44 MEAST^REMENT OF WHOLE WOODS. 



where .s represents the l)asal area at a certain height, Ji the 

 total height and f the form factor of the tree. In all cases, 

 where s, h, and /differ from tree to tree, nothing remains but 

 to ascertain them separately for each tree as indicated under 

 A. In the case of regularly grown woods, however, there are 

 always a number of trees, which show, at any rate approximately, 

 the same basal area, height and form factor, so that they can 

 be thrown together and dealt with in a uniform manner ; in 

 other words, all trees of a wood, which show the same base, 

 height and form factor, are joined into one class ; the volume 

 of one tree (or of a few trees) is ascertained and the volume 

 of the whole class obtained by multiplying the former by the 

 number of trees in the class. If every class is dealt with in 

 the same way, the vokime of the whole wood is obtained by 

 adding together the volumes of the several classes. 



So far, however, little or no advantage is gained, because it 

 would be necessary to ascertain the base, height and form 

 factor of each tree, in order to put it into its proper class, and 

 when this has been done, the volume of each tree may just as 

 well be calculated separately. Moreover, in crowded woods 

 the height is not always easy to measure, and the form factor 

 could only be estimated, unless it is taken from a table. Only 

 the basal area is easily ascertainable by measuring either 

 the diameter or the girth. 



Here, experience had to be called in, which fortunately 

 showed, that, in regularly-grown crowded woods, the height 

 and form factor are approximately functions of the diameter 

 of the tree ; in other words, trees of the same diameter have 

 approximately the same height and form factor. At any 

 rate, this is found to hold good to a sufficient extent, so 

 as to justify a classification according to diameter classes 

 only. 



In open woods, however, the height and form factor vary 

 within much wider limits, so that, besides diameter classes, at 

 any rate also height classes must be formed. Hence, the two 

 cases must be dealt with separately. 



