Dendrochronology (or tree-ring dating) is the scientific method of dating tree rings (also called growth rings) to the exact year they were formed in a tree. As well as dating them, this can give data for dendroclimatology, the study of climate and atmospheric conditions during different periods in history from the wood of old trees. Dendrochronology derives from the Ancient Greek (), meaning "tree", (), meaning "time", and (), "the study of".The term "dendrochronology" was coined in 1928 by the American astronomer Andrew Ellicott Douglass (1867–1962). 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." "
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. 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 2020, securely dated tree-ring data for the Northern Hemisphere is available going back 13,910 BP (years before 1950). A new method is based on measuring variations in oxygen isotopes in each ring, and this 'isotope dendrochronology' can yield results on samples which are not suitable for traditional dendrochronology due to too few or too similar rings. Some regions have "floating sequences", with gaps which mean that earlier periods can only be approximately dated. As of 2024, only three areas have continuous sequences going back to prehistoric times: the foothills of the Northern Alps, the southwestern United States, and the British Isles. Miyake events, which are major spikes in cosmic rays at known dates, are visible in trees rings and can fix the dating of a floating sequence.
History The Greek botanist Theophrastus (c. 371 – c. 287 BC) first mentioned that the wood of trees has rings.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.For the history of dendrochronology, see: (Condensed from:
James H. Speer, Fundamentals of Tree-ring Research (Tucson, Arizona: University of Arizona Press, 2010), Chapter 3: History of Dendrochronology, pp. 28–42. In his 1651 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.See: 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 sawed 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. 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.du Hamel & de Buffon (27 February 1737) "De la cause de l'excentricité des couches ligneuses qu'on apperçoit quand on coupe horisontalement le tronc d'un arbre; de l'inégalité d'épaisseur, & de different nombre de ces couches, tant dans le bois formé que dans l'aubier" (On the cause of the eccentricity of the woody layers that one sees when one horizontally cuts the trunk of a tree; on the unequal thickness, and on the different number of layers in the mature wood as well as in the sapwood), Mémoires de l'Académie royale des science, in: Histoire de l'Académie royale des sciences ..., pp. 121–134. They found that in 1709, a severe winter produced a distinctly dark tree ring, which served as a reference for subsequent European naturalists.du Hamel & de Buffon (4 May 1737) "Observations des différents effets que produisent sur les végétaux les grandes gelées d'hiver et les petites gelées du printemps" (Observations on the different effects that the severe frosts of winter and the minor frosts of spring produce on plants), Mémoires de l'Académie royale des science, in: Histoire de l'Académie royale des sciences ..., pp. 273–298. Studhalter (1956), p. 33, stated that Carl Linnaeus (1745, 1751) in Sweden, Friedrich August Ludwig von Burgsdorf (1783) in Germany, and Alphonse de Candolle (1839–1840) in France subsequently observed the same tree ring in their samples. In the U.S., Alexander Catlin Twining (1801–1884) suggested in 1833 that patterns among tree rings could be used to synchronize the dendrochronology of various trees and thereby to reconstruct past climates across entire regions.Alexander C. Twining (1833) "On the growth of timber — Extract of a letter from Mr. Alexander C. Twining, to the Editor, dated Albany, April 9, 1833" , The American Journal of Science, 24 : 391–393. The English polymath Charles Babbage proposed using dendrochronology to date the remains of trees in peat bogs or even in geological strata (1835, 1838).See: (Anon.) (1835) "Evening meeting at the Rotunda" , 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. Charles Babbage (1838) "On the age of strata, as inferred from rings of trees embedded in them" , The Ninth Bridgewater Treatise: A Fragment, (London, England: John Murray, 1837), pp. 226-234.
During the later 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.See: Jacob Kuechler ( August 6, 1859) "Das Klima von Texas" (The climate of Texas), Texas Staats-Zeitung [Texas state newspaper] (San Antonio, Texas), p. 2. "The droughts of western Texas", The Texas Almanac for 1861, pp. 136–137; see especially p. 137. In 1866, the German botanist, entomologist, and forester Julius Theodor Christian Ratzeburg (1801–1871) observed the effects on tree rings of defoliation caused by insect infestations.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. 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.) By 1882, this observation was already appearing in forestry textbooks.Franklin B. Hough, The Elements of Forestry (Cincinnati, Ohio: Robert Clarke and Co., 1882), pp. 69–70. In the 1870s, the Dutch astronomer Jacobus Kapteyn (1851–1922) was using crossdating to reconstruct the climates of the Netherlands and Germany.Kapteyn, J. C. (1914) "Tree-growth and meteorological factors", Recueil des Travaux Botaniques Néerlandais, 11 : 70–93. In 1881, the Swiss-Austrian forester Arthur von Seckendorff-Gudent (1845–1886) was using crossdating.See: 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. Speer (2010), p. 36. 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.Speer (2010), p. 36–37. One of the earlies attempts to date an archaeological site in the United States was by Manesseh Cutler in 1888. He estimated the date of the Marietta Earthworks by having trees growing on it cut down and counting the number of rings. In 1892, the Russian physicist (1841–1905) wrote that he had used patterns found in tree rings to predict droughts in 1882 and 1891.See: 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.
During the first half of the twentieth century, the astronomer A. E. Douglass founded 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).
Methods
Growth rings
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.