14 THE THEORY AND PRACTICE OF WORKING PLANS 



Using the data in McCarthy's yield table for white pine, 

 and a rotation of 140 years, 



nv=—^l 23,780+ ) =3,044,657 board feet, 



but this is for 140 acres, 



for the average acre = ^' ^ =21,748 board feet 



140 



per acre, which compares well with iiv by yield tables, i.e., 

 21,407 board feet per acre. 



Distribution of the Age Classes 



All the trees in a stand or forest whose age falls within stated 

 limits are spoken of as being in the same age class. Age classes 

 are usually divided in twenty-year periods, but in old stands 

 may be of wider range. Age classes are stated in extent of 

 area or in percentage of the whole stand; in selection forest 

 in terms of number of trees. A stand where the ages of the 

 majority of the trees fall between twenty-one and forty years, 

 would, for twenty-year age classes, be referred to as being in 

 " Age Class II." 



In the selection forest, diameter classes take the place of 

 age classes. A diameter class is a grouping of the trees of a 

 stand on the basis of diameter, the intervals varying usually from 

 I to 4 inches, fractions being rounded off to the nearest full 

 inch of the limit. For example, with a 3-inch Umit all trees 

 from 22.6 to 25.5 inches in diameter would be assigned to the 

 24 -inch class. The diameter classes may be stated by numbers 

 of trees in each class on the unit of area or by the percentage of 

 the total contents of the stand represented in each, or by area 

 occupied or in any other way. 



Distribution of age classes refers to either the local dis- 

 tribution of age classes (Verteilung der Altersklassen) or the 

 percentic or absolute representation of the different age classes 



