PERIODICAL LITERATURE 583 



taken, the percentic distribution in normal stock of timber forest, if 

 all trees of 6 inches and more are calipered, will also on average sites 

 show 75 to 85 per cent of total normal volume for 100 to 120 year 

 rotation. 



A table is appended, showing the ideal distribution of stock in spruce 

 and fir selection forest, provisory values, according to whether 3, 4, 

 or 5 size classes are formed. This table gives an interesting insight 

 into the composition so that we translate it in part. 



Percentic, Ideal Distribution of Stock in Fir and Spruce Selection Forest 

 Timbervvood per cent 



In the estimating of young and middle-aged stands a comparison 

 with normal conditions is all that is needed, so that yield tables and 

 sample plots give sufficiently accurate results. However, care must 

 be taken to avoid errors in comparing the density, in rounding off 

 diameter classes, and in calculating the cubic contents by multiplying 

 the length of logs by the middle area. Young stands should be inven- 

 toried at each revision of the working plan. 



Errors in estimating mature timber arise mainly from the following 

 sources : 



1. Choosing sample trees w^iich are not representative. 



2. Using volume tables not applicable to local conditions. 



3. Improper use of form height and stand form factors. 



The per cent of error from these sources varies from i to 10, usually 

 plus. Minus errors are, however, to be feared in underestimating total 

 heights and log lengths of standing timber. ^Making diameter classes 

 of 2 cm. (i inch) gives errors of less than 2 per cent, so that they are 

 nesrliffible in most cases. 



