Susceptibility of Brewer spruce (Picea breweriana) to dwarf mistletoes (Arceuthobium spp., Viscaceae)

Brewer spruce is an endemic of the Klamath and Siskiyou Mountains in southwestern Oregon and northwestern California. Three species of dwarf mistletoes (Arceuthobium spp.) have been reported to parasitize Brewer spruce, but the susceptibility of this spruce to infection by these parasitic plants has received no attention since the late 1960s. The objective of this study is to improve estimates of the susceptibility of Brewer spruce to these parasitic plants in select areas currently supporting highly infected dominant and co-dominant trees. Three mixed conifer stands infested with one of the three dwarf mistletoes known to infect Brewer spruce (nine stands total) were sampled to evaluate the relative susceptibility of this conifer to each mistletoe species. At each of the nine study sites, 10 to 20 temporary circular plots with a 6-m radius (0.012 ha) were established around large, severely infected trees. The following data were collected for each live tree within a plot: species, diameter, and dwarf mistletoe rating. Based on the incidence of infection (% of trees ≥ 5 cm in diameter infected), Brewer spruce was assigned to host susceptibility classes. Based on our findings, Brewer spruce is more susceptible to dwarf mistletoe infection than previously reported. It is a principal host of both western white pine and Wiens' dwarf mistletoes, and a secondary host of mountain hemlock dwarf mistletoe. Infection of Brewer spruce was frequently severe in the nine stands we sampled, however, additional research is needed on the distribution of dwarf mistletoes on Brewer spruce and the effects these parasites have on mortality, growth, and regeneration of this rare tree.