Page:Climatic Cycles and Tree-Growth - 1919.djvu/125

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99
CYCLES.

conspicuous and general cycles at once apparent in the trees are directly related to the solar period. They are as follows:

5 to 6 years approximate half sunspot period.
10 to 13 " " full " "
21 to 24 " " double " "
32 to 35 " " triple " "
100 to 105 " " triple-triple " "

There are few if any periods over 20 years not in this list, but under 20 years several are fairly persistent, such as 19-, 14-, 10-, and 7-year periods. There is also a period of about 2 years which causes a frequent alternation of size in successive rings, giving a "see-saw" or "zig-zag" effect in the appearance of the curve. The discussion in this chapter, however, will be confined to the solar group of periods above listed and to a preliminary statement regarding the 2-year period. As the larger of these solar periods are very nearly simple multiples of the 11-year period, it is naturally suspected that they are or should be real multiples of the sunspot period. Hence I feel at liberty to speak of the "double sunspot period" or the "triple sunspot period" without committing myself to its exact length.

Locality and solar cycles. — Compared to the multitudes of meteorological districts about the world, the few isolated localities which have here been investigated seem very insignificant. The wet-climate trees near the Baltic Sea show variations following almost perfectly, the curve of sunspot numbers. The Scotch pines just south of the sea have had good care since they were planted about 90 years ago. This care has prevented the excessive competition between individuals which characterize natural forests, and perhaps for that reason they give this remarkable record of external conditions. The trees to the north of the Baltic include spruces as well as Scotch pines, and show the same reaction. Both these groups are in comparatively level country and far from mountains. The group of pines from the Swedish province of Dalarne show the 11-year period somewhat less clearly. They were nearer the backbone of mountains which extends down the Scandinavian peninsula. The older trees of this group show evidence of a triple sunspot period. The groups growing in the mountains and in the inner fjords of Norway show extensive variations and even reversals. Some of the individual trees exhibit the sunspot period very well, while some show it inverted and some divide it into two crests. The older trees show evidence of an inverted double period.

The trees near sea-level, both at Christiania and on the outer coast of Norway, return again to the 11-year period. The former do not cross-identify well and the latter show occasional variations, such as double-crested period, inversion, etc. Variations of this kind were noted in different radii of the same tree. The trees from the south of