706 JOURNAL OF FORESTRY 



The fact that the Arizona yellow pine grows at four elevations in the 

 Santa Catalinas at which the climatic conditions are very dissimilar, 

 and at which the associated trees, shrubs, and other plants are very 

 different, and at the same time fails to show a marked difference of 

 growth rate at three of these elevations, is doubtless due to the oppos- 

 ing action of two sets of factors. Although there is a progressive in- 

 crease of rainfall from 6,000 to 9,000 feet,* there is a shortening of the 

 growing season with this ascent, and it may well be that the oppor- 

 tunity for increased growth which is afforded by a moister soil is offset 

 by the shorter season of favorable temperatures for growth. Nearly 

 all of the sections of trunks from 6,000 feet show a frequent recurrence 

 of broad rings alternating with narrow ones, the two being separated 

 by very light and thin bands of "autumnal wood," whereas the pairs are 

 separated by broad bands of dark autumnal wood. This points to the 

 existence of two periods of growth at 6,000 feet, and this interpretation 

 was used in making the ring counts. The first and greater period 

 doubtless comes at the commencement of the growing season, when the 

 soil is still moist from the rains of late winter and early spring. The 

 mid-year check in growth is occasioned by the arid foresummer, which 

 has an average length of eleven weeks at this altitude. The later and 

 minor period of growth is probably inaugurated by the increase of soil 

 moisture due to the rains of July and August. At higher elevations 

 the arid foresummer is shorter and the minimum soil moisture of that 

 season is higher (see Table i), resulting in a later commencement of 

 growth and an uninterrupted continuance of it. The occurrence of a 

 double season of growth is, therefore, apparently responsible for a por- 

 tion of the effect of nearly identical growth rates at 6,000, 7,000, and 

 8,000 feet. 



The Arizona yellow pine is so closely related to the western yellow 

 pine (Pimis ponderosa) that the relations of the two to environmental 

 conditions may be taken as nearly identical. The geographical dis- 

 tribution of the two species — extending from British Columbia to Du- 

 rango — is so wide as to indicate an ability on the part of these trees to 

 adjust their physiological processes to very dissimilar environmental 

 complexes. At the southern edge of the range of these trees their 

 lower limit is undoubtedly determined by a definite value for the ratio 



*The figure given in Table i for the average summer rainfall at 9,000 ft. is 

 based on a shorter series of records than the other rainfall data. There have 

 been some summers in which the rain was slightly less at 9,000 ft. than at 8,000 

 ft., but it is doubtful if this is the average condition. It is at least certain that 

 the soil moisture is much higher in the early part of the growing season at 9,000 

 ft. than it is at 8,000 ft. 



