In 1929 A.E. Douglass showed how dendrochronology could be used to date archaeological material. By matching the ring pattern of timbers collected from a restricted area in the southwest United States, and therefore controlled by the same climactic variation, he built up a master plot extending from the present back to the period of the pre-Columbian Pueblo villages. Any timber recovered from a site, preserved there by near desert conditions, could be dated exactly by matching its rings against the plot.


The technique relies on the fact that many trees have an annual growing cycle. In the spring and summer large cells are produced which become smaller towards winter, this creates a series of rings. Trees grow outwards and the inner most ring of the tree was produced when it first began growing. Not only do many trees have annual growth rings but they are also sensitive to general climatic changes around them. Thus a tree that suffers a drought one year may produce a very narrow ring. The varying of widths within tree rings can correspond between trees of the same, and even different species, from the same region.


The variation of tree-ring widths is the key to dating wooden items. Trees of a known age, for instance felled in 1999, may have a ring sequence extending back a few hundred years. The pattern of the ring widths can be used to match to other samples of wood with no known age and provide a date for them. These in turn may have earlier rings in their sequence than those found in the first tree and hence the sequence can be extended back further. The creation and extension of regional chronologies has meant that even items originating from prehistoric times can often be dated.


Any date given, however, can only relate to the latest ring in the sequence and artefacts that have had outer rings trimmed from them during manufacture will thus be given a date after which they have been created but not specific to the actual year. Equally the timber may not have been used immediately to produce the tool so dendrochronology gives the date the tree was felled not the date the tool was produced.


The technique is also limited to areas where climactic variation (of rainfall or temperature) was sufficiently large, where timber was much used, and where enough of it has been preserved (by desiccation, waterlogging or charring) to be worked on. Despite these restrictions it has given many useful results, notable in southwestern USA, Alaska and Scandinavia. This technique has allowed useful checks to be made on the validity of radiocarbon dating.