CHAPTER VII. CALCULATION OF THE POSSIBILITY. 



48. Theory and practice. 



If everything were as it ought to be, the yield of the forest 

 should be equal to the normal increment, that is, the maximum 

 possible production per acre per annum under the rotation 

 adopted, that the soil and chmate permit. In highly organised 

 forests where intensive working has been carried out for a long 

 time, and where reUable yield-tables and increment-tables and 

 exact statistical data of aU kinds are available, the calculation 

 of the possibihty can be strictly worked out in accordance with 

 the theory. In everyday practice, however, where such con- 

 ditions rarely obtain, the regulation of the yield is generally 

 calculated in a simpler and less theoretical fashion. This is 

 especially the case when we have agreed to be content with 

 short views, and to fix the jdeld only for twenty or thirty years, 

 with a revision of the calculation every ten years. What actually 

 happens in most cases is that we have a definite area to be worked 

 over during a definite number of years, say twenty years for 

 example, during which time we have to regenerate this area and 

 to remove all the standing stock of old trees. We should then 

 make an estimate of the actual volume of this mature crop, add 

 on a trifle perhaps for future increment during the period, and 

 then divide this total by the number of years, to get the annual 

 yield for the period. This is the simplest method, and there is 

 nothing theoretical in it. 



The regulation of the jdeld mav be by area, by volume, or 

 by both. 



49. Regulation by area. 



First, by area. This is the method appUed to coppice feUings, 

 and to clear-feUings. The area under working is divided by the 

 number of years in the rotation, and this gives the size of the 

 annual coupe. If the site quality varies appreciably, the coupes 

 may be made equiproductive rather than exactly equal in area. 

 In the case of Coppice-with-standards, a further regulation has 

 to be made with regard to the over-wood. To do this an estimate 



