Sound WoodWhen to test a tree for internal decay
You can't test every tree, and you don't need to. A sonic decay test takes setup time and adds cost to a job, so it should answer a real question. The question is almost always the same: there's a reason to suspect decay inside, and the decision about this tree depends on how much there is and where it is.
This guide covers the signs that should make you reach for the detector, the situations where a test pays off even without obvious signs, and how to choose where to test.
Outside signs that point to decay inside
Internal decay rarely stays completely hidden. Look for these during your visual assessment:
- Fungal fruiting bodies. Conks, brackets or mushrooms on the trunk, root flare or surface roots are the clearest sign that a decay fungus is established. The fruiting body is the small visible part of a much larger infection inside the wood.
- Cavities and open wounds. An opening is an entry point, and the decay behind it is often much larger than the hole.
- Old wounds and large pruning cuts. Especially ones that never closed, or that show dark, soft or wet wood.
- Swellings, bulges and odd growth. A tree often adds wood around a weak spot, so a bulge on the trunk or a swollen flare can be the tree compensating for decay behind it.
- Cracks and seams. Vertical cracks, and the ridges of wood that form along them, can come with internal decay or create the conditions for it.
- Dead or loose bark. Sunken or missing bark, or bark that sounds hollow, can mark dead tissue underneath.
- Hollow sound under a mallet. Sounding is quick and useful, but it only tells you something's off near the surface. It doesn't tell you how much sound wood is left.
- Root and soil damage. Trenching, grade changes, soil compaction or root cuts near the trunk can let decay into the root flare and lower trunk.
- Crown signs. Thinning, dieback or small leaves don't prove decay, but combined with any of the above they add weight.
Situations where a test is worth the time
Signs are one trigger. The situation is the other. A sonic test makes the most sense when the answer will change what you recommend:
There's a target
A tree over a house, a playground, a road, a parking lot or a busy path deserves more than a visual check if you see any of the signs above. When the consequence of failure is serious, the client needs to know how much sound wood is left, not just that something is wrong.
The client wants to keep a big or valued tree
Removal is often the easy recommendation when you're unsure. A decay map can show that a tree with a scary-looking cavity still has a solid shell, which can save a heritage tree. It can also show that a healthy-looking tree is in trouble. Either way the client gets a decision based on evidence.
Before major work
Before a heavy reduction, a cabling job or a climb on a tree with signs of decay, it's worth knowing whether the stem can carry the work and the climber. A few minutes of readings can change the plan.
Monitoring over time
If you've decided to retain a tree with decay, a test now gives you a baseline. Test again at the same height and sensor positions next year, or after a storm season, and compare. Growing pockets point one way, stable ones another. Sound Wood keeps every tree in your account so last year's map is there when you come back.
Advanced assessment
Within the ISA's Tree Risk Assessment framework, decay testing falls under an advanced (Level 3) assessment: a closer look at a specific concern that a basic assessment raised. If your basic assessment ends with "possible internal decay, extent unknown," a sonic test is the natural next step.
When a test isn't the right tool
Be honest about where it won't help. If the tree is coming out anyway, a test adds cost without changing the outcome. If decay is obviously extensive, a large open cavity you can see into, for example, a map may add little you can't already see. And if the concern is high in the crown, a trunk test at chest height won't answer it.
Very small stems are also hard to test well, because there's little room for sensor positions and the paths are short. Keep in mind too that a sonic test finds slow wood, not what caused it: cracks and seams can read as slow, so when the outside of the tree shows big cracks, factor that into how you read the map.
Choosing where on the trunk to test
Where you place the sensor positions matters as much as whether you test:
- Test where the concern is. If the conk is at the root flare, test low. If the wound is at 4 feet, test at 4 feet. A map at the wrong height can come back clean while the decay sits a foot below.
- Stay level. Keep all the positions at the same height, so you're measuring one cross-section.
- Spread the positions evenly. Even spacing gives paths that cross the whole trunk. Put a position near any visible wound so you learn whether the decay reaches the bark there.
- Set the nails in sound wood. Go through loose bark, and avoid callus ridges and the lip of a cavity where you can, because poor contact adds time and looks like decay.
- Test more than one height if needed. A tall wound or a fruiting body at the base with a seam running up the trunk can call for two levels. Save each as its own tree in the app so the maps stay separate.
- Record what you did. Note the height, where zero was and which way S1 faced. Sound Wood can store the direction and your GPS location so a retest lines up next time.
Tip: A reference reading on sound wood of the same tree, away from the problem area, is cheap insurance. It shows what normal looks like for that tree on that day.
The decision comes first
The best reason to test is a decision you can't make well without knowing what's inside. If you find yourself thinking "I'd keep it if the shell is solid" or "I'd remove it if the decay reaches the street side," that's when a sonic test pays for itself. Mark the positions, take the readings, and let the map show you which way to go, then make the call with your own judgment, the way you always have.