278 BOTANICAL GAZETTE : [APRIL 
spp. and Symphoricarpos oreophilus A. Gray. Other common 
-shrubs growing on dry hillsides are Kunzia tridentata (Pursh) Spreng., 
Amelanchier elliptica A. Nels., and Holodiscus dumosus (Nutt.) 
Heller. Grassland alternates with the sage in mesophytic level 
places. The hills, except south exposures, are covered with lodgepole 
pine. Engelmann spruce occurs in the deep and narrow canyons, 
even in the lower parts of the park, but it is not abundant below 
gooo feet. Rocky Mountain yellow pine is very uncommon in 
Middle Park.?* 
WESTERN SAGE PLAINS AND LOWER FOOTHILLS 
Under this head will be discussed that portion of the state west of 
the continental divide, which extends up to the montane zone, with 
the exception of Middle Park. 
Precipitation: below 10 inches in the western part and 10-15 inches in the 
eastern part; about 50 per cent. during the growing season. Temperature: 
mean annual 40°50° and above; mean summer 60°70° and above; mean 
annual range above 45; diurnal range great. Relative humidity: generally low. 
The dominant forms of vegetation of this area are sage brush, 
pinyon pine, scrub oak, and Rocky Mountain yellow pine. The 
northwestern part of the state, however, has climatic conditions which 
are appreciably different from those of the middle and southwestern 
part (see jigs. 5.and 6). In the former area the mean annual and 
mean summer temperatures are about 5° cooler than in the south- 
western area, and although the precipitation is about the same in 
amount and distribution, the effect of the lower temperature is 
indicated by the lower altitudinal distribution of typical arborescent 
forms. In northwestern Colorado, Rocky Mountain yellow pine 
grows to some extent on slopes between 6000 feet and 8000 feet; 
scrub oak occurs between the same altitudes; aspen comes in abun- 
dantly at 7500 feet, and scattered groves are found at lower elevations. 
In southwestern Colorado, Rocky Mountain yellow pine is not 
abundant below 8000 feet; scrub oak is seldom abundant below 
8000 feet, and aspens do not occur commonly below gooo feet. 
In southwestern Colorado along streams, Populus angustifolia, 
Lonicera involucrata, and Lepargyraea argentea (Nutt.) Greene are 
. 2t The writer is not sufficiently familiar with conditions in North Park to discuss 
em. 
