126 CLIMATIC CYCLES AND TREE-GROWTH 



with some possible interference for a considerable interval about the 

 end of the seventeenth century." Early in 1922 a letter was received 

 from Professor E. W. Maunder, of England, calling attention to the 

 prolonged dearth of sunspots between 1645 and 1715, and saying that 

 if there were a connection between solar activity and the weather and 

 tree-growth, this extended minimum should show in the weather and 

 in the trees. On receipt of the letter, this period was immediately 

 recognized as the interval referred to in which there was entire failure 

 in attempting to trace effects of the well-known solar cycle. The 

 sequoia record for the last 500 years, as summarized in figure 33, page 

 103, of the previous volume, confirms minutely the result. So also do 

 the Vermont hemlocks and other tree-records. 



Dearth cycles — In 1922 or before it was noticed that when the 

 11-year cycle disappeared from the trees near 1700, two other cycles, 

 one of 10 or 20 years and the other of 7 or its smaller multiples, became 

 prominent in its place in the Arizona pines (see Plate 9 and Fig. 19). 

 Soon after, it was noticed that the Vermont hemlocks and the sequoias 

 of California show similar change at that time. And then it was 

 observed that these three cycles appear generally in the western trees; 

 they are, first, the known sunspot cycle of about 11£ and its double 

 of 23 years; second, 10 or 20 years; and, third, 7, 14, 21, or 28 years. 

 These three cycles, with others mentioned below, have been confirmed 

 in the present study of the 42 western groups. There is some reason 

 to think that all of these cycles come from the sun, for at different 

 times the sunspot cycle itself has changed to one or the other of them. 

 For example, from 1748 to 1788 there were four complete cycles of 

 close to 10 years each; and from 1788 to 1837, 49 years, there were 

 three complete cycles of about 14 years each and one of 7. It seems 

 at least likely that these other two cycles, found in western trees with 

 extraordinary persistence, are also of solar origin. 



Wet and dry climatic effects — In this study of cycles in the western 

 yellow pine it was found that in this dry region, where trees are 

 specially sensitive to rainfall, they show, besides other cycles, a double- 

 crested 11-year variation, just as the rainfall itself does, but in the 

 moist coastal regions this solar cycle has more often a single crest 

 like that of the sunspot numbers. This agrees with the result of 10 

 years ago, in which the wet-climate Scotch pines of North Europe, 

 especially near the Baltic Sea, showed a direct single-crested cycle 

 having a remarkable resemblance to the curve of sunspot numbers 

 (Volume I, p. 77). Their growth gave the solar changes with an 

 accuracy exceeding that of any trees of the southwestern area. (See 

 S-14 in Plate 9.) This remarkable solar record is a wet-climate 

 phenomenon, but it is not yet clear just what causes its accuracy. 

 It seems probable that these trees follow the sunspot cycle more 



