Dendrochronology or tree-ring dating is the method of scientific dating based on the analysis of tree ring patterns, a technique that was invented and developed during the 20th century. Simply stated, trees in temperate zones grow one growth-ring each year. Trees develop annual rings of different thickness depending on weather, rain, temperature, etc. in different years. For the entire period of a tree's life, a year-by-year record or ring pattern is formed that in some way reflects the climatic conditions in which the tree grew. Adequate moisture and a long growing season results in a wide ring. A drought year may result in a very narrow one. Trees from the same region will tend to develop the same patterns of width 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. Following these tree-ring patterns from living trees back through time, chronologies can be built up. We can thus compare wood from old or ancient structures to our known chronologies, match the ring patterns (a technique called cross-dating), and determine precisely the age of the wood used by the ancient builder.
Laboratory analysis of timber core samples measures the width of annual rings. 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 trees are more suitable than others for this type of analysis. Likewise, in areas where trees grew in marginal conditions such as aridity or semi-aridity, the techniques of dendrochronology are more consistant that in humid areas. These tools have been important in archaeological dating of timbers of the cliff dwellings of Native Americans in the arid Southwest.
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. For dating purposes, in some regions sequences of more than 10,000 years are available.