As trees grow they produce annual rings of their growth - relating ultimately to the climate and nutrients they received in the growing season. A tree will therefore store this information in its wood - even if it is cut down this pattern, like a barcode, will be retained for the period of growing.
Comparison of several trees growing in the same area at the same time will produce a similar, but not identical, pattern. If a sequence of 50 tree-rings can be matched to a master sequence of all the growing seasons, then a date for a timber can (sometimes) be established.
Dendrochronology is that tree-ring dating. In Britain a master chronology of all the growing seasons has been obtained from the present to the first Palaeolithic trees in Britain, over 10, 000 years ago. A core of oak can then be taken from a timber, (be it a structural building part such as a cruck) or artefact, and compared to the master chronology - there are several master chronologies for different parts of the country. Statistical comparison between the sample and a master chronology can then be used to provide a date of when the sample started growing and was felled.
However, it is important to note that at least 50 rings need to be matched, that the samples may be of re-used wood and that seasoning of wood may have occurred between the wood being felled and actually used. Furthermore, under circumstances of disease or poor weather no or false rings may be produced causing problems in analysis. Currently there is only a master chronology going back into prehistory for oak - other species were used in the past. In the northeast there have been wood imports from the Baltic area of fir and spruce trees since the Medieval period for structural building work. Current research is ongoing on producing a master chronology for these woods.
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