FOREST RESOURCES OF THE PONDEROSA PINE REGION 



Forest Growth 



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IN a region such as this with extensive areas of 

 unmanaged, mature forests, the forest capital 

 is constantly being depleted by human and 

 natural agencies, at the same time that it is re- 

 plenished to an extent by growth. Growth and 

 depletion are characteristically out of balance 

 during the early period of rapid timber harvest. 

 Some sort of a balance, however, must eventually be 

 attained. If depletion is abruptly curtailed to equal 

 growth, the production of marketable products, 

 recovery of wealth, and the contribution to com- 

 munity support from the forest will fall short of 

 the sustainable amounts. On the other hand, if 

 depletion exceeds growth too long, forest capital 

 will be so reduced that the subsequent rate of 

 possible timber harvest will be very low. One aim 

 of forest management is to bring the two opposing 

 forces of growth and depletion into balance at the 

 optimum level. Hence, a current excess of de- 

 pletion over growth is justified by the region's 

 substantial old-growth timber supply, to the extent 

 it is harvested under forestry practices that will 

 insure continuing yields at this level. 



Although varying widely in character, the com- 

 mercial forest stands may be classified with respect 

 to condition of growth into two broad categories — 

 immature stands and mature or virgin stands. 

 Estimates of gross growth were computed for all 

 commercial stands, but detailed net growth esti- 

 mates were restricted to the immature stands — 

 those not more than 160 years of age — on the 

 assumption that in older stands, on the whole, 

 growth is offset by losses through mortality and 

 decay. 



Growth estimates were made only for the com- 

 mercial forest sites. Rates applied to the immature 

 ponderosa pine stands were derived from two 

 sources — those for even-asred stands from the 



normal yield tables, 5 those for uneven-aged stands 

 from the growth charts for selectively cut forests 

 (77). For the virgin saw-timber types growth rates 

 were derived from 323 samples taken throughout 

 the region and analyzed especially for this study, 

 as detailed in the appendix, p. 91. Rates for the 

 immature stands of other species were derived 

 from the Douglas-fir yield tables ( 10) . 



Three kinds of volume growth were calculated in 

 both cubic feet and board feet. Each kind is 

 useful in describing in part the forest ' growth 

 situation, but none alone adequately portrays the 

 entire situation. These are current annual growth, 

 or the current annual increment of stands in their 

 present condition; periodic growth, or the esti- 

 mated increment within three periods of 10 years 

 each; and potential annual growth, or the average 

 annual increment that could be obtained from all 

 commercial forest land through reasonably inten- 

 sive forestry practice as judged by current local 

 standards. 



Current Annual Growth 



Estimates of current annual growth are based on 

 forest conditions as of date of survey inventory, 

 which averages about January 1936. This ex- 

 pression of growth should not be used as a basis for 

 estimates of volume at a future time, as changing 

 conditions quickly invalidate it. It shows neither 

 the potential productivity of the land nor the 

 relation between character of forest practice and 

 forest productivity. Current growth is directly 

 comparable with current depletion, but conclu- 



5 Yield table adjustment factors were applied to the rates 

 derived from the published tables (72) in order to allow 

 for approach of understocked stands to normal. These 

 factors were based on findings of J. W. Girard and L.J. Cum- 

 mings in connection with the growth phase of the forest 

 survey in northeastern Washington. 



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