V. CORRELATION WITH RAINFALL. 



Result of study of curves. On completing numerous curves of tree- 

 growth in the manner already described, three characteristics were 

 observed: (1) in arid-climate groups the annual rings are approximately 

 proportional to rainfall; (2) in moist-climate groups they vary with 

 the changes of solar activity; (3) in each they are subject to certain 

 cycles or periodic variation. The first of these is the subject of the 

 present chapter. 



Early tests of rainfall correlation. The earliest comparison with 

 rainfall hi this investigation was made between the first Flagstaff 

 subgroup of 6 trees and 43 years of precipitation records at Prescott, 

 67 miles distant. It was not expected that agreement in individual 

 years would be found; accordingly smoothed curves were used, con- 

 sisting of overlapping means of 9-year groups. This produced curves 

 of gentle variation, but similarity in the curves was evident. These 

 early curves are presented in figure 13. The best agreement was found 



1870 I860 1830 



Years 



Fio. 13. Correlation between tree-growth and rainfall in smoothed curves; Flagstaff. 



by placing each mean of 9 years of rainfall at the end of the 9 years 

 as in this figure instead of in its center. This lag of four years seemed 

 inconsistent with the later results of yearly agreement without lag, and 

 in fact for years it has been accepted with some hesitation by the 

 writer. Yet in the present consideration of the subject it appears to 

 have a special significance. This existence of the lag in long periods 

 agrees in principle with the "accumulated moisture" effects observed 

 in the Prescott trees and with the idea of a tree exhibiting a reserve 

 power or vitality which may run low or be built up by varying environ- 

 ment. The principle will be referred to again below; it is sufficient now 

 to state that it seems quite reasonable to find no lag in yearly correla- 

 tion with rainfall and at the same time a very considerable lag in the 

 slower variations. 



The comparison in figure 13 was made with Prescott records because 

 there were not at that time enough Flagstaff records to be of service. 

 But later, when a Weather Bureau station had been established in 

 Flagstaff for several years, the striking comparison shown in figure 14 



65 



