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Dwarf mistletoes (Arceuthobium spp.) are parasitic plants that are widely distributed in coniferous forests of the northern hemisphere. Because the effects of dwarf mistletoes on their host trees include stunted growth, reduced seed production, and death, these mistletoes may have a substantial influence on forest structure. Studies of the effects of dwarf mistletoe on forest communities have focused primarily on their influence on timber production. We studied the effects of southwestern ponderosa pine dwarf mistletoe (A. vaginatum) on the abundance and diversity of bird communities in central Colorado. Four stands, which ranged in level of mistletoe infestation from none to heavy, were selected at each of two locations. Each stand was surveyed approximately once per week during the avian breeding seasons of 1989 and 1990 by spot mapping. The number of bird registrations and bird species richness were positively correlated with the level of dwarf mistletoe, and this pattern was consistent among 24 of 28 avian species. No species had a significant negative correlation with the level of dwarf mistletoe. The relative abundance of bird species (i.e., evenness) did not differ among stands. The number of cavity‐nesting birds detected also was positively correlated with both dwarf‐mistletoe levels and number of snags. The number of snags and dwarf‐mistletoe levels also were highly correlated. Most snags had been infected as live trees by dwarf mistletoe and the mistletoe probably contributed to their death. While dwarf mistletoe has traditionally been viewed as a forest pest because of reductions in timber volume, we suggest that in areas where management goals are not strictly focused on timber production, control of dwarf mistletoe may not be justified, practical, or even desirable. Our data suggest that dwarf mistletoes may have positive influence on wildlife habitat. Consequently, we suggest that eradication efforts be reconsidered given that dwarf mistletoes have been a part of these forest ecosystems for thousands, and possibly millions, of years.

Summary The reported hosts of Viscum album include more taxa, than any other mistletoe, including Dendrophthoe falcata. From a review of the literature, we identified hosts among 452 species, subspecies, varieties, and hybrids in 96 genera of 44 families. Rosaceae has the most hosts with 128 taxa parasitized. Commonly infested genera include Salix, Populus, Acer, Malus, Crataegus, Prunus, Sorbus, Abies, and Pinus. The host range of V. album increased greatly with introduction of susceptible taxa into its native range and with spread of the mistletoe to new regions with rich floras, such as California.

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From 1965 to 1978, mountain pine beetles killed 25% of the pines taller than 4.5 feet in a study area in north-central Colorado.Average basal area was reduced from 92 to 58 square feet per acre. Mortality increased with tree diameter up to about 9 inches d.b.h. Larger trees appeared to be killed at random. Mortality was directly related to number of trees per acre and presence of dwarf mistletoe, but not to site index, elevation, or percent Douglas-fir in the stand.Many ponderosa pine (Pinus ponderosa var. scopulorum EngeJm.) trees of all diameters, as well as relatively intact patches and stands of trees, survive. A few scattered stands will be almost completely killed. If overstocked stands of relatively large tree diameters are allowed to develop over widespread areas, control programs aimed only at treating infested trees are unsound. Trees must also be thinned to some acceptable stocking level.Silvicultural manipulation to favor nonhost trees, such as Douglas-fir (Pseudotsuga menziesii var. glauca (Beissn.) Franco) over ponderosa pine in order to "bug proof stands, seems of little benefit in an outbreak unless one is willing to convert most of the stand to nonhost trees.Dwarf-mistletoe-infected (Arceuthobium vaginatum subsp. cryptopodum (Engelm.) Hawksw. and Wiens) ponderosa pines are more susceptible to mountain pine beetle attacks than are healthy trees. Consequently, forest management practices aimed at reducing stand risk to bark beetles by thinning stands should simultaneously remove dwarf-mistletoe-infected trees.