Dendrochronology

Dendrochronology is a method of dating through the analysis of tree rings. While it has broad applications for geologists, historical environmentalists, and dendroecologists, dendrochronology has proven especially helpful to archaeologists. Prior to the 1930s, archaeologists could assign only relative dates to their material, using, for example, artifact typologies, an object's position relative to artifacts in other stratigraphy, or the artistic tradition with which the object was ornamented. Relative chronologies, however, are especially problematic for pre-historic sites, which often lack any written corroboration not only of the site in question, but also of the civilization.

Dendrochronology emerged from the American Andrew Ellicott Douglass's master chronology of yellow pine in the 1920s for the dating of prehistoric Indian cultures (such as the Anasazi) in the southwestern United States. Other scientific dating methods (archaeometry) soon followed. Thermoluminescence dating, for example, can be used for nonorganic material (stone and pottery), but it is still in its developmental stage, and radiocarbon dating (C-14), which has a functional dating range to 50,000 b.c.e., only has an accuracy of more or less 200 years. Dendrochronology, therefore, is the only archaeometric process through which it is possible in both theory and practice to date to within a range of one year. One drawback, however, is that dendrochronology can date objects no older than the oldest samples of its regional tree-ring scale. Even with the oldest tree-ring scale in the world (south Germany), therefore, it is only possible to provide dates no older than 10,000 years b.c.e.

Dendrochronology is based on the practice of ring-width pattern matching. Each year, most trees grow a new layer, or ring, of wood. The thickness of the ring (ring-width variability) depends on external environmental factors, including temperature, aridity, and soil types. Trees in temperate zones generally display more ring-width variability than trees growing in semiarid regions. In constructing treering scales, dendrochronologists choose trees indigenous to their region of study because they must compare the patterns of living trees with ancient samples. Core samples are collected from old living trees and compared with ring-growth patterns among several tree cross-sections. Missing years can be supplemented with wood from old buildings (such as medieval churches) or ancient charcoal and preserved wood. By overlapping the cross-sections of a large number of samples, dendrochronologists will arrive at a scale with which scholars can date archaeological contexts with preserved wood.

In addition to the American southwest, master chronologies have been developed for the American Midwest and eastern coastal regions, much of Europe, the Aegean, and the Near East. Chronologies are emerging as well for China and parts of South America. Dendrochronology's contributions to early history are many, including uncovering a climatic regression c.540 c.e. that affected the entire northern hemisphere. Archaeologists excavating waterlogged medieval cities (London, Dublin, Bergen, Stockholm, and Novgorod) in northern Europe use the preserved wooden foundations of buildings to reconstruct precise tree-ring chronologies spanning hundreds of years. And dendrochronologists have dated many of the thousands of painted oak wood panels that artists north of the Alps, especially in the Netherlands, favored during the Renaissance. In addition to the dates, the provenance of many of the panels signed by such artists as Rubens and Rembrandt has been traced to Gdansk (Danzig), Poland, thus shedding significant light on the commercial underpinnings of the Renaissance. The science of measuring tree time, therefore, continues to demonstrate applications extending far beyond the absolute dating of pre-historic sites.

...