NATURAL HISTORY OF THE FOREST. 149 



shade endurance assures to them a place in the 

 forest. 



Many of them are characterized by a height 

 growth which, though slow, is persistent ; while the 

 light-needing species, by falling behind in their 

 rate of height growth, often lose in the end what 

 they attained in their youth. As a result the 

 shade endurers finally become dominant, and the 

 light needers occur in the mixed forest only 

 sporadically, the remnants or single survivors of 

 groups, all the outside members of which have 

 perished ; and only when a wind-storm or insect 

 pest creates an opening of sufficient size is a chance 

 for their reproduction given. 



Just as in the mixed forest the species are dis- 

 tributed according to their shade endurance, so in 

 the pure forest of one species, or of species of 

 equal tolerance, will the different-sized or different- 

 aged trees develop side by side according to avail- 

 able light, each crowding the other, the laggards 

 being finally killed by the withdrawal of light. 



In a well-established young growth of white 

 pine, the seedlings, some 50,000 to 100,000 on an 

 acre, with their symmetrical crowns sooner or. later 

 form a dense crown canopy, excluding all light from 

 the soil. After a few years the leaves of the lower 

 branches, no longer able to function under the shade 

 of the superior part of the crown and of their 

 neighbors, fail to develop and the branchlets die 

 and break off ; this natural cleaning, which secures 



