Dendrochronology is a scientific method used to date samples of wood based on the simple observable fact that trees grow by putting on a new ring of sap-wood every year of their lives. Accordingly samples of ancient wood provide us with a record of their growing lives.

Records of individual trees can be compared, matched and overlapped and, ultimately, – when a sufficient number of such samples have been found and examined – continuous tree-ring chronologies can be established against which finds of new wood samples (e.g. from ancient structures or ancient wooden implements) can be compared and hence dated.

When suitable samples are found, dendrochronology has the capacity to be exceedingly accurate, for example it can tell us the precise year in which a tree or a branch stopped living (was felled etc).  While this cannot be taken in all cases to be the same date as the use of the wood in question for the making of an implement or a statue etc., or its incorporation into a building or structure (for instance, an old piece of word could have been so-used), at very least it can tell us the object’s terminus post quem.  This is the earliest possible date (or the date after which) that object or structure was made.

An additional use of dendrochronology has been to provide us with a very precise check on radiocarbon dating, i.e. as an aid for calibration.

The word dendrochronology is derived from the Greek words for a tree-limb and for time. The study was developed originally in the first half of the 20th century in Arizona.

Growth rings, or ‘annual rings’ as they are also called, can be seen easily in the cross-section of any tree trunk. A ring usually marks the passage of one year in the life of the tree and, depending on width and other detectable variations, also reflects the local environmental and climatic conditions in that year. Trees from the same region will tend to develop the same patterns of rings for a given period.  Different types of trees are suitable for dendrochronology in different parts of the world.  In Ireland the long-living oak has proved very suitable.

As with radiocarbon dating, we are very lucky in Ireland that one of the leading laboratories for this kind of study is located at Queen’s University Belfast. The work of Prof. Michael Baillie and his colleagues there has been of exceptional importance. By assembling and studying the annual growth rings of about 10,000 samples originally belonging to oak-trees from this country, a continuous chronology as far back as c. 5,200 BC is now available and being made use of widely in Irish archaeology.