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Effects of Douglas-Fir Dwarf Mistletoe on Trees in Thinned Stands in the Pacific Northwest

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The growth of 450 Douglas-fir (Pseudotsuga menziesii) in thinned stands was assessed over 20 yr. Trees infected with Douglas-fir dwarf mistletoe (Arceuthobium douglasii) were compared with uninfected trees. A significant reduction in radial and height growth was observed when infection progressed beyond half to two-thirds of the volume of a tree crown being composed of witches' broom. Growth rates varied over time, depending on environmental conditions, but heavily infected trees showed significantly reduced growth under all observed conditions. The effect of dwarf mistletoe on diameter growth in heavily infected trees was greater when the trees grew under more rigorous conditions. Tree vigor, as estimated by the ratio of basal area increment to sapwood area, was significantly reduced among trees with more than two-thirds of the crown broomed. The level of infection was measured using broom volume rating, a seven-class system based on a ratio of the volume of broomed to unbroomed branches. Over a 10-yr period, tree infection class increased at the rate of approximately one infection class every 15 yr on average. At high levels of infection, crown decline and dieback contributed to an accelerated rate of change in infection class in this study. For. Sci. 45(3):359-365.
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Keywords: Arceuthobium douglasii; Pseudotsuga menziesii; growth; intensification; vigor

Document Type: Journal Article

Affiliations: Consulting Scientist with Biological Lab Services, 3355 Hiawatha Ave. S., #203, Minneapolis, MN 55406--Phone: (612) 721-6123;, Fax: (612) 729-2360

Publication date: 01 August 1999

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