Table 2. — Differences in estimates of timber growing base area, in thousand acres, between 

 study inventory and previous forest inventory by unit and cause 



Differences due to: 



Total , Unstable Isolated Land 



Study unit difference and patches Land use ^ ^ ^ exchange 



P^od^^- unusable and conflicts^ Deferred ^^^^^^ 



tivity land' stringers error 



Thousand acres 



Lolo Area,^ R-1 



116.1 





23.4 





32.7 



58.9 



+1.1 



Arapaho N.F., R-2 



268.8 



67.2 



2.2 



9.9 



127.5 



64.9 



-2.9 



Coconino N.F., R-3 



224.9 



57.3 



n08.3 



^3 



36.8 



22.2 





Boise Area,^ R-4 



112.0 



23.5 



25.6 



11.2 



43.3 



5.6 



+2.8 



E. Klamath W.C., R-5 



47.8 



7.9 





37.9 



2.3 





-.3 



Gifford Pinchot N.F., R-6 



119.6 



17.2 



10.1 





68.3 



19.3 



+4.7 



Total 



889.2 



173.1 



169.6 



59.3 



310.9 



170.9 



5.4 



^Includes rock talus, rock land, and other unusable Forest soils. 

 ^ Crest and special multiple use zones are included in this category. 

 ^Portion of National Forest. 



^Over 100,000 acres are stable but unusable for timber production because of other soil factors. 

 ^Most isolated patches and stringers on this unit were on unusable Forest soils. 



We found that the standard has not been 

 applied uniformly or accurately. The usual 

 procedure has been to determine soil produc- 

 tivity (site quality) from age-height relation- 

 ships of dominant trees. Growth capability has 

 then been determined from yield tables or yield 

 equations based on the yield tables. Inherent in 

 the procedure is the assumption that the area 

 being evaluated can support as many trees as 

 the land on which the yield table data were 

 collected. This is not always true and generally 

 is not true in stands located on the more arid 

 fringe of the forest. In areas of low rainfall each 

 tree requires more room than is "normal" to 

 fulfill its moisture requirements. Thus, "full 

 stocking" in such instances may be consider- 

 ably below yield table levels. In any locality, 

 rock outcrops, shallow soil, or soil variations 

 may preclude stocking at yield table levels 

 (fig. 4). 



All inventory samples were examined care- 

 fully in areas where it appeared that stocking 

 constraints might exist. This required consider- 

 ation of certain ecological subtleties. For ex- 



ample, in some localities, the presence of a juni- 

 per tree (a noncommercial species) would not 

 necessarily indicate that a commercial species 

 such as ponderosa pine could grow there. Yield 

 table estimates of growth were discounted in 

 proportion to the stocking capability.^ For ex- 

 ample, if the information available indicated 

 that only 50 percent stockabiiity was obtain- 

 able, then the yield estimate derived from the 

 site tree measurements and yield equations was 

 reduced 50 percent. Field samples were classi- 

 fied as unproductive forest land and excluded 

 from the timber growing base if they failed to 

 meet the 20-cubic-foot standard which includ- 

 ed consideration of both site and stocking 

 capability. 



Table 2 shows that many stands formerly 

 considered to be part of the timber growing 

 base don't measure up to the standard when 



^For the lack of a better basis, -'t was generally 

 assumed that the existing tree stocking on marginal 

 areas (or the stocking formerly present if the area had 

 been disturbed) is the maximum the land could 

 support. 



14 



