Page:Climatic Cycles and Tree-Growth - 1919.djvu/42

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32
CLIMATIC CYCLES AND TREE-GROWTH.

sections there is more than usual variation in different radii, an excess of growth starting in one direction and then slanting off in some other direction. Here it was found also that maxima were not always the same in different radii. It was suspected that some radii tried to follow a single cycle and others a double cycle. A photograph of one of these sections is shown in plate 4, a.

The appendix presents a table of mean growth of this group from 1845 to 1912. No. 2 had its center in 1865, and between that date and 1845 extrapolated values have been used in forming the means. These extrapolated or artificial values preserve the average shown by the individual tree during its years of growth, but are made to vary from that average in accordance with the variations of the rest of the trees in the group. From 1828 to 1844 the mean of 3 sections only is given. The actual mean has in this latter case been multiplied by 1.25 to bring the average into accord with the group, for the mean of these 3 for the 11 years from 1845 to 1855 inclusive is only 80 per cent of the mean of the group. In this group one center was in 1865, three in 1844, one in 1842, one in 1840, one in 1836, one in 1827, one in 1800, and one in 1693.

These means are plotted in figure 8. The same corrected to a standard mean and smoothed by Hann's formula will be found in figure 23. No real correction for age has been made in this case, for there seems little change in rate of growth that can certainly be identified as such. The whole, therefore, has been simply reduced to scale for comparison with other groups by dividing every year by 1.25, which is very nearly the average growth in millimeters.

INNER COAST OF NORWAY GROUP.

It is a great help to visit the exact locality in which the trees grew, or to get very near it, as in the groups already described, and especially to obtain personal information in a mountainous country like Norway, where meteorological conditions may vary enormously within a few miles. But it was impossible in the present group, whose sections had mostly been collected some years before for use in the forest service and schools. By courtesy of various officials I was permitted to examine and measure these sections in their offices, and whenever it was possible thin sections were cut off for me to add to my collection. In measuring sections of which samples were not retained, for example B 15, B 16, and N 2, there was no opportunity of cross-identifying rings, and hence unusual precautions were observed in numbering the rings. If at any spot they seemed to be very close together with any chance whatever of mistake by omission or doubling, the numbering was carried to as many other radii as were necessary for a check, and worked over very carefully until the best possible result was obtained