Arceuthobium abietinum

Arceuthobium abietinum, commonly known as fir dwarf mistletoe and as Arceuthobium campylopodum subsp. abietinum, is a species of dwarf mistletoe. It is native to western North America from Washington to New Mexico to northern Mexico, where it lives in coniferous forests as a parasite on various species of fir, particularly white fir, giant fir, and red fir, and less commonly on species of pine and spruce.

Arceuthobium abietinum
Scientific classification
Kingdom:	Plantae
Clade:	Tracheophytes
Clade:	Angiosperms
Clade:	Eudicots
Order:	Santalales
Family:	Santalaceae
Genus:	Arceuthobium
Species:	A. abietinum
Binomial name
Arceuthobium abietinum Engelm. ex Munz
Synonyms
Arceuthobium campylopodum subsp. abietinum (Engelm.) Nickrent

Description

As with other dwarf mistletoes, A. abietinum is parasitic on conifers, extracting water and nutrients from host trees via tissues called haustoria that grow within the branches of the host. After incubating within the host and collecting resources for a period of two to five years, a network of flabellately-branched stems bearing knobby, scale-like leaves emerge above the bark of the host tree, forming a small shrub.^[1] The stems of most varieties of A. abietinum average about 12 cm in length but range from 3.5 cm to 24.5 cm.^[2]

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.^[1] Fruit are sticky berries, 4-5 mm long, green, and glaucous.^[2] Seed dispersal peaks in late summer to early autumn, when hydrostatic pressure in the fruit propels the seed at an initial velocity of nearly 100 km/h.^[3] The average dispersal distances for A. abietinum ranges from 9.2 meters to 10.7 meters, depending on the variety.^[4] A sticky coating of viscin on the seed helps it attach to its host.^[3]

Ecology

Infection by A. abietinum affects the growth and health of host trees, sometimes inducing witch's brooms and reducing longevity.^[2] 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.^[5]

Plants of A. abietinum serve as a food source for squirrels and hairstreak butterflies in the genus Callophrys, including the thicket hairstreak and Johnson's hairstreak.^[1] The spittlebug Clastoptera distincta, a specialist on dwarf mistletoes, has also been found feeding on A. abietinum in Arizona.^[6]

Taxonomy

Treatments of Arceuthobium abietinum within Section Campylopoda

Species boundaries in Arceuthobium Section Campylopoda are difficult to identify, and authorities disagree in their treatments of A. abietinum. Some regional taxonomic authorities such as the Burke Herbarium, Oregon Flora Project (OFP), and Flora of the Pacific Northwest recognize A. abietinum as a species, as does Plants of the World Online (POWO).^[7][8][9] Others, notably The Jepson Manual and the Flora of North America, treat A. abietinum as a synonym or subspecies of a more broadly-circumscribed Arceuthobium campylopodum.^[10][11]

Among authorities that recognize A. abietinum as a species, treatments of the infraspecific taxonomy also differ. For example, POWO recognizes infraspecific taxa at the varietal level, following work of Tiehm,^[12] whereas OFP recognizes infraspecific taxa at the subspecies level.

Recognized varieties

Since dwarf mistletoes in Section Campylopoda are morphologically similar and often sympatric, host preference is important for identification. The five varieties of A. abietinum are distinguished largely by the following preferences.

• Arceuthobium abietinum var. abietinum,^[13] commonly known as white fir dwarf mistletoe, principally infects the Sierra white fir (Abies concolor subsp. lowiana). In areas where infection rates are high, rare cross-infection to other conifers is possible, including to subalpine fir (Abies lasiocarpa), Sierra lodgepole pine (Pinus contorta subsp. murrayana), sugar pine (Pinus lambertiana), and western white pine (Pinus monticola).^[2] Plants of this variety are yellow or yellow-green.

• Arceuthobium abietinum var. grandae (Kenaley) Tiehm,^[14] commonly known as grand fir dwarf mistletoe, principally infects the grand fir (Abies grandis) and the grandicolor fir (Abies grandis × colcolor). Rare cross-infection is also possible, to Pacific silver fir (Abies amabilis), Engelmann spruce (Picea engelmannii), and sugar pine (Pinus lambertiana).^[2] Plants of this variety are yellow or yellow-green.
• Arceuthobium abietinum var. magnificae (Mathiasen & Kenaley) Tiehm,^[13] commonly known as red fir dwarf mistletoe infects the red fir (Abies magnifica) and is not documented on other host species.^[2] Plants of this variety are yellow-green to greenish-brown.
• Arceuthobium abietinum var. mathiasenii (Kenaley) Tiehm,^[14] commonly known as Mathiasen's dwarf mistletoe, principally infects the Rocky mountain white fir (Abies concolor subsp. concolor) and Durango fir (Abies durangensis). It sometimes cross-infects Mexican spruce (Picea engelmannii subsp. mexicana) and, rarely, Southwestern white pine (Pinus strobiformis).^[2] Plants of this variety are yellow-green to blue-green and highly glaucous.
• Arceuthobium abietinum var. wiensii (Mathiasen & C.M. Daugherty) Tiehm,^[15], commonly known as Wiens' dwarf mistletoe, principally infects the red white fir (Abies magnifica) and Brewer's spruce (Picea breweriana). Occasionally, it cross-infects grandicolor fir (Abies grandis × colcolor) and Western white pine (Pinus monticola).^[2] Plants of this variety differ from other varieties of A. abietinum in color, being distinctively reddish-brown or brownish-green.

Type specimens

The type specimen for A. abietinum was collected by J. G. Lemmon in 1875, growing on Abies concolor subsp. lowiana in the Sierra Valley.^[16] It was formally described by George Engelmann in 1880 as Arceuthobium douglasii var. abietinum before being recombined as A. abietinum by Munz in 1935.^[17][18]

The type specimen for the nominate variety (i.e. A. abietinum var. abietinum) is the same as the type specimen for the species. Types for the other varieties are listed below.

• The type specimen for A. abietinum var. grandae was collected by R.L. Mathiasen in 2019, growing on Abies grandis in Deschutes County, Oregon.^[14] It was formally described in 2020 by Shawn Kenaley as A. abietinum subsp. grandae and later recombined as A. abietinum var. grandae by Arnold Tiehm.^[19]
• The type specimen for A. abietinum var. magnificae was collected by Mathiasen and C.M. Daugherty in 2018, growing on Abies magnifica on Echo Summit in El Dorado County, California.^[20] It was formally described in 2019 by Mathiasen and Kenaley as A. abietinum subsp. magnificae before being recombined as A. abietinum var. magnificae by Tiehm.^[19]
• The type specimen for A. abietinum var. mathiasenii was collected by Mathiasen in 2019, growing on Abies concolor subsp. concolor on the Pink Cliffs of Dixie National Forest in Kane County, Utah.^[14] It was formally described in 2020 by Kenaley as A. abietinum subsp. mathiasenii and later recombined as A. abietinum var. mathiasenii by Arnold Tiehm.^[19]
• The type specimen for A. abietinum subsp. wiensii was collected by Mathiasen and Daugherty in 2008, growing on Picea breweriana on Baldy Mountain in Siskiyou County, California.^[15] It was formally described in 2009 as A. abietinum subsp. wiensii before being recombined as A. abietinum var. wiensii by Tiehm.^[19]

Distribution

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.^[1] 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.^[1]
• Arceuthobium abietinum var. magnificae: Distribution is limited to California, throughout the Sierra Nevada.^[1]
• Arceuthobium abietinum var. mathiasenii: This variety has the southernmost distribution, occurring in southern Nevada, southern Utah, Arizona, and Northern Mexico.^[1] Though the distribution of this variety is fairly broad, it consists of a system of isolated populations.^[2]
• 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.^[1][21] Populations of this mistletoe are isolated rather than continuously distributed.^[2]

Conservation status

NatureServe lists A. abietinum var. mathiasenii as imperiled in Nevada and critically imperiled in Utah.^[22] Though the conservation status of A. abietinum var. wiensii has not been assessed, it is one of the rarest and most poorly-documented mistletoe taxa in North America.^[2]