Some tree rings don’t form around the entire trunk. These "wedging rings" are usually caused by poor growth conditions – here, probably by a cold summer.
Wedging rings complicate the exact dating of a sample, as was the case with this disc. Researchers will end up with different values depending on which radius is chosen when counting the annual rings from pith to bark. If researchers only take cores - as is usual when investigating living trees - they risk "missing out" on annual rings.
However, since wedging rings don’t occur simultaneously in all trees of a given site, researchers sample cores from as many trees as possible from the same site.
They then overlay the growth patterns of each tree to locate missing rings. In this way, each annual ring is dated to the correct calendar year.
This basic principle of tree-ring research is referred to as "crossdating".