COMPARISON TO RELATED YIELD 

 ESTIMATES 



Cochran (1979) has pubhshed estimates of net volume 

 yields for natural stands of white (grand) fir and Douglas- 

 fir east of the Cascades in Oregon and Washington. 

 Although his samples were from natural stands, his rigor- 

 ous screening for uniform spacing, minimal past mortal- 

 ity, and even-aged character of the stand would suggest 

 that the yields he presents might also represent planta- 

 tions. Furthermore, because his tables are arrayed by 

 breast-high age, effects of the wide variation in early 

 height growth found in natural stands would be reduced. 

 Cochran's site index was defined as the height of the tall- 

 est tree on the Vs-acre plot at 50 years age at breast 

 height. Although his rate of selection of five trees per 

 acre was less than the 20 trees per acre we used, the dif- 



ference in site index definitions is estimated to be about 

 3 feet for site indices of about 70 feet. To shift our tables 

 to a breast-high age basis, we estimated years to reach a 

 height of 4.5 feet for plantations from the coefficients in 

 table 13 of Ferguson and others (1986). For combinations 

 of habitat types, elevations, slopes, and aspects that pro- 

 duce site index 70, time from regeneration treatment to 

 4.5 feet in height was estimated to be 10 years for 

 Douglas-fir trees at +1 standard deviation in the distribu- 

 tion of heights and 16 years for grand fir. 



A comparison of the yields reported by Cochran and 

 those reported in this study is presented in figure 69. The 

 curves for Douglas-fir are nearly identical, while for grand 

 fir the Prognosis estimates are consistently higher. The 

 difference in yields of grand fir seem reasonable because 

 we expect grand fir in the Inland Empire to reach higher 

 maximum densities than grand fir growing in eastern 

 Oregon and Washington. 



24000 r 



20000 - 



8000 - 



4000 



GF-P 



GF-C 



DF-P 



10 20 30 40 50 60 70 80 90 

 BREAST HEIGHT AGE 



100 110 120 130 140 



Figure 69 — Comparison of Cochran and Prognosis yields for Douglas-fir and grand 

 fir, site index 70. tVlanagement codes indicated on tfiis figure are; GF-P = grand fir- 

 Prognosis; GF-C = grand fir-Cocfiran; DF-P = Douglas-fir-Prognosis; DF-C = 

 Douglas-fir-Cochran. 



55 



