FORM-FACTOR IN CONIFEROUS TREES. 145 



The Form-factor which is in general use, and therefore of 

 most importance to the forester, belongs to Type i and is the 

 Under-bark Breast-height Form-factor, where the volume con- 

 sidered is that of all timber, without bark, up to 3 inches 

 diameter, the basal area is taken at breast-height, i.e. 4 ft. 3 ins. 

 from the ground, and the height is the total height of the tree. 

 It is the behaviour of this type which it is proposed to examine. 

 For the sake of simplicity we shall substitute the Over-bark 

 type for the Under-bark type, and, to assist in the explanation, 

 the following types will also be "considered :— The Breast-height 

 Form-factor with timber measurement to the tip and the base 

 at breast-height; the Absolute Form-factor with timber measure- 

 ment to the tip and the base on the ground ; the Absolute Form- 

 factor with timber measurement to 3 inches diameter and the 

 base on the ground. These are Type i {a) and {b) and Type 

 2 (a) and {h). 



It will be shown that not all of these types are true expres- 

 sions of form, i.e. they are not all accurate expressions of the 

 degree of taper of stems. This is especially true of young 

 trees. In order to demonstrate this, it is proposed to deal 

 throughout with trees of exactly the same form or degree of 

 taper, but of different sizes. The form of tree selected is a true 

 cone and in a longitudinal section through every stem to be 

 considered here, the diameter at the base subtends an angle of 

 2° at the tip. 



I. — Absolute Form-factor with Timber to the Tip. 



Here the base is on the ground and the form-factor expresses 

 the ratio between the whole volume of the tree and the volume 

 of a cylinder with the same base and height as the tree. 



Fig. I shows a relatively small tree, and Fig. 2 a relatively 

 large one. It is obvious that these two figures are exactly 

 alike, except in size. The volume of the cone, that is, stem, in 



, Basal Area x Height ^, , ^ , ,. , 



each case is The volume of the cyhnder 



3 

 is in each case Basal Area x Height, so that the form-factor 

 is in each case one-third or '333. Moreover, this type is a true 

 expression of form or taper. When the stem remains a true 

 cone of uniform taper, the form-factor is always a constant. 

 When, however, there is increased volume in the upper part and 



