48 
The Colorado Experiment Station 
Arkansas River to above Buena Vista. There is a fringe of pinyon 
pine-juniper on the warm slopes that overlook San Luis Valley 
(Fig. 17). The south corner of Archuleta County, the southern 
part of La Plata County, and most of Montezuma County, have 
scattering growths of this community. It extends northward, in¬ 
cluding the western half of Dolores, San Miguel and Montrose 
Counties. The warm slopes in the Grand and Uncompaghre Val¬ 
leys, up to McCoy in Eagle County, above Basalt in Pitkin County, 
and to Ouray in Ouray County, bear stands of pinyon pine-juni¬ 
per, which may be pure in places, but often mixed with sagebrush. 
In the northwest counties, there are a few areas of pinyon pine and 
juniper, particularly along the western border of the State. The 
pinyon pine-juniper belt is best developed below 7,000 feet, but in 
the San Luis Valley and many other sections it may reach close to 
9,000 feet. When found at high altitudes, it is always on south ex¬ 
posures that become very warm. It does not reach the altitudes 
that sagebrush does, and its range of temperature conditions is 
much less than that of sagebrush. However, over thousands of 
square miles, pinyon pine-juniper and sagebrush alternate (Fig. 
13), the former occupying rough, broken country, or shallow, stony 
soil, while sagebrush occurs on the more level ground, which has 
a deeper and finer soil. It is not uncommon to see sagebrush land 
cultivated up to the edge of a rocky knoll, capped with pinyon 
pine and one-seeded juniper. 
Climate, and Relation to Agriculture .—The mean annual pre¬ 
cipitation in the pinyon pine-juniper belt is uniformly under 15 
inches. The summer temperature in a large percentage of the area 
is not below 65°, although in some few localities it may be several 
degrees lower. The average length of the frostless season is 
usually between 100 and 125 days, and the average date of the last 
spring frost about May 20. Pinyon pine-juniper is a plant com¬ 
munity of warm slopes, and warm soil (Fig. 18). Although often 
local in its distribution above 6,500 feet, it is a reliable indicator 
there of a warm habitat, of a warm island, so to speak, in a cool 
area. The pinyon pine-juniper woodland is a mark of tem¬ 
perature conditions which permit the growth of all but the ten- 
derest fruits, all the small grains, flax, sugar beets, potatoes, al¬ 
falfa, and the garden vegetables, excluding melons. In the lower 
part of the belt, peaches, sweet cherries, melons, and other tender 
crops yield bountifully. Much valuable orchard land of Colorado 
is cleared pinyon pine-juniper soil. Scattered pinyons, and less 
often one-seeded junipers, will be found considerably above the 
limits of growth for the crops mentioned, but such individual spec- 
