Dendrochronology dating method
But for the specimen to be useful in extending the tree-ring chronology, the absolute calendar age of its rings must be determined.
The annual growth rings vary in thickness each year depending on environmental factors such as rainfall.
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.
Some critics of dendrochronology suggest that the process of pattern-matching is highly error-prone.
But another independent check came along which was even better than the Douglas fir chronology.
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.
These measurements demonstrated the basic validity of the science of dendrochronology.
If the method had a large component of random error due to inaccurate pattern matching, how could such detailed agreement between the radiocarbon in the rings of two independent dendrochronologies be possible?
The pattern of radiocarbon in the rings showed a maximum divergence, even at very old ages, of only around 40 years.