IMPROVEMENT THINNING. 9 



ally become separated, in the course of their struggle for 

 existence, are of assistance in determining the extent of a 

 thinning. Four classes are usually distinguished. They 

 are as follows: (1) dominant, (2) intermediate, (3) sup- 

 pressed, (4) dead. Dominant trees are those that have 

 their crowns in the light ; they have kept ahead of the other 

 trees in height growth. Intermediate trees are those that 

 still have their crowns in the light, but are somewhat back- 

 ward, and are destined to become suppressed trees in the 

 near future. Suppressed trees are those that stand slightly 

 below the intermediate class, and will probably die within a 

 few years. 



Moderate thinning would involve the removal of such of 

 the intermediate trees as are interfering with the best devel- 

 opment of the dominant trees. Of course the circumstances 

 are often such that an occasional dominant tree must be re- 

 moved ; but care should be exercised in the removal of both 

 dominant and intermediate trees, not to open up the stand 

 to such an extent that undesirable undergrowth will be en- 

 couraged. In the case of shallow-rooted species, like the 

 spruce, the stand should not be opened up too much, or it 

 will become liable to windthrow. On the other hand, the 

 stand must be opened enough to stimulate the growth of 

 the remaining trees, or very little good will have been ac- 

 complished by the operation. The forester must balance 

 one alternative against another, quickly and accurately, and 

 decide in the case of each tree. In no case should the cover 

 be broken into to such an extent that it will not close again 

 in two or three years. 



Whether suppressed and dead trees should be removed, 

 depends principally upon whether they contain enough 

 wood to make their removal along with the rest worth while. 

 Some stimulation will result from the removal of certain of 

 the suppressed trees, but most of them are so far behind the 

 dominant trees that are to compose the final stand that their 

 presence or absence has little effect, one way or the other, 

 upon the development of the dominant trees. Yet it often 

 pays to remove some of the suppressed, and sometimes even 

 a portion of the dead trees, while the more important thin- 



