58 



B. Determination of the mean increment of a crop for the entire period in 

 which it reaches exploitability. 



In ihe ease of all crops that are nearly exploitable tbe mean in- 

 crement at the present age is for all practical purposes the mean 

 increment sought. It is only in very old crops that the present 

 mean increment will be found to be too small. Por very young- 

 crops, and in crops much younger than the age of exploitability, 

 the required mean increment must be obtained from yield tables. 



C. Course of the increment of a crop during the whole period of its life. 



The stem-analysis (see page 53) of the sample stems can give 

 the course of growth solely of the present main crop {i.e., the crop 

 exclusive of the overtopping, overtopped and suppressed stems), and 

 as regards the crop at previous ages it can furnish only the volume 

 and height of the largest trees which the crop then contained. 

 Hence, to be able to trace the course of growth of a crop through 

 its successive ages, the same crop must be successively surveyed at 

 each of those ages. 



D. Which method of determining the increment of crops to employ according 

 to the purpose for which the information is required. 



The determination of the increment of entire crops is generally 

 undertaken for the purpose of framing working-plans, and the ob- 

 ject then is to ascertain either what the yield of the several crops 

 will be when they are felled, or at what respective ages they will 

 severally become exploitable. 



When it is required to determine.the mean annual increment of 

 a crop up to the time it becomes exploitable, yield tables should he 

 used if the crop is young, and the formula ^-^^SST^ '^ ^^® '"'°P 

 is nearly exploitable. 



When it is required to know what a crop will yield if it is felled 

 ■within the next 20 years, we must add to the present volume the 

 probable increment for the ensuing 5 or 10 or even 15 years. This 

 increment may be determined either (1) as a percentage in the crop 

 itself, or (3) by adopting such percentages as usually accrue in 

 similar crops at the age of the highest mean increment, or (3) by 

 adopting the mean annual increment* of the given crop at the pre- 

 sent age, or (4) by using the increment furnished by yield tables. 



In some methods of framing working-plans it is necessary to 

 know what the yield at the time of felling will be of all the crops, 



