vn] CALCULATION OF THE POSSIBILITY 61 



as a maximum, if there is any object in so doing, or given merely 

 as a general guide to the conduct of the annual working. 



This possibility may be calculated as follows : An enumera- 

 tion is made of the whole stock, either by actual counting or by 

 estimate, and the number of stems in each of four or five size- 

 groups is ascertained. Suppose for example that 6 feet in girth is 

 taken as the size of maturity, then all trees of 6 feet or over form 

 Class I. Class II includes all trees with a girth between 4! and 

 6 feet, Class III includes all stems between 3 feet and 4^ feet in 

 girth, and so on. It has also to be ascertained by ring countings 

 how many years the average tree takes to pass from one class 

 to the next. Knowing then the number of trees in Class II, and 

 the number of years it will take the whole of this number to 

 pass up into the exploitable Class I, we can easily calculate the 

 annual rate of production of trees of the exploitable size. The 

 number of trees in Class II has only to be divided by the number 

 of years that it takes a tree to pass through that class. 



This number forms the basis of our calculation of the yield, 

 which, it is to be noted, is not based on the existing number of 

 Class I trees (as might, at first sight, seem natural), because the 

 number of trees in Class I is purely accidental, depending on 

 past fellings, and tells us nothing as to the future rate of pro- 

 duction annually of trees of the required size. 



There is, however, a further point to be considered. We do 

 not propose to fell over the whole area every year, but we 

 have chosen a felling-cycle, of, for example, twenty years, so 

 that we are going to confine our operations each year to one- 

 twentieth of the area, and we can only realise the production 

 of the whole forest off one-twentieth of its area on the con- 

 dition that we have twenty years' accumulation of production 

 standing, waiting for the fellings to come round. It is evident 

 therefore that we must have twenty years' accumulation of 

 Class I trees on one coupe, nineteen years' accumulation on 

 the next coupe, and so on down to one year's production of 

 Class I trees on the area worked over last year. We shall 

 therefore have to keep a considerable stock of Class I trees, 

 in regular gradated succession, always standing. The point 

 that we now have to consider is whether the existing stock 



