Some Historical Background on Dendrochronology

The systematic study of tree rings in western North America began with an intuitive insight by an astronomer, Andrew Ellicott Douglass. He was, before the turn of the 20th century, working at the Lowell Observatory in Flagstaff, Arizona, and was interested in the cyclic nature of solar activity, particularly sunspots, and its relation to terrestrial climate. Since the written record of solar activity extended further back in time than the record of terrestrial weather, he envisioned tree growth as a proxy measure of climate. Douglass’ investigation began in 1901 and was based on the following premises: that the rings of a tree are a measure of its food supply; that the food supply depends largely on the amount of available moisture, especially in drier climates where the quantity of moisture is limited and the life struggle of the tree is against drought rather than competing vegetation; and that therefore the rings are a measure of precipitation (Douglass, 1914, page 321). His method involved first the preparation of a treegrowth curve, and for this purpose pine (Pinus) trees growing in the environs of Flagstaff were chosen. In addition to convenience, these trees had two obvious advantages. First, the moisture available to the trees was primarily in the form of precipitation and, second, the average age of the trees was nearly 350 years, with some more than 500 years old (Douglass, 1914, page 322). This latter quality allowed a large backward extension of the growth curve in the record of a single tree.