PERIODICAL LlTKRATrRI". 581 



of stands. The use of the average increment at felHng age ( Heyer's 

 proposition, which we consider most logical for felling budget calcula- 

 tions) is discouraged without argument. The a^erage increment, 

 volume divided by age, which if the stock is measured and its age 

 determined and not merely estimated, is a truthful statement, is 

 declared of no use in selection forest. Only the current ( or periodic 

 average) increment gives satisfactory indication of the effect of ^il- 

 vicultural measures and fellings, besides being the sustained yield 

 felling budget. To be sure, this current increment can only be secured 

 by repeated measurements. The relation between current, average 

 periodic and average increment at felling age is discussed upon the 

 basis of a curve and table. For mountain spruce on III site for the 

 whole rotation the following average values in cubic feet per acre for 

 main stand are found. 



Timberwood Total 



Average increment at felling age 95.8 107.2 



Average increment (of all age classes) mean for rotation... 94.4 120.0 



Current increment mean for rotation 11 1-5 116.0 



From this the author is forced to admit that the use of the average 

 increment at felling age was after all not so faulty. Nevertheless, he 

 considers that the average increment (secured by dividing the age of 

 each age class into its volume ) represents best the performance of the 

 stands ; its total and mean value represent the annual sustained yield 

 and felling budget in toto and per acre unit. Only when middle-aged 

 stands are prevalent and older stands are deficient it works out too 

 high, but then the normal stock comes also out relatively too high and 

 furnishes a compensating factor. 



The author accentuates the need of proper bookkeeping in order 

 to secure for the revision of working plans more and more reliable 

 increment data. 



An important question from the regulator's point of view in con- 

 trolling sustained yield management is the distinction between main 

 and intermediate yield. In the selection forest based on diameter 

 classes there is no difficulty in this distinction ; all that is cut above a 

 certain diameter is main yield. For timber forest it has been sug- 

 gested to use the same method, but with the many thinnings that are 

 practised in such forest it is too circumstantial and changes the man- 

 agement of stands to a management of trees, and loses the advantages 

 of our knowledge of the development of the stand, as exhibited in yield 

 tables, with their normal stock, given number of trees, etc. The im- 

 portance of booking correctly the intermediate yield is brought out 

 by a statement of the per cents which the customary, medium severe 



