spruce and subalpine fir height growth responds 

 strongly to site quality, although both species are 

 capable of growing on poor-quality sites (cold, high- 

 elevation sites) (figs. 1. 2. and 9). Western larch is some- 

 what less sensitive to site quahty (figs. 1. 2, 7. and 9) 

 and able to grow on all but the coldest subalpine sites, 

 the dry portion of the Douglas-fir series, and the entire 

 ponderosa pine series. Lodgepole pine is less sensitive 

 yet (figs. 1, 3, 7, 8, and 9). The grand fir regression Lines 

 demonstrate unexplained erratic behavior. But figures 1 

 and 8 (Douglas-fir and western larch, for which large 

 samples exist) show grand fir quite responsive to site 

 quality. Overall, Engelmann spruce generally has the 

 highest observed site index (figs. 1, 3, 4, 7, 8, and 9), a 

 result attributed as much to its site index being com- 

 puted on breast height age as to its superior height 

 growth. With the exception of mountain hemlock, grand 

 fir and lodgepole pine show the lowest observed site 

 indexes (figs. 1, 7, 8, and 9). These observations do not 

 reflect on height potential at any age other than index 

 age; in fact, grand fir is often the tallest species on 

 moderate sites. 



Use of height projection equations (table 3) is limited 

 to the recorded age range of the species to be predicted 

 and the observed site index range of the predictor 

 species. The equations are empirically derived from these 

 specific ranges; do not extrapolate equations beyond the 

 given ranges because their behavior may not correspond 

 to expected height growth patterns. 



Figures 10 through 18 portray height growth poten- 

 tials of associated species, using table 3 equations to 

 project height growth for each of the predicted species 

 associated with a single observed species. For example, 

 figure 12 corresponds to a stand in which the observed 

 grand fir site index is 60; projected heights for 



Douglas-fir, Engelmann spruce, and western larch are 

 graphed over only the observed age range of each spe- 

 cies. The following site quality terms, which have been 

 arbitrarily defined in respect to northern Idaho sites and 

 applied to all species, are employed here only to facili- 

 tate the ensuing discussion: low, site indices < 55; 

 medium, site indices ^ 55 but < 70: high, site indices >70. 



Figures 10 and 17 show that grand fir and western 

 white pine have the greatest height growth potential on 

 medium and better sites. Their heights are generally 5 to 

 10 feet taller than those of Douglas-fir, western larch, 

 and Engelmann spruce over the age range 40 to 110 

 years. Douglas-fir. western larch, and Engelmann spruce 

 are similar in height potential, with western larch some- 

 what taller at ages less than 70 years (figs. 11 and 12). 

 Engelmann spruce is taller on high sites (fig. 10), 

 whereas western larch is comparatively taller on medium 

 sites (fig. 13). AU three species demonstrate good height 

 growth on medium to high sites. 



Lodgepole pine is shown to be as tall as or taller than 

 associated species at young ages on poor sites (figs. 15 

 and 16), but not nearly as tall on medium and better 

 sites (figs. 11 and 17). Ponderosa pine also shows rapid 

 early growth, followed by a more rapid deceleration of 

 height growth than its associates (fig. 10). If height were 

 the sole determinant of rotation age, then ponderosa 

 pine apparently lends itself to short rotation manage- 

 ment on warm, dry sites. Lodgepole pine may be 

 similarly managed on cold and'or dry sites. 



Subalpine fir has height growth comparable to 

 associated species at younger ages, but its height is less 

 after 70 years (figs. 11 and 12). Mountain hemlock 

 height growth is consistently slower than aU other 

 species (figs. 11. 13, and 15). 



140 

 120 

 100 



80 



60 

































































































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40 60 80 100 120 140 160 180 200 

 AGE OF PREDICTED SPECIES, YRS 



,140 



o120 



LU 

 X 



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LU 

 V- 



o 



LU 



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40 60 80 100 120 140 160 180 200 

 AGE OF PREDICTED SPECIES, YRS 



Figure 10. — Douglas-fir at site 

 index 70 is used to project the 

 heights of Engelmann spruce, 

 grand fir, lodgepole pine, 

 ponderosa pine, western larch, 

 and western white pine; heights 

 of the associated species 

 (predicted variables) are 

 projected over their sampled age 

 ranges. 



Figure 11. — Engelmann spruce at 

 site index 70 is used to project 

 the heights of Douglas-fir, grand 

 fir, lodgepole pine, mountain 

 hemlock, subalpine fir, and west- 

 ern larch; heights of the 

 associated species are projected 

 over their sampled age ranges. 



8 



