74 



VEGETATION OF A DESERT MOUNTAIN RANGE. 



In order to compare the altitudinal shortening of the frostless season 

 on the Santa Catahnas with the same datum for a series of valley 

 stations located at progressive altitudes, figures were collected which 

 are shown in table 12. These figures are based on the last vernal and 

 first autumnal occurrence of a temperature of 32°, for 1903 to 1912 

 inclusive, without regard to the reports of frost made by the voluntary 

 observers at these stations. In figure 15 the length of the frostless 

 season at the several stations is graphically shown by horizontal lines, 

 and the limits of the frostless season for the Santa Catalinas (for 1909 

 to 1914) are shown by oblique fines. 



Spring opens at an earlier date at 3,000 and 4,000 feet on the Santa 

 Catalinas than it does at Tucson and Benson, but at 4,000 and 5,000 

 feet it does not open at so early a date as it does at Cochise and Fort 

 Huachuca. At all four of the elevations mentioned the close of the 



Table 12. — The altitudinal shortening of the frostless season in southeastern Arizona, as 

 shown by the dates of the last spring occurrence and the first autumn occurrence of a tempera- 

 ture of 33° at eight stations at graduated altitudes, in the decade of 1903 to 1912. 



* The elevation of Chlarson's Mill is reputed by the proprietor to be 8,000 feet, and it is so 

 stated in the publications of the Weather Bureau. Several aneroid determinations by the writer 

 indicate that it is approximately 7,200 feet. 



t These dates are based on an incomplete record. 



growing season comes sooner at the valley stations than it does on the 

 mountains. The length of the frostless season at Flagstaff is notably 

 shorter than it is at the same elevation in the mountains. 



The advent of spring is retarded at Tucson and Benson by the cold- 

 air drainage of the Santa Cruz and San Pedro rivers respectively. 

 Cochise is situated in the middle of the eastern bajada of the Dragoon 

 Mountains and Fort Huachuca at the top of the northern bajada of 

 the Huachuca Mountains. Each of these stations is therefore removed 

 from the operation of cold-air drainage, as is manifested by the failure 

 of their greater altitude to impose upon them shorter frostless seasons 

 than those of Tucson and Benson. 



The length of the frostless season at Marshall Gulch and at the 

 similarly situated mountain station at Chlarson's Mill (Pinaleno Moun- 

 tains) is greater than at Flagstaff, which is at a slightly lower altitude. 

 This is to be attributed partly to the higher latitude of Flagstaff, but 



