60 Forest Mensuration 



the increment in second growth woodlands can be obtained from tabulated 

 statements ("yield tables") showing the rate of growth for woodlands of 

 a given species in a given locality. Under normal yield tables are under- 

 stood such tables which give the rate of growth for even-eged, pure, nor- 

 mally stocked, well thinned woodlots for given localities (compare Para- 

 graph LIII. and XCIV..). 



Such normal yield tables are constructed abroad for beech, pine, spruce, 

 fir and oak. In this country they exist only in Pinchot's and Graves' 

 yield tables for white pine. In America, pure even-aged woods are found 

 in rare cases only (taeda, echinata, rigida, jack and longleaf pines, tama- 

 rack, coppicewood). 



In the construction of normal yield tables the following points require 

 consideration : 



1. The different methods of construction (Paragraph XCII.). 



2. The combination, interpolation, adjustment and correction of the 

 results (Paragraph XCIIL). 



3. The contents and use of yield tables (Paragraph XCIV.). 



PARAGRAPH XCII. 



METHODS OF CONSTRUCTION OF NORMAL YIELD TABLES. 



Normal yield tables may be based on: 



A. Repeated survey of some typical woodlots during their entire life- 

 time. 



B. Repeated survey of different woods standing on an equal quality 

 of soil, during a period of years equal at least to the longest difference in 

 age found amongst them. 



C. One-time, simultaneous survey of a very large number of woods of 

 different ages standing on different qualities of soil. Missing links are 

 here obtained by graphic or mathematical interpolation (Paragraph 

 XCIIL). 



If tables are constructed by repeated survey of several woods (B), it 

 is often found that the links cross one another for unexplainable reasons. 



PARAGRAPH XCIIL 



GATHERING DATA FOR NORMAL YIELD TABLES. 



In order to see whether or not two \\oods, in the case C of the pre- 

 ceding paragraph, belong to the same chain of growth, two methods are 

 in use : 



a. The horn or curve method, after Baur. 



b. The stem analysis method. 



