258 COLLECTION OF STATISTICS. 



by the age of the wood, the mean annual increment is obtained, 

 which indicates the quaKt3^ 



It is evident, that in reahtj' a multitude of different qualities 

 exist, but for practical work they are grouped into a few, 

 generally not more than five, quality classes, which are 

 numbered I. to Y. Of these I. should represent the lowest 

 and V. the best quality, but, unfortunately, the reverse 

 numbering has been largely introduced. A still more con- 

 venient way is to represent the best quality by 1 and the 

 others in decimals of 1. Each of these quality classes 

 represents a distinct yield capacity, which differs with the 

 species and method of treatment. 



The quality can be determined with the help of yield tables ; 

 their preparation has been explained in Forest Mensuration. 

 Such tables represent the progress of volume, or increment, 

 throughout life for each quality class ; hence, assessing the 

 quality means, in this case, the selection of the proper yield 

 table. The difficulty is, that for every species and sylvicultural 

 system a different set of yield tables is required. It may even 

 be desirable to have different sets for different localities, so- 

 called local yield tables ; but such a procedure is likely to lead 

 to confusion, as different standards of the quality classes are 

 introduced into the account. Hence, general yield tables are 

 to be preferred, even if the same degree of accuracy is not 

 obtained as in the case of local tables. The difference is, how- 

 ever, not considerable, as experience has shown, that, within 

 reasonable limits, general tables give sufficiently accurate data 

 for the preparation of working plans. It has, for instance, 

 been proved, that the general yield tables for the Scotch pine, 

 prepared for North Germany, may safely be used for fairly 

 crowded woods grown in the south of England. The fact is, 

 that the sources of inaccurac}^ unavoidable in the best methods 

 of measuring the volume of a standing crop, are greater than 

 those caused by using general yield tables for any particular 

 locality. 



Yield tables for oak, beech, Scotch pine, spruce and silver 



