The Methodology of Dendrochronology (tree-ring dating)

By Kyle Lent (MMA Student)

An instrumental tool in providing dating for archaeologists, dendrochronology is firmly grounded in the principle of cross dating to identify synchronous rings in separate trees. This method provides an absolute date in history, accurate to the calendar year. Defined as “the method of employing tree-rings as a measurement of time…and…the process of inferring past environmental conditions that existed when the rings were being formed…” (Bannister 1963: 161), dendrochronology has undergone a period of explosive growth in the past three decades. Today, more than 100 tree-ring dating programs cover all areas of the globe (Dean 1997: 31).

Dendrochronology Sample. Image Courtesy livius.org

All dendrochronological studies concern trees that are either living now or have lived in earlier times (Eckstein 1984: 5). As trees grow they produce growth rings. These rings result in the change of growth speed through the seasons of the year. Thus, each visible “ring” usually marks one calendar year in the life of the tree, with each “year” being diversely affected by the elements of that point in time. The pattern created is similar amongst trees of the same type in the same region; therefore, the same yearly sequence can be recognised in separate trees (Bowens 2009: 24). Here, cross dating methods can be employed to determine the year of the wood sample. For maritime archaeological purposes, the most prevalent sample for studying tree rings lie in shipwreck timbers, with wooden artefacts, furniture and wooden utensils being important examples as well (Dean 1996: 48).

Methods:

         

Sampling of a fragment of the hull of Chretienne A Shipwreck, France. Image courtesy culture.gouv.fr

Although the basic concepts and principles of dendrochronology have remained the same since the creation of the discipline, the methods used have changed dramatically. Once the data has been gathered, technical development furthers five general objectives: data capture, standardization, data comparison (cross dating), data consolidation and environmental reconstruction (Dean 1996: 39).

Measurements capture the full range of variation in ring samples. These measurements can be statistically evaluated and manipulated, thus it is important to capture accurate data in such samples to provide for cross dating in extremely long tree ring chronologies. The eastern Mediterranean chronology, for example, has the potential to reach beyond 7,000 BC (Kuniholm 1995). Several devices that measure ring widths to the nearest .01mm are currently in use (Dean 1996: 40). Advances and x-ray and densitomatric technology have simplified the process of taking such strategic measurements.

Two basic approaches exist for cross dating, visual-graphical and statistical. The first allows for the visual comparison of wood samples, the latter involves computing the mathematical measurements with software programs designed for the tree rings. Programs include the International Tree-Ring Data Bank (ITRDB) and computer programs such as SNCHR and CATRAS (Dean 1996: 42). Designed for measuring, error checking, managing data and comparing the tree-ring series, these programs have been instrumental in the advancement of dendrochronology.

The decision regarding how much of the deposit or material to recover must be taken in light. What is its archaeological potential? What can it reveal about the archaeology of the site? What will it entail to recover all or part of the sample? All decisions must be based upon careful consideration for the preservation of the site.

A complete cross-section cut perpendicular to the grain is ideal, although v-shaped cuts and coring are also possible. Increment corers have been used in successfully dating boat artefacts, such as a late Saxon log boat from England (Tyers 1989). Such information can also provide correlations that can cut across the boundaries of different cultural areas as well (Muckelroy 1980: 154).

Application within Maritime Archaeology:

Extensive dendrochronological studies have been performed in the Maritime Archaeological environment worldwide, with popular examples including the Viking age ship burials at Oseberg, Gokstad and Thune, Norway, (Bonde and Christensen 1993) and the Tudor warship Mary Rose (Bridge and Dobbs 1996). To determine a date for a structure, a sample cut is required from well preserved and long grown timbers, preferably with sapwood surviving. In general, ring sequences of less than 50 years will not date reliably (Bowen 138). Adequate sampling will give the dendrochronologist sufficient opportunity to  select the most suitable and informative timber samples for dating purposes, thus allowing for the most accurate sample date. Recent studies, such as an analysis of framing timbers from the Princes Chanel wreck in Thames estuary, England, provide an excellent reference for the methodology of dendrochonology. From the report, a total of twelve samples were taken from lifted sections of the hull from sufficient ring samples on oak framing timbers. Tree-ring sequence from ten of these samples cross-matched to form a 306 year period which has since been dated against regional tree ring chronologies in Eastern England. The results are consistent with the vessel having been constructed from oak from Eastern England, soon after AD 1574 (Nayling 2004).

Sampling at the Dendrochronology Lab at Lampeter. Image courtesy University of Whales

Problems with tree-ring dating:

Many submerged sites produce an abundance of wood, which proves to be helpful in the dating of material. However, it is very important to note that the date provided by dendrochronology relates to the growing period of the tree, not its arrival on site (Bowens 2009: 25). While dendrochronology gives an accurate date to the felling of the sample, by no means does that date reflect the exact sinking of the vessel, just a relative proximity.

Conclusion:

When the sample is complete, dendrochronology is a dating method that is not only precise in that a specific ring is related to a specific year, but the method itself is very accurate in that the specified year date is equal to that of the trees felling, which provides a valuable timeframe as to when such shipwreck timbers were harvested. Arguably the most accurate dating method existing (Eckstein 1984: 28), dendrochonology has produced dating methods time and again in the study of underwater archaeology (Bonde and Christensen 1993, Bridge and Dobbs 1996, Nayling 2004, Tyers 1989). Based on the growing level of interest in the field, it is safe to say the discipline will continue to expand its geographic scope in the years to come (Dean 1996: 52). As vast areas of Asia are opening huge dendrochonological potential, South American and Southeast Asian tropics are compiling crossdatable ring records to advance the study. Developments across the world have enormous potential for archaeologists in terms of site dating. I believe improved collection and preparation methods, along with technological advances and increased global interaction will undoubtedly strengthen cross datable ring records, thus strengthening the field of dendrochronology for the modern archaeologist.

References:

Bannister, B. 1963. Dendrochronology. In Brothwell, D. and Higgs E., eds., Science in Archaeology. New York, New York, Basic Books: 161-176.

Bonde, N. and Christensen, A.E. 1993. Dendrochronological dating of the Viking age ship burials at Oseberg, Gokstad, and Thune, Norway. Antiquity 67: 575-583.

Bowens, A., 2009. Underwater Archaeology: The NAS Guide to Principles and Practice. 2nd edition. Blackwell Publishing, West Sussex, UK: pp. 24-25, 137-139.

Bridge, M.C. and Dobbs, C. 1996. Tree-ring studies on the Tudor warship Mary Rose. In Dean, J.S., Meko, D.M and Swetnam, T.W., eds., Tree Rings, Environment and Humanity: Proceedings of    the International Conference, Tuscon, Arizona, 17-21 May 1994. Tuscon, Arizona, Radiocarbon: 491-496.

Dean, J., 1997. Dendrochonology. Plenum Press, New York. In Taylor, R.E., and Aitken, M., 1997. Chronometric Dating in Archaeology. Plenum Press, New York: pp. 31-55.

Eckstein, D., 1984. No. 2 Dendrochronological Dating, Handbooks for Archaeologists. European Science Foundation, Strassbourg: 1-50.

Kuniholm, P.I., Kromer, B., Manning, S.W., Newton, M., Latini, C.E. and Bruce, M.J. 1996 Anatolian tree rings and the absolute chronology of the eastern Mediterranean, 2220-718 B.C. Nature 381: 780-783.

Muckelroy, K., 1980. Archaeology Under Water: An Atlas of the worlds submerged sites. McGraw-Hill Book Company, New York. Pp: 154-155.

Nayling, N., 2004. Tree Ring analysis of framing timbers from the Princes Channel Wreck, Thames Estuary. HARP Dendrochonology Report 2004/02. University of West Lampeter, Ceredigion.

Tyers, I., 1989, Dating by tree-ring analysis, in P. Marsden (ed.), A late Saxon logboat from Clapton, London Borough of Hackney, International Journal of Nautical Archaeology 18.2, 89-111.

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