338 DETERMINATION OF THE YIELD. 



Area to be dealt with during the first ten years = 300 acres. 

 On this area all 120-years-old trees are cut, as well as the 

 necessary number of younger trees, so as to reduce them to a 

 suitable number of mature trees. As it is a laborious matter 

 to ascertain the age of the trees, it is desirable to substitute 

 the diameter (or girth) at maturity for the age. For instance, 

 instead of an age of 120 years, it may be laid down that trees 

 with a diameter of, say, 2 feet at chest-height above the ground 

 shall be considered mature. 



The areas to be dealt with in each period of, say, 10 years, 

 should be marked off on the ground ; in some cases even the 

 annual coupes may be marked. 



A distinction between final and intermediate returns is very 

 difficult in the case of selection forests. 



To attempt a regulation of the expected returns by volume 

 seems of doubtful utility. At any rate, to fix the yield in 

 material only, whether in cubic feet or number of trees, is a 

 risky procedure, which may lead either to over- or under- 

 working of the forest. By far the best plan is to fix the yield 

 by area, and to determine the minimum size of the mature 

 trees to be cut. This area should not be exceeded. With 

 this reservation, the probable amount of final returns to be 

 cut may be estimated according to von Mantel's or Brandis' 

 methods described above. 



3. Change from one Sylvicultural System into another, called 

 a Conversion. 



The number of conversions from one sylvicultural system 

 to another, which are conceivable, is considerable, and it is 

 impossible to give any general rules of procedure. Whatever 

 the nature of the conversion may be, the only sure basis for the 

 determination of the expected yield is the annual cutting area. 

 Hence the consideration of one specific case will bring out the 

 essential points to be considered in each conversion : 



An irregularly-stocked forest of broad-leaved species, partly 

 coppice, or partly coppice with standards, shall be converted 



