32 BULLETIN 460, U. S. DEPARTMENT OF AGRICULTURE. 



tween 5,000 and 8,500 feet (Map Xo. 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 pihon 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 

 light 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 abundant 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 1 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 TOO 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, where 

 good reproduction often occurs. 



