CLIMATIC CYCLES. 249 



Kapteyn (1914: 70) has studied the growth of oak trees in Holland and Ger- 

 many and reaches the conclusion that during fairly long intervals of time they 

 exhibit not only a regularity, but also an actual and fairly constant periodicity 

 in growth. From 1659 to 1784, or for a stretch of 125 years, a period of about 

 12.4 years is clearly indicated. While this period disappears in certain groups, 

 it persists in others, so that its recurrence for two centuries is demonstrated, 

 with only one minimum missing. Huntington (1914: 135) has devoted his 

 attention chiefly to the major sun-spot cycles indicated in the rings of trees 

 and has secured some exceedingly suggestive evidence of cycles of 100 years 

 and more. Douglass (1919: 111) has examined the trees studied by Hunting- 

 ton, in order to obtain evidence from them as to the shorter cycles, especially 

 that of 11 years, and to carry the existence of such cycles back for a period 

 of 3,200 years: 



"The variations in the annual rings of individual trees over considerable 

 areas exhibit such uniformity that the same rings can be identified in nearly 

 every tree and the dates of their formation established with practical certainty. 



"In dry climates the ring thicknesses are proportional to the rainfall with 

 an accuracy of 70 per cent in recent years, and this accuracy presumably ex- 

 tends over centuries; an empirical formula can be made to express still more 

 closely this relationship between tree growth and rainfall; the tree records 

 therefore give us reliable indications of climatic cycles and of past climatic 

 conditions. 



"Certain areas of wet-climate trees in northern Europe give an admirable 

 record of the sun-spot numbers and some American wet-climate trees give a 

 similar record but with their maxima 1 to 3 years in advance of the solar 

 maxima. It is possible to identify living trees giving this remarkable record 

 and to ascertain the exact conditions under which they grow. 



"Practically all the groups of trees investigated show the sun-spot cycle or 

 its multiples; the solar cycle becomes more certain and accurate as the area 

 of homogeneous region increases or the time of a tree record extends farther 

 back; this suggests the possibility of determining the climatic and vegetational 

 reaction to the solar cycle in different parts of the world. 



"A most suggestive correlation exists in the dates of maxima and minima 

 found in tree growth, rainfall, temperature, and solar phenomena. The 

 prevalence of the solar cycle or its multiples, the greater accuracy as area or 

 time are extended, and this correlation in dates point toward a physical 

 connection between solar activity and terrestrial weather. 



"The tree curves indicate a complex combination of short periods including 

 a prominent cycle of about 2 years. " 



In addition, Douglass has made a preliminary study of sections of fossil 

 trees, which show a similar cycle for some of the more recent geological periods 



Considerable preliminary work has been done in tracing the effects of the 

 11-year cycle in plants other than trees and in plant communities. It has 

 been discovered that the dominant shrubs of sagebrush, chaparral, and desert 

 scrub often show this cycle in the growth-rings and that, in some cases at 

 least, the age of the shrub suggests that establishment takes place largely 

 and sometimes only during the wet phase of the cycle. Studies of the ex- 

 tension of forest and woodland into grassland or other arid communities has 

 shown that the entrance of the young trees occurred during the wet phase. 

 Henry (1895: 49) has shown that the height-growth of trees varies greatly 

 from wet to dry periods, and it seems certain that a similar relation exists 



