150 ECONOMICS OF FORESTRY. 



the desirable clear boles, takes place during the 

 period of rapid height growth, which occurs from 

 the tenth to the thirtieth year. At the age of 

 thirty years the trees are slender poles having a 

 diameter of 3 to 5 inches, and a height of from 20 

 to 25 feet, with a few taller ones, the boles bearing 

 a dense conical crown and beset for the greater 

 part of their length with small limbs, the lower 

 ones dead or dying. Not a few trees are seen to 

 fall short of reaching the general upper crown 

 level ; the crowns of these laggards are shorter, 

 more open, with fewer leaves on each twig. Others 

 again will be found dead or scarcely vegetating, 

 with crowns very poorly developed. In other 

 words, we can recognize different vigor in devel- 

 opment according to constitution and accidental 

 opportunity, and can make a differentiation into 

 development classes : the predominant, with their 

 crowns 5 to 10 feet above the general level, which 

 must finally make up the mature stand ; the sub- 

 dominant, still alive and; should accident remove 

 some of the superior class, ready to occupy their 

 air space ; and the dominated or inferior ones, hope- 

 lessly out of the race. 



Of the tens of thousands which started only 

 2000 or 3000 are surviving, and as each tree tries 

 to expand its crown, and secure for itself as much 

 air space as well as root space as it can, the result 

 is a continued diminution of the number of trees 

 occupying the acre. 



