Biomass of witches’ brooms caused by Douglas-fir mistletoe in northern Arizona

Douglas-fir dwarf mistletoe (Arceuthobium douglasii) is a parasitic flowering plant that infects Douglas-fir (Pseudotsuga menziesii) throughout the western United States. Douglas-fir dwarf mistletoe induces the formation of witches' brooms which alters infected host branch biomass and architecture. Little work has quantified the effects of this parasite on branch morphology and biomass. This study obtained pairs of broomed and non-broomed Douglas-fir branches for estimating and comparing the biomass of each branch type. Branch pairs meeting 9 specific criteria were randomly selected from a pool of broomed and non-broomed branches from three mixed conifer study sites on the San Francisco Peaks, Arizona severely infested with Douglas-fir dwarf mistletoe. A total of 17 branch pairs were sampled. Branch pairs were carefully collected, dissected into smaller segments based on branch and twig diameter classes, needles, and loose debris. Branch material was then dried and weighed to determine total biomass and the biomass of specific branch components. Overall, broomed branches had approximately two times the biomass of non-broomed branches. Differences in biomass between broomed and non-broomed branches ranged from as low as 400 g to as high as 1200 g. The greatest increases in biomass between broomed and non-broomed branches were for needles and twigs > 25 mm in diameter. Because of the much greater biomass found in witches' brooms caused by Douglas-fir dwarf mistletoe, particularly needle biomass, brooms provide a structurally diverse habitat for a variety of wildlife species which use brooms for resting, hiding, foraging, and nesting sites.