50 
The Colorado Experiment Station 
the pinyon pine-juniper belt. In the southwestern counties the 
lower altitudinal limit of the forest is about 7,000 to 7,500 feet; at 
this limit it may be mingled with chaparral of oak, chokecherry, 
Juneberry, buckbrush, etc. It extends northward to the Gunnison 
and Grand drainage systems, the growth becoming more scattering 
as it reaches in that direction. In the northwest counties, in Mid¬ 
dle Park and North Park, yellow pine is not an important tree, 
and seldom forms a forest of any extent. 
The principal tree associate of yellow pine is Douglas fir 
(Pseudotsuga mucronata) . These two* trees will alternate through 
out the yellow pine belt; Douglas fir dominates the cooler and 
moister north exposures, while yellow pine more frequently holds 
to the warmer, and drier exposures. This alternation is most no¬ 
ticeable at lower altitudes. Douglas fir extends to higher alti¬ 
tudes than yellow pine, and is often a strong element in the lodge- 
pole pine belt of the eastern slope. 
Other characteristic plants in the yellow pine association are: 
Chokecherry ( Primus m elan o carp a) , buckbrush ( Sy nip h oricarpos 
spp.) , skunk-bush ( Rhus trilobata), sumac ( Rhus glabra ), yellow- 
flowered currant ( Ribes longiflorum) , red currant ( Ribes cereum), 
New Jersey tea ( Ceanothus fendleri, C. pubescens) , wax flower 
(Bdwinia am eric ana), Rocky Mountain maple ( Acer glabrum), 
Kunzia tridentata , Juneberry (Amelanchier) , squaw apple (Per- 
aphyUum ramossissimum) , narrow-leaved cottonwood ( Populus 
angustif olia) , alder (AInns temdfolia) , streami-side birch ( Betula 
fontinalis) , Oregon grape ( Berberis repens), bearberry ( Arctos - 
taphylos iiva-ursi) and spiraea ( Holodiscus dumosus). The quak¬ 
ing aspen is abundant in the yellow pine belt, but it finds a more 
suitable environment above it. 
The yellow pine forest goes to higher elevations on south ex¬ 
posures than on north exposures, and long tongues of vegetation 
more properly belonging to the lodgepole pine belt, or its altitu¬ 
dinal equivalent, penetrate it along cool mountain streams and on 
north exposures. 
Climate .—The yellow pine forest seldom displays its typical 
development in those regions possessing less than 15 inches of 
precipitation annually. In northern Colorado, the 15-inch precipi¬ 
tation line (see Eig. 8), is practically co-extensive with the lower 
limit of the yellow pine zone, while in the southern part of the 
state the 20-inch line quite closely follows the lower limit of the 
belt. The snowfall exceeds that of the grass-steppe, sagebrush 
