112 



THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



in May are in the niidst of their most rapid growth, while those cut in summer may or may 

 not show the double ring just forming. The conditions are shown in table 2. 



Table 2. 



By reference to figure 14, showing the curves of monthly rainfall for 1909 and 1910, 

 it wiU be seen that 1910 would be Ukely to carry its growth right through the year and 

 produce a single line, as in group 3 above. 1909 is of intermediate character, having heavy 

 winter precipitation and also a severe spring drought of 3 months. So in the gi'oups cut 

 at this time 33 out of 43 show a red ring forming in July, August, or September, doubt- 

 less the preliminary ring of a double. This lesser red ring is due to the spring drought. 



so- 

 lo- 



-10 



• 5 



-15 

 -10 . 



• 5 : 



- 13 ' 



-10 . 



- 5 



■ 15 ■ 

 -5 ' 



- 15 



- 10 

 -5 



> 30 - 



: 20 - 



■■30 

 ^20 - 

 1.0- 



30 - 

 20 

 10 ■ 



.^ 



M 





S I 



CZL 



Wl 



JEIL 



Fig. 15. — Monthly and Yearly Precipitation from 1866 to 1909, and Size and Character of Rings. 



and its appearance at this time indicates a lag of a couple of months, more or less, in the 

 response of the tree to rain. The whole matter of the relative thickness of the red and 

 white portions of the rings is illustrated in figure 15. The heavy, sinuous line shows the 

 rainfall month by month at Prescott throughout the 43 years under consideration. The total 

 rainfall for the year is indicated by the dotted rectangles, while the size and character of 

 the rings is shown in the solid rectangles. In these the white portion indicates white tissue 

 and the shaded portion indicates red tissue. 



MATHEMATICAL RELATION OF RAINFALL AND GROWTH. 



All the preceding investigations lead up to the question of the accuracy with which 

 the growth of trees represents the rainfall. The final answer will necessarily require a large 

 amount of work, but even now some definite idea may be obtained. In order to answer 

 this question an effort was made to construct a mathematical formula for calculating the 

 annual growth of trees when the rainfall is known. Any such formula must perform three 



