Sound WoodRetesting a tree for decay over time
Not every tree with decay needs to come down. Often the right call is to keep it, reduce the load if needed, and watch it. Watching only works if you can compare like with like. A map from this year and a map from next year are only comparable if you tested the same way both times.
Here's how to set up a sonic test so it can be repeated, and how to read the change.
Why retest at all
A single test tells you how much decay there is today. It doesn't tell you whether it's spreading. Some decay advances slowly while the tree adds new wood around it; some moves faster. Two or more tests over time are the only way to tell which you're dealing with, and that trend can matter as much as the number itself.
Retesting is also useful after an event: a storm, construction near the roots, a large pruning cut, or new fruiting bodies appearing.
Set up the first test with the second one in mind
Mark the height
Measure the height of your positions from a fixed reference, such as the soil line on the uphill side, and write it down. Note the reference too. "18 in above grade on the street side" can be found again; "about knee height" can't.
Mark zero and direction
Choose a zero mark you can find again, and record which compass direction it faces. Facing a fixed object helps: "zero faces the front door." In Sound Wood, set which way S1 faces, or use the compass button, and the map gets a north arrow.
Record the positions
Write down each position in inches clockwise from zero. Sound Wood stores them with the tree. Using the same positions next time means the same paths, which makes the comparison much more direct. Even spacing makes this easy: the same number of positions on the same circumference lands in the same places.
Photograph the setup
A photo of the tape and nails on the trunk, with the zero mark visible, is the quickest way to put things back where they were. Add it to the tree in the app.
Save the GPS location
On a large property or a street with many similar trees, knowing exactly which tree you tested matters. Sound Wood can save the tree's GPS position with the record.
On the retest
- Same height, same zero, same positions. Re-measure the circumference at that height; the tree will have grown a little, and the app needs the current figure.
- Same species and wood group. So the same cutoff is used.
- All the same pairs. If you took every pair last time, take every pair again.
- Similar season if you can. Wood moisture and temperature affect sound speed. Very cold or frozen wood in particular can change readings. Testing at a similar time of year reduces that variable.
Save the retest as a new tree record, with a name that makes the pair obvious, like "Oak 7, 2027". That keeps both maps intact for side-by-side comparison.
Comparing the maps
Compare the range, not just one number
Look at both the confirmed minimum and the "up to" figure. If the minimum rose from 8% to 15%, the confirmed decay has grown. If only the "up to" figure changed, look at your coverage before drawing conclusions; the uncertainty may simply have shifted.
Compare where, not just how much
Has the pocket moved toward the bark? Has it reached a new position? Decay breaking out to the bark on one side can matter more than a small change in percentage.
Compare path by path
Because you used the same positions, you can compare the same paths directly. A path that went from green to red is a clear signal. A path that's a little slower but still green is within normal variation.
Allow for normal variation
No two tests are identical. Small differences in contact, moisture and tap strength move readings a bit. Look for consistent changes across several paths, not one reading that moved.
Tip: If one path changed a lot and nothing around it did, re-check it before you report it. A loose nail or a typo is more likely than a sudden pocket.
How often to retest
There's no fixed rule. It depends on how much decay there is, how close to your action threshold it is, the species, the target, and how the tree is responding. Many arborists start with a yearly retest for a tree they're monitoring and adjust from there: less often if it's stable, more often if it's changing or the target is serious. Whatever interval you choose, put it in the report so the client knows when you're coming back.
Keeping it all in one place
Retesting gets much easier when last year's record is on your phone at the tree. Sound Wood keeps every tree in your account, synced across devices, with the positions, height, direction, photos and notes. Open last year's tree, copy the setup, take the readings, and compare the two maps right there.