418 



THE USE OF YIELD TABLES 



Age class. 

 Years 



Area 100 per cent 



stocked. 



Acres 



Actual area in age 



Acres 



This method of obtaining the area of separate age classes makes 

 possible the prediction of yields from yield tables based on age for 

 long periods with considerable accuracy, where without such separation 

 this would not be possible and yields could be predicted only for the 

 current decade or two. 



320. Application to Forests Having a Group Form of Age Classes. 

 Forests composed of species which are intolerant and fire-resistant 

 tend to form groups of approximately even age. A yield table based 

 on age can be obtained for such species, which will serve as a 100 per 

 cent standard. But it is very difficult to separate the forest itself into 

 its component age classes by mapping the areas which they occupy, 

 and equally difficult to determine in a practical manner the average 

 actual age of the stand on such areas even if mapped. But the forest 

 can still be separated into these age classes based on area and age, 

 permitting the application of this yield table to predict its growth, 

 provided proper use is made of the laws of averages. (In timber estimat- 

 ing, it is permissible to employ averages known to be subject to error 

 because it is not practicable to attain mathematical accuracy on account 

 of expense.) 



The problem here is, 



1. To determine the trees which belong to each age class so that the 

 volume of the class may be found. 



2. To determine the age of the age class. 



3. To find its area. Given the first two of these elements, the 

 method of finding the third has already been shown (§319). 



By reference to § 275 it is seen that diameter is an indicator of the 

 age of trees, but that a given age class will include a wide range of diam- 

 eters. Where stands are composed of trees of many different ages so 

 that it is not possible to ascertain the age of a given stand by felling 

 one or two trees, nor to map the separate areas in the forest which are 

 occupied by these age classes, the only alternative in obtaining age 

 is through the use of average diameters. The diameters can be meas- 



