Rydberg: Phytogeographical notes 19 



rum, or. as it perhaps is more often known, P. ponderosa scopulorum. 

 It is seldom found in Colorado above an altitude of 10,000 feet, 

 which the writer regards as approximately the upper limit^of the 

 Montane Zone. It descends, however, far below the lower limit 

 of this zone, even below an altitude of 5,000 feet; it is really in fact 

 the only important tree of the foothills. It is a tree of wide 

 distribution found nearly everywhere in the Montane and Foothill 

 Zones of the whole Rocky Mountain region, as treated in this 

 article. The only mountains I know of where it is lacking are the 

 La Sal Mountains in eastern Utah, where the Douglas fir is also 

 missing. The bull pine is a massive tree, sometimes becoming 

 no to 125 feet in height and with a trunk almost 4 feet in diam- 

 eter. A tree 3 feet in diameter is about 250 years old. 



The Douglas Fir, red fir, or red spruce, Pseudotsuga miicronata, 

 is almost as important as the bull pine. It usually grows with the 

 latter and reaches about the same size. It is common at an 

 altitude of between 7,000 and 10,000 feet, but ascends sometimes 

 as high up as 10,500 and very rarely up to nearly 11,000 feet, and 

 descends seldom in the ravines as low as 6,000 feet. In the higher 

 altitudes it mixes with Pinus flexilis and P. aristata, rarely with 

 Picea Engelmannii, in the lower extremes it grows with the latter 

 and Abies concolor. As to its geographical distribution, the 

 Douglas fir has about as wide a distribution as any Rocky Moun- 

 tain coniferous tree, the other widely distributed ones being 

 Picea Engelmannii and Pinus Murrayana, and is surpassed among 

 the trees only by the aspen. It ranges from Alberta and British 

 Columbia to western Texas, northern Mexico and southern 



California. 



The Lodge-pole Pine comes next in importance. This is the 

 most common conifer in the Montane Zone of the Northern 

 Rockies, but is not so common nor so important in the Southern. 

 It often grows in pure stands on gentle slopes or flats, but mixes 

 also with other pines as well as with the Engelmann spruce. 

 As the lodge-pole pine germinates more readily than the other 

 conifers, it often takes possession of burnt-over areas, just as the 

 aspen does. The young trees usually spring up close together 

 and form almost impenetrable thickets. Standing so close to- 

 gether the trees grow very slowly in size and large trees are rare. 



