CHAPTER XI. 



A METHOD OF ESTIMATING RAINFALL BY THE GROWTH OF 



TREES. 



By a. E. Douglass, Sc.D., of (he University of Arizona. 



In the great northern plateau of Arizona, lying at an average altitude of 6,000 feet above 

 the sea, the higher elevations are covered with forests of yellow pine [Pirms -ponder osa) , 

 a fine timber tree with a heavy cyhndrical trunk and rather bushy top. The trees are 

 scattered gracefully over the plains and hills and, with the remarkable absence of under- 

 growth, render travel through their shady midst attractive and delightful. For centuries 

 these magnificent pines have stood there, enduring the vicissitudes of heat and cold, flood 

 and drought. They have not been subjected to a mild chmate for, contrary to common 

 opinion, northern Arizona has really a cold chmate. Several feet of snow lie on the ground 

 during the winter, and the summer days, though hot in the sun, are cold in the shade. 

 Hence the growth of the trees is sharply hmited to the warmer season. The climate of 

 Arizona presents not only a strong contrast between summer and winter, but between 

 successive years, the rainfall in some years being no more than a quarter as much as in 

 others. This being the case, it would seem that the trees must contain some record of the 

 cHmatic variations through which they have lived. Other methods of studying this matter 

 enable us to go back only from twenty to sixty years to the beginning of meteorological 

 records in Arizona. The trees, however, if they prove to convey any information at all, 

 will yield data covering two to five centuries. 



The possibility that the trees might serve as indices of the chmate of the past led the 

 author to begin investigation of the matter in 1901. His line of reasoning was as follows: 



(1) The rings of a tree measure its food supply. 



(2) Food supply depends largely upon the amount of moisture, especially where the 

 quantity of moisture is hmited and the hfe struggle of the tree is against drought rather 

 than against competing vegetation. 



(3) In such countries, therefore, the rings are likely to form a measure of the precipi- 

 tation. 



In planning the work three fundamental steps were anticipated. First, to prepare a 

 curve of tree growth ; second, to find if there exists in this any connection with precipitation ; 

 third, by carrying this back through long periods to find whether meteorological variations, 

 if discovered, show association with astronomical phenomena. 



Note. — Throughout the present investigation our purpose has been to employ as many different methods as 

 possible and to apply them in as many places as possible. Our danger has been that the framer of a theory, having 

 developed new lines of reasoning, is apt to become so convinced of their validity that he sees everything from a biased 

 standpoint. Fortunately, however, we are able to neutralize this danger by means of a new method of investigation, 

 a method entirely independent of those hitherto discussed, and one so exact in character that the personal opinion 

 of the investigator has little influence upon the main results. This method was suggested by Professor A. E. Douglass, 

 of the University of Arizona, in an article published in the Monthly Weather Review for June 1909, under the title 

 "Weather Cycles in the Growth of Big Trees." It does not, to be sure, shod light on the problem of the influence 

 of climatic changes upon human actions and history, but it enables us, by means of actual measurements, to ascer- 

 tain exactly what kinds of changes have taken place and at exactly what dates. The final determination of these 

 things is, of course, a long process, and can not be completed for many years, but important results can be obtained 

 at once. In order that the reader may have a first-hand statement of the matter, I have asked Professor Douglass 

 to contribute to this volume a chapter which shall embody not only his original work as described in 1909, but certain 

 measurements which he has since made, and upon which he bases fuller conclusions. Professor Douglass's contribu- 

 tion is inserted without further comment. — E. H. 



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