Effects of Douglas-fir dwarf mistletoe on trees in thinned stands in the Pacific Northwest

The growth of 450 Douglas-fir (Pseudotsuga menziesii) in thinned stands was assessed over 20 yr. Trees infected with Douglas-fir dwarf mistletoe (Arce uthobium douglasii) were compared with uninfected trees. A significant redu ction in radial and height growth was observed when infection progressed be yond half to two-thirds of the volume of a tree crown being composed of wit ches' broom. Growth rates varied over time, depending on environmental cond itions, but heavily infected trees showed significantly reduced growth unde r all observed conditions. The effect of dwarf mistletoe on diameter growth in heavily infected trees was greater when the trees grew under more rigor ous conditions. Tree vigor, as estimated by the ratio of basal area increme nt to sapwood area, was significantly reduced among trees with more than tw o-thirds of the crown broomed. The level of infection was measured using br oom volume rating, a seven-class system based on a ratio of the volume of b roomed to unbroomed branches. Over a 10-yr period, tree infection class inc reased at the rate of approximately one infection class every 15 yr on aver age. At high levels of infection, crown decline and dieback contributed to an accelerated rate of change in infection class in this study.