THE ROLE OF ASPEN IN REFORESTATION 259 



20 the aspen covered plots again showed a higher percentage due 

 to the smaller water loss after the summer rains ceased. 

 , The effect of the aspen on soil moisture appears to be about 

 as follows: Other conditions being equal, the greater humus 

 content of the soil in the aspen stands should increase its water 

 holding capacity, and therefore at the end of the spring thaws 

 when the soils are fully saturated we should expect to find a 

 higher moisture content in the aspen than in the open. Direct 

 evaporation from the soil is undoubtedly less under the aspen 

 because of the protection afforded by the tree crowns and the 

 leaf Htter. On the other hand, the loss by transpiration through 

 the aspen foliage may more than offset the difference in direct 

 evaporation from the ground. 



The records of evaporation (table 3) show that this factor is in 

 every instance decidedly more active in the openings than under 

 the aspen. This is due partly to higher solar radiation and partly 

 to greater wind movement in the openings. That wind alone 

 has an appreciable effect is shown by the fact that on June 27, 

 1913, when the sky was overcast all day but the wind velocity 

 in the open was twice as great as in the aspen, the evaporation 

 was 60% greater in the open (see table 3). The combined 

 effect of wind and sun is shown on June 26, 1913, when the evap- 

 oration in the open was 90% higher than under the aspen. The 

 records show no constant relation with respect to relative 

 humidity. 



CONCLUSIONS 



The success of Douglas fir under aspen as compared with the 

 open situations may be attributed mainly to a reduction in trans- 

 piration. Abundant soil moisture, while important, can only in 

 a measure compensate for excessive transpiration. It is neces- 

 sary to maintain the proper balance between water absorption 

 and expenditure. As shown in the open plots of 1913, heavy 

 losses may take place despite the presence of an abundant mois- 

 ture supply in the soil if transpiration is not controlled. The 

 same condition has been experienced in nurseries where Douglas 

 fir transplants exposed to full sunlight, though well watered, 



