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 in 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 
ob I 
~20 wA a oars Growth/—~ oe , § 
ue 2th /f- smooth | 
A 
ry Sa a : 
Fe lL \A AL a erren) | ATR NE 
7 : aa" a ae 
7870 7880 7890 7300 
Fig. 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 
