638 forestry Quarterly. 



vations for special reasons can then be made from time to time. 



In silvicultural direction there is also difference of opinion. 

 The tropical forest, according to Mayr, if only the most valuable 

 species are exploited continuously deteriorates in composition 

 and value (so does the mixed forest everywhere!), while the 

 pure forest is easily reproduced, hence Mayr recommends this 

 form, but, according to his well-known theory, in small areas of 

 not over 25 acres for the single stand, to avoid the dangers of 

 pure stands generally (mixture by stands). Jentsch pleads for 

 the retention of the natural mixed forest in selection form. 



The difficulty in the absence of annual rings to determine the 

 admissible felling budget, based on increment, Jentsch proposes to 

 overcome by confining (as Brandis did originally in India) the 

 exploitation to a diameter limit. This, tentatively upon the basis 

 of a few sample areas, merely to illustrate the calculation, he 

 places at 24 inches, when, finding that about two thirds of the 

 total volume is formed by trees above that size, he formulates the 

 volume increment per cent, for n years {In) : 



V 67 



2/7V : V = In : 100; In = ■ L_ioo 



/J y 



The question of the length of n, the period of return, needed to 

 replace the 2/^ V , or the / for a given n, can be found only by 

 experience after the first n has passed. If n were placed at 10 

 years the annual increment per cent, would have to be 6.7%, 

 which the author does not think out of the way, corresponding to 

 a production of 540 to 960, average 758 cubic feet per acre. By 

 lengthening the period of return, say to 20 years the increment 

 needs to be only 370 cubic feet, a very reasonable expectation. 

 Applying to the sample areas the proposition to remove two thirds 

 of the volume, it is found that in the virgin timber of the 361 to 

 668 trees present (per hectare) only 29 to 48 or 4.6 to 9.2% are 

 removed, hence from 90.8 to 95.4% remain. In the secondary 

 forest areas similar results appear. It would, therefore, be safe, 

 even to cut into the second size class. 



The author advocates the exploitation by private enterprise 

 under concessions. The time for which such concessions are to 

 run must be determined not by silvicultural considerations alone, 

 which would make 10 to 20 years sufficient, but by business con- 

 siderations, especially the size of the capital required for the 



