Methods of dating from tree rings
Growth rings, also referred to as tree rings or annual rings, can be seen in a horizontal cross section cut through the trunk of a tree.Growth rings are the result of new growth in the vascular cambium, a lateral meristem, and are synonymous with secondary growth.Thus wood from ancient structures can be matched to known chronologies (a technique called cross-dating) and the age of the wood determined precisely.Cross-dating was originally done by visual inspection, until computers were harnessed to do the statistical matching.Many trees in temperate zones make one growth ring each year, with the newest adjacent to 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.
These tools have been important in archaeological dating of timbers of the cliff dwellings of Native Americans in the arid Southwest.
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.
By taking samples from different sites and different strata within a particular region, researchers can build a comprehensive historical sequence that becomes a part of the scientific record; 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.
Some genera of trees are more suitable than others for this type of analysis.