6 BULLETIN 1405, U. S. DEPARTMENT OF AGBICULTUBE 



zone during June and September. During July and August there 

 is only a slight difference in the number of hours of growing tem- 

 peratures in the three zones, although, of course, the highest and 

 undoubtedly most effective temperatures are much more frequent 

 in the lowest zone. 



Favorable temperatures occur in sufficient duration for inception 

 of plant growth in the oak-brush zone about May 1 and some- 

 times earlier. Frequent heavy frosts and many hours of unfavor- 

 able temperature, however, retard the early development. Begin- 

 ning about the middle of May, the temperature becomes very favor- 

 able for growth and continues favorable until about October 1, 

 when frosts are apt to arrest further development. On the whole, 

 the plants generally found in the oak-brush type require more heat 

 units (potential growth hours) to complete their life cycle than 

 do the plants in the aspen-fir and spruce-fir types. 



Table 2. — Average monthly precipitation, period 1914 to 1922, inclusive 





Oak-brush type 



Aspen-fir type 



Spruce-fir type 



Month 



Years 

 observed 



Average 

 precipi- 

 tation 



Years 

 observed 



Average 

 precipi- 

 tation 



Years 

 observed 



Average 

 precipi- 

 tation 





3 

 3 

 3 



4 

 5 

 8 

 8 

 8 

 8 

 6 



1.21 

 1.16 

 2.41 

 2.03 

 1.68 

 .54 

 1.30 

 1.61 

 1.15 

 1.73 

 1.83 

 2.23 



4 

 4 

 4 

 6 

 6 

 8 

 9 

 9 

 9 

 8 

 8 

 5 



3.88 



3.48 

 4.04 

 4.22 

 2.88 

 .70 

 1.88 

 2.00 

 1.25 

 2.25 

 1.97 

 2.71 



4 

 4 

 4 

 5 

 5 

 8 

 9 

 9 

 9 

 8 

 8 

 5 



3.09 





2.73 



March . - - - 



4.38 





3.21 



May. -- 



1.91 





.66 



July 



1.96 





1.82 





1.26 





1.89 



November 



December 



6 

 4 



2.12 



2.65 





18.88 





31.26 





27.68 



I 









In the aspen-fir type favorable growing temperatures occur about 

 May 15, but because of snow which lies on the ground until about 

 May 25 the vegetation is delayed considerably in starting new 

 growth. However, by the time the snow has disappeared the season 

 has advanced, so that development is very rapid. Freezing tempera- 

 tures sometimes check the new growth, but the hours of temperature 

 unfavorable to growing which occur after the beginning of growth 

 are much fewer in this zone than in the oak-brush zone, and conse- 

 quently the plant development is more rapid. 



Except on the wind-swept ridges, snow usually lies on the 

 ground in the spruce-fir zone until about June 20. The tempera- 

 tures prevailing at so late a date are very favorable for plant 

 growth and the vegetation is seldom delayed by freezing tempera- 

 tures. 



PRECIPITATION 



There is considerable variation in the average monthly and the 

 average yearly precipitation in the three zones. The oak-brush 

 has the lowest yearly average and the aspen-fir the highest, while 

 that of the spruce-fir nearly equals that of the aspen-fir (Table 2, 



