98 



groups, instead of forming compact blocks, are scattered and 

 intermixed in the most irregular manner throughout tlie 

 forest. It is usually impossible to ascertain what area each 

 group occupies ; so that it is necessary to determine whether 

 the yolumes of material in each group are in normal pro- 

 portion to each other. It has been ascertained^ that^ in a 

 normally-stocked high forest divided as explained above 

 into three age-groups, the volume of material in the crops 

 of the second group is equal to three-fif ths of the volume of 

 the crops in the first group.* 



Therefore, whenever in a selection-worked forest the 

 volume of the material in the group ofsfjill-aged crops 

 exceeds by two-thirds the volume of the group oi^medium- 

 aged crops it may be admitted that these two groups cor- 

 respond to the two similar age-groups of a normally-stocked 

 high forest ; and if it is arranged to exploit the group of 

 full-aged trees in one-third of the number of years corre- 

 sponding to the age of the exploitable tree, there should be 

 no decrease in the yield during the following period when 

 the trees which at the outsej; constituted the group of 

 medium-aged crops will be felled. Furthermore, all risk 

 of overfelling may be avoided by omitting to take into 

 account the increment of the full-aged crops during the 

 period prescribed for their exploitation. 



In practice it is convenient to base the classification on 

 some more easily ascertainable factor, such as the circum- 

 ference or diameter of the trees, and thus prescribe the 

 maximum size for trees in the groups of medium-aged and 

 young crops. Usually the operation can be still further 

 simplified by determining the size corresponding to the age 

 of exploit ability, and by assuming that the full-aged and 

 medium-aged crops comprise respectively trees exceeding 

 two-thirds the size of the exploitable tree, and trees from 

 one to two-thirds of that size. The method is very easy to 

 apply in calculating the yield of . selection- worked forests. 

 First, the age of exploitability and the circumference or 



• That this ia very approximately oorreot may bo seen from fig. S, page 24, by aotaally 

 counting the tectanglea representing the production, or in a similar figure by aesuming the 

 average annual growth per acre to be uniform and equal from year to year. If " a " be the 

 area, v " the average annual production perunit of area, and "r " the number of years in 

 the exploitable age, the volume of wood in the first group would be — 



■ 2 X -g J and in the second group .^^ * i ^. x ~ 



Hence Volume of wood in first group _ f + 1 _ ^ 



Volume of wood in second group | + J "~ ^' 



