132 



CLIMATIC CYCLES AND TREE-GROWTH 



shown in the trees, was made and analyzed. The major dry periods 

 came at 1440, 1580, 1735, and 1880 to 1900, or an average of about 

 150 years apart. Also, the single tree which gives a record beginning 

 at 1285 shows a great depression at 1295 to 1300, which conforms to 

 this 150-year spacing. Thus the major droughts give a cycle which 

 was long since (1914) noted as occurring in the Arizona record. Cyclo- 

 grams 9 and 10 show analyses at 14.6 and 20.2 years as the best to 



18.) 



IS. I 



Cycle identity across 200 miles 



1650 



4 



Vermont 

 hemlocks 



Dearth cycles at 1700 A.D. 



Flagstaff cycles near 7,l4and 22 



1442 1580 1735 1900 



M \* »« 4 # ,,(* ****** 



/*x 



10 



»>", 





Arizona drouth cycles 



Fig. 19 — Details of cyclogram patterns in Plate 9 



cover this 575-year lapse of time. These cycles are near the 14.0 

 and 21.0 values and may be identical. It will be seen that there is a 

 tendency to group the droughts at intervals of something under 50 

 years. This could be 42 years, the interval at which 14- and 21-year 

 cycles have their major effect on each other. Probably the 150-year 

 effect emphasizes whichever 21-year multiple is nearest, with some 

 modification from the 14-year cycle. 



