Tree ring dating, called dendrochronology, is a fascinating science that uses the growth rings of long-lived trees as a record of climatic change in a region. Tree-ring dating was one of the first developed absolute dating methods, and it was invented by astronomer Andrew Ellicott Douglass and archaeologist Clark Wissler in the first decades of the 20th century, culminating in research at the pueblo town of Showlow Arizona in 1929. Tree ring dating works because growth rings are established annually in trees, caused by seasonal changes like temperature and moisture availability. Dendrochonology measures a tree's cambium, the layer of cells that lies between the wood and bark and from which new bark and wood cells originate. Environmental inputs into the cambium include nonchronological changes such as regional climatic variations, which establish recognizable patterns in the rings, encoded as variations in the width of a particular ring, in the wood density or structure, and/or in the chemical composition of the cell walls. Many tree ring sequences have been developed over the past 100 years, including an 8,700 year-long bristlecone pine sequence in California, and a 10,000 year-long sequence of oak trees in central Europe. But building a chronology of climate change in a region, while interesting and undeniably useful, is not the only thing a study of ancient wood can tell us.
Tree Rings and Medieval Lübeck, Germany
An article by Dieter Eckstein in the February 2007 issue of Dendrochronologia called Human time in tree rings describes the variety of possible routes of research available to scholars today. Eckstein, a wood biologist at the University of Hamburg, uses as his example, research into wood buildings and objects from the Medieval town of Lübeck, Germany.
The town of Lübeck was founded about 800 AD by Slavic tribes as a rampart in the Schleswig-Holstein area of northern Germany, within the dense beech and oak forests between the North and Baltic seas. The old town of Lübeck was called Liubice, and it is 6 km north of today's Lübeck; today this place is located amid open farmland. About 350 years later, modern Lübeck was founded on its present location by Germanic tribes, who took the old name Liubice from the Slavic fortification. Lübeck's medieval history includes several events that are pertinent to the study of tree rings and forests, including laws passed in the late 12th and early 13th century establishing some basic sustainability rules, two devastating fires in 1251 and 1276, and a population crash between about 1340 and 1430 resulting from the Black Death. A Wider Application of Tree Rings
In his article, Eckstein used dendrochronological data to record evidence of several important characteristics of Lübeck.- Construction booms and busts. Boom periods in building construction (such as immediately after the fires) are marked by the extensive use of younger trees, which resulted from demand outpacing the ability of the forests to recover. Conversely, busts (such as after the Black Death decimated the population of Lübeck) are denoted by a long periods of no construction, followed by the use of very old trees.
- Presence of a timber market. While collecting dates on the rafters of buildings in Lubeck, researchers noticed that sometimes all the rafters in a given house were cut down at the same time, while in other houses, the dates of the rafters spanned a year or more, representing several cuttings. In general, it was the more expensive houses that had the consistent rafter dates. Eckstein surmises that if all the rafters in a house are of the same age, they were likely from trees cut down at the time the house was built. When the rafters range in dates, wood for the house may have been obtained at a timber market, where the trees would have been cut and stored until they could be sold. Upper classes likely had proprietary interest in the woods, or could contract individually with the owners, while less wealthy individuals would not have had such access.
- Evidence of long-distance timber trade. Since tree ring formation is tied specifically to local climatic variations, tree ring patterns vary on a regional scale. Thus, imported wood can be identified by its variation from the regional ring pattern. At Lübeck, pieces of art such as the Triumphal Cross and Screen at the St. Jacobi Cathedral were identified as having been constructed out of wood that had been specifically shipped in. Comparisons of the wood rings to other regional reference chronologies allows researchers to identify the provenance of the wood. Thus, the screen from St. Jacobi was made in the late 15th century from planks taken from 200-300 year old trees from the Polish-Baltic forests, probably along established trade routes from Gdansk, Riga, or Konigsberg harbors. Other buildings in Lübeck that have imported lumber include the organ in the St. Jacobi church and the screen in the Holy Ghost Hospital, both dated to the 15th century. Further evidence of long-distance trade can be found in objects made from wooden crates or barrels used to ship other materials.
Tree Rings and Cultural Heritage
While dendrochronology as a dating technique has been around for almost a century, it is clear from Eckstein's research that the application has much more to teach us about how people in the past managed their scarce resources.Sources
Dieter Eckstein. 2007. Human time in tree rings. Dendrochronologia 24:53-60. (published in 2006)A bibliography of tree rings and archaeology has been assembled for this project.
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