I thought that was on the ocean’s floor where the changes reflect a zebra-striped pattern. So the tree rings have iron cells or compounds that line up?
Paleomagnetic Dating uses dendrochronological sequences where available and apropos to track the shifting locations of the magnetic pole based on remanent magnetic fields expressed in baked clay objects and long slow deposits of magma. The magnetic pole shift recorded in layers of rock at the major plate boundaries was instrumental in the so-called plate tectonics revolution in the 1950s-60s. This particular sample is huge, and right near the RC limit, although I'm not too sure this is absolutely the first time it's been done this way. There's obviously an advantage to having the magnetic field lines recorded in the actual sample that is getting RC dated.