106 



CLIMATIC CYCLES AND TREE-GROWTH. 



immediately after their completion, and the period at once suggested 

 the Bruckner cycle of 35 years. This interesting tree has been men- 

 tioned on pages 34 and 41. 



A 2-year cycle. In the cross-identification of the trees used in this 

 investigation, a constantly recurring feature has been a marked alter- 

 nation in size of successive rings, giving them an appearance of being 

 arranged in pairs. In the plotted curves this produces a zig-zag or 

 see-saw effect. Usually such effect lasts a few years and then disap- 

 pears or reverses, but the example illustrated in figure 39 shows unusual 

 persistence. It is taken from D-22 from 750 B. C. to 660 B. C. The 

 even dates show less growth than the odd almost continuously for 

 60 years, but for the next 30 years the reverse is the case. This is 



mm. 



- 1.5 



I5OO 



IfcOO 



I70O 



I8OO 



I9OO 



FIG. 38. Triple sunspot cycle in a single tree from northern Norway. 

 Lower curve, a 34-year cycle. 



evidently due to a short period of about 2 years in length. It has not 

 yet been fully studied, but it is prominent in the European groups and 

 in the Vermont group. It frequently shows a duration of a little less 

 than 7 years in one phase, with odd dates greater in growth than even 

 dates, and then for the next 7 years reverses its phase. This 14-year 

 cycle is the series of beats the 2-year cycle produces by interfering with 



10 700 



Year- B.C. 



FIG. 39. D-22 at 750 to 660 B. C., showing a 2-year period. 1 



the exact annual and biennial effects in the tree. Hence, by a simple 

 process, its length is found to be in effect frequently 21 or 28 months. 

 Comparison has been made with the rainfall records near the Vermont 

 group (Douglass, 1915 : 181) and a variable period has been found 



!. The corrections found in 1919 make these dates one year earlier. 



