RATE OF GROWTH OF ARIZONA YELLOW PINE 703 



tions of the six hectares the adult trees were grouped into size classes. 

 Those between lo and 20 cm. in diameter were grouped in the 15 cm. 

 class, those between 21 and 30 cm. in the 25 cm. class, et cetera. The 

 graphs in figure 4 show the make-up of the six populations with respect 

 to the size of the component trees. The area at 9,000 feet shows the 

 most sustained count of larger trees. The south-facing area at 6,000 

 feet is now seen to be made up largely (40 per cent) of trees between 

 10 and 20 cm. in diameter, with only a small proportion (10 per cent) 

 of its trees exceeding 25 cm., regarding yellow pine only. If the data 

 for the four south- facing areas were converted into the customary 

 terms of total basal area they would show a steady decline with de- 

 crease of altitude. A few measurements of height were made in each 

 area, which showed practical uniformity between 8,000 and 7,000 feet 

 (50 to 70 feet), with a slightly smaller stature for 9,000 and 6,000 feet 

 (40 to 60 feet). 



On again comparing the south slope at 8,000 feet with the north 

 slope at 7,000 feet, and the south slope at 7,000 feet with the north slope 

 at 6,000 feet, a general similarity is discovered, but not so close a corre- 

 spondence as exists between the populations alone. 



Through the kindness of the officers of District 3 of the Forest Serv- 

 ice and of the Coronado Forest, permission was secured to cut some 

 trees in each of these areas for determination of the relation of age to 

 diameter. From 11 to 17 trees were cut on each hectare, the selections 

 being made with a view to securing various sizes, and giving equal rep- 

 resentation to excurrent rapidly growing trees and more mature indi- 

 viduals with flat-topped crowns. 



The results of the determinations of age in relation to size are shown 

 in graphic form in figure 5. The diameters used in placing the dots 

 are those secured by calipering the tree without removal of bark, and 

 the age used is that found by ring count on stumps about 2 cm. high 

 or by use of a Swedish increment borer at about i m. in height. Owing 

 to the variable nature of the results, the paths of variation are indi- 

 cated as well as the mean line representing increase of diameter with 

 increase of age. 



Table 4.— Average Diameters (in cm.) of Aricona Yellow Pine to be Expected 

 at Three Ages in Six Habitats of the Santa Catalina Mountains 



so years 



9,000 S 19 



8,000 S 8 (?) 



7,000 S 15 



7,000 N 14 



6,000 S 12 



6,000 N 16 



