RATE OF GROWTH OF ARIZONA YELLOW PINE 707 



of soil moisture to the evaporative power of the air, as appears to be 

 true of all trees in the desert mountain ranges. In critical years this 

 climatic feature becomes important to the trees which are in critical 

 stages of their establishment or are occupying unfavorable spots. The 

 upper limit of the western yellow pine appears to be reached at about 

 10,000 feet on the San Francisco peaks in northern Arizona and on Mt. 

 Graham, in the Pinaleno Mountains of southeastern Arizona. 



The environmental conditions change so greatly on ascending from 

 the lower to the upper limit of the yellow pine that a much greater dif- 

 ference might be expected in the growth rate, which may be regarded 

 as a measure of the collective physiological processes of the tree. Even 

 the small number of age determinations and the small size of the areas 

 used in this work should have been sufficient to discover the differences 

 of growth rate if they were as great as the differences in climatic con- 

 ditions would lead us to expect. The possibility of the action of com- 

 pensating factors in equalizing the growth rate, at least at the three 

 lowest elevations, opens up a field for further investigation of this 

 problem which seems to the writer to be an important one in the man- 

 agement of our southwestern forests. 



