CLIMATIC CYCLES AND TREE-GROWTH.
The investigation described in the subsequent pages bears close relation to three sciences. It was approached by the author from the standpoint of astronomy and a desire to understand the variations of the sun. It was hoped that these variations could be more accurately studied by correlation with climatic phenomena. But the science of meteorology is still comparatively new and supplies us only with a few decades of records on which to base our conclusions. So botanical aid was sought in order to extend our knowledge of weather changes over hundreds and even thousands of years by making use of the dependence of the annual rings of trees in dry climates on the annual rainfall. If the relationship sought proves to be real, the rings in the trunks of trees give us not only a means of studying climatic changes through long periods of years, but perhaps also of tracing changes in solar activity during the same time. Thus astronomy, meteorology, and botany join in a study to which each contributes essential parts and from which, it is hoped, each may gain a small measure of benefit.
It is entirely natural that the yellow pine, Pinus ponderosa, common on the western Rockies, should have been the first tree studied, since it was an intimate and extensive acquaintance with the forest and with the climate of northern Arizona that led the writer to the thought of possible relation between the two. The climate had been sought for astronomical reasons because its limited rainfall of about 22 inches gave many clear nights and superb skies. The forest with its great extent and stately trees proved wonderfully attractive and the absence of undergrowth or of other species of trees was its most noticeable feature to anyone accustomed to moist climates. Evidently the absence of undergrowth was related to the dryness, and the critical problem of the tree was to survive periods of drought rather than to compete successfully with other species in the struggle to obtain food supply. The following argument, therefore, was naturally suggested: (1) the rings of trees measure the growth; (2) growth depends largely upon the amount of moisture, especially in a climate where the quantity of moisture is limited; (3) in such countries, therefore, the rings are likely to form a measure of precipitation. Relationship to temperature and other weather elements may be very important, but precipitation was thought to be the controlling factor in this region and for the sake of simplicity it is the element fundamentally considered throughout the present study.
In the very beginning of the work it was expected that only in large averages would a relationship be found between tree-growth and climate. Accordingly, something like 10,000 measures had been made