CLIMATE AND PLANT GROWTH. 69 



may be economically employed, should be confined to soils of high 

 water-holding capacity and subject to minimum run-off, so that the 

 soil may return to the plant a high percentage of the rainfall. 



SUMMARY. 



The data reported, pertaining as they do (A) to the climatic 

 characteristics of three distinct vegetative associations, (B) to com- 

 parative instrumentation and methods of summarizing and express- 

 ing climatic factors, and (C) to correlations between environmental 

 factors and plant growth and other physiological activities, may best 

 be summarized under three heads. 



A. CLIMATIC CHARACTERISTICS OF THE PLANT ASSOCIATIONS. 



1. The mean annual temperature increases gradually from the 

 highest to the lowest type, and this results in the longest growing 

 season in the lowest type and a gradual decrease in the period of 

 growth with increase in elevation. Thus from the time of the begin- 

 ning of growth to the occurrence of killing frosts there are about 

 120 days in the oak-brush type, 105 in the aspen-fir type, and TO 

 in the spruce-fir type. 



2. The normal annual precipitation is greatest in the aspen-fir 

 association but is only slightly heavier in this association than in the 

 spruce-fir. Less than half as much precipitation is recorded in the 

 sagebrush- rabbit-brush as in the aspen-fir association; and in the 

 oak-brush type it is only slightly greater than in the sagebrush- 

 rabbit-brush type. The precipitation is rather uniformly distributed 

 throughout the year. 



3. Of the three associations^rlticaiiy studied^ the evaporation dur- 

 ing the main growing season is greatest in the oak-brush type: but 

 owing to high wind velocity in the spruce-fir type the evaporation 

 is nearly as great as in the oak-brush type. In the aspen-fir type 

 the evaporation factor is notably less than in the types immediately 

 above and below. This is accounted for largely by the lack of high 

 wind velocity, which is due to the luxuriant vegetation and to topo- 

 graphic features. 



4. The wind movement is greater by about 100 per cent in the 

 spruce-fir association than in the types immediately below. Not un- 

 commonly the velocity of the wind exceeds 40 miles per hour for sev- 

 eral hours in succession. In the lower types the velocity averages 

 slightly less than half that recorded in the spruce-fir type. 



5. Sunshine duration and intensity are practically the same in all 

 types studied. 



6. There is considerable difference in barometric pressure in the 

 respective types, but the daily seasonal fluctuations within a station 

 are slight. 



