4 THE THEORY AND PRACTICE OF WORKING PLANS 



or normal. However, regulation of cut is based upon actual, 

 not normal increment, hence actual increment becomes the basis 

 of the normal forest. Where the real increment is to be taken 

 from yield tables,* the values given in the table must be reduced 

 by the actual factor of density, since yield tables are always 

 for fully stocked stands. 



Where yield tables are not available, the increment must 

 be determined on the ground, either by applying the increment 

 per cent of representative trees of the stand, or else by calipering 

 sample areas and figuring their increment by means of diameter- 

 age and diameter-volume tables. The former (and yield tables) 

 is better for nearly even-aged stands; the latter method for 

 uneven-aged stands. 



Where diameter-age tables are lacking, stump analyses 

 can be made. 



For determining current annual increment the use of an 

 increment borer is deserving of wider popularity than it has 

 heretofore enjoyed in America. Where no increment borer is 

 obtainable, the representative trees, selected according to any 

 of the standard methods (Draudt, Urich, etc.) can be cut into 

 at breast height and the rings on the last inch of radius counted 

 on the horizontal under-cut. Either Schneider's or Pressler's 

 formulae may then be applied. 



Schneider's formula: 



p = ^—j (or 450 or 500 according as the height-growth of the 

 nd 



tree is poor, average, or good), 



where /) = the current annual increment per cent; 



n = number of rings of annual growth in the last inch of 

 radius; 

 and d = diameter breast high, in inches. 



* Yield table: A tabular statement of the volume of a stand of specified char- 

 acter per unit of area. This is usually constructed for units of one acre and 

 for intervals of ten years. 



