What is dendrochronology

A.� It's the study of tree rings and is used mainly as a dating technique for archaeology. This technique was first used in the 1920s by A.E. Douglass, who worked at the University of Arizona, in his dating of the Pueblo Bonito, a prehistoric Native American settlement in New Mexico.

�

Q.� What's so special about tree rings

A.� As a tree grows it puts on a layer of new wood under the bark. The thickness of that layer, the tree-ring, depends on various factors, particularly climate. In a good growing year, wider rings are formed, in a bad year, narrower rings are produced, but each ring is relatively unique, so when seen together distinct patterns are apparent.

�

Q.� What are tree-ring chronologies

A.� They are reference data, built up using overlapping ring patterns from trees grown in the same general climatic region, usually plotted onto graphs, from a number of trees of varying age.

�

First you start with a tree of known age, then work back matching the patterns of successively older trees, a technique known as cross dating. Chronologies around the world going back for thousands of years have been built up in this way.

�

Q.� How are timbers from archaeological sites dated using tree ring chronologies

A.� By matching the pattern of ring growth from the ancient sample with a reference data chronology, rather like finger printing. From this archaeologists can tell, to the exact year. when the tree was felled.

�

Q.� Are tree rings studied for reasons other than dating

A.� Yes, although dendrochronology is mainly used as a dating technique, tree rings are also represent the seasonal conditions that existed year on year through a tree's life. By studying the patterns of tree rings, scientists can trace back rainfall, wind, snow, droughts, fire and volcanic activity for thousands of years.

�

Do you want to know how any other scientiifc dating technique works Just click here to ask.

�

by Lisa Cardy

�