50 THE PRINCIPLES OF HANDLING WOODLANDS 



no trees at all from 60 to 80 years of age, and the place 

 which would normally be occupied by this age-class is 

 covered with trees less than sixty years old. It is clear 

 that after the expiration of 20 years and the oldest age- 

 class is removed, there would be no trees at all from 80 

 to 100 years old, and it would be necessary to wait another 

 20 years before cuttings could be made. Unless, there- 

 fore, all age-classes are normally represented the cuttings 

 must be at irregular intervals. 



The selection system is applied in this country chiefly 

 in virgin forests. The normal form of a virgin forest is 

 of a selection character; that is, it consists of trees of dif- 

 ferent ages mingled together promiscuously. The forests 

 of the United States, however, have in the past been seri- 

 ously damaged by destructive fires. It is common, there- 

 fore, to find in virgin forests stands of great age which 

 have a regular or even-aged form. This occurs where 

 past fires have cleared the ground and a new stand has 

 come in on the clearing. Even in forests where there 

 have been no destructive fires in the past, frequent small 

 clearings have been made by windfall, insects, and other 

 destructive agencies, and the distribution of age-classes is 

 therefore irregular. 



The range of age-classes in most virgin forests is very 

 great. In northern Idaho, for example, the oldest trees 

 may be over 400 years old. In such forests, if there 

 have been no disturbances by fire or otherwise, all ages 

 may be represented. But in classifying the trees with a 

 view to practical management all the mature trees would 



