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The use of the annual rings of trees to date historical events. The dating of archaeological sites depends largely on cross dating. This involves taking small cores from old living trees and comparing them with the rings of timbers at the site. The year the timbers were felled is determined by finding the point at which ring patterns of the living tree correspond to those of the archaeological specimens. Bristlecone pines (Pinus longaeva) are often used as they can live for over 4000 years. If sufficiently aged living trees cannot be found in a particular region then chronologies may be built up successively further back in time by matching the ring patterns of a number of wood samples whose ages overlap.
Annual rings also provide a record of past environmental conditions. Thus ring width is normally positively correlated with water abundance. The presence and level of certain pollutants, e.g. lead, may also be recorded in the rings. It is necessary in such work to ensure that only those tree species that produce one growth ring a year are used. Certain trees, e.g. many juniper species, produce multiple rings each year, while in others it may not be possible to distinguish one season's growth from the next.
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