18 THE THEORY AND PRACTICE OF WORKING PLANS 



A convenient way to express the age limits and average age 



in an uneven-aged stand is by the expression ^^ where, in 



the example above, the age varies from 50 to 100 years and the 

 average has been determined as 71 years (strictly 71.4 years). 

 Where the average age has not been arithmetically determined 

 the approximate age figures will, at least, serve as a valuable 



guide. Or even the letters O, Y, M may be used, e.g., 



would be a stand Mature to Overmature with the average 

 Overmature, i.e., in excess of the rotation age. 



Nor should it be forgotten that certain species, such as fir 

 and spruce, often withstand decades of suppression during which 

 their growth is almost nil. In determining their age this " core 

 of suppression " should, therefore, be disregarded. 



Areas that are being regenerated by shelterwood methods 

 fall into two age classes, divided according to what remained 

 of the original stand. For example, a shelterwood cutting 

 in a ninety-year old stand covering 200 acres of which only 

 40 per cent of the stand remained uncut would be apportioned: 

 80 acres to the higher-age class and 120 acres to the lowest or 

 to the " blanks " if no reproduction was on the ground. Where 

 less than 20 per cent of the original stand remains on a cutting 

 area or burn and the density of stocking is less than .3 and there 

 is no reproduction the area, is temporarily at least, classed 

 with the " blanks." 



The age classes are differentiated by species only if there is 

 a marked difference in their value. 



There are two graphic ways of comparing the actual with 

 the normal distribution of the age classes. One is by plotting 

 the normal and the actual area of each age class on cross- 

 section paper, using the ordinates for age and the abscissas for 

 area. The normal distribution will, of course, be a straight 

 line; the actual a zigza-g, now rising above, now falling below 

 the horizontal line of normality. 



The other method is that of rectangular blocks, the 



