134 CLIMATIC CYCLES AND TREE GROWTH 



can only be checked in an immensely long series of data such as thousands of 

 years in which internal evidence can be developed. Because of the discon- 

 tinuity of climatic cycles we can not, for example, with 49 years of records, 

 tell what the 50th year will be, but with 300 we have a much better chance 

 of estimating the 301st and, better still, with 3000 we can judge the 3001st 

 much more closely. The absolute need of these long sequences has been met 

 to an important extent by the thousands of years of dated tree-ring records 

 of tangible, and, in one case at least, of high rainfall value. With these aids 

 our procedure is to examine first the results in a region where they are 

 best known, and that is in Arizona. 



THE ARIZONA RESULTS 



These tree-ring records show remarkable similarity to each other over 

 great areas. This is called cross-identity and is due to meteorological effects. 

 The relationship of the ring growth of these trees to winter rainfall is very 

 evident over the period of time in which precipitation records are available. 

 Because of this development of tree-ring records in northern Arizona and New 

 Mexico, the climatic cycles as expressed in rings have been subjected to 

 special study. They form a group called the common "cycle complex" be- 

 cause in analyzing thousands of tree-ring curves both modern and ancient the 

 same cycle lengths are largely found to dominate. These are approximately 

 5f , 8-f, 10, 11£, 14, 17, 19 to 20 and 23 years, not all coming at once. We find 

 that certain of these prevail over the others in certain areas, perhaps due to a 

 selection resulting from degree of water conservation. In Arizona, a 19-20 

 year cycle has been very strong; the next is near 14 years. When the great 

 Arizona averages, including 50 or 60 trees (Flagstaff Area Mean Curve — 

 FAM), have these two cycles subtracted from them, the double-crested 11?- 

 year cycle that is called the Hellmann cycle is the chief one left. Some of the 

 Arizona groups, such as the Grand Canyon group, show the Hellmann cycle 

 without its being necessary to subtract the longer ones mentioned. 



It seems probable that three or four cycles have formed the chief basis 

 of Arizona variations in the last 100 years, at least as far as winter rainfall 

 is concerned, and their immediate application in prediction might be ad- 

 vantageous. There are still three obstacles to the desired fullest solution of 

 the problem in Arizona. 



First: These cycles are not permanent and we are not yet certain when 

 any one is going to change. That special phase of the problem is called 

 cycle sequence or recurrence. 1 There is evidence of a long control cycle of 

 about 270 years, in which the shorter cycles tend to recur in orderly sequence. 

 It is especially important to note that these cycles in their discontinuity do 

 not change all at once but rather one at a time; hence, for short intervals of 

 a few years the effect of possible change of one of several becomes actually 



1 See Proceedings National Academy of Sciences, vol. 19, No. 3, 359-360, March 

 1933. 



