GRAZING AND FORAGE PRODUCTION ON NATIONAL FORESTS 



fig. 3). The lowest monthly average occurs in June and the high- 

 est usually in March for all types. 



Air temperature and precipitation are so closely related that 

 it is impossible to segregate them and determine the specific influ- 

 ence of each on the development of vegetation. The growing season 

 in the oak-brush zone is longer than that in either of the others, 

 partly because air temperatures are favorable for plant growth 

 both earlier and later, and partly because of the influence of the 

 snow in the higher zones. Not only does precipitation influence 

 temperature, but the amount of precipitation and the time of its 

 occurrence also have an important influence on plant growth and 



» 



16- 



5— 



3"~ 



2— 



MMHMMM MMIMAMM BMHgMi 



y 



OAK-BRUSH TYPE. ASPEN- FIR TYPE SPRUCE-FIR TYPE 



Average year/y Preclp'rfofi'on /9/-+—Z922 inc/us/ve 



OaK-Brush 

 lAspen-Fi'r 

 Spruce -Ffr 

































mm IUHW1 





1 



L_l _ 



1 1 1 1 1 1 i i i 



1.1 1 



3™i i, 



O 5" tO" 15" 20" 2& 30" 



Fig. 3. — Average monthly precipitation, 1914-1922, inclusive 



development. From November 1 to May 1 precipitation is usually 

 in the form of snow, especially in the aspen-fir and spruce-fir types. 

 Precipitation is relatively very high during these months and there 

 is a much larger accumulation of snow in the two higher types 

 than in the lower one. This partly explains the delay in the be- 

 ginning of plant growth in the spring in these upper areas. Tem- 

 peratures favorable for plant growth may occur early in May in 

 the aspen-fir and spruce-fir types, but at that time they have little 

 if any effect on plant development because of the deep covering 

 of snow. On many of the steep north exposures in the upper reaches 

 of the aspen-fir zone and in the spruce-fir zone snowdrifts may 



