So maybe similiar to a ringbark?
Maybe 3/16” deep with a sharp razor?
Maybe down to cambium levels....makes sense
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I had done some JBP air layers back in the 1980's and 1990's, a couple lifetimes ago. One would form lots of callus, but no roots. When toward the end of the 2nd growing season I cut exactly as you have the red line (without severing the layer from the understock). Re-wrapped, and the following spring there was a nice set of roots. 2 things with my example, the cutting off of the bottom side of the callus, and the allowing the tree to go dormant, experience frosts and freezes, and cold storage, then a warm up in spring. Which worked? Cutting the callus? or the cold dormancy?
I would take a sub-set of these, move them to pots, acclimate them to growing outdoors, let them vernalise in autumn (get cold) store them for winter just barely above freezing. See if that stimulates rooting.
I had a JWP that I tried air layering. 4 years, it formed a nice disk of callus. each summer, I would slice off the bottom 25% of the callus, re-wrap and hope for the best. At the end of the 4th growing season for the air layer there was a big knot of callus but zero roots. In frustration I terminated the experiment.
Last thought, we (family) own a blueberry farm. We take cuttings in autumn, beginning a few weeks before leaf drop. We continue to take cuttings after leaf drop. Strike cuttings in peat, with zero hormone use. Leave cuttings outdoors, in bright shade, but otherwise fully exposed to the elements. By spring all the cuttings that will make it will have formed callus. The cuttings then push just a small number of small leaves. They then sit looking like nothing is happening until August. Pulled out before August, the cuttings are just callus at the base. Then in August a flush of roots develop, and the cutting begins to add more leaves and a few short branches. August is when night temperatures drop on the farm. Beginning of August, nights are above 65 F most nights. By August 15 or 20, nights are dropping below 65 F for 4 out of 7 nights per week. I believe environmental triggers are responsible for the callus switching to root formation, as
@River's Edge mentioned. Cuttings were moved to individual nursery pots to size up before planting in the field, the following spring, so 14 to 18 months after taking the cuttings they had enough roots to move to nursery pots. A few cuttings required a second winter before their callus formed roots, so they took 26 to 30 months. A few still had not formed roots and they were discarded. We started with about 100 cuttings ended up with 40 or so plants to stick back in the "holes" in the blueberry rows.
With the cutting method described, we were getting about 50% successful strike rate. This was just "letting the the flats sit in the wild". With more controlled situation, controlled humidity, etc, might easily get higher strike rate.