RATE OF GROWTH OF ARIZONA YELLOW PINE 701 



An inspection of figure 3 will show the relation existing between the 

 adult stands of all trees and of yellow pine on the six areas investigated. 

 A comparison of the populations of the four south-facing slopes shows 

 the largest number of trees (352) at 9,000 feet, a smaller number (202) 

 at 8,000 feet, and a still smaller number (131) at 7,000 feet, with an 

 unexpected rise in the number (to 249) at 6,000 feet. The population 

 at 7,000 feet is about one-third that at 9,000 feet, and the curve of de- 

 crease would lead us to expect about 90 to 100 adult trees per hectare 

 at 6,000 feet, instead of the 249 trees that were found. This stand con- 

 tained more associated species than any of the others, the number of 

 yellow pines being only 171. It is possible that the density of the popu- 

 ulation of this area is influenced by the location of the hectare that was 

 enumerated, which is on the low slopes of the north wall of Bear Canyon, 

 near the lower end of an alluvial flat which has formed as a result of the 

 narrowing of the canyon by a dike of particularly resistant gneiss. The 

 hectare is entirely above the influence of the favorable soil moisture 

 conditions of the lower end of the alluvial flat, but is partly within the 

 influence of the cold-air drainage, which gives this locality lower mini- 

 mum temperatures than those recorded at 7,000 feet. Adjacent locali- 

 ties of the same slope exposure are either extremely precipitate and 

 rocky or have been burned over within the last twenty-five years. Al- 

 though there is some possibility that the south- facing area at 6,000 feet 

 bears an, abnormally heavy population, the writer is not willing to regard 

 this as a wholly typical phenomenon. 



The populations of the north slopes at 7,000 and 6,000 feet show 

 about the same reduction with altitude that is shown between the 

 south slopes at 8,000 and 7,000 feet. One of the most interesting feat- 

 ures displayed by a comparison of the populations on the north and 

 south slopes is the close similarity between the total number of trees on 

 the south slope at 8,000 feet and on the north slope at 7,000 feet, and 

 the corresponding similarity between the figures for the south slope at 

 7,000 feet and the north slope at 6,000 feet. In the previous publica- 

 tion which has been cited, the author has shown that the vertical limits 

 of the types of vegetation and of very many species are about 1,000 

 feet lower on north slopes than they are on south slopes. In other 

 words, two slopes facing north and south at the same elevation exhibit 

 the same differences of vegetation that will be found on two north 

 slopes or two south slopes which are 1,000 vertical feet apart. It has 

 also been found that the ratio between soil moisture and evaporation 

 differs between north and south slopes at the same elevation by nearly 

 the same amount that separates the readings on the same slope exposure 



