INTRODUCTION 



Computer programs for such purposes as the compilation of forest inventory data 

 must contain instructions for operations that were done by hand only a few years ago. 

 At that time, tables, graphs, and alinement charts were indispensable to the forester. 

 The information which was contained therein is still indispensable, but today we must 

 be able to store this information in a computer program in a form readily accessible 

 for use. Computer use of charts or graphs to describe site or yield functions is 

 impractical, but information stored in tabular form could be used. However, this 

 would require either a very extensive table for each species or interpolation within a 

 small table. The core storage required for a number of extensive tables, in addition 

 to the rest of the program and the data being processed, might well be more than is 

 available on smaller computers, while interpolation can lead to inaccuracy. 



The existence of an alinement chart or a smooth curve, or tables made therefrom, 

 implies the existence of a mathematical relationship between the variables involved. 

 Sometimes the relationship is known, as when a table is made by repeated solution of 

 an equation. Some charts and tables were derived altogether by graphic methods. When 

 this is the case, an approximate mathematical relationship can usually be derived. 

 However, the resulting approximation equation, complex and sophisticated as it may 

 appear, can never be any better than the hand-drawn curve that it is supposed to 

 represent . 



Data collected 40 years ago represent our only source of information on site 

 quality and yield capability for some species. Yield capability, as used by Forest 

 Survey, is defined as mean annual increment of growing stock attainable in fully 

 stocked natural stands at the age of culmination of mean annual increment. Yields may 

 be substantially higher with thinning and other intensive management. Yield capability 

 is measured in cubic feet per acre per year. 



Site index, together with associated yield information, provides a means of 

 assessing yield capability or, in other words, productive capacity of the land. 

 However, all variation in yield capability from one site to another cannot be explained 

 by conventional "site index." Short of direct measurement of stand growth itself, it 

 is not generally known what stand parameter(s) could be used to account for variation 

 not explainable by site index. 



Despite its shortcomings, site index does provide a means of rating sites in terms 

 of volume yield capacity--a more meaningful expression than site index alone. 



Another factor may affect yield capability of a stand--the extent to which an area 

 may be stockable. It may be that available soil is interspersed with areas of rock or 

 gravel that will not support tree growth. Any yield capability figure expressed in 

 this report on a per-acre basis will refer to an acre of tree-supporting soil, not to 

 an acre of land containing some nonstockable area. Where full stocking is not possible, 

 the yield capability estimate should be adjusted. This might be done by multiplying 

 the yield capability estimate by the ratio of productive land area to total land area. 

 The result would be a yield capability estimate for the entire area, including non- 

 stockable portions. 



Growing stock is defined, for Forest Survey purposes, as the live, noncull trees 

 of commercial species in the stand 5.0 inches d.b.h. and larger. However, the sources 

 of information from which the material presented in this report was derived used 

 differing size standards, so the definition of growing stock that was necessarily used 

 for each species will be given with the equations for that species. 



