Evaluating effects of thinning on wood quality in southeast Alaska

We examined the effect of thinning on wood quality of western hemlock (Tsuga heterophylla) and Sitka spruce (Picea sitchensis) located on Prince of Wales and Mitkof Islands in southeast Alaska. Sample trees came from paired plots (thinned versus unthinned) in eight naturally regenerated, mixed stands of young-growth western hemlock and Sitka spruce in an effort to examine a range of thinning densities, from 10 X 10 ft to 20 X 20 ft spacing. The stands, which had been thinned in the late 1970s and early 1980s, ranged in age from 36 to 73 years at the time they were selected for this study in 2003. The main focus of the stand selection in this retrospective study was to find individual stands that included a thinned plot and an adjacent unthinned plot from the same original stand. A random sample of trees from both thinned and unthinned plots was selected, stratified by tree dbh. About 12 trees per species per stand (461 trees in total) were selected for the study. Selected trees were harvested, and a lumber recovery study was conducted, enabling us to relate log volume to recovered lumber volume by product grade. Surfaced dry dimension lumber was produced, graded, and nondestructively tested using the transverse E-vibration standard test for stiffness. We did not evaluate the resource for appearance-grade products. For each species, the overall treatment effect (thinned versus unthinned) on lumber grade recovery and transverse E-vibration modulus of elasticity by vertical-log position (butt log, middle log, or top log) were analyzed using a mixed-effects procedure. Results suggest that there were no significant differences in product recovery or value between the thinned plots and the untreated control plots in the manufacture of structural lumber products.