Photo 1: Removing a tarp from beneath the mulch below a struggling pine to reduce stress and increase water to the soil. All photos courtesy of the author.

I’ve had a number of consultations in Pleasant Ridge, a small community with many mature oak and early veteran trees, after a hollow tree failed onto someone’s house (nobody was injured). Around the same time, another oak nearby had a significant failure that totaled a neighboring garage.

Horrible things for everyone involved in both of those events, I am sure. And the knee-jerk reaction a lot of their neighbors had was to kill/remove their own trees out of fear. Opportunistic and predatory tree companies in the area were thrilled to accommodate them.

The more thoughtful folk of Pleasant Ridge have been asking me to come to their properties and assess the health of their trees. I praise those clients for not being led by recency bias. But sometimes what a client is actually asking me for is not a health assessment but a risk assessment. A fair and important thing to do, but a different thing than a health assessment.

This article touches on the differences between a tree-health assessment and a tree-risk assessment. It is common for homeowners, tree workers and even arborists to get these two things confused. But their differences are meaningful.

We’ll also touch on tree hollowing, because that is what the people of Pleasant Ridge have on their minds, and, therefore, that’s what’s on my mind, too. The oak that crushed the home was hollow, according to those in the neighborhood. I never saw this tree or its aftermath myself.

As I assess a tree, I am mentally categorizing my observations into either the “stress” domain or the “risk” domain. This is because stress indicators and risk factors are often side by side in a tree. And, of course, there can be a relationship between tree stress and tree risk. But the relationship between the two is like a Venn diagram, with overlapping circles; the differences are not black and white. A high skill in assessing trees is to be able to identify when to focus on a certain domain, and why you should do so.

Tree stress
If a tree is at an extreme end of the health spectrum, quite healthy or quite unhealthy, we can visually assess that pretty easily. If a tree is quite healthy, no stress-reducing procedures might be necessary at all. On the other hand, if a tree is extremely unhealthy, finding the primary stressors and addressing those is paramount to a tree’s improving health. Once the primary stressors are being addressed, move on to the secondary stressors. After all, once the primary stressors are alleviated, it isn’t as if those secondary issues just vanish.

When alleviating stress, the primary goal is often the improvement of the environment the tree is growing in. This is mostly achieved by focusing on soils, putting the “culture” in arbori-culture. (Photo 1)

Here are some things I’m looking for while visually assessing a tree’s stress:

• Leaf area of crown.
• Live-crown ratio.
• Patterns of stem death/distribution of dead stems.
• Apical growth of primary stems.
• Patterns and distribution of epicormic stems.
• Cankerous regions.
• Fine-root growth near the trunk (assessed by light digging).

If a tree’s health is somewhere in the middle of that spectrum, sometimes you’ve got to use more tools to get some details you can’t tell just by looking at a tree:

• Increment borer.
• Photosynthesis meter.
• Sap analysis.
• Soil moisture meter.
• Soil penetrometer.
• Soil microbe test.

These are just some of the things I’m thinking about or using. It would take more pondering to create a list of every possible metric when assessing a tree’s health. But I digress.

Photo 2: A sonogram (tomography scan) taken by Tree First Arboriculture.

Stem breakage
Just like in the Pleasant Ridge example above, I’m usually called for a “health inspection” once something breaks from a tree.

Healthy stems can break. Stem breakage is not necessarily caused by a stem’s decline in health. It can be to some degree, but even healthy, normal stems without dysfunction can break. It happens to many trees with formations that have certain tendencies. It can happen during extraordinary storms, or even if the wind pattern is moderately different than usual.

Trees with lots of inclusions, for example, have an increased likelihood of experiencing a breakage (think mature Freeman maples). Not because of their health, but because of the formation of inclusions. And a tree can be perfectly healthy, have no inclusions and still have breakages. Sometimes the conditions align perfectly to break a healthy and solid stem – it happens.

A tree also can be very healthy by most metrics and be very hollow in its base. A tree can be totally solid and be extremely stressed. A tree can be half hollow and very healthy, or can be half hollow and very unhealthy.

Do you see what I’m getting at here?

Separating and making clear the differences between health and risk enriches the way we all think about trees, for both arborists and our clients. And making these distinctions makes the nuances of looking at trees evident. Sometimes you can’t look at a tree for just five seconds and reach a thorough conclusion.

Tree risk
The Pleasant Ridge folk are mostly concerned about risk at this point. And I think often it is wise to consider risk before stress when managing urban trees. Risk can be a tricky thing to assess because, after all, even perfectly healthy stems can break. When communicating about risk, never say never. It is critical to identify the boundaries of what is knowable and what is not.

Pruning is the major method of risk reduction regarding trees, if you don’t count killing/removing them. Reducing leverage and end weight brings a branch’s center of mass closer to its fulcrum. Doing so also reduces the load borne on old wounds or tears. That is to say that, when done properly, we reduce the risks of trees via physics.

When I am assessing a tree, I’m not only looking for stressors, I’m also looking for formations or features that pertain to tree risk:

• Broken/cut roots.
• Internal decay or hollowed regions in critical areas.
• Broken stems.
• Dead stems.
• Wildlife presence.
• Stem deformations of the trunk.
• Common crown formations.
• Mushrooms.

And when I need a higher degree of certainty when assessing a tree’s risk, I’ll use some of these tools:

• Sonic tomography.
• Resistance drills.
• Pull testing.
• Bending-resistance software.

I want to emphasize that tomography and resistance testing are not stress-diagnostic tools, despite being described as such by many arborists. These tools are used to determine the state of the inside of a tree – whether a space is hollow or not, and to what degree. Or whether an interior space is decayed, and to what degree. The information we get from these tools is used in a stem-breakage analysis, not a health analysis. (Photo 2)

Hollowing and tree health
Hollowing is a normal part of the tree aging process, and managing hollow trees requires a bit more nuance than solid trees. Hence why determining the extent of a tree’s hollowing is useful to know.

From here on we use “hollow” as both a verb and a noun. A tree can be nearly totally hollow and be nearly totally healthy at the same time. Healthy trees can hollow and unhealthy trees can hollow.

Hollowing is not a quality only unhealthy trees possess. And healthy trees can be high risk and unhealthy trees can be low risk. Hollow trees can be low risk; hollow trees can be moderate or high risk, too. All the same is true for solid trees. It is also true that hollow trees fail, and so do solid trees.

In other words, there is a lot more to tree failure and tree risk than just whether a tree is hollow or not. But it is a great place to start thinking about tree stem strength and resistance to bending.

Photo 3: A tree that had a major inclusion – half of the tree failed away from the other.

Notes on hollowing
This section could be an entire article, as the subject is very complex and interesting. I am really resisting the urge to elaborate as deeply as I want to.

The reason a tree can be hollow and healthy is because of physiological differences between sapwood and heartwood.

As a tree hollows, it is experiencing what could generally be referred to as “heart rot.” Trees undergoing heart rot will have their heartwood decay and eventually hollow out in time. Organisms that cause heart rot generally do not or cannot (exceptions exist, of course) cause dysfunction to the sapwood of a tree. A different group of organisms mess with sapwood, but we’re not talking about those here.

Sapwood is doing a lot of health-related stuff. Carrying sugars and water around the tree. Structurally, too, the sapwood is very important, as the outer surface of a shape plays a large role in resisting bending forces, similar to a pipe. This is one reason why sap rots found in wood can be a bit more concerning than heart rots when it comes to tree risk.

Asymmetric hollowing, or deformities, resulting in the tree’s cross section at the base being largely incomplete, tend to be more concerning than centrally located trunk hollows. (Photo 3)

Sapwood contains more living (or more recently dead) cells than heartwood, and the conditions within said cells are less favorable for pathogenic decay in healthy trees.

Heartwood also adds to the bending resistance of a stem. As it hollows, some degree of strength and flexibility is lost. But less strength is lost than our intuition might suggest.

Physiologically, heartwood essentially is a place where the byproducts of cellular respiration of the xylem cells in the sapwood end up. Embolizing heartwood, or allowing air inside of those vessels, is one way this type of hollowing can be initiated within a tree, i.e., by excessively large pruning or natural wounds.

Hollowing also can initiate from the ground, via embolizing of large-root tissue. This happens, again, through the hands of humans and/or naturally with time. Endophytic fungi (or their spores) can become opportunistic when conditions inside the wood change – another way heart rot can initiate.

Some trees do not form heartwood and instead form “ripewood,” which is essentially a sort of in-between state between sapwood and true heartwood. So this whole explanation is not universal, it is just a summary.

Photos 4, 5 & 6: Trees with roots forming within their own decaying stems.

Adaptations
Trees can retrench as they hollow, naturally reducing the likelihood they self-destruct under their own weight. Arborists also can reduce the likelihood that a hollow tree self-destructs or breaks by reducing the maximum height of the crown, among other things. We can’t forget that trees add strength and increase resistance to bending annually with new rings of sapwood.

There are some other cool adaptations trees have as they begin to hollow. The one I think is the coolest is the growth of roots inside of their own decaying bodies. Adventitious roots can grow from within the tree, inside of the trunk and wherever else it hollows, utilizing the moisture and nutrients from their own decaying heartwood. Gnarly. (Photos 4, 5 & 6)

The fact that trees can survive hollowing is quite self-evident. Hollow trees can remain standing for very long periods of time, both in forests and neighborhoods. I see them almost daily and assess them frequently. Many more trees are hollow than we ever can know, simply for not assessing them all that closely.

Hollowing is a normal phenomenon that occurs as trees enter later life stages. It is not an evil force – it is a force for good. It is part of the great forest system, and many animals, large and small, depend on the existence of hollows. Good arboriculture contextualizes the tree in its environment. A tree is never just a tree. Fearing and condemning trees on the basis of a bit of hollowing alone is not an act of responsible arboriculture. It is a crime against nature.

More nuance
What an experienced person means when they describe a tree being hollow is the hollowing at a key location. They aren’t insinuating the entire tree is hollow. Hollowing or extreme deformities within either the base or primary union(s) are both critical considerations when thinking about tree risk.

Having some hollows and cavities within old trees is normal, and plenty of trees with those features are managed. Plenty of trees with unknown and non-managed hollows are fine.

To be fair, a tree can sometimes justifiably be condemned even if most of the tree is solid and healthy. The base of a tree bears all of the forces produced by the crown. If the base is significantly deformed, hollowed or decayed, yet the tree has a healthy crown, the base may not be able to build rings fast enough to replenish its bending resistance before conditions arrive that could cause a major breakage. But sometimes they can!

Similarly, I’ve heard plenty of non-tree folk complain that the base of a stump was totally solid despite being told it was hollow by the tree cutter who condemned it.

There are many explanations to this. One explanation is, indeed, that the tree cutter was predatory or simply did not know a whole lot about trees. This is sort of common in my neck of the woods.

Another possible explanation is that the critical deformity could have been 5 feet off the ground and not necessarily at the base of the trunk. For risk-management purposes, condemnation of the tree can be justifiable in that case, because the outcomes of a worst-case scenario are nearly the same as a failure at the base.

Final cut
I don’t expect a client to know these nuances themselves – that’s why they call me. It is why, however, when I’m asked to check out a tree, I have to ask certain questions to hone in on what the client is actually concerned about. Not necessarily as an “upsell,” but to actually give the client what they’re asking for, or to better determine what kind of assessment makes sense. Tree-stress management and tree-risk management are two sides to the same coin. Both need to be addressed if one is to be doing arboriculture in a complete way.

In order for us and our clients to exist with badass and amazing old trees, we need to communicate that not all hollowing is equal. Not all hollowing is grounds for condemning and killing a tree, and we ought to stand up for trees, even against paying customers. Keeping hollow trees around is possible, and it’s done all the time, whether those working on the trees know it or not. You have likely unknowingly signed off on a hollow tree being low risk. There are ways to assess tree hollowing without a chain saw. Pros do it all the time.

Working with and retaining less than perfect trees is how you actually level up as an arborist. Communicating on behalf of trees is perhaps the most important aspect of our job. That’s where tree care begins. These are things that responsible and curious arborists should be thinking about, learning about, communicating about and implementing in their practices.

Jeremiah Sandler, BCMA, is owner of Tree First Arboriculture in Ferndale, Michigan.