86 



those to become exploitable during the felling rotation by 

 the number of years in that rotation. 



The calculation can, however, be made in a less rude 

 fashion. Supposing that the ages are evenly graduated, 

 the average number of trees that can attain exploitable 

 dimensions each year in the immediate future is the total 

 number in Class II (the size-class next below that of which 

 all the trees are exploitable) divided by the number of years 

 required for trees of the lowest dimensions of this class to 

 become exploitable. This is then the possibility of the crop 

 for the time being ; and it must be estimated — from the 

 average production of other similar forests for instance — 

 whether the result so obtained is above or below the normal 

 or potential possibility, and the number of trees to be felled 

 should be increased or diminished accordingly. With a 

 view to estimating whether the age-classes are suitably 

 graduated or not, the number of trees in Class II may be 

 compared with those in the lower and higher classes. Finally, 

 as regards the sufficiency or insufficiency of the stock already 

 exploitable, it must be remembered that, in a normal forest in 

 which the age-classes occupied equal areas, there would be no 

 trees above the exploitable size on the ground immediately 

 after a felling, and only one year's growth immediately before 

 the next felling. In a selection- worked forest, owing to the 

 trees of different ages being distributed all over the area, 

 this would only be the case where the whole area of the forest 

 was worked over each year. Ordinarily, however, as already 

 explained, the entire forest is gone over in a number of years ; 

 and consequently it is only on the portion of the area felled 

 over twelve months previously that there is one yearns 

 growth of exploitable trees, on the next area there is two 

 years' growth, on the next three, and so on up to the limit of 

 the felling rotation. Knowing, therefore, the number of 

 trees of Class II which annually attain exploitable dimensions, 

 we can calculate the normal exploitable stock and thus ascer- 

 tain whether the actual exploitable stock is sufficient, insuffi- 

 cient or superabundant relatively to the stock in Class II. If 

 superabundant, the excess can be utilised at once or in several 

 years according as the lower stages of growth are sufficient 

 or insufficient. If insufficient, less than the normal pos- 

 sibility should be removed. 



The crop already taken as an example will serve to illnstrate this method of 

 analysis. The number of trees below exploitable dimensions, 18 to 24 inches in 



