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. As of 1995, the maximum reported fully anchored chronologies is ~11,000 years before the 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, critical for the title method, one ring generally 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.
Many trees in temperate zones make one growth ring each year, with the newest adjacent to the bark. Hence, for these, for the entire period of a tree's life, a year-by-year record or ring pattern is formed that reflects the age of the tree and the climatic conditions in which the tree grew. Adequate moisture and a long growing season result in a wide ring, while a drought year may result in a very narrow one.
Direct reading of tree ring chronologies is a learned science, for several reasons. First, contrary to the single ring per year paradigm, alternating poor and favorable conditions, such as mid-summer droughts, can result in several rings forming in a given year. In addition, particular tree species may present "missing rings", and this influences the selection of trees for study of long time spans. For instance, missing rings are rare in oak and elm trees.
Critical to the science, trees from the same region tend to develop the same patterns of ring widths for a given period of historical study. These patterns can be compared and matched ring for ring with trees growing at the same time, in the same geographical zone (and therefore under similar climatic conditions). When these tree-ring patterns are carried back, from tree to tree in the same locale, in overlapping fashion, chronologies can be built up—both for entire geographical regions and sub-regions. Moreover, wood from ancient structures with known chronologies can be matched to the tree ring data (a technique called cross-dating), and the age of the wood can thereby be determined precisely. Cross-dating was originally done by visual inspection; computers have been harnessed to do the task, applying statistical techniques to assess the 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, a process 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 extending back more than 11,000 years exist for river oak trees from South Germany (from the Main and Rhine rivers) and for pine from Northern Ireland. The consistency of these two independent dendrochronological sequences has been supported through comparison of 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).
Sampling and dating
Dendrochronology makes available specimens of once-living material accurately dated to a specific year. Dates are often represented as estimated calendar years B.P., for before present, where "present" refers to 1 January 1950.
Timber core samples are sampled and used to measure the width of annual growth rings; by taking samples from different sites within a particular region, researchers can build a comprehensive historical sequence. The techniques of dendrochronology are more consistent in areas where trees grew in marginal conditions such as aridity or semi-aridity where the ring growth is more sensitive to the environment, rather than in humid areas where tree-ring growth is more uniform (complacent). In addition, some genera of trees are more suitable than others for this type of analysis. For instance, the bristlecone pine is exceptionally long-lived and slow growing, and has been used extensively for chronologies; still-living and dead specimens of this species provide tree-ring patterns going back thousands of years, in some regions more than 10,000 years. Currently, the maximum span for fully anchored chronology is a little over 11,000 years B.P.
Dates from dendrochronology can be used as a calibration and check of radiocarbon dating, see the related article section. In addition, ancient building timbers found 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, e.g., to support archaeological dating of cliff dwellings of Native Americans in the arid U.S. Southwest.
In 2004 a new calibration curve, INTCAL04, was internationally ratified to provide calibrated dates back to 26,000 B.P. (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 spans 12,410 calendar B.P., and a further 14,700 calendar years prior to that.
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 and ship timbers.
In areas where the climate is reasonably predictable, trees develop annual rings of different properties depending on weather, rain, temperature, soil pH, plant nutrition, CO
2 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.
While archaeologists can date wood and when it was felled, it may be difficult to definitively determine the age of a building or structure in which the wood was used; 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.
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 related technique isused to analyse fish stocks through the analysis of growth rings in the otolith bones of fish.
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- In distinction to the use of free ring chronologies to calibrate and validate 14C-derived chronologies, IntCal04 is constructed from 14C-derived dates—of foraminifera from Venezuela's Cariaco basin, corrected for a constant reservoir age of 405 years. See Reimer Paula J, Baillie Mike GL, Bard Edouard, Bayliss Alex, Beck J Warren, Bertrand Chanda JH, Blackwell Paul G, Buck Caitlin E, Burr George S, Cutler Kirsten B, Damon Paul E, Edwards R Lawrence, Fairbanks Richard G, Friedrich Michael, Guilderson Thomas P, Hogg Alan G, Hughen Konrad, Kromer Bernd, McCormac Gerry, Manning Sturt, Ramsey Christopher Bronk, Reimer Ron W, Remmele Sabine, Southon John R, Stuiver Minze, Talamo Sahra, Taylor FW, van der Plicht Johannes, Weyhenmeyer Constanze E (2004). "INTCAL04 Terrestrial Radiocarbon age calibration, 0–26 cal kyr BP" (PDF). Radiocarbon 46 (3): 1029–58.
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