CLIMATIC AND EDAPHIC INDICATORS. 347 



cipitation than the pine, and if all sites had the same aspect and gradient the 

 amount of precipitation might determine the type. There are, however, too 

 many influences affecting the final value of precipitation to make this element 

 a safe criterion. 



"From the above it is readily seen that the measurement of soil temperature 

 affords the simplest means for determining the quaUties of the site. In this 

 measurement are involved the effects of the slope and aspect; the direct or 

 indirect solar insolation; the effect of retained snow or precipitation which 

 cool the soil; the effect of wind movement and humidity as they may cause 

 evaporation from the soil, and the effect of wind movement as it may bring 

 heat or cold from areas of different temperature. " 



Sampson (1918:69) has determined the physical factors of the chaparral, 

 montane, and subalpine associations of the Wasatch Mountains in central 

 Utah, employing standard plants as well as instruments for habitat analysis, 

 and showing the differences with respect to the various factors and responses 

 in graphic fashion. His general conclusions are as follows: 



"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 beginning 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 70 in the spruce-fir type. 



"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. 



"Of the three associations critically studied, the evaporation during 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 vegeta- 

 tion and to topographic features. 



"In the case of all species employed, the total, and, indeed, the average 

 leaf length and total dry weight produced are notably greatest in the aspen- 

 fir association, these activities being rather similar in the spruce-fir and oak- 

 brush types. The decreased production in leaf length and the production of 

 dry matter in the respective types are in direct proportion to the evaporation. 



"The elongation of the stem is greatest in the oak-brush type, intermediate 

 in the central type, and least in the aspen-fir type. Thus stem elongation 

 appears to be determined largely by temperature and seems to be little in- 

 fluenced by the intensity of the evaporation. 



"The efficiency of the leaves per unit area as manufacturing agents, that 

 is, in the production of dry matter, appears to vary inversely with the evapor- 

 ation, though, indeed, temperature appears to be one of the important factors. 

 The largest amount of dry matter per unit of leaf area is produced in the aspen- 

 fir type and the least in the oak-brush type, while in the spruce-fir type, where 

 the evaporation is only shghtly less intensive than in the oak-brush type, the 

 dry matter produced is only slightly greater than in the oak-brush type. " 



