106 CLIMATIC CYCLES AND TREE-GROWTH. 
immediately after their completion, and the period at once suggested 
the Briickner 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 
NAVA AAV AVA 
Ldn 
L 1 
1500 1GOO 1700 1g00 1900 
Fia. 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 
aN! 1 salle A elec 
Aw W\nw W a 
fven bers (gh ~“ 
- ~ 
+ -/- : 
| 4£ven pumbers ow 
a = he 
750 40 30 20 0 700 90 80 70 ? 
Yeor-8.C. 
Fia. 39.—D-22 at 750 to 660 B. C., showing a 2-year period.* 
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 
1, The corrections found in 1919 make these dates one year earlier. 
