434 MEASUREMENTS FOR FOREST MANAGEMENT 



rate, until their efficiency has been assured by collecting the 

 average form factors for innumerable crops of normal density 

 in this country. 



At present the average sample trees must be measured in 

 the ordinary way, except that an hypsometer should be used 

 for accurately gauging the height. 



Where possible, the sample trees should be felled and 

 accurately measured, and their form factors ascertained, and 

 a careful record of them kept for future use. 



3. Estimation by Reference to Average Yield Tables. 



This is a method which will give approximately correct 

 results if average yield tables be obtainable. 



It is, therefore, very useful in the making of working plans, 

 or in drawing up a report on any woodland area, especially 

 for estimating the contents of the younger crops. But, when 

 great accuracy is necessary, their use cannot be recom- 

 mended. 



There are no average tables in general use, at present, for 

 crops grown in this country. 1 Any attempt to apply con- 

 tinental tables cannot be expected to give more than a rough 

 estimate, as the conditions under which they are grown, both 

 as to culture and climate, vary very greatly from those that 

 obtain in Great Britain. 



The application of the tables to an existing crop is not an 

 easy matter. It is necessary to know the age of the crop and 

 the quality of the land for the species of crop that is growing. 



In order to ascertain the age of a crop, it is necessary to 

 fell a few trees and count the annual rings, except that, in the 

 case of young coniferous crops, it is nearly always possible to 

 count the annual whorls, thus avoiding the necessity of felling 

 any trees. 



The quality of the land is much more difficult to estimate. 

 It is, of course, necessary, because the average yield tables are 

 referable to land of a particular quality for the particular crop 

 under consideration. 



For practical application there ought to be average yield 

 1 But vide Chapter XI. 



