Fir Dwarf-Mistletoe

As with other dwarf mistletoes, A. abietinum is a shrub that grows on and parasitizes conifers. For the first few years of the plant's life, it grows tissues called haustoria within the branches of the host; these tissues tap into the host's xylem and phloem to extract water and nutrients, respectively. After accumulating resources for a period of two to five years, the mistletoe grows a network of stems that emerge from the host tree, forming a small shrub. The stems have a fan-shaped branching pattern and average 12 cm in length but range from 3.5 cm to 24.5 cm. While the aerial shoots contain chlorophyll and bear small, scale-shaped leaves, their photosynthetic rate is low, and the mistletoe continues to rely on its host for the vast majority of its carbohydrates. Plants of A. abietinum are dioecious, with male and female mistletoe plants producing spikes of staminate and pistillate flowers, respectively. Male flowers have 3 or 4 petals, average 3 mm in diameter, and bloom in mid- to late summer, usually peaking in August.

Fruit are sticky, oblong berries, 4–5 mm long, green, and glaucous. Seed dispersal peaks in late summer to early autumn, when hydrostatic pressure in the fruit propels the seed at an initial velocity of 27 m/s (89 ft/s). The average dispersal distances for A. abietinum ranges from 9.2 meters to 10.7 meters, depending on the variety. …

A. abietinum has a wide distribution compared to most other dwarf mistletoes, occurring across much of western North America in Washington, Oregon, California, Nevada, Arizona, and Utah. The geographic ranges of its varieties are described below.

Arceuthobium abietinum var. abietinum: Distribution is limited to California, from the southern Cascade Range through the Sierra Nevada with a disjunct population in the San Bernardino Mountains. Arceuthobium abietinum var. grandae: Distribution ranges from southern Washington, through the Cascade Range, and throughout the Siskiyou Mountains and Klamath Mountains of Southern Oregon and Northern California. Arceuthobium abietinum var. magnificae: Distribution is limited to California, throughout the Sierra Nevada. Arceuthobium abietinum var. mathiasenii: This variety has the southernmost distribution, occurring in southern Nevada, southern Utah, Arizona, and Northern Mexico. Though the distribution of this variety is fairly broad, it consists of a system of isolated populations. Arceuthobium abietinum var. wiensii: The distribution of this variety is limited to the Klamath-Siskiyou region in southwestern Oregon and Northwestern California, which has been identified as an epicenter of dwarf mistletoe biodiversity. Populations of this mistletoe are isolated rather than continuously distributed.

Infection by A. abietinum negatively affects the growth and health of host trees, reducing longevity and seed production. A. abietinum also regularly induces abnormal growth patterns in its host, including the production of witch's brooms. While these brooms are typically small, flattened, and nonsystematic, A. abietinum var. wiensii often induces large brooms in Picea breweriana. These brooms increase the availability of nutrients to the mistletoe and provide valuable habitat for wildlife, but they also decrease the vigor of the host tree. Unlike in some other mistletoe-conifer relationships, the brooms that A. abietinum induces in its hosts do not significantly increase the host's susceptibility to fire. On true firs infected by dwarf mistletoes, further infection by the canker fungus Cytospora abietis is common, resulting in flagging and death of infected branches. Plants of A. abietinum serve as a food source for squirrels and hairstreak butterflies in the genus Callophrys, including the thicket hairstreak and possibly Johnson's hairstreak. Because hairstreak larvae take on the color of their food and have a knobby texture, they are camouflaged on mistletoe shoots while feeding. The spittlebug Clastoptera distincta, a specialist on dwarf mistletoes, has also been found feeding on A. abietinum in Arizona.