dendrochronology - Overview, More detail, Scientific value

The construction of archaeological chronologies from annual tree-ring sequences. Rings vary in width and structure from year to year, depending on the prevailing climatic conditions; overlapping patterns observed in preserved timbers can therefore be matched and linked to form an accurate and absolute chronology extending back unbroken from the present day. Notable sequences derived from the long-lived Californian bristlecone pine (Pinus aristata, to c.6700 BC) and Irish bog oaks (to c.5300 BC) have proved a crucial check on radiocarbon dates.

Dendrochronology or tree-ring dating is the method of scientific dating based on the analysis of tree-ring growth patterns.

Overview

Many trees in temperate zones grow one growth ring each year, the newest ring being under the bark. For the entire period of a tree's life, a year-by-year record or ring pattern is formed that reflects the climatic conditions in which the tree grew. Trees from the same region will tend to develop the same patterns of ring widths for a given period. These patterns can be compared and matched ring for ring with trees growing in the same geographical zone and under similar climatic conditions.

To eliminate individual variations in tree ring growth, dendrochronologists take the smoothed average of the tree ring widths of multiple tree samples to build up a ring history. A tree ring history whose beginning and end dates are not known is called a floating chronology. It can be anchored by cross-matching either the beginning or the end section against the end sections of another chronology (tree ring history) whose dates are known.

In areas where the climate is reasonably predictable, trees develop annual rings of different properties depending on weather, rain, temperature, etc.

More detail

Timber core samples are used to measure the width of annual growth rings. for example, ancient timbers found in buildings can be dated to give an indication of when the source tree was alive and growing, setting an upper limit on the age of the wood.

Scientific value

A benefit of dendrochronology is that it makes available specimens of once-living material accurately dated to a specific year to be used as a calibration and check of radiocarbon dating. The bristlecone pine, being exceptionally long-lived and slow growing, has been used for this purpose, with still-living and dead specimens providing tree ring patterns going back thousands of years.

The dendrochronologist faces many obstacles, however, including some species of ant which inhabit trees and extend their galleries into the wood, thus destroying ring structure. These are used for dating in a manner similar to dendrochronology, and such techniques are used in combination with dendrochronology, to plug gaps and to extend the range of the seasonal data available to archeologists.

While archaeologists can use the technique to date the piece of wood and when it was felled, it may be difficult to definitively determine the age of a building or structure that the wood is in. The wood could have been reused from an older structure, may have been felled and left for many years before use, or could have been used to replace a damaged piece of wood.