10 BULLETIN 193 



Clements (3), studying the conditions in mature forests of lodge- 

 pole pine, found that the intensity of light in these forests varied from 

 0.12 to 0.05. He says "the values most frequently found were 0.08 and 

 0.07 which may well be taken as the normal." His lowest value was 

 0.016. Germination and growth were fairly good from 0.2 to 0.14, de- 

 creasing rapidly below the last named figures. When the light in- 

 tensities were .08 to .05 there were no vigorous seedlings. 



Pearson (12) found western yellow pine seedlings growing in 

 habitats with average light intensities of 0.630 to 0.309. Only in in- 

 tensities of 0.414 or better were the seedlings marked "good." Es- 

 pecially interesting in the data given for Abies concolor, where there 

 seems to be little connection between the values given and the condi- 

 tion of the growing trees. In an intensity of 0.02 the trees were sup- 

 pressed, while in light only a little stronger, 0.027 and 0.028, they were 

 marked "good." The trees were doing well in light intensities of 

 0.048 and 0.068. This lack of uniformity in light requirement for the 

 seedlings in each of these species must be due to some factor other than 

 light, or else the method used is not trustworthy. 



The following light values were taken from Pearson's readings, 

 as given by Zon and Graves (16). 



TABLE VI LIGHT VALUES FOE WESTERN TREES (PEARSON) 



Hesselman (9) noted great variations in light intensity under the 

 shade of the plant groups which he studied. In some cases these 

 variations amounted to as much as 300 percent. In groups of old 

 spruces the light intensity was reduced to 0.044 and 0.037; in young 

 spruce these values were 0.020 and 0.016. 



Ciesler (2) measured light intensities in several types of forests, in 

 each of which he took measurements after three grades of thinning. 

 He found that forests which had been only lightly thinned held back 

 a surprisingly large proportion of the light. The crowns of a severely 

 thinned stand of fir absorbed about 80 percent, a beech stand 80 to 90 



