PINE TREES OF ROCKY MOUNTAIN REGION. 15 
are found at as low a level as 5,500 feet (Map No. A). 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 medeuate seeder, the amount of seed 
borne varying with the elevation. Seed Maioon 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 Zucearini. 
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 “ pifion ” 
and “nut pine.” ‘The name Mexican pifion, 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). 
