40 



SITE INDEX 



30 



20 



10 



100 

 90 

 80 



70 



60 



50 

 40 



10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 



AGE 



Figure 1 5 — Quadratic mean diameter vs. age for Douglas-fir precommercial 

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

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

 same. 



10 20 30 40 50 60 70 80 90 100 110 120 130 140 150 



AGE 



Figure 16 — Quadratic mean diameter vs. age for Douglas-fir precommer- 

 cial 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. 



18 



