How to date an archaeologist: Dendrochronology - Ep 29
September 10, 2024·23 min
Dr. Matilda Zeebrecht hosts this solo training episode on the archaeological dating method of dendrochronology, or tree-ring dating. She explains the biological process behind tree-ring formation, focusing on the vascular cambium layer and its production of xylem and phloem cells. The episode details how alternating seasons create distinct light "early wood" in spring and dark "late wood" in summer, forming a visible ring.
Andrew Ellicott Douglas, an astronomer studying sunspots, pioneered the method by using tree rings as a proxy for past environmental conditions like droughts. Archaeologists Clark Wissler and others later collaborated with Douglas to date prehistoric sites in the American Southwest using his pine tree chronologies. Douglas founded the Laboratory of Tree-Ring Research at the University of Arizona in 1937, and his original reference collection is still used today.
In practice, dendrochronologists take a cross-section or core sample of wood, sand it smooth, and match its ring pattern against a long, overlapping chronology of samples. The method's accuracy relies on regional reference collections because trees in the same area produce similar ring patterns due to shared climate conditions. Dendrochronology primarily dates when a tree was cut down, which requires careful archaeological interpretation of how soon the wood was used.
A surprising insight is that tree rings can record environmental events almost immediately, providing a precise historical climate record. The "divergence problem" notes that some modern trees, stressed by global warming, have altered growth responses, potentially complicating climate reconstructions but not yet affecting dating. Digitizing the pattern-matching process has proven difficult, as the human eye and expert interpretation remain superior to software.
This reliance on specialist knowledge and physical reference collections makes dendrochronology a potentially fragile discipline vulnerable to lost expertise or destroyed samples. The episode has an educational and conversational tone, blending clear scientific explanation with casual asides about tea. Listeners interested in archaeological science methods or environmental history will find this episode thoroughly engaging. Those seeking narrative stories about specific artifacts or sites might prefer this show's guest interview episodes.
