oe BULLETIN 460, U. S. DEPARTMENT OF AGRICULTURE. 
tween 5,000 and 8,500 feet (Map No. 10). It is very moderate in 
soil requirements, growing on all soils from glacial drift and vol- 
canic ash to deep, loose sands and stiff clays. Dry, well-drained 
sandy or gravelly soils are, however, most characteristic. Once es- 
tablished, it requires very little moisture in the upper layers of 
earth; for the enormously deep roots enable it to thrive in soils ap- — 
parently as dry as those in which pinon pines and junipers grow. 
Pure extensive stands are formed; and it occurs also in mixture 
occasionally with broad-leafed trees, but more commonly with west- 
ern larch, Douglas fir, and white fir, and sometimes with Arizona 
pine and Mexican white pine. Open grassy park lands are often 
interspersed, and, as a rule, little or no underbrush, or even grass, 
occurs in these forests because of frequent fires. 
Pinus ponderosa demands an abundance of light throughout its 
life, particularly after the pole stages. Young stands may remain 
dense for from 10 to 15 years, but subsequently they thin out rapidly. 
Trees above 20 feet in height require almost unbroken light. Those 
in mature stands are rarely closer than 30 feet, and the crowns sel- 
dom touch. In the South, however, seedlings do not endure intense 
hght and heat, usually coming up in the shade of old trees, or in 
openings near logs, bowlders, and brush which afford slight protec- 
tion. In the North, where the light is less intense, they grow com- 
monly in unprotected openings. | 
Pinus ponderosa is a frequent and abaidact seeder. Cones are 
locally produced about every year, so that there is always some seed 
in a forest; but specially good seed years occur only at intervals of 
from 3 to 5 years. Germination from natural dissemination, in 
which much of the seed fails to come in contact with mineral soil, is 
usually about 50 per cent; while germination from artificial sowing, 
in which the seed is in close contact with the soil, is from 60 to 80 
per cent. Well-drained, fresh soils, and a moderate daily range of 
temperature are necessary for germination. Trees from 20 to 25 
years old produce seed; but generally the quantity is limited and 
the quality poor until the trees are about 50 years old. Large, 
thrifty trees produce over 1,000 cones, the average amount of seed 
per.tree amounting to from i to 6 pounds. The seed is not wafted 
far from the mother trees in the closer stands, but in open ones it 
may be carried by the wind for from 500 to 700 feet. A mature tree 
can seed about one-fourth of an acre in a good seed year. Squirrels 
and birds eat considerable quantities of seed. The rodents, however, 
disseminate some seed as a result of their habit of storing it for 
food; but reproduction from this source is accidental. Much seed is 
washed down steep slopes to stream beds and depressions, Ribs 
good reproduction often occurs. 
