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Is dendrochronology absolute dating
technique for measuring time intervals and dating events and environmental changes by reading and dating the pattern (number and condition) of annual rings formed in the trunks of trees.
In a very dry year growth will be restricted, and the ring narrow, while a wet and humid year will produce luxuriant growth and a thick ring.
By comparing a complete series of rings from a tree of known date (for example, one still alive) with a series from an earlier, dead tree overlapping in age, ring patterns from the central layers of the recent tree and the outer of the old may show a correlation which allows the dating, in calendar years, of the older tree.
The central rings of this older tree may then be compared with the outer rings or a yet older tree, and so on until the dates reach back into prehistory.
Problems that arise are when climatic variation and suitable trees (sensitive trees react to climatic changes, complacent trees do not) are not be present to produce any significant and recognizable pattern of variation in the rings.
Another problem is that there may be gaps in the sequences of available timber, so that the chronology 'floats', or is not tied in to a calendrical date or living trees: it can only be used for .
Also, the tree-ring key can only go back a certain distance into the past, since the availability of sufficient amounts of timber to construct a sequence obviously decreases.
Only in a few areas of the world are there species of trees so long-lived that long chronologies can be built up.
This method is especially important in the southwestern United States, Alaska, and Scandinavia, dating back to several thousand years .
Since timber can be dated by radiocarbon, dates may be obtained from dendrochronologically dated trees.
It has been shown that the radiocarbon dates diverge increasingly from calendrical dates provided by tree-rings the further back into prehistory they go, the radiocarbon dates being younger than the tree-ring dates. Douglass first showed how this method could be used to date archaeological material.