110 THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



MONTH OF BEGINNING ANNUAL MEANS. 



Before passing on to other matters, a word of explanation must be added as to the 

 method of calculating the rainfall. That it must take some time for the transmutation 

 of rain into an important part of the organic tissue is evident. It has often been asked of 

 the writer how soon the rains affect the trees. There is evidence, as will be shown later, 

 that the summer rains often have an almost immediate effect. The winter precipitation, 

 however, is more remote in its action. Much of the first growth in the spring must come 

 from precipitation long past, and a large part of the yearly growth comes from the melting 

 of the fall and winter snows. It seems reasonable, therefore, to consider any snowfall as 

 applying to the following yearly ring. 



At Flagstaff the precipitation of November is almost always in the form of snow, and 

 therefore that month should certainly be considered as falling after the arboreal new year 

 of that locaUty. In view of the uncertainty as to the exact month when the precipitation 

 begins to have an influence upon the growth of the following season, and in view of prob- 

 able variations in different years, it seemed wise to test the matter by a purely empirical 

 method. The annual rainfall was ascertained for yearly periods beginning (1) with July 

 1 of the preceding year, (2) with August 1, and so on to (9) with March 1 of the current 

 year. Another method involved a separating of the suimner rains, one-half to apply on 

 each adjacent winter, while a final method involved a similar division of the winter rains. 

 This was done for 12 years at Flagstaff and 43 at Prescott. Part of the Flagstaff curves 

 are given in the lower part of figm-e 11, where the rainfall can be compared with the growth 

 of the trees. The 11 curves plotted from these figures were found to have substantial 

 disagreements, although, of course, the smoothed curves of all of them would be practically 

 identical. A comparison of the growth of the tree with these 11 curves showed that the 

 use of the year beginning November 1 at Flagstaff and September 1 at Prescott gave the 

 closest agreement between growth and rainfall. At Flagstaff the majority of the trees 

 came from a thin clay derived from decomposed lava, and so there was little depth for the 

 storage of moisture. At Prescott half the sections of group 5, whose curve, it will be remem- 

 bered, is shown in figure 10, came from trees growing in a porous soil of decomposed granite 

 in a rather flat depression with retarded drainage, so that conservation would have a greater 

 influence. Perhaps this explains why the year beginning September 1 gives the best results 



there. 



THE TIME OF YEAR OF RING FORMATION. 



Among the problems connected with the relation of the growth of trees and the amount 

 of rainfall, one of the most interesting was suggested by Mr. R. H. Forbes, of the Arizona 

 Experiment Station. The problem is to determine the time of formation of the red or 

 autumn portion of the rings, and the causes for the formation of double rings. Apparently 

 the red cells are due ultimately to a decreasing absorption of moisture during the cold 

 period of winter when the ground is frozen. This study is the more necessary because 

 many rings in the Prescott series (although very few in the Flagstaff series) show a faint, 

 preUminary red ring forming a double. The first test was designed to determine the 

 character of the rainfall in the years producing such double rings. The half-dozen most 

 persistent cases were selected, and in each of these the red ring was found double in the 

 following number of cases: 4 out of 10 in 1896; 5 out of 10 in 1891; 7 out of 10 in 1881; 

 4 out of 10 in 1878, 1872, and 1871. The average width of all the rings was 1.5.5 mm. 

 The mean rainfall by months for the years above selected was found and is plotted m the 

 soUd Une of the upper diagram of figure 13. Six other rings showing one double in ten 

 trees in 1898, but no doubles in 1897, 1885, 1884, 1876, and 1874, and averaging 1.54 mm. 

 in thickness, were then selected and the curve of rainfall by months for the year durnig 

 which they grew has been plotted as the upper dotted line in figure 13. The curves seem to 



