Cavity and Included Bark Field Inspection Technique
Why this matters
The treat-remove-monitor call on a cavity or an included-bark union is only as good as the measurement underneath it, and both defects are easy to characterize badly from a single angle. Climbing to inspect a suspect union is itself working near a structure whose integrity is the exact question you are there to answer, so treat every suspect cavity or union as potentially compromised until your own probing says otherwise, and never load a union you have not yet assessed with body weight or rigging tension. Get the sounding and probing wrong, shallow when it needed a full circumference, one angle when the shell was thinnest on the far side, and the decision built on top of it is wrong no matter how sound the decision logic is.
What this covers and what it does not
This covers the hands-on field technique for characterizing a suspect cavity or included-bark union: sounding, probing, and measuring what you find. It does not cover what to do with the finding, remove, treat, or monitor; that decision logic lives in Treat vs Remove vs Monitor: Included Bark on Codominant Stems Decision Matrix, and this article's job ends at handing that matrix accurate inputs. It does not cover the escalation ladder to a resistograph or an air spade when field probing is not enough; that sequencing lives in Which Test First On A Suspected Hazard Tree, and this article is the technique used at the "sounding" and "field probe" rungs of that ladder, before an instrument gets involved. It also does not repeat the general risk-walk sounding covered in Tree Risk Assessment Field Walkthrough Technique, which is a coarse screen across a whole tree; this is the deeper technique applied once that screen has already flagged a specific zone.
Sounding technique for a cavity
Once a suspect zone is flagged, sound it on a tight grid, strikes a few inches apart, rather than a handful of taps confirming the dull tone is real and moving on. The grid's job is to map the boundary where tone transitions from sharp to dull, and that boundary is the edge of whatever is behind the bark, not the visible opening if there is one. Distinguish three tone qualities as you go: a sharp, ringing tone is solid wood; a dull, flat thud with some resistance felt back through the mallet is punky or partially decayed wood, not yet a true void; and a hollow, almost empty-sounding ring with little felt resistance is an open cavity with little or no wood behind the shell at that point. Mark the grid boundary as you sound it, chalk or flagging tape directly on the bark, so the mapped zone stays fixed while you move to probing rather than being re-estimated from memory afterward.
Probing technique: depth and wall condition
Where sounding flags punky or hollow tone, probe it. Push an awl, a screwdriver, or a purpose-built arborist probe into the wood at several points across the mapped zone, not just the center or the most obviously affected spot. Firm resistance that gives at a specific, repeatable depth tells you where sound wood starts and lets you estimate remaining shell thickness at that point. A probe that sinks freely with no resistance at all means you have gone straight through the shell into open cavity, or the wood at that exact point is fully punky with nothing solid left to feel.
Probe around the full mapped boundary, not only the point closest to a visible opening, since a cavity's remaining wall is almost never uniform in thickness. Research on hollow-tree biomechanics (Mattheck and others) found that a stem's resistance to breaking drops off sharply once the sound shell at its thinnest point falls below roughly 30 percent of the trunk radius at that height; treat that ratio as a flag that pushes the finding toward escalation or removal consideration in the decision matrix, not as a removal trigger by itself, and only trust the ratio if you actually probed the thinnest point around the full circumference rather than the point that happened to be easiest to reach.
Included bark inspection technique
Start visual: look at the union from below and from each side for a raised bulge or rib in the bark just under the crotch, a sign the union is already under compression from the wood behind it pushing outward, which reads as more advanced than a smooth, unremarkable union even before any probing happens. Then probe the seam itself: work a thin blade or a probe into the top of the union where the two stems meet, feeling for how far it slides before contacting solid wood-to-wood connection between the stems. A shallow seam stops the probe quickly, meaning true wood connects the stems close to the surface. A deep seam lets the probe slide well down into the union before hitting resistance, meaning bark, not wood, extends that much further between the stems, and the union is holding together on a correspondingly smaller real wood connection than its outward size suggests.
Measure that seam depth against the union's total diameter and record it as a fraction, not just "deep" or "shallow" in a note. That fraction is one of the inputs the treat-remove-monitor matrix scores the defect on; this article's job is producing an accurate number, not deciding what the number means for that particular tree, target, and age.
Measuring what you find, not just naming it
Every finding gets a physical record, not a verbal impression carried back to the truck. Photograph the mapped sounding boundary with a tape measure in frame for scale. Record probe depths at each point around the circumference, not just the single deepest or most dramatic reading, since the matrix downstream needs the thinnest point, and you cannot know which point that is without recording all of them. Note the clock position of every probe point relative to a fixed reference (facing the street, facing the trunk's most prominent lean) so a second visit, or a resistograph technician brought in later, can find the exact spots already probed instead of re-mapping from scratch.
One gate, two outcomes: two cavities, same technique
Two different jobs, two similar-looking cavity openings on lower limb stubs, the same sounding-and-probing technique applied to both, resolving in opposite directions.
Case A. A small opening on a shade tree's lower limb stub, roughly the size of a fist, sits over the back corner of a large yard with no structure, path, or parking within the fall zone. Sounding maps a dull zone extending a short distance around the opening; probing at eight points around that zone's full circumference finds sound-shell thickness comfortably above the 30 percent flag at every point, including the point directly opposite the visible opening. Target value is minimal and the shell measurement is consistently favorable at every probed point. This case resolves to monitor, on a documented interval, per the matrix's target-and-severity logic.
Case B. A visually similar small opening on a different property's shade tree, this one twenty feet from a walkway used daily. Sounding maps a similarly modest dull zone from the visible opening's angle, and a probe at that opening alone would have shown adequate shell thickness, the same reassuring reading Case A gave. But probing the full circumference, not just the visible face, finds the shell thinning sharply on the far side of the union, invisible from the opening's angle, dropping to just under the 30 percent flag at one point roughly opposite the opening. That single point, found only because the full circumference was probed rather than just the visible face, combined with a moderate-occupancy target below, is what separates this case from Case A. Both openings looked the same from the angle a driveway glance would use; only the full-circumference probe distinguished them.
When findings trigger the next test
Case B's marginal shell reading at one point is exactly the situation Which Test First On A Suspected Hazard Tree exists to route: field sounding and probing localized and roughly quantified the defect, but a single marginal probe reading on a target this consequential is not enough certainty to stop at. That article's ladder sends a finding like this to a resistograph for a quantified reading across the suspect line, rather than either accepting the field estimate as final or excavating with an air spade, which would be the wrong next rung for a defect that is above grade and already localized. This article's job stops at handing that ladder an accurate, fully-mapped starting point; which instrument comes next is not re-derived here.
Verify your findings before you leave
Before leaving either property, confirm four things: the full circumference of the suspect zone was sounded and probed, not just the side visible from the ground or from a single approach angle; the mapped boundary is marked and photographed with a scale reference in frame; probe readings are recorded at multiple points around that boundary, with the thinnest point identified rather than assumed; and every measurement, shell thickness at its thinnest point, included-bark seam depth as a fraction of union diameter, is written down in a form that feeds directly into the appropriate decision matrix rather than left as a recollection from the climb.
References
- See related: Treat vs Remove vs Monitor: Included Bark on Codominant Stems Decision Matrix
- See related: Which Test First On A Suspected Hazard Tree: Sounding Vs Resistograph Vs Air Spade Decision Tree
- See related: Tree Risk Assessment Field Walkthrough Technique
- Mattheck, C. and Breloer, H. - The Body Language of Trees (sound-shell ratio and hollow-stem mechanics)
- ISA Best Management Practices: Tree Risk Assessment