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A key silvicultural decision in managing young conifer plantations is determining the number and timing of release treatments to control competing vegetation. Three coastal Douglas-fir ( Pseudotsuga menziesii (Mirb.) Franco) plantations were treated under eight alternative herbicide regimes during the first 5 years after planting to test treatment effects on vegetation dynamics and seedling growth. After termination of herbicide treatments, competing vegetation developed at a rate similar to that of check plots, reaching 40%–60% cover in the first growing season and approaching 100% by the third. Recovery of competing vegetation was slightly more rapid with greater number of previous releases. Annual volume growth of seedlings was negatively correlated with current cover of competing vegetation, but competitive effects from previous years were fully accounted for by initial tree size. Under 4 years of release, delaying treatment by 1 year reduced volume attained at the end of 5 years by about 15%. Plots receiving 5 consecutive years of weed control reached the 5 year volume of check plots in only 3.9 years, implying an age shift of 1.1 years. Increasing the number of operational release treatments significantly improved seedling growth in the short term, but long-term growth effects must be monitored to determine the economically optimal regime.

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Coastal Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) response to eight weed control treatments was measured 12 years after planting at two Oregon sites. Treatments included four areas of weed control around individual trees (0.375, 1.49, 3.35, and 5.95 m 2 ), no weed control (check), total vegetation control, control of herbaceous competition only, or control of woody competition only. Douglas-fir growth and woody-species invasion differed between the Coast Range site (Summit) and the Cascade Range foothills site (Marcola). Woody species reinvasion was more intense at Summit, with Douglas-fir cumulative mortality in the check treatment reaching 23% in year 12. Woody-only control improved Douglas-fir growth at Summit but had no significant effect on growth at Marcola. Total vegetation control had a profound effect on stem volume growth 12 years after planting. At Summit, total vegetation control resulted in a 355% increase in volume per hectare relative to the check. At Marcola the increase was only 63%. At Summit, growth increased with each increase in area of weed control, whereas at Marcola growth increased with increasing area of weed control up to 3.35 m 2 of control. Results suggest that much of the gain in volume growth attributable to weed control may be lost if weed-control treatments are not highly efficacious. The differential response to woody control indicates that its benefit at a given site is strongly related to the abundance of competitive hardwood species, which may be predicted from the preharvest stand structure and vegetation community.Résumé : La réaction du douglas vert (Pseudotsuga menziesii (Mirb.) Franco var. menziesii) typique à huit traitements de maîtrise de la végétation compétitrice a été mesurée 12 ans après la plantation dans deux stations de l'Oregon. Les traitements incluaient la maîtrise de la végétation compétitrice sur des superficies de quatre dimensions différentes autour d'arbres individuels (0,375, 1,49, 3,35 et 5,95 m 2 ), un témoin (pas de traitement de maîtrise de la végétation), la maîtrise de toute la végétation, la maîtrise de la végétation herbacée seulement et la maîtrise de la végétation ligneuse seulement. La croissance du douglas vert et l'invasion par les espèces ligneuses différaient entre la station de la chaîne côtière (Summit) et la station située dans les contreforts de la chaîne des Cascades (Marcola). L'invasion par les espèces ligneuses a été plus intense à Summit où la mortalité cumulative du douglas vert atteignait 23 %, 12 ans après la plantation. La maîtrise de la végétation ligneuse seulement a augmenté la croissance du douglas vert à Summit mais n'a eu aucun effet significatif sur la croissance à Marcola. La maîtrise de toute la végétation a eu un effet prononcé sur la croissance en volume des tiges 12 ans après la plantation. À Summit, ce traitement a produit une augmentation de 355 % du volume à l'hectare par rapport au témoin. À Marcola, l'augmentation n'a été que de 63 %. À Summit, la croissance a augmenté avec chaque augm...