58 



BULLETIN 1105, U. S. DEPARTMENT OF AGRICULTUEE. 



Obviously the effect of a cut-over stand is less than that shown in 

 the above investigation. In June and July, 1912, it was found that 

 in a forest in which about 75 per cent of the stand had been re- 

 moved, wind movement was 61.8 per cent of that in the open park, 

 whereas in a virgin forest it was only 52.3 per cent of that in the 

 park. No comparisons of temperatures are available, but it seems 

 safe to assume that, except in situations subject to strong air drain- 

 age, temperature extremes are seldom modified more than a degree 

 or two. 



Recent investigations have placed greater emphasis upon varia- 

 tions within the forest itself due to local variations in forest cover. 

 In this connection it should be borne in mind that the trees in the 

 virgin western yellow pine stands of this region ^^ are not uniformly 

 distributed, but are mostly arranged in groups from 50 to 7o feet 

 in diameter, separated by somewhat larger openings. In cut-over 

 stands the tree groups are fewer and the openings larger (Fig. 1). 



A rough index of solar radiation is furnished by soil tempera- 

 tures. Although soil temperature does not represent the total radi- 

 ation, it is of value for comparative purposes. Table 15 shows the 

 mean soil temperature at a depth of 1 foot under cover of a group 

 of veterans, under partial shade, and in an opening which is reached 

 by shadows only early in the morning and late in the afternoon. 

 During the winter months the difference is slight, because the blan- 

 ket of snow tends to equalize the temperature. When the ground 

 is bare of snow, however, there is a pronounced difference, which is 

 greatest in May, June, September, and October, the months with 

 the greatest percentage of clear days. In May the soil was 12.2° F. 

 warmer in the opening than in the shade of the groups of pines. 



Table 15. — Soil temperature at a depth of 1 foot in the yellow pine forest. 



Month. 



January.. 

 February 



March 



April 



May 



June 



July 



August... 



Cover. 



o J, 



31.4 

 29.9 

 34.4 

 39.6 

 44.4 

 56.1 

 58.4 

 57.4 



Partial 

 shade. 



" F. 

 32.1 

 30.4 

 35.2 

 41.6 

 47.9 

 58.6 

 60.2 

 60.3 



Open. 



' F. 



33.5 

 31.1 

 35.6 

 46.7 

 56.6 

 65.4 

 65.3 

 65.9 



Month. 



September 



October 



November 



December 



Mean, annual . . 

 Mean, May-Get 



Cover. 



o Jf, 



54.4 

 48.0 

 37.3 

 33.1 



43.7 

 53.1 



Partial 

 shade. 



' F. 



68.0 

 51.9 

 39.2 

 32.7 



45.7 

 56.2 



Open. 



F. 



63.6 



58.0 



41.1 



32.7 



49.6 

 62.5 



Surface temperatures bring out the above effects still more strik- 

 ingly. During December, 1918, the mean maximum temperature at 

 the surface of the soil in the opening was 20.6° F. higher than in 

 the shade of the pines. On June 11, 1920, the surface soil at a 



w This applies particularly to the Coconino and Tusayan Forests, 

 the distribution is more uniform. 



In other localities 



