SITE INDEX 



20 



10 



90 



80 

 70 



10 20 30 40 



50 60 



70 



80 



90 100 110 120 130 140 150 



Figure 37 — Quadratic mean diameter vs. age for grand fir precommercial 

 thinning to 250 trees per acre at 20 years and commercial thinning to 100 trees 

 per acre at 50 years such that the mean d.b.h. before and after thinning is the 

 same. 



30 



20 



10 



SITE INDEX 



90 

 80 

 70 



60 

 50 



40 

 30 



10 20 30 40 50 60 70 SO 90 100 110 120 130 140 150 



AGE 



Figure 38 — Quadratic mean diameter vs. age for grand fir precommercial 

 thinning to 250 trees per acre at 20 years and commercial thinning from above to 

 100 trees per acre at 50 years in Inland Northwest forests. 



34 



