THE ROLE OF ASPEN IN REFORESTATION 255 



the air stratum occupied by the stems. Readings were made 

 hourly for periods of from 8 to 24 hours in both clear and cloudy 

 weather. The evaporimeters were thoroughly tested and are 

 known to give rehable results for purposes of comparison. 

 • Wind movement was measured by means of Standard United 

 States Weather Bureau anemometers. The instruments were 

 set in a hole so that the cups revolved about 8 cm. (3 inches) 

 above the level of the ground. 



The results of planting and measurement of physical factors 

 are given in the following tables. 



Some idea of the relative intensity of solar radiation in the 

 open and under the aspen is furnished by the records of two 

 thermographs exposed on the ground without shelters (see 

 figs. 1 and 2). When the sun was under a cloud the records of 

 the two instruments were practically the same, but when the 

 sun shone the reading of the thermograph in the open was usually 

 several degrees higher. This was not always the case, however, 

 because at times the thermograph under the aspen, as well as the 

 one in the open, received direct -sunUght. The maximum tem- 

 peratures recorded are as follows: 



June 21, 1912 Open 30.0°C. (86.0° F.) 



12 m. to 5 p.m. partly cloudy Aspen 25.5°C. (77.8° F.) 



Julys, 1912 . Open 41. 1°C. (106.0° F.) 



10 a.m. to 4 p.m. clear Aspen 36.2°C. (97.3° F.) 



In comparing the aspen-covered and open plots in tables 1, 

 2 and 3, it should be borne in mind that the counts and measure- 

 ments in the various years were on entirely different plots; thus 

 the variation due to peculiar local conditions is to a large extent 

 overcome. It will be noted that the measurements of physical 

 factors are confined largely to the month of June or early July. 

 This period represents the early summer dry season which is 

 considered the critical period for plants not fully established. 



The results of the planting (table 1) furnish convincing evi- 

 dence of the superiority of the aspen-covered areas over the open- 

 ings. The dead or injured plants almost invariably have the 

 appearance of suffering from drouth. The tops turn brown and 



