180 



PRINCIPLES OF AMERICAN FORESTRY. 



volumes found by multiplying their respective cross- 

 sectional areas at that point by the length of the topless 

 trunk. 



A more careful stem analysis of a tree affords detailed 

 measurements from which the volume at any time during 

 its lifetime may be determined very accurately. The fol- 

 lowing table cf measurements of a Pine will furnish data 

 for the calculation of its volume at different ages, and 

 its progressive development is graphically illustrated in 

 Fig. 62. 



The Accretion of a Forest for a given number of years 

 is found by multiplying the accretion of the sample tree 

 for that period by the number of trees per acre and the 

 number of acres in the tract. If the trees are arranged 

 in diameter classes, the accretion of each class is determined 

 and the sum of accretions of all the classes taken as the 

 accretion of the forest. 



The Working Plan of a forest contemplates the economic 

 management of the growing crop, so that there may be 

 cut each year not to exceed the amount of the annual 

 accretion; or, if worked on the rotation plan, so that there 

 may be cut at any one time not more than the accretion 



