Tree-Ring Dating

The Ancient Bristlecone Pine, Pinus longaeva, found in the White Mountains of California, made possible the sequencing of its rings showing that climatic fluctuations provide a recognizable pattern. The Ancient Bristlecone Pine lives to a great age and thrives in the relatively dry southwest. The latter factor permits analysis of annual growth in chronological terms since such growth results from naturally occurring patterned climatic conditions—patterning must remain dependent upon climatic conditions for cross-reference purposes.

A bristlecone pine tree from the Ancient Bristlecone Pine Forest in the White Mountains of California. Some of the trees are more than 4,000 years old. A BibArch Photo by John Palmer.

Comparison of many bristlecone pine rings from small core samples obtained from over the entire region facilitates cross-referencing. Increasingly older trees provide additional data. Matching of wood samples from various archaeological sites provides a means for the absolute dating of the sample. The present chronology covers over 7,000 years in a continuous scale. The Laboratory of Tree-Ring Research at the University of Arizona continues this work.

As a rule, researchers must find trees of the same species with distinguishable rings capable of comparison to rings of known date. A control set of tree ring samples provides an index for comparative purposes. Gaps in ring sequences occur for various reasons.

When trees grow in moist soil a wide ring results. Water deficiency produces a narrow ring. Drought conditions or extreme dryness may prevent ring growth altogether. Samples may contain injury rings or false rings. Local climate variations can result in different ring growth for trees in the same general area.

A bristlecone pine tree from the Ancient Bristlecone Pine Forest in the White Mountains of California. A BibArch Photo by John Palmer.

Not all tree species lend themselves to tree ring analysis because some tree species do not react to climatic conditions in a way that is helpful to discerning their age. Other environmental factors, e.g., chemical pollutants, insect infestations, volcanic eruptions, and the like, also affect ring growth.

Present day tree-ring research has expanded on a global scale. Of significance for archaeology in the Near East is the possibility of a master chronology for cedar tree-rings based upon the work of Pierre Michel Bikai. He developed a limited scale using the cedars of Lebanon (Bikai 1991).]

A wealth of cedars of Lebanon artifacts in Egypt provides a vast archive for investigation. What remains for the future is the determination of a scale that provides an accurate chronological framework. Even when wood sample dates are clear, archaeologists have to use care in drawing conclusions for wood samples must be closely associated with the artifacts under analysis and the wood could be in a secondary use.