Tree Rings and History
A tree's age can be easily determined by counting its growth rings, as any Boy or Girl Scout knows. Annually, the tree adds new layers of wood which thicken during the growing season and thin during the winter. These annual growth rings are easily discernible (and countable) in cross-sections of the tree's trunk.
In good growing years, when sunlight and rainfall are plentiful, the growth rings are broad, signifying a period of rapid growth. But in periods of drought, they become very thin. Thus, it is possible to count backward among the rings of a tree to determine what the weather was like in years past (it is not necessary to cut down the tree--core samples do as well).
Because few varieties of trees live beyond several hundreds of years, it might be expected that that would be the limit to the chronology that can be established from growth rings, but it isn't. Since the width of the rings is determined by climate, all members of any regional species exhibit identical patterns of wide and narrow rings.
Because not all trees are the same age, it is not difficult to find cases where ages overlap. Therefore, a continuous record can be extended backward in time indefinitely by matching the earliest rings of a living tree with the later rings of an older dead tree or pieces of preserved wood, such as a beam. By repeating the process with even older pieces of wood, investigators can produce a chronology covering much of the history of civilization. For instance, it is possible to determine the year when a tree used in building a structure was cut and presumably utilized.
Tree ring specialists, or dendrochronologists, as they are called, are finding that climatic conditions of the past may have influenced many historic events.
As reported in an article by Thomas Maugh in the June, 1981 issue of Science 81, Samuel K. Eddy of Syracuse University has found that the size of tax payments (a measure of agricultural production) in the ancient Greek town of Byzantion was proportional to the thickness of tree rings. The chronologies also show that each year of constitutional crisis in Athens between 632 B.C. and 510 B.C.--a time when democracy was being forced on the aristocracy--was preceded by one or two dry years.
The expansion of the early Christian church seems also to be closely related to dry periods. "At least," Eddy says, "it is a remarkable fact that nearly all the martyrdoms of the second century that can be accurately dated coincided with years of thin rings. It was as the Christian lawyer Tertullian wrote: Persecution was most likely when the Nile failed to irrigate Egypt, or the Tiber flooded Rome, or when drought set in."
To put it in a slightly more callous vein: people get nasty when they're hungry, and tree rings bear the testimony.
