212 FORM CLASSES AND FORM FACTORS 



Volumes of trees can thus be obtained from the vokmies of cyhnders, 

 when once the average form factor is known. 



The form factor is therefore, in theory, a direct expression of the 

 relative volume of a tree compared with a standard or constant volume, 

 and tables of such factors were expected to give the key to universal 

 volume tables showing form classes. But the diameter of the cylinder 

 which is to serve as the unit or basic volume must first be obtained and 

 must equal that of the tree. If this diameter is taken at the stump or 

 at ground, the butt swelling gives an abnormally large irregular vari- 

 ation in the cylindrical volume. This method is known as the Absolute 

 Form Factor. 



But the diameter can be shifted to B. H. with the cylinder equaling 

 the total height of tree as before. Form factors so calculated give uniform 

 or consistent results from which cubic volumes can be calculated, 

 and are termed Breast-high Form Factors. These form factors in turn 

 vary not only with the form of the tree, but with the total height as 

 well, hence could not be used to indicate absolute form. The reason 

 is that the diameter of the basic cylinder is taken, not at a height pro- 

 portional to the total height of the tree, but at the fixed height of 4| 

 feet. For short trees this point falls proportionally nearer the tip, 

 with relatively smaller cylinder, than for tall trees of identical form. 

 The breast-high form factor therefore decreases as height of tree 

 increases. 



In an effort to overcome this drawback and express form directly 

 by means of form factors, the so-called Normal Form Factor was devised, 

 in which the basal area is measured at a point on each tree represent- 

 ing a fixed ratio to the height of the tree. This plan has not proved 

 practical, owing to the difficulty of determining this point rapidly and 

 accurately. 



By comparing only the portion of the tree above B.H. with the 

 volume of a cylinder of equal height, the form factor for this portion 

 alone corresponds directly with variations in form for the tree. This 

 is known as Riniker's Absolute Form Factor. 



The Riniker form factor of trees of each form class was calculated by Jonson 

 from the normal form or tapers of trees of each D.B.H. and height class, taking 

 the diameters at points representing one-tenth of the stem above B.H. Then 



/=— - for the bole above B.H. only. 

 Bh 



Since form quotients indicate correctly the relative forms of trees, absolute 

 form factors of trees whose form quotients are equal should also be equal. That 

 this is true is indicated by the following test, e.g., from investigations of Claughton- 

 Wallin and F, McVicker: 



