Dendrochronology and Anthropology: What Trees Can Teach Us About History

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Dendrochronology demonstrated at Museums of Liverpool Conservation Centre. Photo by Val Williamson

Anthropologists are, among other things, detectives – piecing together evidence to work out how people behave and survive in certain conditions. Archaeology is anthropology of the past, so a proven timeline for dating archaeological artefacts is a great investigative tool. Dendrochronology supplies physical evidence for a timeline compiled through international data-sharing between practitioners. This fascinating branch of science provides anthropologists with accurate dates, and other information, and has roots going back millennia.

What is Dendrochronology?

Dendrochronology is otherwise known as tree-ring dating. Researchers take a slice from across a tree trunk, and gain information by looking at the distinctive rings formed by the annual growth of the tree. Each ring records a response to climatic conditions, and forms a unique timestamp matched on all trees in the same species and region. In modern technique, an increment borer is drilled into a tree or a wooden object and the resulting core is carefully withdrawn with ring patterns intact. Thus, a picture is preserved of the tree-ring sequence that can be compared to reference samples, or to drawings or photographs of samples.

One type of dendrochronology bore, with sample cores. Photo by Hannes Grobe

Astronomer A. E. Douglass developed the process as a tool for analysis in the 1890s as he worked at the Lowell Observatory in Arizona. He used tree-ring dating from dead very long-lived trees and from wooden beams in the structures of old buildings to create a chronology for the effects of solar (i.e. climatic) variations. Douglass established standard techniques and procedures while developing the first long comparison data.

Dendrochronology Databases

Dendrochronological information has been gathered by thousands of researchers donating and collating their results since the International Tree-Ring Data Bank was established in Colorado in 1974. The addition in 2011 of a Digital Collaboratory for Cultural Dendrochronology, based in Utrecht, provides a data library of historical wood analyses for use in Humanities research.

Dendrochronology as a Standard Method in Anthropology
Dendrochronology did not become a standard research method until the 1980s, when the first extensive chronologies were published, but it has remained interdisciplinary. Using both natural live wood and seasoned crafted wood to form chronologies proved helpful to both climateology and archaeology.

Dendrochronology dated the cedar foundation beams in the Doucet Hennessy House in New Brunswick to 1808. (See above video) Tree-ring analysis gives archaeologists information about building materials, local forest resources, and the kinds of tools and techniques craftsmen used hundreds, even thousands of years ago.

Tree-Ring Analysis for Archaeology

Early 16th century oak screen in late medieval oak-framed hall, Lancashire, UK. Photo Credit Val Williamson

Oak dating is crucial to European archaeologists, because medieval builders mainly used oak, and within the first year or two after cutting down the tree. Dendrochronological analysis of large internal architectural features or buried archaeology in northern Europe offers an accurate dating strategy since Baillie published a full oak timeline for Ireland back as far as 5000 BC.

Wood Provenance

In Medieval Archaeology (2003) Christopher Gerrard labels dendrochronology an archaeological science. Despite the innovations of other scientific processes, he says, “no single dating technique has had the same impact on later medieval archaeology as dendrochronology. Large regional samples of timbered buildings have been systematically analysed and the technique continues to have important implications for the dating of artefacts and sites.”

Gerrard describes how dendrochronologies have altered interpretations of distinctive pottery found in relation to wood samples. This shifted the record of how people interacted and traded to centuries earlier than previously thought. Since pottery itself is used as a means of dating archaeology, and of tracing demographics of production, distribution and usage, this one example impacts significantly on anthropological knowledge.

Dendrochronologies are 95% accurate, and were used to calibrate the radio carbon dating timelines that are another essential tool for archaeologists.

Oldest on the right: Photo Credit Val Williamson

Medieval castle gates at Hay Castle, oldest on the right. Photo Credit Val Williamson

Dating Oak Castle Gates

Analysis of thin samples is not so reliable. An electric-powered microbore was developed for use on examples such as the great doors at Hay Castle, Wales, where timbers are less than two inches thick. Similar latticed castle gates at Chepstow were dated 1159-1189, while an older Anglo-Saxon door at Westminster Abbey was dated 1050 by the same dendrochronologist. The oldest door at Hay proved undatable from the samples taken during a survey by the Royal Commission on the Ancient and Historical Monuments of Wales, so the possibility remains that it is the oldest hanging, hinged door in Wales.

Tree-ring Dating in Anthropology

Tree-ring growth is sensitive to climatic conditions, so the study of ring patterns promotes understanding of the human effects of past events like volcanic eruptions, droughts and floods.

1783 the Kauwerak survivors, Napauruhk and her son, trekked along the south coast of the Seward Peninsula to find food. Image courtesy of demis.nl

There are ample written cultural records of the effect of the Laki volcanic eruption in 1783 for Europe and North and South America, yet only oral accounts of the concurrent double winter, resulting famine, and population collapse of Inuit communities in the Alaskan northwest. Deserted settlements were documented in the notebooks of early European explorers making the first ethnographic observations of the region at the time, but they had no comparative data. Only ten survivors from the Kauwerak village are known, the stories of four persisting in the oral tradition, written down in the 1970s.

This account of the collective catastrophe motivated Jacoby to research tree-rings across Alaska (as Porter notes, in those extreme conditions, the characteristic low growing trees tend to endure for 300 years or more).  The tree-ring density measured in the Alaska northwest for 1783 showed the coldest growing season for over 900 years. Oral history of the dark summer had been verified as related to the Iceland volcanic eruption by reading dendroclimatological data collated by Jacoby et al in their interdisciplinary study.

Inupiat Family, King Island Alaska 1917. Photo by George R. King

Consulting the tree chronology for a region can shed light on previously unexplained events, such as sudden migrations of populations, or the emergence of powerful mythologies that influence behaviour and belief systems, as in the Inuit story of Napauruhk and her son.

Dendrochronology is an applied science, applicable in many contexts, but for historians and anthropologists, its results are directly comparable with deductions from historical and biological record.

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