JOURNAL OF FORESTRY 



Vol. XV OCTOBER, 1917 No. 6 



THE DENSITY OF STAND AND RATE OF GROWTH OF 



ARIZONA YELLOW PINE AS INFLUENCED 



BY CLIMATIC CONDITIONS^ 



By Forrest Shreve 

 Desert Laboratory, Tucson, Arizona 



The work described in this paper was carried out in the Santa Cata- 

 lina Mountains, which are situated in southern Arizona, about fifteen 

 miles northeast of Tucson, and form a portion of the Coronado Na- 

 tional Forest. In the general features of their forest cover and other 

 vegetation they are similar to nearly a score of other small ranges in 

 southern Arizona and New Mexico, The writer has been working for 

 several years on the climatic conditions and vegetation of these moun- 

 tains, and has already described the salient features of their vegetation 

 and given a digest of the climatological data which it had been possible 

 to secure up to the end of 1914.^ The Santa Catalinas are surrounded 

 by desert plains, out of which they rise from a basal altitude of about 

 3,000 feet to the summit of Mt. Lemmon at 9,150 feet. The vegetation 

 of the lower slopes is very similar to that of the surrounding desert, 

 and it is only between elevations of 4,000 and 4,500 feet that the cacti 

 and thorny bushes of the desert begin to disappear. At these eleva- 

 tions are seen the first individuals of juniper and the evergreen oaks 

 which form the dominant element of the vegetation up to 6,500 feet. 

 These middle elevations, which may be designated as woodland, or as 

 encinal, are further characterized by Manzanita, Garrya, Dasylirion, 

 Nolina, Yucca, and a large number of small shrubs and root perennials. 

 While the lower portions of the encinal region are extremely open, the 

 upper portions from 5,500 to 6,500 feet are nearly closed, and at 6,500 

 feet appear the first individuals of the large forest trees that form the 



\The substance of this paper was read at the San Diego meeting of the Eco- 

 logical Society of America, August lo, igi6. 



^ Shreve, Forrest : "The vegetation of a desert mountain range as determined 

 by climatic conditions." Carnegie Inst., Wash. Publ. 217. 1915. 



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