vi] METHODS OF TREATMENT 45 



able size, and capable of effective supervision. Extraction and 

 transport should be cheap and easy. Then taking into considera- 

 tion the light-requirements of the principal species, the cycle 

 should be such that no too great interval of years will lapse 

 between two successive operations on any part, nor should it be 

 so low that the crop will be fatigued and damaged by too frequent 

 fellings. It will be noted that if a felling-cycle of twenty years, 

 for example, be applied, we are going to harvest twenty years' 

 accumulated production on one-twentieth of the area, instead 

 of taking each year's production off the whole area. 



The determination of the girth limit which is adopted as the 

 size of maturity will result directly from the definition of the 

 object of management, taken in conjunction with the size 

 (easily ascertainable) of the firstly physical, and secondly com- 

 mercial, maturity of the average tree. 



The age corresponding to the size adopted as the exploitable 

 size is obtained as the result of a large number of ring-countings 

 of sample trees. The exploitable age, in round numbers, fixes 

 the rotation, of which the felling-cycle will for convenience be 

 generally taken as a sub-multiple. 



Now although the constitution of the timber crop is hidden 

 from our eyes, because the trees which form it are growing one 

 above another, all mixed up in the utmost irregularity, there are 

 two outstanding conditions which are necessary to enable the 

 realisation of an equal annual yield in perpetuity: firstly, re- 

 generation must be taking place every year without intermission 

 on every acre of the area, and secondly there must be a complete 

 and regularly graduated succession of all age-classes year by 

 year, all occupying equal areas, all over the forest. This equal 

 series of age-classes must exist although we cannot see it. In 

 addition, for the annual yield to be a maximum, it is of course 

 necessary that the forest should be fully stocked. 



The only way in which the constitution of the crop can be 

 ascertained is by making enumerations, and counting the number 

 of stems in each group of size-classes. If the area of the working- 

 circle, or felling-series, is small, we might make a complete 

 enumeration, or if the area is too big, we should make a partial 

 enumeration, either by sample plots, or, better, by linear surveys, 



