NORTHERN IDAHO FOREST RESOURCES AND INDUSTRIES 



Forest-Management Possibilities 



-^>- 



Forest Increment 



ESTIMATES of forest growth in northern Idaho 

 have been made numerous times in the past, but most 

 ot these were based on general opmion and te 

 wide of the mark on the low side. According to the 

 Forest Survey, growth adds each year 636 million board 

 teet to the commercial saw-timber volume. That figure 

 includes the volume added to saw-timber trees, and also 

 the volume of those smaller trees which during the year 

 have moved up into the saw-timber class through increase 

 in diameter. It is a net estimate after normal losses from 

 shading, insects, disease, and miscellaneous causes (except 

 fire) have been deducted. In the tabulation below" the 

 current annual growth is shown by species: 



Million board 

 feet {tog scale) 



Western white pine 164.9 



Ponderosa pine 92.7 



Western larch 83.7 



Douglas-fir 126.3 



Grand and alpine firs 85.7 



Western redcedar, including poles 27.3 



Western hemlock 9.3 



Engelmann spruce 24.4 



Lodgepole pine 22. 1 



Total 636.4 



The present rate ot accretion of 636 million board feet 

 is by no means the maximum to be expected from this 

 commercial-forest area. There are approximately 1 mil- 

 lion acres of nonstocked land and 1 million acres of over- 

 mature stands on which no net growth is taking place. If 

 all the nonstocked and understocked areas were properly 

 stocked, if the nongrowing saw-timber stands were re- 

 placed by growing stands, and if the age classes were 

 properly distributed, a maximum increment of 1.6 billion 

 board feet annually might be achieved — 2.5 times the 

 current rate. Although no such high figure is likely to be 

 realized it is within reason to expect with proper manage- 

 ment a much larger current annual growth than at present. 



'1 CUMMINCS.L. J., AND KemP, P. D. FOREST INCREMENT IN NORTHERN 



IDAHO. North. Rocky Mountain Forest and Range Expt. Sta. Forest 

 Survey Release No. 18, 74 pp., illus. 1940. [Processed.] 



Fire and cutting together are taking each year 715 

 million board feet of saw timber, or 79 million board 

 feet more than the current annual growth. 



11 Deterniiniifion of the Allowable Cut 



The primar)' question behind all the growth and drain 

 calculations is this: What is the maximum cut of timber 

 products which can be maintained permanently, and to 

 what extent does present production overrun or underrun 

 this figure.^ Unfortunately, the difference between the 

 current annual board-foot increment and the losses from 

 fire or other causes do not necessarily indicate the volume 

 of timber which should be cut each year. Suppose, for 

 instance, that a given forest property is entirely covered 

 with saw-timber stands 200 years of age and older. At 

 this age the net growth is negligible, yet it is obvious that 

 the permissible and desirable cut would be large, and that 

 any comparison of current annual increment and drain 

 would not be of practical significance. On the 1 million 

 acres of overmature stands in northern Idaho, a current 

 utilization far greater than the present replacement by 

 growth is desirable. 



Also, proper allowance is not made in this type of calcu- 

 lation for fire losses. For example, a fire in a seedling and 

 sapling stand results in no board-foot loss, since the trees 

 are below saw-timber size; and it would appear from the 

 calculation increment minus fire drain equals allowable cut 

 that no reduction in the saw-timber cut would be necessary 

 for fire damage in these young stands. Yet it is obvious 

 that for every acre of such forest land burned, the present 

 cut in saw-timber stands must be reduced by some fraction 

 of an acre if a stable cut is to be maintained. 



In view of the errors this formula invites, the allowable 

 annual cut has been calculated for northern Idaho upon a 

 different basis, which might be termed area regulation. 

 This method has the advantage of simplicity. Therein 

 also lies its greatest weakness, for being simple it is not 

 precise. However, despite its crudeness, this calculation 

 does weigh all of the major factors and is admirably suited 

 for the purpose at hand. The following discussion is 

 primarily devoted to the results obtained by this method, 

 but the reader may check the calculations and reasoning 

 by referring to the appendix. 



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