Dendrochronology
(tree-ring dating)

Dendrochronology, from the Greek dendron (tree) and chronology (time) is the process of dating historic timbers through the scientific study and comparison of tree rings to establish a felling date or date range. 

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The basis of dendrochronological, or tree-ring, dating is that trees of the same species, growing at the same time, in similar habitats, produce similar ring-width patterns. These patterns of varying ring-widths are unique to the period of growth. Each tree naturally has its own pattern superimposed on the basic ‘signal’, resulting from genetic variations in the response to external stimuli, the changing competitive regime between trees, damage, disease, management etc. Anyone familiar with finding a tree stump from a recently felled tree, will notice that this has certain characteristics, in particular, the lighter band of wood around a darker centre, or heartwood. This lighter area is the sapwood, or the living part of the tree, is important when seeking to ascribe a precise date or date range, and will be covered in detail below.

 

In much of Britain the major influence on the growth of a species like oak is, however, the weather conditions experienced from season to season. By taking several contemporaneous samples from a building or other timber structure, it is often possible to cross-match the ring-width patterns, and by averaging the values for the sequences, maximise the common signal between trees. The resulting ‘site chronology’ may then be compared with existing ‘master’ or ‘reference’ tree-ring chronologies. These include chronologies made by colleagues in other countries, most notably areas such as modern Poland, which have proved to be the source of many boards used in the construction of doors and chests, and for oil paintings before the widespread use of canvas.

 

This process can be done by a trained dendrochronologist using plots of the ring-widths and comparing them visually, which also serves as a check on measuring procedures. It is essentially a statistical process, and therefore requires sufficiently long sequences for one to be confident in the results. There is no defined minimum length of a tree-ring series that can be confidently cross-matched, but as a working hypothesis most dendrochronologists use series longer than at least fifty years.

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One can develop long reference chronologies by cross-matching the innermost rings of modern timbers with the outermost rings of older timbers successively back in time, adding data from numerous sites. Data now exist covering many thousands of years, and it is, in theory, possible to match a sequence of unknown date to this reference material.

 

The chances of matching a single sequence are not as great as for matching a tree-ring series derived from many individuals, since the process of combining individual series will remove variation unique to an individual tree, and reinforce the common 'signal' resulting from widespread influences such as the weather. However, a single sequence can be successfully dated, particularly if it has a long ring sequence.

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When interpreting the information derived from the dating exercise, it is important to take into account such factors as the presence or absence of sapwood on the sample(s), which indicates the outer margins of the tree. Where no sapwood is present it may not be possible to determine how much wood has been removed, and one can therefore only give a date after which the original tree must have been felled. Where the bark is still present on the timber, the year, and even the time of year of felling can be determined. In the case of incomplete sapwood, one can estimate the number of rings likely to have been on the timber by relating it to populations of living and historical timbers to give a statistically valid range of years within which the tree was felled. For this region the estimate used is that 95% of oaks will have a sapwood ring number in the range 11 – 41 (Miles 1997).

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Ross Cook works as an Associate Dendrochronologist of the Oxford Dendrochronological Laboratory, with Dr Martin Bridge and Dr Dan Miles, with whom all dendrochronology is undertaken. We offer tree-ring dating services across England and Wales. Website: www.oxford-dendrolab.com