Method of dating based on the analysis of patterns of tree rings
Drill for dendrochronology sampling and growth ring counting
The growth rings of a tree at Bristol Zoo, England. Each ring represents one year; the outside rings, near the bark, are the youngest
Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed. As well as dating them this can give data for dendroclimatology, the study of climate and atmospheric conditions during different periods in history from wood.
Dendrochronology is useful for determining the precise age of samples, especially those that are too recent for radiocarbon dating, which always produces a range rather than an exact date, to be very accurate. However, for a precise date of the death of the tree a full sample to the edge is needed, which most trimmed timber will not provide. It also gives data on the timing of events and rates of change in the environment (most prominently climate) and also in wood found in archaeology or works of art and architecture, such as old panel paintings. It is also used as a check in radiocarbon dating to calibrate radiocarbon ages.
New growth in trees occurs in a layer of cells near the bark. A tree's growth rate changes in a predictable pattern throughout the year in response to seasonal climate changes, resulting in visible growth rings. Each ring marks a complete cycle of seasons, or one year, in the tree's life. As of 2013[update], the oldest tree-ring measurements in the Northern Hemisphere are a floating sequence extending from about 12,580 to 13,900 years. Dendrochronology derives from Ancient Greek: δένδρον (dendron), meaning "tree", χρόνος (khronos), meaning "time", and -λογία(-logia), "the study of".
The Greek botanist Theophrastus (c. 371 – c. 287 BC) first mentioned that the wood of trees has rings. In his Trattato della Pittura (Treatise on Painting), Leonardo da Vinci (1452–1519) was the first person to mention that trees form rings annually and that their thickness is determined by the conditions under which they grew. In 1737, French investigators Henri-Louis Duhamel du Monceau and Georges-Louis Leclerc de Buffon examined the effect of growing conditions on the shape of tree rings. They found that in 1709, a severe winter produced a distinctly dark tree ring, which served as a reference for subsequent European naturalists. In the U.S., Alexander Catlin Twining (1801–1884) suggested in 1833 that patterns among tree rings could be used to synchronize the dendrochronologies of various trees and thereby to reconstruct past climates across entire regions. The English polymath Charles Babbage proposed using dendrochronology to date the remains of trees in peat bogs or even in geological strata (1835, 1838).
During the latter half of the nineteenth century, the scientific study of tree rings and the application of dendrochronology began. In 1859, the German-American Jacob Kuechler (1823–1893) used crossdating to examine oaks (Quercus stellata) in order to study the record of climate in western Texas. In 1866, the German botanist, entomologist, and forester Julius Ratzeburg (1801–1871) observed the effects on tree rings of defoliation caused by insect infestations. By 1882, this observation was already appearing in forestry textbooks. In the 1870s, the Dutch astronomer Jacobus C. Kapteyn (1851–1922) was using crossdating to reconstruct the climates of the Netherlands and Germany. In 1881, the Swiss-Austrian forester Arthur von Seckendorff-Gudent (1845–1886) was using crossdating. From 1869 to 1901, Robert Hartig (1839–1901), a German professor of forest pathology, wrote a series of papers on the anatomy and ecology of tree rings. In 1892, the Russian physicist Fedor Nikiforovich Shvedov (Фёдор Никифорович Шведов) (1841–1905) wrote that he had used patterns found in tree rings to predict droughts in 1882 and 1891.
Diagram of secondary growth in a tree showing idealised vertical and horizontal sections, a new layer of wood is added in each growing season, thickening the stem, existing branches and roots, to form a growth ring
Horizontal cross sections cut through the trunk of a tree can reveal growth rings, also referred to as tree rings or annual rings. Growth rings result from new growth in the vascular cambium, a layer of cells near the bark that botanists classify 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. Removal of the bark of the tree in a particular area may cause deformation of the rings as the plant overgrows the scar.
The rings are more visible in trees which have grown in temperate zones, where the seasons differ more markedly. The inner portion of a growth ring forms 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" (sometimes termed "summer wood", often being produced in the summer, though sometimes in the autumn) and is denser.
Many trees in temperate zones produce one growth-ring each year, with the newest adjacent to the bark. Hence, for the entire period of a tree's life, a year-by-year record or ring pattern builds up 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 complex 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 chronological study. Researchers can compare and match these patterns ring-for-ring with patterns from trees which have grown at the same time in the same geographical zone (and therefore under similar climatic conditions). When one can match these tree-ring patterns across successive trees in the same locale, in overlapping fashion, chronologies can be built up—both for entire geographical regions and for 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. Dendrochronologists originally carried out cross-dating by visual inspection; more recently, they have harnessed computers 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.
A fully anchored and cross-matched chronology for oak and pine in central Europe extends back 12,460 years, and an oak chronology goes back 7,429 years in Ireland and 6,939 years in England. Comparison of radiocarbon and dendrochronological ages supports the consistency of these two independent dendrochronological sequences. Another fully anchored chronology that extends back 8500 years exists for the bristlecone pine in the Southwest US (White Mountains of California).
A typical form of the function of the wood ring width in accordance with the dendrochronological equation
A typical form of the function of the wood ring (in accordance with the dendrochronological equation) with an increase in the width of wood ring at initial stage
The dendrochronological equation defines the law of growth of tree rings. The equation was proposed by Russian biophysicist Alexandr N. Tetearing in his work "Theory of populations" in the form:
where is width of annual ring, is time (in years), is density of wood, is some coefficient, is function of mass growth of the tree.
With the neglection of natural sinusoidal oscillations in tree mass,
the formula of the changes in the annual ring width is:
There , , and are some coefficients, and are positive constants.
The formula is useful for correct approximation of samples data before data normalization procedure.
The typical forms of the function of annual growth of wood ring are shown in the figures.
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.
In 2004 a new radiocarbon calibration curve, INTCAL04, was internationally ratified to provide calibrated dates back to 26,000 B.P. For the period back to 12,400 B.P., the radiocarbon dates are calibrated against dendrochronological dates.
Dendrochronology practice faces many obstacles, including the existence of species of ants that inhabit trees and extend their galleries into the wood, thus destroying ring structure.
European chronologies derived from wooden structures initially found it difficult to bridge the gap in the fourteenth 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.
Dendroclimatology is the science of determining past climates from trees primarily from the properties of the annual tree rings. Other properties of the annual rings, such as maximum latewood density (MXD) have been shown to be better proxies than simple ring width. Using tree rings, scientists have estimated many local climates for hundreds to thousands of years previous.
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.
A portrait of Mary Queen of Scots, determined to date from the sixteenth century by dendrochronology
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 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 fourteenth to seventeenth 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 eighteenth-century copy. However, dendrochronology revealed that the wood dated from the second half of the sixteenth century. It is now regarded as an original sixteenth-century painting by an unknown artist.
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 sixteenth 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 nineteenth and twentieth centuries.
The dating of buildings with wooden structures and components is also done by dendrochronology; dendroarchaeology is the term for the application of dendrochronology in archaeology. 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. The dating of building via dendrochronology thus requires knowledge of the history of building technology. Many prehistoric forms of buildings used "posts" that were whole young tree trunks; where the bottom of the post has survived in the ground these can be especially useful for dating.
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, as wood was not seasoned before use in building at that time in New England. 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 .
The burial chamber of Gorm the Old, who died c. 958, was constructed from wood of timbers felled in 958.
Veliky Novgorod, where, between the tenth and the fifteenth century, numerous consecutive layers of wooden log pavement have been placed over the accumulating dirt.
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.
^The term "dendrochronology" was coined in 1928 by the American astronomer Andrew Ellicott Douglass (1867–1962). Douglass, A.E. (1928). Climatic Cycles and Tree Growth. vol. II. A Study of the Annual Rings of Trees in relation to Climate and Solar Activity. Washington, D.C., USA: Carnegie Institute of Washington. p. 5. From p. 5: "One can see that in all this we are measuring the lapse of time by means of a slow-geared clock within trees. For this study the name "dendro-chronology" has been suggested, or "tree-time." "
^Theophrastus with Arthur Hort, trans., Enquiry into Plants, volume 1 (London, England: William Heinemann, 1916), Book V, p. 423. From p. 423: "Moreover, the wood of the silver-fir has many layers, like an onion; there is always another beneath that which is visible, and the wood is composed of such layers throughout." Although many sources claim that Theophrastus recognized that trees form growth rings annually, this is not true.
R. A. Studhalter (April 1956) "Early history of crossdating", Tree-Ring Bulletin, 21 : 31–35. Available on-line at: University of ArizonaArchived 2016-03-04 at the Wayback Machine (Condensed from: Studhalter, R. A. (1955) "Tree growth: some historical chapters", Botanical Review, 21 : 1–72.)
Studhalter, R. A. ; Glock, W. S. & Aoerter, S. R. (1963) "Tree growth, some historical chapters in the study of diameter growth", Botanical Review, 39 (3) : 245–343.
James H. Speer, Fundamentals of Tree-ring Research (Tucson, Arizona: University of Arizona Press, 2010), Chapter 3: History of Dendrochronology, pp. 28–42.
Leonardo da Vinci, Trattato della Pittura ... (Rome, (Italy): 1817), p. 396. From p. 396: "Li circuli delli rami degli alberi segati mostrano il numero delli suoi anni, e quali furono più umidi o più secchi la maggiore o minore loro grossezza." (The rings around the branches of trees that have been sawn show the number of its years and which [years] were the wetter or drier [according to] the more or less their thickness.)
Sarton, George (1954) "Queries and Answers: Query 145. — When was tree-ring analysis discovered?", Isis, 45 (4): 383–384. Sarton also cites a diary of the French writer Michel de Montaigne, who in 1581 was touring Italy, where he encountered a carpenter who explained that trees form a new ring each year.
(Anon.) (1835) "Evening meeting at the Rotunda"Archived 2015-05-14 at the Wayback Machine, Proceedings of the Fifth Meeting of the British Association for the Advancement of Science held in Dublin during the week from the 10th to the 15th of August, 1835, inclusive, pp. 116–117.
^J. T. C. Ratzeburg, Die Waldverderbniss oder dauernder Schade, welcher durch Insektenfrass, Schälen, Schlagen und Verbeissen an lebenenden Waldbäumen entsteht. [The deterioration of forests or lasting damage that arises from feeding by insects, debarking, felling, and gnawing on living forest trees.], vol. 1, (Berlin, (Germany): Nicolaische Verlag, 1866), p. 10.Archived 2015-10-01 at the Wayback Machine From p. 10: "Die beiden, auf Taf. 42, Fig. 6 (mit dem Durchschnitt Fig. 7) und Fig. 1 (mit dem Durchschnitt Fig. 2) dargestellten Zweige hatten in dem Frassjahre 1862 einen doppelt so starken Jahrring als in dem vorhergehenden angelegt, und auch der (hier nicht abgebildete) Ring des jährigen Triebes war bei den gefressenen stärker as der eines nicht gefressenen." (Both branches that are presented in plate 42, fig. 6 (with the cross-section in fig. 7) and fig. 1 (with the cross-section in fig. 2) had produced, in the defoliation year of 1862, a growth ring that was twice as strong as in the preceding one, and so was the ring of the year-old shoot (not illustrated here) stronger in the case of the defoliated tree than one that was not defoliated.)
Seckendorff, Arthur von (1881) "Beiträge zur Kenntnis der Schwarzföhre Pinus austriaca Höss" [Contributions to our knowledge of the black pine Pinus austriaca Höss], Mitteilung aus dem forstlichen Versuchswesen Oesterreichs [Report from the Austrian Department of Forestry Research] (Vienna, Austria: Carl Gerold Verlag, 1881), 66 pages.
Шведов, Ф. (Shvedov, F.) (1892) "Дерево, как летопись засух" (The tree as a record of drought), Метеорологический Вестник (Meteorological Herald), (5) : 163–178.
Speer (2010), p. 37.
^"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 Merranean species.
^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. doi:10.1017/S0033822200032999. Archived from the original on 2013-06-30.
^Spronk, Ron, More than Meets the Eye: An Introduction to Technical Examination of Early Netherlandish Paintings at the Fogg Art Museum, Harvard University Art Museums Bulletin, Vol. 5, No. 1, Autumn, 1996
^Towner, Ronald H. (2002-04-05). "Archeological dendrochronology in the southwestern united states". Evolutionary Anthropology: Issues, News, and Reviews. 11 (2): 68–84. doi:10.1002/evan.10009. ISSN1060-1538.