NORTHERN IDAHO FOREST RESOURCES AND INDUSTRIES 



Appendix 



j^^ — — 



Timber Yield Calculations 



The management calculations in this report have been based on area 

 regulation. The theory of area regulation is this: With a 120-year ro- 

 tation in which the one-cut method of logging is used, approximately 

 1/120 of the total stocked area should be cut yearly on the average. 

 Multiplying this acreage by the volume of saw timber per acre to be 

 obtained at rotation age provides an estimate of the allowable cut in 

 board feet. The concept itself is easy to grasp. However, unmanaged 

 areas are usually not easily converted into managed areas, primarily 

 because there is a shortage of certain sizes of timber. This fact, to- 

 gether with the necessity for making adjustments for fire losses and the 

 limitations of the basic data, make the practical application a bit 

 more complicated than the simple theory. 



Ideal Age Distribution 



Certain hillsides now denuded of timber will, in the course of time, 

 become restocked with a consequent reduction of the deforested area. 

 On the other hand, some green areas will be swept by fire, and this will 

 tend to add to the total deforested area. However, because of a con- 

 tinually improving job of fire protection, it is to be expected that the 

 over-all trend will be toward a gradual reduction of the deforested area. 

 Since this will be a long-time change, it is safest to assume for the immedi- 

 ate calculations that there will be no major change in this acreage and 

 that the cut can be based on the present stocked area. Therefore, the 

 plan will assume the logging of 1/120 of the stocked area each year and 

 returning again to this year's acreage in 120 years. If this is to be pos- 

 sible, the stands to be logged in 20 years should be at least 100 years 

 old today, the stands to be cut in 50 years should be 70 years old, and so 

 on. 



In figure 43 the proper cumulative distribution of area by age class 

 is shown by the step-like solid line. The stands on one-sixth of the area 

 should be 101—120 years (or older), on two-sixths (including the first one- 

 sixth) 81—120 years, on three-sixths 61—120 years, with three more similar 

 steps. It will be noted in figure 43 that the actual stand distribution 

 for the stocked commercial forest area in northern Idaho exceeds these 

 minimum requirements. For the purpose of illustration, however, 

 suppose the stands shown in figure 43 to be 41—60 years old were just 

 21—40 years of age. In that case the bar for stands 41 years and older 

 would be short of the ideal, and if logging were to proceed at the rate 

 of 1/120 of the total area per year, a time would come when for a period 

 there would be no timber of rotation age for lodging. The only alterna- 

 tive (without shortening the rotation) would be to log the stands now 

 older than 40 years at a slower rate to spread the production over the 

 years required for the underage stands to reach the loggable stage and 

 thus build up the growing stock. 



Reduction for Fire Damage 



With the adequate distribution of age classes as shown in figure 43, 

 1/120 of the stocked acreage could be logged annually only if no losses 

 from fire and other causes occurred. Since fire annually destroys a very 

 large volume of timber ot all sizes, this loss cannot be allowed for by a 



(^ 



straight reduction of the area cut by the acreage burned over. The 



RGE 



(Years) 



PLL commgRciflL siflnos 



101 + 

 81 + 

 61 4- 

 41 + 

 21 + 

 1 4- 





1 



,;:r7 















1 A, 



q' ACTUAL 

 ^IDEAL 







^ 







1 



> 



1 













1 1 





1 1 





1 



1 1 1 I 





1 



1 1 1 



L 









1 1 1 1 1 



C 



) 1 2 3 4 5 f 



MILLION ACRES 



5 



Figure 43. — Present age distriiution {cumulative) on all commercial Jorests 

 and compared with ideal pattern for a 120-year rotation on a one- 

 cut basis. Total stocked area equals 5.9 million acres. 



■destruction of a pole stand or a seedling and sapling stand, while im- 

 portant, is not so serious as the loss of a saw-timber stand. Suppose 

 for example, that in a particular area there is only enough saw timber to 

 care for the sustained-yield cutting budget for a few years. In that 

 case, every acre of saw timber destroyed requires a like reduction of 

 logged area (except for such salvage as is possible). What has been 

 lost on each acre, as far as the rotation calculations are concerned, is 120 

 years of growth. The average pole stand is roughly 60 years old — 

 just half grown. So, when an acre of poles is destroyed by fire, only 60 

 years of growth are lost. Assuming restocking is no harder in one case 

 than another, from a management standpoint, this loss is just half as 

 serious as an equal area loss of saw timber. This can be expressed in 

 terms of cut by saying that for every 2 acres of pole stand destroyed by 

 fire, 1 acre less of saw timber should be logged. In the same way a fire 

 loss in the average seedling and sapling stand, estimated to be about 20 

 years in age, can be expressed in terms of saw timber by the ratio ol six 

 to one. 



In the case of an operating unit with an excess of older stands, as in 

 figure 43, the reduction for fire losses in saw timber must also be modified. 

 For illustration, suppose fire consumes a portion of the mature saw- 

 timber stand which, according to the cutting plan will not be reached 

 for SO years. To keep the area cut in balance, it will be theoretically 

 necessary to reduce the cut no more than if a 70-year-old stand had been 



62 



