696 JOURNAL OF FORESTRY 



predominant vegetation from 7,000 feet to the summit. The common- 

 est of the pines at the lower edge of the forest is the Chihuahua pine 

 (Pinus chihuahuana) , which is, however, not a common tree above 

 6,500 feet. The bulk of the foresi is formed by the Arizona yellow 

 pine (Pinus arisonica) . 



The gradual but striking changes of vegetation that may be seen on 

 passing from the base to the summit of the mountains are accompanied 

 '"by an equally striking change of climatic conditions. It has been possi- 

 ble to work out in a preliminary manner the gradients of vertical change 

 for precipitation, soil moisture, evaporation, and certain elements of 

 the temperature. In Table i are shown the values for some of these 



Table i. — Important Elements of the Climatic Conditions Through tJie Vertical 

 Range of Arizona Yellow Pine in the Santa Cataliim Mountains 



Elevation (feet) 6,000 7,000 8,000 9,000 



Summer rainfall (inches) 18 30 33 20+ 



'I<^ngth of frostless season (weeks) 26 21 17 16; 



I^ength of arid foresummer_ (weeks) 11 9 7 6? 



Lowest soil moisture of arid foresummer at 



15 cm. (per cent dry weight) 2.6 4.0 9.3 18.6 



Average daily evaporation in arid foresum- 

 mer (cc, atmometer) 50-68 49-65 29 



Depression of minimum temperature below 



^. that of Tucson (° F.) 9.2 13.7 20.1 25.9 



climatic elements for the forested elevations of the mountain. Particu- 

 lar significance attaches to the sharp differences encountered in soil 

 moisture and evaporative power of the air, by virtue of which the 

 moisture conditions for plants at the base of the mountain are ren- 

 dered thirty times more difficult than for plants growing at 9,000 feet." 

 At all elevations from the base to the summit, these mountains exhibit 

 the pronounced differences due to slope exposure which are to be found 

 in all mountains and hills in the western United States. These differ- 

 ences have been found to be accompanied by striking differences in soil 

 moisture and evaporation conditions on north and south slopes, as will 

 be seen from the data given in Table 2. 



Table 2. — p^alues for Evaporation and Soil Moisture on North and South Slopes 



in the Arid Foresummer Through the Vertical Range of Arizona 



Yellow Pine in the Santa Catalina Mountains 



6,000 ft. 7,000 ft. 8,000 ft. 9,000 ft. 



S. N. S. N. S. N. S. N. 



Evaporation (daily average in 



cubic centimeters) 53 47 55 44 29 29 



Soil moisture (at 15 cm. in per 

 . cent of dry weight) 1.8 3.5 2.6 5.5 7.4 11. 3 9-4 27.9 



" Op. cit., p. 92. 



