Douglas-fir (Pseudotsuga menziesii) trees with and without dwarf mistletoe (Arceuthobium douglasii) brooms were examined for evidence of use by wildlife. Evidence of foraging occurred in 51% of the broomed trees and in 29% of the trees without brooms. Evidence of nesting by mammals occurred in 18% of the broomed trees and in none of the trees without brooms. Brooms used and those not used by wildlife were significantly different in type and volume. These findings suggest that retaining mistletoe brooms in stands may be important to provide nesting, foraging, resting and roosting habitat for mammals and birds. West. J. Appl. For. 14(2):100-105.

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Rates of dark respiration were determined for foliated twig tips of branches from Pinuscontorta and for aerial shoots of Arceuthobiumamericanum. Tree samples were obtained from twigs of branches on uninfected trees, twigs from uninfected branches on infected trees, and from uninfected twigs of branches having localized infections. Rates of dark respiration were significantly lower for twigs from infected trees as compared with twigs from uninfected trees. The rate of dark respiration for aerial shoots of A. americanum was significantly higher than the host samples. The biomass of plant parts from infected and uninfected P. contorta was measured. We found that the ratio of relative bole to branch mass changed from 3.8:1 for lightly infected trees to 1.6:1 for heavily infected trees. This apparent shift in mass allocation from stem to branch wood would account for a portion of the reduction in timber volume that is associated with infection by Arceuthobium.

We investigated the importance of lodgepole pine dwarf mistletoe (Arceuthobium americanum Nutt.) in determining the height to crown top (HCT), height to crown base (HCB), and live crown ratio (LCR) of 2025 lodgepole pine (Pinus contorta var. murrayana (Grev. & Balf.) Engelm.) growing over a 24-km2 study site in central Oregon. We compared the effects of infection and associated witches' brooms with those of site topography, soil type, shrub cover, stand density, and the abundance of mature ponderosa pine (Pinus ponderosa Dougl. ex P. Laws. & C. Laws). using multiple regression and path analysis. The density of dominant-size P. contorta was consistently the most important factor influencing HCT, HCB, and LCR across the study site. In dense stands, trees tended to have elevated crown bases due to self-pruning and, hence, lower values of LCR. Dwarf mistletoe and related witches' brooms uniquely explained 6.9% of the variance in LCR, which was close to that of dominant P. contorta (7.1%) and more than that of soil type (3.0%), but explained only 2.6% of the variance in HCB, which was less than that of dominant P. contorta (6.5%) and soil type (4.6%). Regression models suggest that heavily infected trees should be 18% shorter and have crown bases 37% lower than uninfected trees, while moderately infected trees should have an LCR over 20% larger than that of uninfected and heavily infected trees. We also found that the largest 25 heavily infected trees sampled were approximately 19% shorter and 11–13% smaller in diameter than the largest 25 uninfected trees. The results suggest that dwarf mistletoe can be an important factor in determining the crown dimensions of P. contorta but that these effects may be interpreted only in the context of site characteristics and stand structure.

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