Page:Climatic Cycles and Tree-Growth - 1919.djvu/88

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CLIMATIC CYCLES AND TREE-GROWTH.

3. From An an approximate Rn is found by the formula

R_n=M+A_n-A_{n-1}

4. This series of approximate rainfall Rn is smoothed and becomes the Sn of the formula.

5. Final values are then found by the proportion

\frac{0.90M+\tfrac{1}{4}A_n}{250}:S_n::T_n:R_n

It should be emphasized that the above formula for conservation is the one found to apply under dry climatic conditions. In moist climates the trees, so far as observed, seem to depend on other meteorological elements or combinations of elements.

The Prescott trees, as we have seen, even without correction give a record of rainfall with an accuracy of about 70 per cent. It is possible that the Flagstaff trees with their higher elevation, more certain rainfall, and more central location in the zone occupied by this species give somewhat more accurate records. They are probably much less often subjected to extremes of dryness, which throw the tree out of its equilibrium and cause it to produce an abnormally small set of rings. It seems likely, also, that the less compact soil, combined with a more abundant precipitation, produces a yearly growth more nearly proportional to the rainfall than at Prescott.

Summary. — In considering this reduction it seems fairly clear that (1) there is a strong correlation between rainfall and tree-growth; (2) the accuracy may be increased by introducing a conservation correction; (3) in dry soils this factor enters as a coefficient; (4) this coefficient depends on the state of activity of the tree; (5) in the Prescott trees this state of activity follows the curve of accumulated moisture.

Although the moisture-content of apparently dry ground may be a most important item, it is by no means certain that the observed accumulated moisture effects consist in actual moisture in the ground. It may be that they represent some vital condition of the tree. The matter is a very interesting one for future study.

Sequoia correlation with rainfall. — On his return from the big trees in 1912, Professor Huntington supplied me with a curve of sequoia growth obtained from many comparatively young trees which had been cut in the lower edge of Redwood Basin near Camp 6. On comparing these with his curve of rainfall in the San Joaquin Valley, compiled from records at Fresno and San Francisco, a close relation was not evident, but an additive formula

T_n=k\cdot{\frac{R_n^2+R_{n-1}^2+R_{n-2}^2}{R_n+R_{n-1}+R_{n-2}}}

was used with encouraging results. This formula was designed to allow for strong conservation in the soil, not of the static type as in a