From the Greek dendron, tree limb, and khronos, time, dendrochronology is the scientific method for establishing the age of timber. By "reading" the annual growth rings visible in the cross section of a tree trunk the exact year of its felling can be established.
The technique was first developed by the astronomer A.E.Douglass as a means of studying sun spot activity. Douglas hypothesized that the earth’s climate was affected by solar activity which would be recorded in the differing rates of growth of trees. In order to further this area of research he founded the Laboratory of Tree Ring Research, at the University of Arizona.
Trees produce new layers of wood for each year of their life cycle. These annual additions leave visible rings which mark the new growth. Several factors affect the size of the rings, most notably climate. In favorable conditions wider rings are produced whilst narrower ones reflect such natural phenomena as drought. Normally one growth ring is produced annually with only one known instance of this not happening. 1816, known as the year with no summer, failed to produce rings in oak trees. Undoubtedly the result of the massive volcanic eruption of the Indonesian island of Tambora; the year was more evocatively remembered by New Englanders as, "Eighteen hundred and frozen to death."
A useful tool for paleoecologists, who study past ecological systems, and archaeologists. Not only does the record indicate the year of a tree’s felling, it shows where the tree originated; thus giving critical historical clues to patterns of trade. With current concerns regarding the world’s weather systems it is surprising that there are no histories of climate. When an enterprising Ph.D. candidate decides to rectify this shortcoming dendrochronolgical findings will undoubtedly provide major sources of statistical evidence..
The exact year that a tree was chopped down is not necessarily the year that it was fashioned into an artifact. As green wood is usually not suitable for construction, or shipbuilding, it is normally seasoned first. Dendrochronologists can’t supply data for this. Similarly, as wood was an expensive commodity, it was always possible to reuse salvaged timber. The massive timbers in ships’ keels make perfect roofing materials.
As wood is perishable it isn’t possible to take the record too far back in time. About 11,000 years seems to be the maximum so far.
Used in conjunction with other technologies, such as ice core sampling, dendrochronology has added to our knowledge of the world; in the field of historical scholarship and, what might prove to be vital for future survival, it provides clues to climatic patterns.