commercial thinning is a potential opportunity for forest managers to realize mid rotation wood volume while stimulating increased growth of residual trees for the final harvest. the practice of commercial thinning is, however, not a common practice in ontario due to lack of experience and scientific evidence. this thesis evaluates the effects of four different thinning treatments on two different black spruce (picea mariana) boreal forest sites10 years following commercial thinning. the four treatments were analyzed from a commercial thinning operational study conducted in northwestern ontario in 2007. analysis included; diameter at breast height (dbh), basal area, height and volume. two factor anova tests indicated significant effects of treatment or treatment by site interactions showed direct and strong evidence that the thinning treatments had effects. significant site effects from the anova tests also tells us that there is a difference in the variables we measured between sites. commercial thinning does show promise and these legacy sites are a unique research opportunity deserved of ongoing study to further understand the treatment effect thru to final rotation age.