CLIMATE AND PLANT GROWTH. 31 



On the basis of the beginning of growth and the occurrence of 

 killing frosts the growing periods in days for the respective types are 

 approximately as follows : 



Oak-brusli type 120 



I Aspen-fir type 105 



Spruce-fir type TO 



PRECIPITATION. 



The monthly precipitation from October, 1915, to September, 1916, 

 is summarized in figure 14. 



In view of the higher temperature, the longer growing season, and 

 the higher evaporation in the oak-brush type, it is significant that 

 the annual precipitation is less in that type than in any of the more 

 elevated types in which plant studies were conducted. The annual 

 averages of precipitation of the types, including the untimbered 

 type below the oak-brush, as recorded from 1914 to 1916, inclusive, 

 are: 



Inches. 



Sagebrush-rabbit-brush type _ 11. 15 



Oak-brush type 13. 25 



Aspen-fir 27. 18 



Spruce-fir 25.40 



During the growing period in 1916 the aspen-fir type, as in the 

 case of the three-year average, received the heaviest precipitation, 

 nearly the same amount, however, being recorded in the type imme- 

 diately above. 



EVAPORATION. 



Monthly evaporation and precipitation are represented in the same 

 figure (fig. 15). Owing to the occurrence of freezing temperatures 

 in June, particularly in the two higher types, unbroken evaporation 

 records were obtained only from July to September, inclusive. 



Figure 15, based upon the records of the porous cup atmometer 

 (see fig. 16), shows that the highest evaporation occurs each month 

 in the* oak-brush type. In the spruce-fir type the evaporation is 

 nearly as great as in the lowest association, while in the aspen-fir 

 type it is much less than in either of the others. Table 7 and figure 8 

 also indicate that the evaporation in the oak-brush and spruce- fir 

 types is much greater than in the aspen-fir type. 



In the case of growth studies begun in the stations at a later period, 

 the summation of evaporation is quite as contrasted, as is shown in 

 figure 9. While the records in this series of experiments cover a 

 shorter period than those in the original study, being 52, 70, and 65 

 days in the oak-brush, aspen-fir, and spruce-fir types, respectively, 



