230 



TIMBER RESOURCES FOR AMERICA'S FUTUBE 



cent, the composition factor was computed as fol- 

 In brief, the composition on this 



lows: t^=.80. 



area was 80 percent of the standard. 



Literal application of this procedure might, in 

 some cases, result in a zero composition factor. 

 This could lead to the imrealistic implication that 

 the growth of a forest stand consisting of "accept- 

 able" species only would have no utility whatever. 

 Hence, a minimum composition factor of 0.5 was 

 adopted. No composition factor lower than this 

 minimum limit was applied. 



Effect of Felling Age or 

 Premature Cutting 



Forest stands grow in natural cycles. These 

 cycles or natural growth trends have been defined 

 by study of the average annual growth of many 

 species. First it is necessary to define average or 

 mean annual growth. The term refers to the 

 growth calculated by dividing the volume of a 

 stand of timber by its age in years. Usually 

 mean annual growth is expressed in units of volume 



per acre at a given age. Bj^ calculating the mean 

 annual growth in stands of a given species or forest 

 type for a series of ages, the changes of growth with 

 advancing age can be determined. All past inves- 

 tigations of this kind have shown that from the age 

 at which volume can be measured in usable prod- 

 ucts the mean annual growth increases rapidly 

 with age, reaches a peak, and declines. 



This basic growth cj^cle is illustrated in figure 78. 

 Using it as an example, we see that the peak of 

 mean annual growth is reached at 125 years. If 

 clear cut, then the yield will represent the accumu- 

 lation of annual growth amounting to an average 

 of 100 volume units per year for 125 j^ears, the 

 maximum attainable. But if clear cut at 75 years 

 an average annual growth of only 80 volume imits 

 or 80 percent of maximum would be realized. 

 Partial cuttings such as thinnings or improvement 

 cuttings made at ages younger than those of peak 

 annual growth tend to maintain or increase subse- 

 quent growth of the stand and add to the total 

 volume harvested during a complete growth cycle. 

 Such partial cuttmgs therefore have beneficial ef- 

 fects upon productivity, while clear cutting at 

 young ages reduces it. 



average annual growth per acre (units of volume) 



ja. OS OO O NJ 



















'5 







// 





~^ 





t 



/ 











/ 













25 50 75 1( 



ageO 



)0 125 150 1; 

 ^ears) 



Figure 78 



