Effects of dwarf mistletoe on the response of young Douglas-fir to thinning

Abstract

Four sites from two forests were examined to determine the effect of various levels of infection by Arceuthobiumdouglasii on the growth of Pseudotsugamenziesii in precommercially thinned stands. We found less than 1% mortality among the trees that we examined. Changes in level of infection did occur; we estimate that changes to levels of infection sufficient to cause significant reductions in diameter growth occurred among 19% of the infected trees that we studied. Height growth was significantly reduced in both forests, while diameter growth was reduced by infection in one forest. Trees of lower infection rating (dwarf mistletoe rating 0–2) showed a significant increase in radial growth following thinning in both forests, while more heavily infected trees (dwarf mistletoe rating 5 and 6) did not. Trees that were heavily infected and had spike tops consistently showed significant reductions in diameter growth in both forests.