166 



THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



4, and 5 years. In either case it is obvious that the most favorable growth comes at the 

 end of a series of 4 or 5 years of increasing rainfall, while the slowest growth follows a similar 

 series of years of cUminishing rainfall. In the case of slow growth we see evidence that a 

 slight improvement in the amount of rainfall is not able to overcome the harmful effects 

 of a preceding series of bad years. Otherwise the curve of the slow years would not hook 

 up at the right-hand end, showing that after a series of bad years, even though the rainfall 

 increases somewhat, the trees do not respond at once. 



Av. rainfall 

 in inches 



Years preceding year of growth 

 5 4 3 2 1 



Mean rainfall in relationto 

 15 years of rapid growth 



Mean rainfall for 62 years 



Mean rainfall in relation to 

 14 years of slow growth 



Av. rainfall 

 in inches 

 12 



11 



10 



Years preceding year i 

 4 3 2 



Fig. 46. — Conservation Factor in the Relation 

 of Growth and Rainfall, Method I. 



9 

 Fig. 47. 



f growth 

 1 



Mean rainfall in relation to 

 15 years of rapid giowth 



Mean rainfall for 62 years 



Mean rainfall in relation to 

 14 years of slow growth 



Conservation Factor in the Relation 

 of Growth and Rainfall, Method II. 



If we are right as to the relation of the rainfall of past years to the growth of the sequoia 

 during any particular season, it ought to be possible to reduce this relation to a formula, 

 just as in the case of the yellow pines of Arizona. Professor Douglass has kindly consented 

 to work out the formula. It is given below, together with his comments: 



"A trial of the 'accumulated moisture' formula of the yellow pines in Arizona shows that 

 it does not apply to the sequoias of California, presumably because the precipitation is heavier 

 among the Sierras than in the plateaus of Arizona. An 'additive' formula, on the other hand, 

 gives an encouraging result, as is shown in the accompanying diagram (figure 48). This formula 

 allows for strong conservation by the soil, not of the static type, as in a pond, but of the moving 

 type as if a belated supply from the snows came to hand and then passed on. The tree, then, 

 has moisture from the current year and from the first and second preceding years; and whichever 

 of the three is greater, that one has the more effect. The formula is 



r„ = K 



Rn + Rn-l + Rn-? 



This is of course empirical and will be improved. It is worthy of study as illustrating what 

 appears to be a difference in type of formula for different climates. Without doubt the reversal 

 of this formula to ascertain rainfall from tree growth is much more difficult than that of the Arizona 

 formula, for the tree automatically smooths the rainfall variations, but variations of a longer period 

 than three years will be evident." 



The gist of the relation of the growth of the sequoias to precipitation may be stated in 

 a few words. In the regions whence our measurements have been obtained the growth 

 depends primarily upon the amount of rainfall, but almost equally upon its monthly dis- 

 tribution. Owing to the conservation factor the rainfall of any single year is only one of 

 the factors which determine the amount of growth. Only by taking a period of 3 or more 

 years can we form an accurate judgment as to the actual amount of growth which corre- 

 sponds to a given rainfall. Where a longer period than 5 years is concerned we may say 

 with confidence that, if due allowance is made for age, longevity, and other factors, the 

 thickness of the rings of growth is dependent upon the amount and season of the rainfall. 

 Excessive precipitation may, perhaps, in some cases check growth, but as yet no evidence 



