Dendrochronology for medieval studies
By Lynn T. Courtenay
Avista Forum Journal, Vol.13:1 (2003)
Introduction: Dendrochronology, or Tree-ring dating, has emerged in recent decades of the 20th century as one of the most important dating tools for a number of disciplines such as archaeology, climatology, botany, and the history of art and material culture. While there are numerous publications and informative websites concerned with the technical aspects of dendrochronology and its application, this introduction is designed as a guide for medievalists with a general interest in the subject.
Briefly stated, dendrochronology creates a statistically based calendar of tree felling dates derived from the meticulous measurement of the variation of annual growth rings. Each ‘ring’ represents a single year’s growth. Since tree rings reflect seasonal changes, only trees from temperate and arid regions have identifiable annual rings. Species like oak, chestnut, elm, beech, juniper, and conifers produce annual rings whose width and character depends mainly on climatic conditions, such as moisture and temperature. Thus, one would expect to find similar ring patterns for the same species in wetland habitats of similar altitude and growing conditions. When measurements of the variations in the size of the annual rings have been accumulated for large numbers of trees from the same species and region, a chronology of ring widths derived from these dimensions can be established.
This methodology, for “dendro dating” by means of a reference chronology was developed in the early twentieth century by A.E. Douglass who used yellow pine from ancient pueblos in Flagstaff, Arizona. Oak, however, was the premier building material for large-scale carpentry in pre-modern Europe, and early buildings have furnished a considerable amount of specimens. Oak chronologies for Europe were developed mainly in the last quarter of the twentieth century, such as: Hollstein’s north German oak chronology, Becker and Delorme’s south German oak chronology; there are exceptionally long chronologies for Ireland, extended via fossil material and anchored by living trees; numerous chronologies were developed in Britain, but the sequences for oak are generally shorter, e.g. Fletcher and Laxton and Litton, East Midlands. These tree ring chronologies, including others from Scandinavia, Holland and Belgium span most of the medieval period and have been used as the foundation oak chronologies for those developed later for Europe, including parts of France, such as the early Northern French Chronology AD 1274- 1979 and in eastern France. Of particular interest for the exploitation of Baltic oak is the north Polish chronology. It should be pointed out, however, that while there are hundreds of dated tree-ring sequences throughout the world, many are designed for post-medieval climatic and ecological studies and involve species other than oak.
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