614 JOURNAL OF FORESTRY 



Again we find the statement that "good soil," meaning, no doubt, in 

 most cases a constantly wet soil, decreases the light requirement. 

 Gayer tells us that birch on good soil produces a dense crown and 

 that spruce on a poor soil is intolerant ; that the tolerance of the spruce 

 increases with humWity of the air and on a good soil. Heyer says 

 that the elm is cultivated as "unterholtz" on a fresh soil which is deep 

 and fine, allowing a good root development, a thing that would be 

 impossible on sand. In the very wet years of 1886, 1888, and 1896 

 some of the intolerant trees in Germany were heavily leaved like the 

 tolerant or shade-loving species. 



Mayr explains the increased tolerance of old trees by the fact that 

 they have a larger reservoir of stored water which enables them to be 

 independent of slight fluctuations in water supply and temperature. 



Hartig analyzed the so-called light increment which we get after 

 thinning. His researches show that the limiting factor in the trees 

 with which he worked was not light. That the chloroplasts were not 

 working to their full capacity with the light present, but that he could 

 reduce the leaf surface by 40 per cent without decreasing the rate of 

 growth. He found that after a thinning trees which were not exposed 

 to more light thereby, produced much greater amount of wood and 

 this can only be explained by the decrease in root competition for water 

 and mineral foods. The classic experiment of Fricke is known to all 

 and tends to the same conclusion, but his work has never received the 

 recognition it deserves. 



Heyer, the author of our tolerance theory, says that all trees are 

 tolerant in a garden soil. I hardly believe he would recognize his 

 theory as it is at present stated in its extreme form. 



We have already spoken of the standards by which foresters judge 

 tolerance and intolerance of trees. We have pointed out that leaf 

 structure is not a safe standard, since it may represent a water rela- 

 tionship rather than light relationship. Further, we have seen that 

 the dying of leaves in the center of the crown, even in the birch, is 

 governed by water supply and not by light intensity. I have con- 

 ducted some experiments with the white pine in which it was evident 

 that the dying of the leaves was not due to light but to water supply. 

 Three miles from the University on a light sandy soil a small grove of 

 white pine was planted 4 by -4 feet. The trees were about 15 feet high 

 and the lower limbs and leaves were dead up to a height of about 

 8 feet. 



