Figure 3. -Reductions in the timber base because of low productivity . 



stockability is taken into account. As would be 

 expected, the submarginal forest is most com- 

 mon in the central and southern Rockies. 



LAND INSTABILITY 



Each cooperating Region either made or had 

 already completed a detailed or extensive soil 

 survey for the National Forest included in this 

 study (fig. 5). As part of the study, Forest Serv- 

 ice soils-and-hydrology specialists then classi- 

 fied the forest land according to suitability for 

 timber use, taking account of the basic struc- 

 tural stability of the slope, susceptibility of the 

 soil surface to erosion, and the capacity of the 

 land to receive, store, and discharge water. 

 From such information they delineated the fol- 

 lowing classes of forest land on maps: 



( 1 ) Stable forest land usable for timber pro- 

 duction with present timber growing 

 and harvesting technology. 



(2) Unstable forest land potentially usable 

 for timber if there are improvements in 

 timber growing and harvesting tech- 

 nology requiring fewer roads and less 

 on-the-ground skidding of logs. 



(3) Unstable forest land completely unusa- 

 ble for timber production because it is 

 extremely susceptible to mass land 

 slumps, flows, and slides or because the 

 soil surface would wash away too read- 

 ily if the vegetation were seriously dis- 

 turbed. Snow avalanche or potential 

 avalanche areas, rock talus, and other 

 forms of rock land are also included in 

 this category though the land itself is 

 not unstable. On the portion of the 

 Lolo National Forest studied, for ex- 

 ample, much of the land in the "un- 

 stable" category is rock land and rock 

 talus. On the Arapaho, mass land move- 



15 



