136 



THE CLIMATIC FACTOR AS ILLUSTRATED IN ARID AMERICA. 



yellow pine, one in New Mexico (the dotted line which we have already considered) and 

 the other the curve for 217 trees in Idaho. The two curves were prepared in exactly the 

 same way, and to both the corrections for age and longevity were applied according to 

 the same mathematical processes. Nevertheless, they are almost diametrically opposed. 

 ^Vllere one rises to a maximum the other is depressed to a minimum, and the oi)position 

 is evident in practically every case. Between 1600 and 1700 it is especially noticeable. 

 During the next centurj', when climatic conditions were less extreme, the difference in the 

 curves becomes less marked, and they run along together in a medial position. Then, 

 in the nineteenth century, when drought was severe in New Mexico, the two curves are 

 again in marked opposition. Moreover, the general trend of the New Mexican curve is 

 downward, indicating that, on the whole, conditions during the past three centuries have 

 liecome less favorable to the growth of the yellow pine in that region. The Idaho curve, 

 on the contrary, tends upward, suggesting that conditions have there become more and 

 more favorable. Much stress must not be laid on this last point, however, for the opposed 

 trends may be due partly to errors in the determination of the corrective factors. 



The relation between the two curves of the yellow pine is susceptible of two inter- 

 pretations. In the first place, it may indicate that rainy periods in New Mexico are syn- 

 chronous with dry periods in Idaho; or, in the second place, it may mean that droughts 

 are synchronous in the two regions, but that Ihe trees of New Mexico, growing in a warmer, 

 drier region than the others, are stimulated by long winters, heavy snowfall, and late, 

 moist springs, while those of Idaho are hindered by the same conditions. An examination 

 of the rainfall of the two regions seems to confirm the first possibility. For purposes of 

 comparison I have taken all the stations in Idaho, five in number, and in western and 

 central New Mexico, including El Paso on the border of Texas, seven in number, which 

 have continuous rainfall records since 1894. The Idaho stations range from 1,665 to 4,191 

 feet in altitude, with an average altitude of 2,700 feet. Their mean annual temperature 

 ranges from 43.4° F. to 51.3° F., with an average of 46.9° F. The New Mexican stations 

 vary in altitude from 3,760 to 7,000 feet with an average of 5,150 feet, while their mean 

 annual temperature ranges from 49.2° F. to 63.7° F. and averages 56.5°. In both cases 

 the meteorological stations are 2,000 to 3,000 feet lower than the region where the pines 



1894 1895 1896 1897 1898 1899 1900 1901 1902 190r! 1904 190.-, 1900 1907 1908 Year 



Inches ^ 

 of rain ji 

 25' 



g Inches 

 [-: of rain in 

 Idaho 



nches 

 _^of rain in 

 j'New Mexico 



Fig. 33.— Rainfall of Idaho (solid line) Compared -nath that of New Mexico (dotted line). 



grow, but the relative conditions are approximately the same. The trees of Idaho certainly 

 grow where the chmate is cooler and more rainy than in the parts of New Mexico where 

 their species flourishes. Nevertheless, in both cases moisture seems to be the factor of 

 chief importance. Figure 33 illustrates the matter; it shows the average rainfall of the 

 five Idaho stations year by year compared with the seven New Mexican stations. In 

 only 4 cases out of 14 does the line connecting one year with the next slope the same way 



