CLIMATIC CYCLES AND TKEE-GKOWTH, 



I. INTRODUCTION. 



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 



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