The awkward part of counting tree rings is that the tree may have left one out.[1] A sample in the University of Arizona's guide to ring-growth anomalies has four full rings along its lower portion and only three along the upper portion: one ring narrows away until it is locally absent.[1] Counting carefully in the wrong place still gives the wrong answer.
The opposite problem exists too: growth within a single year can contain a false band that looks like another annual boundary.[1] The guide shows a dark band fading back into earlywood, rather than marking the start of a new year.[1] A tree has no particular obligation to make its paperwork easy.
Dendrochronologists handle these problems through crossdating: comparing sequences of growth across samples.[2] Trees responding to a shared regional climate can record recognizable patterns of relatively wide and narrow rings.[2] Researchers combine crossdated material into a dated reference, a master chronology, against which another sample can be checked.[2]
The pattern does more work than a single conspicuous ring. If a sample's sequence agrees with the reference and then slips out of alignment, that discrepancy tells the researcher to look for a missing or false ring.[2] Matching also makes it possible to date preserved dead wood whose final year of growth wasn't already known.[2]
The reason to be this fussy is clearest in Sonya Odsen and Cameron Naficy's account of crossdating for fire-history research. When fire scars in trees across an area date to the same year, they provide evidence of a fire affecting that area in that year.[2] Let missing or false rings shift the dates, and the same collection can instead suggest scattered trees burning over several years.[2]
That is a much bigger error than getting a tree's birthday slightly wrong. It changes the event you think happened.
There is an unexpectedly useful quality in an uneven record. Arizona's explanation of sensitive and complacent growth describes why a varied sequence of wide and narrow rings is easier to match than a long run of nearly identical rings.[3] A short or sufficiently uniform sample may not be crossdateable at all.[3] More confident counting cannot supply a pattern that isn't there.
This is the part worth keeping when the schoolroom explanation gets compressed to “one ring, one year.” The count is a starting point. Assigning calendar years takes comparisons, attention to the wood itself, and sometimes a decision that the sample does not support an answer. The other trees help establish what happened to this one.
Sources
[1] University of Arizona: Ring-growth anomalies
[2] fRI Research: The truth is in the tree rings, if you crossdate them
[3] University of Arizona: Sensitive vs. complacent tree-ring growth