18 CLIMATIC CYCLES AND TREE-GROWTH. 



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 soil derived in place from decomposed lava, and so there was little 

 depth for the storage of moisture. At Prescott the sections of group 5, 

 shown in the solid line of figure 7, came from trees growing in a porous 

 soil of decomposed granite in a rather flat depression with reaarded 

 drainage, so that conservation would have a greater influence. Perhaps 

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

 In the region of the great sequoias nearly all the precipitation in the 

 mountains (and quite all in the valleys where comparative rain records 

 are found) comes in the winter months. For these trees, therefore, 

 the winter precipitation is compared with the growth for the succeeding 

 year and the month of beginning annual means is in the autumn. 



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 sug- 

 gested by Director R. H. Forbes, formerly of the Arizona Experiment 

 Station. This was to determine the time of formation of the red or 

 autumn portion of the rings and the causes for the formation of double 

 rings, which were very numerous in the Prescott group. It seems 

 evident at once that the growth of red cells is related to the decreased 

 absorption of moisture as winter approaches. A number of tests were 

 made on the Prescott group. The first was designed to determine 

 the character of the rainfall in the years producing 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 1.55 mm. The 

 mean rainfall by months for the years above selected was found and 

 is plotted in the solid line of the upper diagram of figure 1. Six other 

 rings showing one double in 10 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 during 

 which they grew has been plotted as the upper dotted line in figure 1. 

 In each curve the 6 months preceding and the 2 months following the 

 year are included. The curves seem to indicate clearly that the chief 

 cause of doubling is a deficiency of snowfall in the winter months, 

 December to March. This appears to mean that if the winter pre- 

 cipitation is sufficient to bridge over the usual spring drought, the 

 growth continues through the season, giving a large single ring which 

 ends only in the usual red growth as the severity of winter comes on. 

 If, however, the preceding winter precipitation has not been entirely 

 adequate, the spring drought taxes the resources of thetree and some red 

 tissue is formed because of deficient absorption in the early summer before 

 the rains begin. When these rains come the tree continues its growth. 



