66 CLIMATIC CYCLES AND TREE-GROWTH. 
was made. In this the lower curve represents the average annual 
growth of 25 trees and the upper curve is the precipitation 12 miles 
distant. The latter is taken from November 1 to November 1 in order 
to carry the snowfall into the following season of growth. This study 
suggested the investigation of the time of 
19 
year to begin annual means of rainfall, fn ee | 
which has already been presented in  $ j7|F4 infall | \ MN 
Chapter II. Figure 4 gives acomparison & #|-< AH VI los 8 
between Flagstaff rain and the two Flag- 15 rea growth 07% 
staff groups, and also shows how the best _ AT \ 0.6 N 
time of beginning the year was deter- 05% 
mined. It proved to be November 1 at 7900 1905 
Flagstaff and September 1 at Prescott, Fic. 14.—Early test of correlation 
where the nature of the ground gives oy soared Deen aud relat 
more chance of conserving moisture. The 
great difference between individual trees in response to rain is also 
shown in figure 5. It is evident that quick-growing trees serve as 
better indicators. 
THE PRESCOTT CORRELATION. 
Five subgroups, numbering in all 67 trees, were obtained from 
different points in the vicinity of Prescott. These all cross-identified 
among themselves with entire success, both as individuals and as 
groups. The group curves are shown in figures 6 and 7, but in com- 
parison with the Prescott rainfall they differed greatly, the group 
nearest the city showing much the best accordance. Accordingly this 
group is plotted by itself in figures 7 and 15 with the rainfall curve. 
On the whole there is much agreement, as may be seen by comparing 
the crests and troughs of one with those of the other. The most con- 
spicuous discrepancy is in 1886, where the rainfall decreases and the 
growth of the trees increases. In 1873 the growth seems to have 
responded to the decrease in rainfall, but to a greatly diminished 
degree. The tree maximum of 1875, one year behind the extreme 
maximum of 1874 in the rainfall, is entirely reasonable, since the 
ground may become so saturated that the effects last until the following 
year. On the whole, the curves shown in figure 7 support the idea of a 
proportional relation between annual rainfall and annual growth. 
Aecuracy.—The accuracy with which the pine trees near Prescott 
represent the rainfall recorded in that city for 43 years is, without 
correction, about 70 per cent. By a provisional correction for con- 
servation of moisture by the soil this accuracy rises to about 82 per 
cent. The nature of this conservation correction is very simple; it 
makes use of the “accumulated moisture” of the meteorologist. It 
signifies that the rings in these dry-climate trees vary not merely in 
proportion to the rainfall of the year, but also in proportion to the sum 
