Forest Management 7 



years. Each class has an age differing from that of any other class. 

 The youngest class is one year old; the next class is two years old; 

 the third class is three years old, and so on to the oldest class the age 

 of which equals the rotation. 



In the case of natural seed regeneration, the normal number of age 

 classes at hand is expressed by the fraction 



wherein r stands for rotation, and wherein s stands for the number of 

 years normally elapsing between successive seed years. Since a single 

 seed year is rarely sufficient to secure a complete stand of seedlings, a 

 wood raised by natural seed regeneration is usually composed of two, 

 three or more age classes appearing in mixture and forming distinct 

 aggregates. 



Where the rotation comprises loo years, and where the period of 

 regeneration comprises 20 years, and where seed years occur every s 

 years, there a "normal gradation of age classes" contains, in the fall 

 succeeding a seed year, the following aggregate of age classes: 



Amongst all age classes under 21 years old, some mother trees are 

 still at hand up to 'lio years old; and beneath all age classes over 90 

 years old, some seedlings are found up to 20 years old. 



In the ideal selection forest, all age classes are represented on every 

 acre of ground. 



The separation of the age classes (allotting to each age class separ- 

 ate areas) facilitates logging and transportation; it increases, on the 

 other hand, the dangers threatening the forests. 



If a proper gradation of age classes exists in a forest it does not 

 necessarily follow that the age classes are properly grouped and ar- 



