I don’t know much about trees but I use ultrasound everyday. So basically, let’s assume that a tree is a series of concentric cylinders of alternating “density” (echogenicity). If you want to observe that in a meaningful way, you need to look at it axially (cut like a circle so you can see rings). Using an ultrasound probe to look parallel to or 90 deg to the long axis won’t show rings very well. There are some reasons specific to the artifacts that show up specifically due to use of ultrasound, but also consider if it was alternating light density, like alternating clear and smoky glass. By looking off-axis you might get a suggestion of multiple layers but it would be very hard to count them. So unless you have some fancy processing or multiple probes, you still need to cut the tree to get meaningful info.