The North American Forest 23 



branches as possible on the lower part of their 

 trunks. 



When the tree has succeeded in growing above 

 the heads of competitors and begins to spread its 

 crown, it changes its economy in various ways. 

 For one thing, as it is now enabled to provide itself 

 with more leaves, it has a chance to produce greater 

 amounts of wood : for each leaf is a laboratory 

 where the material is distilled out of which wood 

 and other vegetable tissue are formed. This in- 

 creased formation of wood results in an increase of 

 the diameter of the trunk, while the height growth 

 is no longer as rapid as before. At the same time, 

 the character of the wood changes, especially in 

 those trees which have two kinds of wood, an inner 

 core of heart-wood, and a surrounding layer of sap- 

 wood. This is the case with most of our lumber- 

 producing trees. The greater the diameter of such 

 a tree trunk, the smaller the proportion of sap-wood, 

 while a tall tree of very small diameter is nearly all 

 sap-wood. As heart-wood is much more valuable, 

 for timber purposes, it follows that to make his trees; 

 most valuable the forester allows them to follow up 

 the period of rapid height growth by a period of 

 prevalent diameter increase. In other words, he 

 now cuts away the weaker, half-suppressed trees, so 

 that the remaining ones get the benefit of an open 

 stand. Where this happens naturally in the wilder- 

 ness, by one cause or the other, the result is, of 

 course, the same as where the new condition is pro- 

 duced artificially. The forester's art in silviculture 



