June 14, 1924 
Factors Affecting Reproduction after Logging 
1155 
Fig. 3.—Correlation of relative humidity and soil moisture, Priest Kiver Experiment Station 
But since moisture alone does not induce germination, the variations in tem¬ 
perature must also be tested. With this in view, soil thermographs were used in 
the 1920 series, one bulb being placed directly under the surface of the bare soil 
and another similarly placed under a duff surface. Continuous records were 
obtained from April 15 to June 30, 1921. This covers the germination period. 
Results from June 1 to 30, the period of most active seed germination, are given 
in Table V. 
Between 9 a. m. and 3 p. m. from June 1 to 30, the soil surfaces average from 
2° to 5° F. higher temperature than the duff. These are averages for clear and 
cloudy days alike; during sunny days there were differences from 7° to 10°. 
This higher temperature on soil surfaces is naturally explained by the lower 
specific heat of soil, 0.2, as compared with duff, 0.3. The greater heating during 
the day is a condition which greatly stimulates germination on bare soil and ash 
surfaces over that of duff and litter. Soil and ash surfaces are, therefore, more 
