54 



VEGETATION OF A DESERT MOUNTAIN RANGE. 



5,000, and 8,000 feet, and between June 20 and July 18 at 6,000 and 

 7,000 feet. In similar manner the less frequent readings of 1912 and 

 1913 have been divided into the early summer and late summer falls, 

 by the latest July reading, and the averaged curves for early summer 

 and late summer rain are of nearly the same shape, but the late sunamer 

 curve is not so steep. This short record does not seem, therefore, to 

 corroborate the suggestion of Smith. 



Table 6. — Intraseasonal distribution of summer rainfall at the Desert Laboratory and at 6 

 elevations in the Santa Catalina Mountains for 1911. 



The increase of rainfall which accompanies increase of altitude is a 

 phenomenon of general occurrence throughout the southwestern 

 United States. The curves by which such increase may be expressed 

 differ from each other most strikingly, according to the horizontal dis- 

 tance of the successive stations from each other, according to the 

 coastal or continental position of the series of stations, or according 

 to the size of the mountain range on which the successive elevations 

 are secured. Although it is possible to deduce mathematical formulae 

 for the vertical increase of rainfall, it is necessary to introduce into all 

 such formulae a constant for the particular region or mountain involved, 

 and the figures thus secured are merely in the nature of hj^Dothetical 

 means near which the normal conditions may fall. It would be of very 

 great interest in the extension of plant geography to possess data on 

 the actual amounts of rainfall at successive elevations in a large number 

 of mountains and shelving plains throughout the southwest. The mean 

 rainfall conditions which are expressed in a gradient based on a long 

 climatological record are of great importance in connection with vege- 

 tation, but only when consideration is also giveu to the extremes of 

 rainfall, and particularly to the lower extremes, if a semi-arid country 

 is under consideration. The securing of typical normal gradients of 

 altitudinal increase of rainfall is not of so much importance in plant 

 geography, therefore, as a knowledge of the actual oscillations of the 

 rainfall conditions from year to year throughout the series of stations 

 or localities involved. 



