Because radiocarbon is everywhere the same in the atmosphere at any given time, tree rings which grew in the same year should have the same amount of radiocarbon.
By matching ring-width patterns in a specimen of known age (starting with living specimens) to ring-width patterns in an older specimen, the proper placement of the older specimen is determined.
Some critics of dendrochronology suggest that the process of pattern-matching is highly error-prone.Are the long tree-ring chronologies inaccurate due to the inability of dendrochronologists to accurately match tree-ring patterns?This is accomplished using wood specimens found preserved, for example, in historic buildings, or on the forest floor, or in peat bogs.The rings in a non-living specimen can be counted to determine the number of years the specimen spans.
We could discuss the details of pattern-matching technique or the probability of error, but there is another, more quantitative way, to determine if the long tree-ring chronologies are accurate or not.
One can use the amount of radiocarbon in the individual tree rings.