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To determine the absolute age of wood and organic artifacts.
A scientific date is either absolute (specific to one point in time) or relative (younger or older than something else). Dendrochronology, or tree-ring dating, provides absolute dates in two different ways: directly, and by calibrating radiocarbon results.
Direct Dating of Wood
Cross-dating determines the age of undated wood by directly matching ring patterns with trees of known age.
Greatly simplified, the process samples living and dead trees in a given area. The tree-ring patterns are matched, and laid down in series, building a continuous timeline of known dates. Once the timeline exists, the age of similar wood (e.g., from a nearby house) can be established by pattern-matching.
The ultimate tree-ring chronology is the 'master' timeline of bristlecone pines - a chronology spanning more than 9,000 years.
Methuselah is a bristlecone pine, and the world's oldest living thing. His growth rings document nearly 47 centuries of survival. Bristlecones grow so slowly that a century of tree rings adds less than an inch of girth. The precise, extended chronology of these trees is directly responsible for the accuracy of radiocarbon dating.
Founded on a false belief that levels of carbon isotopes never vary, initial radiocarbon dates were commonly off by hundreds of years. Because wood can be dated directly and by radiocarbon, scientists used bristlecone pines to calculate a new calibration curve, and convert radiocarbon results into accurate calendar dates.