400 forestry Quarterly. 



then fall into the same form class independent of height, the "stem 

 curve" of all trees within the same form class will show the same 

 curvature, etc. 



To find what form class should be used is not difficult on felled 

 trees. The diameter at a point situated halfway between breast 

 height and the top of the trees is measured. If this diameter — 

 in the table called "upper diameter" — is divided with the diam- 

 eter at breast height, the quotient — called "absolute form quotient" 

 — gives us the form class. According as the upper diameter is 

 50, 52.5, 55, 57.5, 60, etc. per cent, of the breast height diameter 

 we get in the table form classes 0.50, 0.525, 0.55, 0.575, O-^o 

 to 080. 



The upper diameter corresponding to each diameter breast 

 height for each form class is given in the table, which lessens the 

 work considerably. For instance : a tree with a d.b.h. of twenty- 

 one centimeter and an upper diameter of 13.7 centimeter belongs 

 to form class 0.65. If the height of the tree is twenty meter 

 we find its cubic content to be 0.328 cubic meter. 



The heights of standing trees are measured with the help of 

 special instruments. In this case, to decide what form class 

 should be used is more difficult, however, as the upper diameter 

 cannot as a rule be measured. There are three or four methods 

 described by Jonson for the guidance of the estimator in determin- 

 ing the proper form class. 



The method considered most accurate is described as follows : 



The position and form of the crown is a very good indirect 

 measure of the form of the bole. It has been proved that the 

 pressure of the wind on the tree crown constitutes a force which 

 compels the tree to construct its stem in such a manner that the 

 same relative resistance to breakage acts in all parts, the smallest 

 possible amount of material being used. * As the concentrated 

 force of the wind strikes a point situated lower or higher on the 

 tree we get larger or smaller taper which means poor or good 

 form class. 



With the help of the equation for the stem curve and recognized 

 mechanical laws governing stresses, it has been possible to calcu- 

 late the normal relation between crown and stem form. 



* See Metzger : Der Wind als massgebender Faktor f iir das Wachsthum 

 der Baume, Heft 3 ; and Studien iiber den Aufbau der Waldbaume und 

 Bestande nach statischen Gesetzen, Heft 5, 6, 7, Miindener Forstliche 

 Hefte. 



