58 



VEGETATION OF A DESERT MOUNTAIN RANGE. 



higher elevations are derived from a higher cloud level, probably from 

 convectional clouds which form at times when the atmospheric con- 

 ditions cause condensation at a greater distance from the earth. When 

 a long series of records shall have been secured from the 9,000-foot 

 station it will probably show that its average rainfall is greater than 

 that at 8,000 feet, but the 9,000-foot record for 1913 indicates that 

 there will be occasional years, at least, in which the maximum for the 

 mountain is recorded at 8,000 feet. This probably means that at 

 10,000 feet on adjacent mountains there is a constantly lower rainfall 

 than at 8,000 or 9,000 feet. 



The check in the vertical increase of rainfall which has been described 

 as occurring between 4,000 and 6,000 feet appears to be absent from 

 all curves derived from widely separated valley stations. The writer 

 has seen no such plateau in any 

 curves derived from southwest- 

 ern data, but there is always the 

 possibility that a plateau has 

 been smoothed out of the curves 

 or that the data have been sub- 

 jected to the influence of a 

 straight-line equation. The 

 character of the increase of pre- 

 cipitation with altitude in a sin- 

 gle small range of mountains is 

 no more a special case than is the 

 increase in a widely separated 

 series of stations in any locations 

 whatsoever. In so far as con- 

 cerns the study of meteorological 

 dynamics, such a mountain 

 range as the Santa Catalinas offers exceptional opportunities for investi- 

 gation, and much more might be learned in a single summer of intensive 

 meteorological study on its slopes than could be ascertained by an exami- 

 nation of records of rainfall covering a period of a thousand years. 



As regards vegetation, the most important feature of the study of 

 rainfall conditions is the determination of the extremes of variation in 

 the amount and seasonal distribution of rain, and the ascertaining of 

 the effect of these extremes upon the conditions of soil moisture. Years 

 of heavy precipitation are important for the maintenance of the forest 

 which clothes the higher mountain slopes and for the general restora- 

 tion of the supplies of soil moisture and ground water. The years of 

 low precipitation, and especially the series of consecutive years with 

 deficient rainfall, are of first importance to the vegetation which occu- 

 pies the Encinal region of the mountains. During such years, and 

 particularly during the arid fore-summer of such years, the lowest 



FiQ. 9 



Graph showing lack of relation between 

 summer rainfall at Marshall Gulch (7,600 feet) 

 and at Desert Laboratory (2,663 feet) from 1907 

 to 1914. 



