Tree cross section

The reference chronology may cover several thousand years for some species of trees. Ring patterns from wooden objects of different types, (including paintings on panel) can be compared against this master chronology record, and if the patterns of ring widths match up, an estimate of their likely age can be given.

Dendrochronology

For an oak tree trunk section felled at a known date, its age can be determined by simply counting the rings back from the last one under the bark. For many objects made from wood, however, dating is not this straightforward. If the sequence of measured ring widths in the object can be matched to a dated chronology, only then can we hope to establish a date.

In most cases the bark edge no longer remains on wooden objects, as it would be trimmed off when the trunk was cut into planks. Many of the outer rings might be missing. Also, we may have no idea when the tree was cut down or how old the wood is, other than a general interpreted date from what the item is – for instance if it is part of a mediaeval barn roof, or an 18th century piece of furniture.

When an oak tree was cut up and made into things, both the bark and sapwood (still present on the tree trunk section) would be removed, so even if a match for the measured ring widths can be established with a dateable chronology, there is no surviving outer ring under the bark giving a reference to when the tree was actually felled. An estimate must be made of the number of missing rings. Once this is done, we can suggest an earliest felling date for the tree from which the items were made. This has to be estimated for wood from a particular region at a particular time, and a likely date range is all that can be estimated.

As an added complication, we may not know how long the felled wood was stored for before it was used. Each year as a tree grows it forms a new ring of water conducting pipes under its bark. In oak the pipes are larger in spring than later in the year. The rings are thicker in good growth years and thinner in poor growth years.

Did you know?

The world’s oldest living tree is thought to be a bristlecone pine in the White Mountains of California, USA. It is 4,723 years old. It was nicknamed ‘Methusela’ by Dr Edmund Schulman, the scientist who identified its age.