4 BULLETIN 193 



"root competition"was at fault rather than light shortage in cases of 

 suppression. Hartig (8) says that, on a good soil, a given number of 

 leaves will produce more substance than will the same number of 

 leaves on a poor soil. "The growth of a tree may suddenly double 

 or treble without increase in leaf surface or light intensity if the nour- 

 ishment of the roots is greatly increased." He thus clearly recog- 

 nizes soil conditions as a limiting factor in the growth of trees. 



Tolerance, then, in the strict and proper sense of the term, repre- 

 sents the ability of a tree to thrive under the conditions of soil mois- 

 ture, light, humidity, etc., found in any given habitat. However, this 

 word is so established in our literature as expressing a light relation- 

 ship only, that it will be difficult to superimpose the meaning just in- 

 dicated. A clear understanding of the problem of forest succession 

 and forest reproduction would be more easily attained in all probabil- 

 ity if the word were elided from forestry literature. 



LIGHT MEASUREMENTS IN FORESTS 



The present bulletin attempts to determine the value of "light 

 measurements in the forest" and to determine their significance in a 

 study of the light requirements of our forest trees. 



Light reaches the forest floor in four ways, through the tree 

 crowns, between the tree crowns, reflected from leaves and twigs, and 

 through the leaves themselves. The light which passes through the 

 leaves is altered in composition ; the rest is white light. Obviously, 

 then, the kind and intensity of light vary greatly in forests of dif- 

 ferent species of trees. 



The study of light intensities in the forests has been made along 

 two distinct lines, according as the workers have emphasized the im- 

 portance either of weakened white light or of filtered light in forest 

 growth and reproduction and in the development of the vegetation of 

 the forest floor. 



FILTERED LIGHT IN FORESTS 



Zederbauer (15) made a study of the light which passed through 

 the leaves of several different species of plants and found that there 

 was some variation in the rays absorbed. He argued therefore that the 

 light in the forest must be altered in composition as it passes through 

 the crowns. Accordingly he undertook to study the light in forests 

 with a spectroscope. The stations selected were located in pure stands 

 with the first branches ten meters from the ground and were situated 



