Dendrochronology (from δένδρον, dendron, "tree limb"; χρόνος, khronos, "time"; and -λογία, -logia) or tree-ring dating, is the scientific method of dating based on the analysis of patterns of tree rings, also known as growth rings. Dendrochronology can date the time at which tree rings were formed, in many types of wood, to the exact calendar year. This has three main areas of application: paleoecology, where it is used to determine certain aspects of past ecologies (most prominently climate); archaeology and the history of art and architecture, where it is used to date old panel paintings on wood, buildings, etc.; and radiocarbon dating, where it is used to calibrate radiocarbon ages (see below).
In some areas of the world, it is possible to date wood back a few thousand years, or even many thousands. Currently, the maximum for fully anchored chronologies is a little over 11,000 years from present.
Dendrochronology was developed during the first half of the 20th century originally by the astronomer A. E. Douglass, the founder of the Laboratory of Tree-Ring Research at the University of Arizona. Douglass sought to better understand cycles of sunspot activity and reasoned that changes in solar activity would affect climate patterns on earth which would subsequently be recorded by tree-ring growth patterns (i.e., sunspots → climate → 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 layer of cells near the bark that is classified as a lateral meristem. This growth in diameter is known as secondary growth. Visible rings result from the change in growth speed through the seasons of the year, thus one ring usually marks the passage of one year in the life of the tree. The rings are more visible in temperate zones, where the seasons differ more markedly.
The inner portion of a growth ring is formed early in the growing season, when growth is comparatively rapid (hence the wood is less dense) and is known as "early wood" or "spring wood" or "late-spring wood". The outer portion is the "late wood" (and has sometimes been termed "summer wood", often being produced in the summer, though sometimes in the autumn) and is denser. "Early wood" is used in preference to "spring wood", as the latter term may not correspond to that time of year in climates where early wood is formed in the early summer (e.g. Canada) or in autumn, as in some Mediterranean species.
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. Adequate moisture and a long growing season result in a wide ring. A drought year may result in a very narrow one. Alternating poor and favorable conditions, such as mid summer droughts, can result in several rings forming in a given year. Missing rings are rare in oak and elm trees—the only recorded instance of a missing ring in oak trees occurred in the year 1816, also known as the Year Without a Summer. 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, until computers were harnessed 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 a section against another chronology (tree ring history) whose dates are known. Fully anchored chronologies which extend back more than 11,000 years exist for river oak trees from South Germany (from the Main and Rhine rivers) and pine from Northern Ireland. Furthermore, the mutual consistency of these two independent dendrochronological sequences has been confirmed by comparing their radiocarbon and dendrochronological ages. Another fully anchored chronology which extends back 8500 years exists for the bristlecone pine in the Southwest US (White Mountains of California). In 2004 a new calibration curve INTCAL04 was internationally ratified for calibrated dates back to 26,000 Before Present (BP) based on an agreed worldwide data set of trees and marine sediments. The part of the new calibration curves that relies on tree-ring evidence (IntCal04) dates back to 12,410 calendar (cal) yr B.P. Beyond that and back to 14,700 cal yr B.P., IntCal04 is mainly constructed from 14C dates of foraminifera from Venezuela's Cariaco basin that are corrected for a constant reservoir age of 405 years.
Sampling and dating
Timber core samples 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.
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, through the estimation of a date range formed through the interception of radiocarbon (B.P., or 'B'efore 'P'resent, where present equals 1950-01-01) and calendar years. 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. In some regions dating sequences of more than 10,000 years are available.
Similar seasonal patterns also occur in ice cores and in varves (layers of sediment deposition in a lake, river, or sea bed). 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. Some columnar cactus also exhibit similar seasonal patterns in the isotopes of carbon and oxygen in their spines (acanthochronology). 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 archaeologists and paleoclimatologists.
A similar technique is also used to analyse fish stocks through the analysis of similar growth rings in the otolith bones of fish.
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.
European chronologies derived from wooden structures initially found it difficult to bridge the gap in the 14th century when there was a building hiatus which coincided with the Black Death, however there do exist unbroken chronologies dating back to prehistoric times, for example the Danish chronology dating back to 352 BC.
Given a sample of wood, the variation of the tree ring growths provides not only a match by year, it can also match location because the climate across a continent is not consistent. This makes it possible to determine the source of ships as well as smaller artifacts made from wood but which were transported long distances, such as panels for paintings.
In areas where the climate is reasonably predictable, trees develop annual rings of different properties depending on weather, rain, temperature, soil pH, plant nutrition, CO2 concentration, etc. in different years. These variations are used in dendroclimatology to infer past climate variations.
Dendrochronology has become important to art historians in the dating of panel paintings. However, unlike analysis of samples from buildings which are typically sent to a laboratory, wooden supports for paintings usually have to be measured in a museum conservation department, which places limitations on the techniques that can be used,
In addition to dating, dendrochronology can also provide information as to the source of the panel. Many Early Netherlandish paintings have turned out to be painted on panels of "Baltic oak" shipped from the Vistula region via ports of the Hanseatic League. Oak panels were used in a number of northern countries such as England, France and Germany. Wooden supports other than oak were rarely used by Netherlandish painters.
Since panels of seasoned wood were used, an uncertain number of years has to be allowed for seasoning when estimating dates. Panels were trimmed of the outer rings, and often each panel only uses a small part of the radius of the trunk. Consequently, dating studies usually result in a "terminus post quem" (earliest possible) date, and a tentative date for the actual arrival of a seasoned raw panel using assumptions as to these factors. As a result of establishing numerous sequences, it was possible to date 85% - 90% of the 250 paintings from the 14th to 17th century analysed between 1971 and 1982; by now a much greater number have been analysed.
A portrait of Mary, Queen of Scots in the National Portrait Gallery, London was believed to be an 18th-century copy. However, dendrochronology revealed that the wood dated from the second half of the 16th century. It is now regarded as an original 16th century painting by an unknown artist.
On the other hand, dendrochronology was applied to four paintings depicting the same subject, that of Christ expelling the money-lenders from the Temple. The results showed that the age of the wood was too late for any of them to have been painted by Hieronymus Bosch.
While dendrochronology has become an important tool for dating oak panels, it is not effective in dating the poplar panels often used by Italian painters because of the erratic growth rings in poplar.
The 16th century saw a gradual replacement of wooden panels by canvas as the support for paintings which means the technique is less often applicable to later paintings. In addition, many panel paintings were transferred onto canvas or other supports during the 19th and 20th centuries.
The dating of buildings with wooden structures and components has also been done by using dendrochronology. An example is the Fairbanks House in Dedham, Massachusetts. While the house had long been claimed to have been built circa 1640 (and being the oldest wood-framed house in North America), core samples of wood taken from a summer beam confirmed the wood was from an oak tree felled in 1637–8. An additional sample from another beam yielded a date of 1641, thus confirming the house had been constructed starting in 1638 and finished sometime after 1641 as wood was not seasoned before use in building at that time in New England.
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|Wikimedia Commons has media related to Growth rings.|
- Oxford Tree-Ring Laboratory
- Nottingham Tree-Ring Dating Laboratory
- Bibliography of Dendrochronology
- Multilingual Glossary of Dendrochronology
- Digital Collaboratory for Cultural Dendrochronology (DCCD)
- International Tree-Ring Data Bank
- Ultimate Tree-Ring Web Pages
- Laboratory of Tree-Ring Research University of Arizona