(328) 



The atmometers were placed so that the spheres were approx- 

 imately from six to eight inches above the ground. In this way 

 they show the conditions to which the tree seedlings are subjected 

 in becoming established. This is perhaps the most important 

 level under a forest, because the seedlings are the future genera- 

 tion, on which the forest depends for its perpetuation. 



As a further measure of moisture, standard Weather Bureau 

 rain gauges were installed in the forest. Tests by Horton ^ have 

 shown that tree crowns intercept considerable amounts of 

 precipitation which is evaporated and never reaches the soil. 

 The proportion varies with the density of the crown and the 

 duration and intensity of the rain. But he has found an average 

 of about 25 per cent for most trees in heavy rains of long duration. 

 In Europe, observations by Matthieu ^ have shown an intercep- 

 tion of 5.8 per cent, in winter and 11 per cent, in summer. 

 Reigler ' found that beech intercepted 21.8 per cent, oak 20.7 

 per cent, maple 22.5 per cent and spruce 58.8 per cent. It 

 would obviously be extremely interesting to find out how much 

 hemlock intercepts. There are indications that the slow growth 

 of vegetation under a forest is due not only to shade, but also 

 partly at least to lower moisture resulting from the competition 

 of tree roots and from the interception of precipitation by the 

 crowns. 



There are certain difficulties in measuring the interception of 

 precipitation by a forest canopy, because the rain which reaches 

 the forest floor is not uniformly distributed. Probably more drips 

 off the end of the branches than comes through the crown, so that 

 a part of the forest floor may receive more than the open, and 

 another part under the crown considerably less. Obviously it 

 would require a large number of rain gauges distributed with 

 reference to the crowns to determine just the amount received by 

 the forest floor. This was impracticable in the present study. 



The temperature conditions were measured by taking weekly 

 readings of the maximum and minimum temperature of the air 

 about 8 inches above the ground, and of the soil at depths of 6 

 inches and 18 inches. Each reading represented the coldest and 



' Horton, R. E. Rainfall interception. Mon. Wcath. Rev. 47: 603-623. 1917 

 •Matthieu, A. "Mcteorologie comparee agricole ct forestiere," 1878. 

 •Quoted by B. E. Fcrnow, "Forest Influences." U. S. Dcpt. Agriculture, 

 Forestry Division, Bull. 7: 131. 1902. 



