232 



TIMBER RESOURCES FOR AMERICAS FUTURE 



where small trees were cut for cord wood 

 products, the effect of felling age was based 

 upon the age of peak growth measured in 

 terms of cords or cubic feet. Where saw- 

 logs were removed, the effect of felUng age 

 was based upon the age when growth is at a 

 maximum, in terms of board-feet. Hence 

 the procedure included neither direct nor 

 indirect judgment as to the desirability of 

 either present or future requirements for 

 different products. A free choice of products 

 objectives was assured. 



The standards by product size classes are 

 included in the appendix section Criteria, 

 together with the local modifications pro- 

 vided for and examples of detailed methods 

 and calculations for application of felling-age 

 factors in both even-aged and many-aged 

 stands. 



Basic Level of Standards 



The standards of measurement chosen for each 

 of the four elements represent what was judged 

 to be the most productive condition currently 

 attainable under prevailing operating conditions 

 and the status of knowledge available for each 

 forest type and region or subregion. Thus the 

 standards represent conditions practically attain- 

 able. Tliey are not related to any specified portion 

 of tlie growth obtainable by full application of all 

 know^n technology. Standards developed on the 

 basis adopted are likely to be high in comparison 

 to those practical of attainment a decade or more 

 ago. They will likely prove to be low in the future 

 as economic situations and technological advances 

 favor the development of forestry. Considerable 

 emphasis in developing standards therefore was 

 placed on the e.xercise of judgment as to the 

 desirable condition of recently cut areas that was 

 currently practical of attainment. The ways in 

 which judgment was applied in arriving at 

 standards under this concept is previousl}^ de- 

 scribed under the subtitle How Concepts Were 

 Developed. 



The Productivity Index and Glass 



The four elements used in appraising the 

 productivity of recently cut lands were integrated 

 into a single productivity index. The entire 

 possible range of indexes, 6-100, were subdivided 

 into three broad productivity classes, upper, 

 medium, and lower, as presented in the summary, 

 page 71. The index calculated for each area 

 examined was assigned to tlio appropriate produc- 

 tivity class. 



Methods of Calculating Productivity Index. — The 

 following discussion will explain the methods used 



in calculating the productivitj- index and the 

 reasoning basic to the methods. 



The first two elements closely related to growth, 

 i. e., existing stocking and prospective stocking, 

 together obviously constitute the total stocking 

 which will provide the next cut of forest products. 

 The first step in deriving the index was simply 

 addition of the stocking percentages for existing 

 and prospective stocking. The result is a rating 

 of total stocking expressed as a percentage of the 

 standard chosen to represent 100 percent stocking. 



The previous discussion on species composition 

 has shown how poor composition reduces the 

 utility and value of the current and expected 

 growth. A composition standaixl was presented. 

 Also, for situations where field examination show^ed 

 that the standard was not met, a method w^as 

 presented for calculating a composition factor. 

 This factor appraises the limitation placed upon 

 the utility and value of the growth due to sub- 

 standard composition. It is expressed as a 

 proportion of growth attainable by a standard 

 composition for the total stocking found. Thus 

 the factor for standard composition is 1.0, but for 

 substandard compositions is 0.95, 0.90, 0.85, or 

 some other decimal not lower than 0.5. Expressed 

 in this w^ay, as a proportion, the mathematical 

 relation of total stocking to composition is one 

 of multiplication. As an example, assume a total 

 stocking of 80 percent and a composition factor 

 of 0.9. The second step in deriving a final pro- 

 ductivity index then is the calculation 80X0.9 = 72. 

 The result, 72, is the rating for total stocking 

 modified by composition. 



The effects of felling age or premature cutting 

 in limiting growth on clear-cut areas have also 

 been described. The growth cycle shown in figure 

 78 has been used to illustrate how the efi^ects on 

 mean annual growth of cutting at a given age can 

 be expressed as a proportion of the growth 

 attainable at the age of peak growth. The relation 

 between total stocking modified by composition 

 and the final element of felling age is again one 

 of multiplication. In the event that a felling age 

 proportion or factor of 0.80 was found applicable 

 to the example used in the discussion of composi- 

 tion, the calculation w^ould be 72X0.8 = 58, the 

 final productivity rating. 



In aggregating areas for final results, the 

 infiuence of the factor for premature cutting is to 

 reduce the area of a given stocking and composi- 

 tion rating by the area on which the crop chosen 

 for production did not reach the age of maximum 

 growth. 



More detailed examples and sample calculations 

 are included in the Criteria portion of the ap- 

 pendix. Here also will be found the variations in 

 procedures and standards which were adopted 

 in various sections of the country. 



