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CLIMATIC CYCLES AND TREE-GROWTH, 
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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