Dendrochronology: Tree-Ring Dating

Dendrochronology is just a fancy way of describing the way scientists use tree-rings to date structures and events or to reconstruct past environmental conditions. Dates have been obtained on a wide variety of objects and events, such as past volcanic and glacial activity, avalanches, earthquakes, frosts, epidemics of tree disease, forest fires, floods, historical architecture and other archaeological remains and wood panels used for oil paintings.

Astronomer Andrew Ellicott Douglass established the principles of dendrochronology. Observing that certain tree rings are wide in wet years and narrow in dry years, he demonstrated that ring-width patterns are unique for particular time periods; thus the ring widths can be used to date the time that a piece of wood was a growing tree and to deduce past climatic changes that influenced its growth. Applying the tree-ring dating method to archaeological ruins in the Southwest U.S., he dated 40 major ruins by linking ring-width patterns in ancient wood with those in living trees.

In 1937, Douglass established the Laboratory of Tree-Ring Research at the University of Arizona, Tucson, which is the largest institute of its kind in the world. Tree-ring dates have since been assigned to more than 50,000 archaeological specimens gathered from over 1,200 separate sites in the Southwest. Bristlecone pine, Pinus aristata, from California, which lives to an age of more than 4,500 years, supplies the longest American tree-ring record, almost 8,700 years. A continuous series of about 7,300 years has been established for buried oak wood in Europe. European oak about 9,000 years has been dated through the technique of radiometric age-dating, but gaps in the chronology are not yet filled in for these specimens.

Major reinterpretations of European prehistory have resulted from a revision of the radiocarbon time scale by dendrochronology. When the radiocarbon scale was checked against dated bristlecone pine wood, a flaw was discovered in the radiocarbon dating method, and dates on European sites older than 500 BC were found to be too recent, with maximum discrepancies of 700 years occurring at about 5000 BC.

TREE RINGS

Dendrochronology was developed using the well-defined rings of temperate and subpolar gymnosperm and angiosperm species. The annual ring is formed by cell division in the cambial tissues under the bark. The cambium first produces large, thin-walled cells called earlywood. Subsequently formed wood cells are smaller, denser, and have thicker walls. This dense wood on the outside of the ring is called latewood. The ring boundary is formed by the distinct structural change between the dark and dense latewood on the outside of one ring and the light-colored, low-density earlywood on the inside of the next ring. Dendrochronology has not been successfully applied in the tropics, because rings in many tropical species are usually poorly defined, or else climatic conditions are insufficiently limiting to produce regional variations in ring width.

Environmental interpretation of wood samples implies that conditions such as low or high precipitation or temperature influence the processes that control ring growth. Sunshine, wind, carbon dioxide, antecedent soil moisture, snow, ice, and other conditions can also influence ring character. The same variable can be limiting in different ways at different times throughout the year. Thus, a variety of past environmental conditions can be deciphered from tree rings.

Statistical studies indicate that ring widths may be directly correlated with temperature, precipitation, or sunshine in certain months, inversely correlated in other months, and unrelated in still other months. Nevertheless, moisture often is most limiting in dry regions, and temperature most limiting in cold regions, with sunshine or other conditions limiting if temperature and moisture are neither too low nor too high. The density of wood and its chemical composition, including isotopes of carbon, oxygen, hydrogen, and various pollutants, are additional sources of past environmental information contained in tree rings.

METHODS OF TREE-RING DATING

Living trees are sampled with a tool that extracts a core 4 mm (0.16 in) in diameter and of varying length without harming the tree. Experienced dendrochronologists search for the oldest trees and take 20 to 100 cores at each site.

Cross dating is the time-consuming laboratory procedure of comparing the ring-width patterns on all samples from a site. When all rings are clearly annual and only one exists per year, cross dating is relatively straightforward. In the most limiting sites, however, such as in arid America, some trees may not grow a ring every year, whereas others will occasionally produce two or more rings in certain years. The dendrochronologist identifies these problems by examining the synchroneity of the ring-width patterns among samples. A discrepancy in synchroneity occurs where either an extra ring exists or a ring is missing. The dendrochronologist determines the cause of the discrepancy, makes the changes necessary to remove it, and then rechecks the synchroneity. Cross dating is complete when all such problems are resolved and the relative ring-width patterns coincide among all specimens. A date is assigned to each ring, and the chronology of the ring-width variation is checked against the chronology of variation from trees in neighboring sites. Discrepancies are rarely found, but when one is, all materials are rechecked until agreement is reached. A chronology from living trees is extended backward in time by cross-dating with it older samples of unknown age from the same site.

Individual ring widths or other characteristics are measured, entered onto computer files, and processed to minimize extraneous nonclimatic information. The ring widths are converted to relative values called indices, which are averaged for each year to obtain a chronologic record. Other computer techniques are used to calibrate the indices with climatic factors, such as temperature and precipitation. Calibration is analogous to marking the glass tube of a mercury thermometer so that the height of the mercury column can be read as temperature.

Several chronologies can be calibrated with climatic data from a nearby weather station, or an array of chronologies throughout a region can be calibrated. Past growth values can then be used to construct regional maps of past climatic variations. Such reconstructions have greatly extended our knowledge of past climates, including droughts, unusually warm and cold periods, volcanic eruptions, earthquakes, floods, and other environmental conditions.

Source: Henri D. Grissino-Mayer's Ultimate Tree Ring Web Pages