206 



CHAPTEE III. 



THE NORMAL AGE CLASSES. 



When, under the system of working for a sustained annual 

 yield, the rotation has been fixed, it is necessary, that, year 

 after year, or period after period, the required mature woods 

 are forthcoming, so that the calculated annual yield may be 

 obtained. This involves the establishment of a normal series 

 of age gradation. By that term is, therefore, understood a 

 series of age gradations so arranged, that at all times, when 

 cuttings are to be made, mature woods of the normal age are 

 available and so situated, that no obstacles to their cutting 

 exist. This means, that each age class must be of the proper 

 extent, and that the several age classes must be properly 

 grouped, or distributed, over the forest. 



If a forest is to be managed according to the system of a 

 sustained annual yield and the clear cutting system, it must 

 contain a series of age gradations equal to the number of years 

 in the rotation ; the oldest age gradation must, immediately 

 before cutting, have the age of the rotation, the youngest nuist 

 be one year old, with a dilierence of one year in the age of 

 every succeeding two gradations. 



Example. — Assuming the rotation of a coppice wood to 

 be 20 years, and the height which the oldest wood reaches 

 in that time = 28 feet, then the 20 age gradations may be 

 represented as in the appended figure, 46. 



If the annual returns are to be equal in volume and the 

 quality of the locality is the same throughout, then all age 

 gradations must be of the same extent ; if different qualities 

 occur, the areas of the coupes must be in inverse proportion 

 to the quality of the locality. A series of age gradations so 

 arranged is called a normal working section. This subject 



