Early thinning in mixed-species plantations of Douglas-fir, hemlock, and true fir affected by Armillaria root disease in westcentral Oregon and Washington: 20 year results

Four 10- to 20-yr-old plantations were treated to determine the effects of precommercial thinning on tree growth and mortality caused by Armillaria root disease in the Cascade Range of western Oregon and Washington. One plantation was Douglas-fir and noble fir, one Douglas-fir and western hemlock, one Douglas-fir alone, and one Shasta red fir and mountain hemlock. After 20 yr, differences in crop tree mortality between thinned and unthinned plots were not significant (P = 0.9768). Quadratic mean diameter growth of crop trees, however, was significantly (P = 0.0053) greater in thinned than in unthinned plots. Crop tree basal area/ac growth was significantly (P = 0.0008) greater in thinned plots. There were no significant (P = 0.6647) differences in basal area/ac growth of all trees between thinned and unthinned plots. Apparently, from a root-disease perspective, precommercial thinning does not affect incidence of crop-tree mortality after 20 yr, but individual and per acre tree growth of crop trees increase significantly.