66 



CLIMATIC CYCLES AND TREE-GROWTH. 



1900 



1905 



FIG. 14. Early test of correlation 

 between tree-growth and rainfall 

 by years ; Flagstaff. 



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 

 year to begin annual means of rainfall, 

 which has already been presented in 

 Chapter II. Figure 4 gives a comparison 

 between Flagstaff rain and the two Flag- 

 staff groups, and also shows how the best 

 time of beginning the year was deter- 

 mined. It proved to be November 1 at 

 Flagstaff and September 1 at Prescott, 

 where the nature of the ground gives 

 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. 



Accuracy. 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 





