Influence of Dwarf Mistletoe and Western Spruce budworm on growth and Mortality of Douglas-Fir in Unmanaged Stands

Permanent inventory plots in 94 unmanaged stands of primarily Douglas-fir (Pseudotsuga menziesii) on three national forests in Oregon and Washington were examined for growth suppression caused by dwarf mistletoe (Arceuthobium douglasii) and western spruce budworm (Choristoneura occidentalis). Ten-year increments of diameter and basal area were calculated from measurements in 1977-1987. Severity of dwarf mistletoe was scored using the six-class system. Severity and duration of defoliation were determined from aerial maps drawn over a 20-yr period. Dwarf mistletoe significantly decreased 10-yr diameter increment. Western spruce budworm significantly reduced diameter increment and basal area increment. No significant interactions between defoliation and dwarf mistletoe were found. Ten-year mortality of Douglas-fir due to dwarf mistletoe averaged 5% and 2% of the infected trees and basal area, respectively, for stands with moderate dwarf mistletoe severity and 4% and 3% of the infected trees and basal area, respectively, for stands with severe dwarf mistletoe infestation. Mortality was highest in stands with the most dwarf mistletoe and in stands with the most severe defoliation. There were no significant differences in diameters of dead trees among severity classes for dwarf mistletoe. Deadtree diameters were significantly smaller in stands with the most severe defoliation during the measurement period