Page:Climatic Cycles and Tree-Growth - 1919.djvu/26

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20
CLIMATIC CYCLES AND TREE-GROWTH.

were cut during various months from May to November. Naturally, those cut in May are in the midst of their most rapid growth, while those cut in summer may or may not show the double ring just forming. The conditions are shown in table 2.

Table 2.

1 feet.
6,125
1911 May, June 9 out of 10 show white tissue only.
2 and 4 6420 1909 July to Sept. 30 out of 33 show red ring just forming, probably a doubling.
5 5,800 1909 Summer 3 or 4 out of 10 show red ring just forming, probably a doubling.
3 6,800 1910 Oct. and Nov. All 12 show white without red, probably a large single.

By reference to figure 1, showing the curves of monthly rainfall for 1909 and 1910, it will be seen that 1910 would be likely to carry its growth through the year and produce a single line, as in group 3 above. The year 1909 is of intermediate character, having heavy winter precipitation and a severe spring drought of 3 months. In the groups cut at this time 33 out of 43 show a red ring forming in July, August, or September, doubtless the preliminary ring of a double. This lesser red ring is due to the spring drought, and its appearance at this time indicates a lag of a couple of months, more or less, in the response of the tree to rain. The whole matter of the relative thickness of the red and white portions of the rings is illustrated in figure 2. The heavy sinuous line shows the rainfall month by month at Prescott throughout the 43 years under consideration. The total rainfall for the year is indicated by the dotted rectangles while the size and character of the, rings is shown in the solid rectangles. In these the white portion indicates the white tissue and the shaded portion indicates red tissue.

Significance of subdivisions in rings.—The normal ring consists of a soft, light-colored tissue which forms in the spring, merging into a harder reddish portion which abruptly ends as the tree ceases growth for the year. The present subject (namely, the time of year of ring formation) indicates that the red tissue appears as the tree feels lack of sufficient moisture. Therefore, the great diversity in relative size of the red tissue and the occasional appearance of false rings undoubtedly has a real significance as to distribution of precipitation during the growing-season. This subject is a very promising one, but has received little attention in the present work. The trees of the Prescott group offer a few interesting examples of two or three false red rings in one year; they also have exceptionally many cases of omitted rings; both of these peculiarities are explained by the fact that these trees are close to the lowest elevation at which the climate permits them to live; they are therefore greatly affected by rainfall distribution and probably exaggerate its changes.