RATE OF GROWTH OF ARIZONA YELLOW PINE 



705 



An examination of the paths in figure 5 shows that the 9,000-foot 

 area stands rather sharply apart from the other areas in the growth 

 rate of its trees. The rates at the lower elevations are so similar that 

 their differences might well fall within the bounds of the errors of the 

 method and the limitations of the amount of material. In order to 

 compare the rates on the different hectares more exactly the readings 

 have been made which are given in Table 4. These readings are based 

 on the median lines of average growth rate, and show the mean ex- 

 pectancy of size for trees 50, 100, and 150 years old. The rapid growth 

 at 9,000 feet is again emphasized in these figures. The average size at 



6. — Superposed graphs of growth rate for the Arizona yellow pine on south slopes at 

 four altitudes 



50 years at 8,000 feet is obviously much too low, owing to the absence 

 of any data for trees of about that age in the composition of the growth 

 path. The expectancy of size at 150 years at 8,000 feet is the same as 

 that at 100 years at 9,000 feet, and is well above the expectancy for 150. 

 years at 7,000 and 6,000 feet. The expectancy of size at 50 years 

 at 9,000 feet is not greatly in excess of that at 7,000 and 6,000 feet, 

 and a. true value for 8,000 feet would doubtless be of about the same 

 amount. The expectancies for all three of the sizes at the 7,000-foot 

 arid 6,000-foot areas are very similar. Not only does the altitude fail 

 to exert an influence, but the effects of slope exposure are completely 

 absent. A still closer comparison of the growth rates on the south 

 slopes at the four elevations is made by superposing the four curves of 

 average rate (fig. 6) . The curve for 9,000 feet is then seen to stand 

 well apart from the other three, which follow nearly the same path. 



