PINE TREES OF ROCKY MOUNTAIN REGION. 15 



are found at as low a level as 5,500 feet (Map No. 4) . The largest 

 and best-formed trees are found on moist, gravelly flats and canyon 

 slopes at elevations between 7,000 and 8,000 feet. They are com- 

 monly much scattered, growing singly or occasionally in small 

 groups, and generally associated with Arizona pine and Douglas 

 fir at higher elevations. 



This pine can endure moderate shade only during the seedling and 

 small sapling stages of growth. After this it requires full overhead 

 light. Older overtopped trees show greatly reduced vigor in the 

 density of foliage and also in the rapidity of their height and 

 diameter growth. 



Mexican white pine is a moderate seeder, the amount of seed 

 borne varying with the elevation. Seed production is greatest at 

 the middle elevation, even the better grown trees in protected places 

 at lower altitudes producing smaller quantities of seed. Good crops 

 of seed are borne at intervals of about 3 years, although some seed 

 is produced nearly every year. The nutlike practically wingless seeds 

 fall near the trees and soon afterwards many of them are devoured 

 by birds and squirrels. The latter also eat the seeds of a large 

 number of the ripe closed cones, which they " cut down." This de- 

 struction of perhaps the major part of the season's crop doubtless 

 accounts for the sparse reproduction of the species. Germination 

 occurs most often in mineral soil held in pockets or otherwise pro- 

 tected from washing. In some instances mountain flood waters carry 

 the seeds to lower levels than they would ordinarily reach, thus ex- 

 tending the vertical distribution of the tree to exceptionally low 

 elevations (5,500 to 6,000 feet). 



LONGEVITY. 



Pinus strobiformis is a moderately long-lived tree and of fairly 

 rapid growth. Trees from 14 to 20 inches in diameter are approxi- 

 mately from 80 to 110 years old, and those from 28 to 30 inches 

 through are from 150 to 180 years old. It is probable that this pine 

 reaches an age of 250 years or more in protected situations. Further 

 determinations are required to fix the extreme limit of age. 



MEXICAN PINON. 



Pinus cembroides Zuccarini. 



COMMON NAME AND EARLY HISTORY. 



Of all of our nut pines this species is perhaps the least known to 

 laymen. It has no distinctive common name, being called " pihon " 

 and " nut pine." The name Mexican pihon, here used to distinguish 

 the tree from other nut pines, would appear to be appropriate, be- 

 cause the species occurs almost wholly in Mexico (Map No. 5). 



