6 VEGETATION OF A DESERT MOUNTAIN RANGE. 



topographic situations in the Santa Catalina Mountains, to give the 

 results of the climatological instrumentation which has been carried 

 on, and to indicate in so far as possible the manner and degree in which 

 the successive altitudinal stages of vegetation are dependent upon the 

 gradients of climatic change by which they are accompanied. 



Some of the instrumental records date from the summer of 1907, 

 the first year in which members of the Desert Laboratory became 

 interested in the mountains, but the principal part of the data to be 

 presented were secured in 1911 and subsequent years. The operation 

 of instruments and the study of vegetation have been made on visits 

 of 5 to 10 days, at intervals between April and October. From three 

 to nine such visits have been made to the mountains in each of the 

 summers since 1910. 



The practical exigencies of the work have limited the character of 

 the instrumentation which could be carried out, but have not impaired 

 the accuracy of the data which it was possible to secure. There is no 

 respect in which the results herewith presented may be considered as 

 more than a mere outline of a large and widely ramifying botanical 

 problem. The central interest of the writer has been to determine 

 which of the major environmental factors are responsible for the chief 

 distributional features of the vegetation, and to ascertain something 

 regarding the intensities of the factors responsible for the distributional 

 limits of individual species, and thereby for the limitation of the types 

 of vegetation themselves. Such an inquiry into the correlations exist- 

 ing between physical conditions and the occurrence and activity of 

 plants may do much to explain general vegetistic phenomena, but it 

 does far more to open up the innumerable physiological problems which 

 must be well known at the outset to underlie these correlations. 



GEOGRAPHY AND TOPOGRAPHY OF THE SANTA 

 CATALINA MOUNTAINS. 



The Santa Catalina Mountains occupy the drainage divide between 

 the San Pedro River, a tributary of the Gila, and the Santa Cruz, a 

 river which seldom has sufficient flow to reach an outlet in the Gila. 

 The position of the mountains is between 110 30' and 111 east longi- 

 tude and 32 15' and 32 35' north latitude. The general outline of the 

 range is roughly triangular (see plate 40), its southern base being at 

 about 3,000 feet (915 m.) elevation, its northeastern base (parallel to 

 the San Pedro River) lying at approximately 3,500 feet (1,065 m.). 

 To the northwest a broad grassy plain, 3,500 to 3,800 feet in elevation, 

 connects the Santa Catalinas with the lower Tortilla Mountains. To 

 the southeast a narrow pass, 4,300 feet (1,310 m.) in elevation, connects 

 with the closely adjacent El Rincon Mountains, which reach an eleva- 

 tion of 8,465 feet (2,580 m.). Southward from El Rincon range a pass 

 of 4,000 feet (1,220 m.) elevation leads to the Santa Rita range (9,432 



