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G. A. PEARSON 



aspen and two in the open, each plot being 66 feet square and 

 containing 100 plants, excepting in 1911 when smaller plots were 

 used. The plantings were repeated each spring and examina- 

 tions of the old plots were continued from year to year. 



The methods used in measuring physical factors are as follows : 



Soil moisture was determined from samples representing a 



depth of 5 to 25 cm. (4 to 10 inches) which is the stratum occu- 



Fig. 2. A typical aspen thicket 



pied by practically all the roots of planted trees during the first 

 year. From 2 to 4 samples were. taken on each plot at one time. 

 Evaporation was measured by means of the Piche evaporimeter. 

 Four instruments were placed in each of the two situations com- 

 pared. The evaporimeters were suspended from an inclined 

 stick (see fig. 1) so that the evaporating surface was about 8 cm. 

 (3 inches) above the ground, in order to represent conditions in 



