladders, the boom on a bucket truck, eyebolts, laptops, pens and so on.
Arborists regularly assess the likelihood of failure. Clients ask arborists to assess tree risk. Climbing arborists inspect a tree before ascending and continue to inspect it as they are working in the tree. Climbing arborists inspect gear before ascending and while working. Drivers and operators inspect vehicles and equipment before use.

Photo 2: Fruiting bodies, a sign of possible root decay, are another indicator as to the likelihood of failure.

One thing common to all these actions is looking for defects. Inspecting a climbing or rigging line involves looking for signs that the line may be compromised. This inspection might include damaged or cut strands, glazed or melted fibers, discolored or flat spots and other indications. The same approach applies to other load-bearing components of the climbing system, such as carabiners. We inspect them to make sure the gate works properly and there aren’t badly worn or damaged surfaces. When inspecting pulleys, we need to make sure the sheave rotates freely and the metal doesn’t show signs of overloading or misuse. For hitch cords, we inspect splices or termination knots. There are dozens more examples of defects that arborists look for on gear, equipment, vehicles and trees.

Manufacturers test to meet performance standards

With manufactured products such as ropes, carabiners, blocks, chippers, bucket trucks and the like, there are usually clear instructions on when to retire or downgrade the product. Guidelines for when to retire or downgrade products are based on experiments and experience. Manufacturers conduct extensive tests to make sure their products meet industry performance standards. For example, if a cordage manufacturer wants to say that a rope meets the ANSI Z133 Safety Standard for climbing lines, they must conduct standardized tests to make sure the rope has a minimum breaking strength (MBS) of 5,400 pounds (24.02 kN) and a working elongation of no more than 7%.

The more widely a product is used and the longer it’s been used in different industries, the more we’ll know about how defects affect its load-bearing capacity. Ropes are a good example, since many industries use them and have used them for a long time. Manufacturers and users have plenty of experimental results and experience with ropes in service. This has made it possible for an industry association like the Cordage Institute to develop guidelines for when to take a rope out of service. The Cordage Institute also developed test methods for manufacturers to ensure consistency among test results.

Applying experience to the likelihood of tree failure

The same amount of experience and experimental evidence doesn’t exist for trees, unfortunately. This doesn’t mean we can’t predict how defects can reduce the load-bearing capacity of a tree or tree part. It means that our predictions will be more uncertain – it’s harder to predict. We do have a good idea of what defects to watch out for – decay, weak unions, cracks, cut roots – but it’s less clear how severe a particular defect must be to reduce load-bearing capacity enough that failure is more likely.

Defects reduce load-bearing capacity in a variety of ways. Common defects that arborists assess include decay, dead branches, weak unions, cracks and root damage. (Photos 1 and 2) Decay is a common defect that’s also a good example of how defects can reduce load-bearing capacity. Decay is a long-term process that involves microorganisms (mostly fungi) digesting the cellular components of wood. Over time, parts of a stem or branch can become hollow as fungi digest all cellular components.

It’s also possible, however, that only some components are digested. If the stem or branch cross section becomes hollow, there is less wood to bear the loads the tree experiences, and the likelihood of failure increases. Even if the stem doesn’t become hollow (or partially hollow), if fungi have digested the cellulose but not the lignin, or the lignin but not the cellulose, the wood that remains is too weak to bear loads. Most arborists have probably seen both situations on the job.

Evaluating decay

Focusing on decay, how much is too much? At what point does the amount of decay present too great a likelihood of failure? Both questions miss the mark. More than just how much decay is present – we often think of this as the percent of the stem cross section that has been decayed or hollowed – the location of decay is equally important. And remember that looking at only the loss in load-bearing capacity ignores the loads the tree experiences.

Photo 3: The extent of decay – how much there is and where it’s located in the cross section – affects the loss in load-bearing capacity. For the same proportion of decay or hollow in the cross section, concentric decay reduces load-bearing capacity less than eccentric decay, as shown in Photo 4.

Let’s say a client doesn’t have a high-risk tolerance. They might request a removal of a tree that has noticeable decay, because the tree is in an exposed location and close to their house. But the climbing arborist might inspect the tree before ascending and decide that even though the tree has noticeable decay, they can climb and rig it in such a way as to minimize the loads the tree experiences during the removal.

Let’s return to the importance of looking at both the amount and location of decay. When a stem is bent, the outer fibers of the cross section contribute most to the stem’s load-bearing capacity. This is why stems can be quite hollow without losing too much load-bearing capacity, as long as the decayed area or hollow is centrally located in the stem. But as the location of decay moves farther from the center of the stem, more of the outer wood fibers are affected, and the stem loses more and more load-bearing capacity.

Cross-section analysis

Imagine a cross section that is 50% hollow. (Photos 3 and 4) If the hollow area is in the center of the cross section (Photo 3), the loss in load-bearing capacity is only 10%. This is a relative measurement; the stem has 10% less load-bearing capacity because of the 50% hollow in the center compared to its load-bearing capacity assuming it had no decay. If the 50% hollow is located near the perimeter of the stem, say just inside the cambium (Photo 4), the loss in load-bearing capacity is 30%. The stem has 30% less load-bearing capacity even though it’s the same area of decay (50% of the cross section).

Photo 4: More of the outer fibers are missing or weakened in eccentric decay.

As a general rule of thumb, 30% loss in load-bearing capacity is a good starting point for considering a greater likelihood of failure. This doesn’t mean you should remove any stem that has lost 30% of its load-bearing capacity. It’s just a starting point for a closer assessment of the particular situation. In one study, my colleagues and I showed that trees with more than 30% loss in load-bearing capacity were more likely to experience stem breakage, but trees with less than 30% loss in load-bearing capacity were more likely to uproot.

Determine a suitable tie-in-point for in-tree inspection

If you need to work in a tree that has noticeable decay, it’s essential to inspect it carefully. That might not be possible from the ground, where it can be challenging if not impossible to see the upper side of branches and stems higher in a tree’s crown. If that’s the case, it’s safer to ascend into the tree in shorter steps (branch by branch) rather than setting a line high in the crown with a throwline.

Ascending in shorter steps will enable you to investigate the decay close up before ascending to the next higher branch. When you reach a suitable tie-in-point (TIP), check it carefully for decay as well. Remember that when you’re moving laterally through the crown, the tension in your climbing line can exert bending moments on the stem well below the TIP. For this reason, it’s critical to assess the extent of decay well below the actual TIP. It might be necessary to use more advanced decay-detection devices.

Weak unions

Weak unions are another common defect. The load-bearing capacity of the attachment can be reduced by half in some situations, especially when included bark is present. Decay or cracks associated with the weak union will reduce the load-bearing capacity even more. If you choose a TIP on a stem that has a weak union, even if it’s well below the TIP, make sure to inspect the weak union carefully. The weak union may need to bear the loads (forces and bending moments) you exert on the TIP as you work in the tree.
Insect impact on likelihood of failure.

The presence or signs of insects also can indicate reduced load-bearing capacity. Carpenter ants, for example, nest in decayed wood. Small piles of sawdust often accumulate at the base of a tree that has carpenter ants nesting in it. Bark beetles and wood-boring insects often indicate unhealthy trees that may be more likely to have weakened defenses. They also may have more dead or dying wood, which is not as strong as live wood. If you notice exit holes, frass or other signs of such insects, make sure to investigate more carefully before climbing.


As noted at the end of the article in the February issue, there’s still much to learn about biomechanics and the effect of defects. Learning more about the loads a TIP experiences (see the list of references at the end of this article) and the load-bearing capacity of TIPs will help reduce the odds of injuries and incidents.

Brian Kane, Ph.D., is the Massachusetts Arborists Association Professor in the department of environmental conservation at the University of Massachusetts in Amherst, Massachusetts.


Kane, B. and S.R. Arwade. 2022. The effect of climbing line and ascent technique on the magnitude and frequency of loads associated with arboricultural climbing. Arboric. Urb. For. 48(6):309–318.

Kane, B. 2022. A comparison between battery-powered and human-powered ascents by a climbing arborist. Urb. For. Green.

Kane, B. 2021. Forces and motion associated with arboricultural climbing.  Urb. For. Green.

Kane, B., E. Brigham, and S.R. Arwade. 2020. The effects of ascent technique and the presence of leaves on loading of a tie-in point during climber ascents. Urb. For. Green.

Kane, B. 2020. Loads borne by a tie-in point during ascents and descents on a basal-anchored stationary rope system. Urb. For. Green.

Kane, B. 2020. Loads borne by a tie-in point (TIP) during arboricultural climbing. Urb. For. Green.

Kane, B. 2018. Loading experienced by a tie-in point during ascents. Urb. For. Green. 34:78–84.
Cetrangolo, I., S.R. Arwade, and B. Kane. 2018. An investigation of branch stresses induced by arboricultural operations. Urb. For. Green. 30:124–131.

Kane, B. 2014. Determining parameters related to the likelihood of failure of red oak (Quercus rubra L.) from winching tests. Trees Struct. Func. 28:1667–1677.

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