20 



CLIMATIC CYCLES AND TREE-GROWTH. 



were cut during various months from May to November. Naturally, 

 those cut in May are in the midst 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 1, showing the curves of monthly rainfall for 

 1909 and 1910, it will be seen that 1910 would be likely to carry its 

 growth through the year and produce a single line, as in group 3 above. 

 The year 1909 is of intermediate character, having heavy winter 

 precipitation and a severe spring drought of 3 months. In the groups 

 cut at this time 33 out of 43 show a red ring forming in July, August, 

 or September, doubtless the preliminary ring of a double. This lesser 

 red ring is due to the spring drought, 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 2. 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 the white tissue and the shaded portion indicates red tissue. 



Significance of subdivisions in rings. The normal ring consists of a 

 soft, light-colored tissue which forms in the spring, merging into a 

 harder reddish portion which abruptly ends as the tree ceases growth 

 for the year. The present subject (namely, the time of year of ring 

 formation) indicates that the red tissue appears as the tree feels lack 

 of sufficient moisture. Therefore, the great diversity in relative size 

 of the red tissue and the occasional appearance of false rings undoubt- 

 edly has a real significance as to distribution of precipitation during the 

 growing-season. This subject is a very promising one, but has received 

 little attention in the present work. The trees of the Prescott group 

 offer a few interesting examples of two or three false red rings in one 

 year; they also have exceptionally many cases of omitted rings; both of 

 these peculiarities are explained by the fact that these trees are close 

 to the lowest elevation at which the climate permits them to live; they 

 are therefore greatly affected by rainfall distribution and probably 

 exaggerate its changes. 



