128 



CLIMATIC CYCLES AND TREE GROWTH 



1.50 



1.00 



0.50 



.*.' 



20 30 40 50 56 



Years 



Fig. 54— 57-year cycle in Arizona trees, A.D. 1168 to 1503. 



FREQUENCY PERIODOGRAMS AND THE CYCLE COMPLEX 



In previous pages we have dwelt upon concurrent variations in solar 

 and terrestrial data that suggest physical relationship; we now consider 

 another sort of evidence in favor of such a relationship. It is the "cycle 

 complex" or the group of cycles commonly found both in tree-ring data and 

 in the sun. This complex is shown in the form of a frequency periodogram 

 which gives the occurrence and weight of various cycle lengths observed. It 

 does not require exact amplitudes and epochs and other data discussed in 

 Chapter V. Between most of our groups and individuals in the western 

 areas, the constants so omitted are in excellent common agreement among 

 themselves wherever cross-identification has been done, because cross-identifi- 

 cation itself depends on many of the similarities that determine the length 

 of cycles. The simplification of data in the frequency periodogram greatly 

 enlarges the available material, because we can apply this degree of analysis 

 equally to geologic material whose dating and cross-dating are unknown and 

 to groups of modern specimens whose dating is uncertain. 



