Dendrochronology
Dendrochronology or tree-ring dating is the method of scientific dating based on the analysis of tree-ring growth patterns. This technique was invented and developed during the 20th century originally by A. E. Douglass, the founder of the Laboratory of Tree-Ring Research at the University of Arizona. The technique can date wood to exact calendar years.
Overview
Many trees in temperate zones grow one growth ring each year, the newest ring being 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. Adequate moisture and a long growing season result 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 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. Following these tree-ring patterns from living trees back through time, chronologies can be built up, both for entire regions, and for sub-regions of the world. 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. Nowadays, computers are used to do the statistical matching.
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. This process is termed replication. 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. Fully anchored chronologies which extend back more than 10,000 years exist for river oak trees from South Germany (from the Main and Rhine rivers). A fully anchored chronology which extends back 8500 years exists for the bristlecone pine in the Southwest US (White Mountains of California).
In areas where the climate is reasonably predictable, trees develop annual rings of different properties depending on weather, rain, temperature, etc. in different years. These variations may be used to infer past climate variations — see dendroclimatology.
More detail
Timber core samples are used to measure the width of annual growth 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 genera of 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 consistent than in humid areas. These tools have been important in archaeological dating of timbers of the cliff dwellings of Native Americans in the arid Southwest. Template:Clr
Scientific value
A benefit of dendrochronology is that it makes available specimens of o nce-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.[1]
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.
Similar seasonal patterns also occur in ice cores and in varves (layers of sediment deposition in a lake or river). The deposition pattern in the core will vary for a frozen-over lake versus an ice-free lake,and with the fineness of the sediment. 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.
See also
- Dendroclimatology
- Dendroarchaeology
- Ice cores
- Varve
- Dating methodology (archaeology)
- Post excavation
External links
- Henri D. Grissino-Mayer's Ultimate Tree Ring Web Pages
- The Laboratory of Tree-Ring Research
- Wiener Laboratory for Aegean and Near Eastern Dendrochronology at Cornell University website
- Dendrochronology on musical instruments and art objects
- University of Tennessee Dendrochronology
- Oxford Dendrochronology Laboratory
- University of Victoria Tree Ring Laboratory
- Online Glossary of Dendrochronology
- Mount Allison University Dendrochronology Laboratory
