Chapter 16—Formal Taxonomy

Weir (1918a) successfully inoculated seedlings of the European Pinus mugo (as P. montana) with Arceuthobium americanum, and this dwarf mistletoe has been found on planted P. sylvestris in Washington (Graham and Leaphart 1961) and in Alberta (Powell 1968).

Distribution: Canada (British Columbia, Alberta, Saskatchewan, Manitoba, and Ontario) (fig. 16.12 and 16.13) and the United States (Washington, Idaho, Montana, Oregon, California, Utah, Wyoming, and Colorado) (fig. 16.14). It probably occurs in Nevada in the vicinity of Lake Tahoe, but no specific locations are known (Kartesz 1988). Arceuthobium americanum has the most extensive distribution of any North American dwarf mistletoe (fig. 16.12). Its latitudinal range of about 2,800 km is exceeded only by A. douglasii. It occurs from Lake Athabasca in northern Saskatchewan south to the southern Sierra Nevada in California (Kern County) and to southern Colorado (Saguache County). Its longitudinal distribution of nearly 2,400 km extends from western Ontario to western British Columbia (Elliot and others 1967, Kuijt 1963, Larsen and Gross 1970, Zalasky 1956). Arceuthobium americanum extends to nearly latitude 60°N. Arceuthobium tsugense in Alaska and Viscum album in Sweden (Wallden 1961) reach similar latitudes, but both of these mistletoes occur in maritime environments that are much less extreme than the continental climate in which A. americanum is distributed.

There is one report of Arceuthobium americanum on Pinus banksiana in the Northwest Territories at about latitude 63°N, about 190 km north of Fort Providence (Moody and Cerezke 1985). However, this has not been confirmed (Y. Hiratsuka, personal communication and our observations). We observed brooming to be common in P. banksiana south of Fort Providence, but the brooms were not caused by dwarf mistletoe, and we suspect the report by Moody and Cerezke (1985) is in error.

The distribution of Arceuthobium americanum is centered within that of its principal host Pinus contorta, particularly varieties latifolia and murrayana. However, variety murrayana in southern California and Baja California is not infected. The latter is about 600 km south of the known range of the parasite in the southern Sierra Nevada.

Arceuthobium americanum rarely occurs within the distribution of Pinus contorta var. contorta (shore pine), but it has been observed on this host in coastal British Columbia (Smith and Wass 1979). As previously mentioned, Weir (1917) reported this variety as a host in the Oregon Mountains in southwestern Oregon, and he indicated it was associated with infected P. jeffreyi and P. attenuata. Although P. contorta var. contorta is common in this area, the only dwarf mistletoe occurring on it is A. siskiyouense, and then only rarely. Finding A. americanum in the Oregon Mountain area would extend the western limit of distribution by approximately 100 km from its known distribution in the Cascade Mountains.

Arceuthobium americanum occurs in outlying populations of Pinus contorta var. latifolia in the Cypress Hills of southeastern Alberta and several isolated mountain ranges in north central Montana (Dooling and Eder 1981)—the Little Rocky Mountains (Phillips County), Bearpaw Mountains (Hill County) (Dooling 1973), and Sweetgrass (Whitlash) Mountains (Liberty County) (Thompson and Kuijt 1976). Dooling and Eder (1981) report that this dwarf mistletoe is known from all the isolated P. contorta populations in central Montana, except those in the Highwood and Snowy Mountains.

In central and northern Alberta, Arceuthobium americanum occurs on Pinus banksiana, and it is common on this host in Saskatchewan and Manitoba. The reports of A. americanum at White Otter Lake in western Ontario (Hord and Quirke 1956) and near Sprague in extreme southeastern Manitoba (Zalasky 1956) are misidentifications based on a rare infection of P. banksiana by A. pusillum (Laut 1967, Sippell and others 1968).

Arceuthobium americanum has been confirmed in Ontario (Larsen and Gross 1970) at the Sioux Lookout area (latitude 50°30' N, longitude 96°30' W), nearly 240 km east of the previously known eastern limits of the species near Belair at the south end of Lake Winnipeg, Manitoba. However, Sioux Lookout was severely burned by a wildfire in 1976, and the mistletoe was apparently eradicated (Myren and Gross 1977). The only other known populations in Manitoba south and east of Belair have also apparently been extirpated—Wallace Lake (latitude 49°50' N, longitude 95°30' W) by wildfire and Milner Ridge (latitude 50°N, longitude 96°10' W) by clearcutting (M. Slivitsky and T. Meyer, personal communication).

Arceuthobium americanum only occurs in the western part of the range of Pinus banksiana, so the spread of the parasite onto this tree has probably been relatively recent. Arceuthobium americanum likely evolved as a principal parasite of P. contorta, then spread to P. banksiana through central Alberta where these two species co-occur and frequently hybridize. Yeatman (1967) suggests that P. banksiana first became parasitized by A. americanum after the Wisconsin Glaciation, but available evidence indicates an earlier association (Zavarin and others 1969).

Arceuthobium americanum has been found on Pinus contorta x banksiana hybrids near Grand Prairie and Peace River, Alberta (Hawksworth and Wiens 1972). Also, A. americanum occurs on P. contorta that was planted in a naturally infested stand of P. banksiana at Prince Albert, Saskatchewan (Hawksworth and Wiens 1972).

In the central Rocky Mountains, Arceuthobium americanum occurs at the lower elevational limits of Pinus contorta, but its upper limits are usually about 200 m below the upper limit of stands dominated by P. contorta (Hawksworth 1956b and see fig. 7.2).

Arceuthobium americanum distribution maps have been published for western Canada (Kuijt 1963), Alberta (Baranyay 1970), British Columbia and adjacent southwestern Alberta (Baranyay 1975), British Columbia (Wood 1986), Manitoba and Saskatchewan (Zalasky 1956), Montana (Dooling and Eder 1981), Utah (Albee and others 1988), Colorado (Hawksworth 1987c), and California (Kuijt 1960a).

This dwarf mistletoe varies in elevation from 200 m near Lake Athabasca in northern Alberta and Saskatchewan to 3,350 m in central Colorado.

Discussion: Gill (1935) discusses the typification problem in Arceuthobium americanum. Engelmann believed that the specimen he was describing was collected by Nuttall, and it was designated as the type (Oregon, on Pinus). Gill states, however, that this specimen was actually collected by Douglas in the Blue Mountains of Oregon; we have designated this latter collection as the type specimen.

Despite the extensive geographic distribution of Arceuthobium americanum, we find no criteria for subspecific division. In general, the plants are larger in the Cascade Mountains of Oregon and in northern Idaho than elsewhere, but this is probably associated with the more vigorous host growth in these areas. Arceuthobium americanum induces characteristic systemic witches’ brooms (fig. 16.15) on Pinus contorta (Kuijt 1960b) and produces the same type of broom formations on P. ponderosa (Hawksworth 1956a, Weir 1916c). The witches’ brooms formed on Picea engelmannii, however, are non-systemic (Hawksworth and Graham 1963). Kuijt (1960a) noted that A. americanum cannot perpetuate itself for long periods of time on Pinus jeffreyi or P. ponderosa var. ponderosa in California. In northern Colorado and southern Wyoming, however, the parasite is aggressive in pure stands of P. ponderosa var. scopulorum, some of which are several kilometers from the nearest infection on P. contorta. Most areas where A. americanum occurs in pure stands of P. ponderosa are outside the range of A. vaginatum subsp. cryptopodum, which is the typical parasite on P. ponderosa in the Rocky Mountains (Hawksworth 1968b, 1969).

4. A. apachecum Hawksworth & Wiens, Brittonia 22: 266, 1970. TYPE COLLECTION: ARIZONA: Pima County: Santa Catalina Mountains near the summit of Mt. Lemmon at 2,800 m, on Pinus strobiformis, Hawksworth, Lightle, & Gilbertson 1110, September 13, 1968. (Holotype US! Isotypes: ARIZ, COLO, DS, FPF, MO, RM, UC, UT.)

Description: Mean shoot height 3–4 (max. 9) cm. Shoots yellow, green, or reddish, flabellately branched and densely clustered (fig. 16.16). Basal diameter of dominant shoots 1–2 (mean 1.8) mm. Third internode 5–10 (mean 7.2 ± 2.0) mm long, 1–2 (mean 1.5) mm wide (12 collections), length/width ratio 4.8:1. Flowers axillary. Staminate flowers 2.7 mm across; perianth, 3- to 4-merous; same color as shoots; segments ca. 1.3 mm long, 0.9 mm wide. Mean anther diameter 0.5 mm, centered 0.7 mm from tip of segment. Pollen polar diameter 16–23 (mean 19) µm; equatorial diameter 18–23 (mean 21) µm; polar/equatorial diameter ratio 1:1.11; spine height (1.5 µm), slightly greater than the wall thickness (ca. 1.1 µm) (4 collections). Mature fruit 4 mm long, long 2.5 mm wide; proximal portion ca. 2.5 mm long. Seeds 2.8 x 1.2 mm. n = 14.

Phenology: Meiosis in July. Anthesis from late July to mid-September, with a peak in mid-August (Mathiasen 1982) (fig. 16.17). Fruits mature from mid-August to mid-October with a peak in September (Mathiasen 1982); maturation period averages about 13 months.

Host: Known only on Pinus strobiformis, the only member of subgenus Haploxylon (except pinyons) that occurs within its distribution.

Distribution: United States (Arizona and New Mexico) and Mexico (Coahuila) (fig. 16.18). This dwarf mistletoe has a limited distribution in southern Arizona and central New Mexico, with an outlier in the Sierra del Carmen in northern Coahuila, Mexico. In Arizona, it occurs in the White, Pinaleno, Santa Catalina, Santa Rita, and Chiricahua Mountains and in New Mexico in the Mangas, San Mateo, Magdalena, and Capitan Mountains. The report of a dwarf mistletoe on Pinus strobiformis on Mt. Livermore in the Davis Mountains of west Texas (Powell 1988) requires confirmation. The specimen on which the report was based (Coleman s. n. in 1936 at SRSC) is Phoradendron juniperinum, but there is no host material with the collection, and this mistletoe is not known to parasitize pines. A distribution map for Arceuthobium apachecum in New Mexico was published by Martin and Hutchins (1980). Elevational range is 2,000–3,000 m.

Discussion: We consider Gill’s Arceuthobium campylopodum f. blumeri to comprise 4 allopatric species—A. apachecum, A. blumeri, A. californicum, and A. monticola, all of which parasitize pines of subgenus Haploxylon. These species differ in a number of characteristics including morphology, hosts, phenology, and distribution. Our previous numerical analyses support the classification of these population systems as species (Hawksworth and Wiens 1972), although they may exhibit superficial similarities.

The exclusive occurrence of two dwarf mistletoes (Arceuthobium apachecum and A. blumeri) on a single host species (Pinus strobiformis) is unique in Arceuthobium. Originally, we suspected these taxa might represent a single, variable species. However, the occurrence of geographically consistent morphological differences (table 16.1) indicated that separate taxonomic status was warranted. This conclusion has been confirmed by Mathiasen (1982). The populations of A. apachecum in the Santa Rita Mountains tend to have taller shoots (up to 9 cm) than do those in other portions of its distribution.

When grown under common conditions in a greenhouse at Fort Collins, Colorado, the two species maintained morphological integrity. They are not sympatric, but they do approach to within about 60 km of each other in southern Arizona. Arceuthobium apachecum is abundant north and east of the Santa Rita and Chiricahua Mountains; A. blumeri occurs from the Huachuca Mountains south into Mexico. Unlike A. blumeri, A. apachecum frequently induces witches’ brooms (fig. 16.19).

5. A. aureum Hawksworth & Wiens, Brittonia 29: 414, 1977. TYPE COLLECTION: GUATEMALA: Alta Verapaz, 5 km west of San Cristóbal Verapaz, on Route 7W to Huehuetenango, on Pinus pseudostrobus, Hawksworth, Wiens, & Player 1596, in 1975 (Holotype US! Isotypes: ENCB, FPF, INIF, MO, UNAM, UT).

Description: Shoot height 12–40 cm yellow to golden brown, flabellately branched. Basal diameter of dominant shoots 3–24 mm. Third internode 16–35 mm long, 2–8 mm wide. Staminate flowers ca. 3 mm across, 3- or 4-merous. Mature fruits 4–5 mm long, 2.5 mm wide. Parasitic on pines of subgenus Diploxylon. n = 14.

Discussion: This taxon was originally considered to be part of the Arceuthobium globosum complex (Hawksworth and Wiens 1972), but further field studies in Mexico and Guatemala have shown that it is a distinct species (Hawksworth and Wiens 1977, see discussion under A. globosum). Arceuthobium aureum is comprised of two subspecies.

Key to the subspecies:

5a. A. aureum Hawksworth & Wiens subsp. aureum.

Description: Shoots 12–30 (mean 16) cm tall, golden, flabellately branched (fig. 16.20). Basal diameter of dominant shoots 3–14 (mean 5) mm. Third internode 16–30 (mean 20) mm long, 2–4 (mean 3) mm wide. Mature fruits ca. 4 mm long, lightly glaucous; pedicel ca. 1.5 mm long. n = 14.

Phenology: Anthesis and fruit maturity apparently continuous throughout the year, or at least in the dry season.

Hosts: Pinus montezumae, P. oaxacana, and P. pseudostrobus.

Distribution: Guatemala (fig. 16.21). A common taxon of the low-elevation pine forests in Guatemala. Elevational range is 900–2,000 m.

Discussion: This dwarf mistletoe was formerly considered to be part of the Arceuthobium globosum complex (Hawksworth and Wiens 1977) and is discussed under that species in greater detail.

5b. A. aureum Hawksworth & Wiens subsp. petersonii Hawksworth & Wiens, Brittonia 29: 415, 1977. TYPE COLLECTION: MEXICO: Chiapas, near km-136 (52 km southeast of San Cristóbal de las Casas) on Panamerican Highway, 2,250 m, on Pinus pseudostrobus, Hawksworth, Wiens, & Player 1598 in 1975 (Holotype US! Isotypes: ENCB, FPF, MO, UNAM, UT).

Description: Shoots 14–40 (mean 24) cm tall, golden to yellow-brown, flabellately branched. Basal diameter of dominant shoots 14–35 (mean 23) mm. Third internode 14–35 (mean 23) mm long, 2.5–8 (mean 5) mm wide. n = 14.

Phenology: Anthesis in September. Fruits mature June–July; maturation period of ca. 9–10 months. This is several months less than is common for many dwarf mistletoes.

Hosts: Pinus michoacana, P. montezumae, P. oaxacana, P. oocarpa, P. patula, and P. pseudostrobus are the principal hosts. Pinus michoacana is somewhat less susceptible and is infected only when it grows in association with the other principal hosts.

Distribution: Mexico (Oaxaca and Chiapas) (fig. 16.21). This dwarf mistletoe is common in Chiapas between San Cristóbal de las Casas and Teopisca. Its distribution in Oaxaca is poorly known, but there are a number of collections from the Miahuátlan–Suchixtepec area. Elevational range is 2,200–2,450 m.

Discussion: This subspecies is characterized primarily by its tall, slender, brown to golden shoots, slightly longer fruit (5 mm), long pedicels (4 mm), and general tendency to form witches’ brooms (fig. 16.22). The subspecies is named for R. S. Peterson, who first discovered these populations.

6. A. bicarinatum Urban, Symbolae Antillanae 7: 204, 1912. TYPE COLLECTION: DOMINICAN REPUBLIC: Constanza, on Pinus occidentalis, von Turckheim 3241 in 1910 (Holotype Z! Isotypes: BM, F, ILL, K, MO, NY, S, US). Razoumofskya bicarinata (Urban) Tubeuf, Naturwissenschaftliche Zeitschrift für Forst -und Landwirtschaft 17: 195, 1919.

Description: Mean shoot height ca. 10 (max. 17) cm, dark brownish red, terminal branches of living plants conspicuously glaucous (character lost after drying), shoots frequently twisted when dry, flabellately branched (fig. 16.23). Basal diameter of dominant shoots 2–4 (mean 3) mm. Third internode 6–14 (mean 10.5 ± 2.1) mm long, 1.5–4.0 (mean 2.0) mm wide (6 collections), length/width ratio 5.2:1. Staminate flowers ca. 3 mm across; perianth 3- or 4-merous, reddish, segments ca. 1.5 mm long, 1.0 mm wide, apex acute. Mean anther diameter 0.5 mm, centered 1.0 mm from tip of segment. Pollen polar diameter 21–25 (mean 23) µm; equatorial diameter 25–28 (mean 26) µm; polar/equatorial diameter ratio 1:1.14; spine height (ca. 3 µm) 3 x wall thickness (ca. 1 µm) (3 collections). Mature fruits ca. 4 x 2 mm; proximal portion ca. 2.5 mm long. Seeds 2.5 x 1.2 mm. n = 14.

Phenology: Meiosis in August. Anthesis usually in September. Fruits mature in late August or September of the year following pollination; maturation period averages ca. 12 months.

Hosts: Known only on Pinus occidentalis, the only native pine within its distribution.

Distribution: Dominican Republic and Haiti (fig. 16.24). Arceuthobium bicarinatum is common on the island of Hispaniola. In Haiti, this dwarf mistletoe is common in the Morne la Selle and the Morne des Commissaires, in the southeastern part of the country. In the Dominican Republic, A. bicarinatum occurs on all three major mountain ranges where Pinus occidentalis is present—Cordillera Central, Sierra de Neiba, and Sierra de Bahoruco. More than half of the pine forests in the Dominican Republic are apparently infested by A. bicarinatum, and damage is particularly severe in Santiago and San Juan Provinces (Etheridge 1971). We commonly observed A. bicarinatum in La Vega Province between 1,000–2,600 m. Etheridge (1971) gives a distribution map for the species in the Dominican Republic. Thomas Zanoni of the Jardin Botánico Nacional, Santo Domingo, provided additional information on the distribution of this dwarf mistletoe in the Dominican Republic. Elevational range is 800–2,800 m (Darrow and Zanoni 1991, Etheridge 1971).

Discussion: This highly distinctive species is restricted to Hispaniola. Characteristic features are the dark brownish-red shoot color, sterile nodes near the tips of the pistillate shoots, and glaucous terminal internodes. Internodes on dried specimens often show a twist of about a quarter turn, but this trait is not discernible in living plants.

The witches’ brooms caused by Arceuthobium bicarinatum are mostly non-systemic (as illustrated by Marie-Victorin 1943). Near Valle Nuevo in the Cordillera Central, however, some witches’ brooms were anomalous. These brooms appeared to be systemic (fig. 16.25), but dwarf mistletoe shoots were not regularly produced on the infected branches, as is normally the case for systemic infections in other host–parasite combinations. Detailed studies are needed on the unusual witches’ brooms induced by A. bicarinatum.

This is a serious parasite of Pinus occidentalis in the Dominican Republic, where from 15–85% of the trees in the northern Cordillera Central are infected (Darrow and Zanoni 1991). At higher elevations in Haiti and the Dominican Republic, a leafy mistletoe (Dendropemon pycnophyllus, Loranthaceae) is also common on P. occidentalis and frequently occurs on trees that are also infected by Arceuthobium bicarinatum.

7. A. blumeri A. Nelson, Botanical Gazette 56: 65, 1913. TYPE COLLECTION: ARIZONA: Cochise County: Huachuca Mountains, on Pinus strobiformis, Blumer in October 1910 (Holotype RM No. 78604!). Razoumofskya blumeri Standley, Proceeding Biological Society Washington 29: 86, 1916. A. campylopodum Engelm. forma blumeri (Engelm.) Gill, Transactions Connecticut Academy Arts and Sciences 32: 207, 1935 (in part).

Description: Mean shoot height ca. 6–7 (max. 18) cm, gray to straw or light green, flabellately branched (fig. 16.26). Basal diameter of dominant shoots 1–3 (mean 2.1) mm. Third internode 5–14 (mean 9.1 ± 2.5) mm long, 1–2 (mean 1.6) mm wide (8 collections), length/width ratio 5.5:1. Staminate flowers ca. 2.5 mm long, 2.5–3.0 mm across; perianth 3- to 6-merous (mostly 3- or 4-merous), segments ca. 1.3 mm long, 1.0 mm wide, apex acute. Mean anther diameter 0.6 mm, centered 0.4 mm from tip of segment. Pollen polar diameter ca. 19 µm and an equatorial diameter ca. 20 µm; spine height and wall thickness each ca. 1 µm (1 collection, few grains available). Mature fruit ca. 4 x 2.5 mm, proximal portion ca. 2.5 mm long. Seeds 2.7 x 1.0 mm. n = 14.

Phenology: Meiosis in July. Anthesis from mid-July to late-August, with a peak in early August (Mathiasen 1982). Fruits mature from late August to early October, with a peak in mid-September (Mathiasen 1982); maturation period averages 13–14 months.

Hosts: The host affinities of Arceuthobium blumeri are not clear because of the taxonomic confusion surrounding Pinus strobiformis and the closely related P. ayacahuite var. brachyptera in the Sierra Madre Occidental, Mexico (Eguiluz 1991, Hawksworth 1991b, Perry 1991). However, most host populations of this dwarf mistletoe in the Sierra Madre Occidental are best referred to the latter taxon. On Cerro Potosí in Nuevo León, it parasitizes a different white pine that has been called "Pinus flexilis" (Martínez 1948, Perry 1991), but is perhaps best classified as P. strobiformis var. potosiensis (Silba 1990).

Distribution: United States (Arizona) and Mexico (Sonora, Chihuahua, Durango, Nuevo León, and Coahuila) (fig. 16.27). This dwarf mistletoe extends southward from the Huachuca Mountains in southern Arizona through the Sierra Madre Occidental in Chihuahua and Sonora to southern Durango. In the Sierra Madre Oriental it is known only from Cerro Potosí, Nuevo León, and San Antonio de las Alazanas, Coahuila (Cibrián and others 1980), but it probably occurs elsewhere over this extensive distribution. Elevational range is 2,150–3,250 m.

Discussion: The parasitism of Arceuthobium blumeri and A. apachecum on Pinus strobiformis is discussed under A. apachecum and the two species are compared in table 16.1.

Mathiasen (1982) studied 13 populations in 3 areas of Arizona, Chihuahua, and Durango and found that the perianth segments of the staminate flowers were distributed as follows: 3 segments, 42%; 4 segments, 52%; 5 segments, 6%, and 6 segments, trace. The northern populations (Arizona) had the highest number of segments per flower: 3 segments, 30%; 4 segments, 53%; and 5 segments, 16%. The Durango population had the lowest: 3 segments, 55%; 4 segments, 44%; and 5 segments, 1%.

Distinctive features of Arceuthobium blumeri include its gray-colored shoots, 4- to 6-merous staminate flowers, and rare formation of witches’ brooms. The populations in the southern portions of its distribution in Durango tend to have taller shoots (up to 18 cm).

8. A. californicum Hawksworth & Wiens, Brittonia 22: 266, 1970. TYPE COLLECTION: CALIFORNIA: Mariposa County, Fish Camp 0.4 km west of State Route 41 on Summit Camp Road, on Pinus lambertiana, Hawksworth & Hawksworth 1147, November 6, 1968 (Holotype US! Isotypes: ARIZ, COLO, DS, FPF, MO, RM, UC, UT). A. campylopodum Engelm. var. cryptopodum (Engelm.) Jepson, Manual of Flowering Plants of California 284. 1925.

Description: Mean shoot height ca. 8 cm (max. 12) cm, greenish to bright yellow, turning brown at base of older shoots, flabellately branched (fig. 16.28). Basal diameter of dominant shoots 1.5–4.0 (mean 2) mm. Third internode 6–16 (mean 10.5 ± 2.9) mm long, 1–2 (mean 1.5) mm wide (8 collections), length/width ratio 7.0:1. Flowers axillary. Staminate flowers 3.3 mm across; perianth 3- or 4-merous, segments ca. 1.5 mm long, 1.1 mm wide. Mean anther diameter 0.5 mm, centered 0.7 mm from tip of segment. Pollen polar diameter 18–23 (mean 20) µm; equatorial diameter 19–25 (mean 23) µm; polar/equatorial diameter ratio 1:1.12; spine height (ca. 1.6 µm), approximately 2 x wall thickness (ca. 0.8 µm) (4 collections). Mature fruit 4 x 2.5 mm; proximal portion ca. 2.0 mm long. Seeds 3.2 x 1.2 mm. n = 14.

Phenology: Meiosis in July. Anthesis usually in mid-July to mid-August, with extremes from early July to late August (fig. 16.29). Fruits mature from mid-September to mid-October, with extremes from late August to early November; maturation period averages ca. 13–14 months.

Hosts: The only principal host is sugar pine, Pinus lambertiana, but P. monticola is sometimes parasitized when it grows in association with infected P. lambertiana. This dwarf mistletoe induces large, compact witches’ brooms in P. lambertiana (fig. 16.30).

Distribution: United States (California) (fig. 16.31). This species is distributed from Mt. Shasta southward through the North Coast Range and through the Cascade Range south to Lake County and the west side of the Sierra Nevada, ultimately reaching the Cuayamaca Mountains in San Diego County. Elevational range is 600–2,000 m.

Discussion: Jepson’s (1925) variety cryptopodum, which was intended to include parasites of Pinus lambertiana and Abies grandis, is based on a misapplied name, Arceuthobium cryptopodum Engelm., a Rocky Mountain parasite of P. ponderosa. As discussed under Arceuthobium apachecum, Gill’s A. campylopodum f. blumeri includes four species—A. apachecum, A. blumeri, A. californicum, and A. monticola. A comparison of A. californicum and A. monticola is presented in table 16.2.

Concerning this dwarf mistletoe, Gill (1935) commented that "infected trees are infrequent and were found only in the presence of other infected species." Our studies do not support this observation. Arceuthobium californicum is common in many areas, rarely infects other species, and is a serious pathogen of Pinus lambertiana in California (Scharpf and Hawksworth 1968). Surveys found the parasite on 22% of 274 P. lambertiana plots in California (Anonymous 1968). In many instances A. californicum is the only dwarf mistletoe present in a stand, but it is sympatric with A. campylopodum or A. abietinum (this may have been the basis for Gill’s statement).

Arceuthobium cyanocarpum did not parasitize Pinus lambertiana in the San Jacinto Mountains of southern California, the only instance we know of where P. lambertiana has been exposed to this dwarf mistletoe. These two dwarf mistletoes are morphologically distinct, and each is restricted to its respective host.

9. A. campylopodum Engelmann in Gray, Boston Journal Natural History 6: 214, 1850. TYPE COLLECTION: "Oregon," on Pinus ponderosa, Geyer 577 in 1843 (Lectotype GH! Isotype US). A. campylopodum Engelm. var. macrarthron Engelm., loc. cit. Razoumofskya campylopoda (Engelm.) Kuntze, Revision of Genera of Plants 2: 587, 1891. A. campylopodum Engelm. forma typicum Gill, Transactions Connecticut Academy Arts and Sciences 32: 185, 1935 (in part).

Description: Mean shoot height ca. 8 (max. 13) cm, olive green to yellow, flabellately branched (fig. 16.32). Staminate plants brownish, and pistillate plants greenish (particularly in northern populations). Basal diameter of dominant shoots 1.5–5.0 (mean 3) mm. Third internode 7–22 (mean 11.3 ± 3.8) mm long, 1.5–2.5 (mean 2.0) mm wide (27 collections), length/ width ratio 5.6:1. Staminate flowers 3.0 mm across; perianth 3-merous (occasionally 4-merous), segments ca. 1.4 mm long, 1.0 mm wide. Mean anther diameter 0.5 mm, centered 0.8 mm from tip of segment. Pollen polar diameter 18–25 (mean 20) µm; equatorial diameter 20–27 (mean 24) µm; polar/equatorial diameter ratio 1:1.18; spine height (ca. 2.2 µm) about 2.5 times wall thickness (ca. 0.8 µm) (5 collections). Mature fruit 5.0 x 3.0 mm. n = 14.

Phenology: Meiosis in July. Peak anthesis usually from mid-August to early October, with extremes from early August to late October (fig. 16.33). Fruits usually mature from early September to mid-November, with extremes from late August to late November; maturation period averages ca. 13 months.

Hosts: The principal and most commonly infected hosts are Pinus ponderosa var. ponderosa and P. jeffreyi. According to Wagener (1965) and R. F. Scharpf (personal communication), P. jeffreyi is somewhat more susceptible than P. ponderosa, but both species incur considerable damage and are classed here as principal hosts. Other trees frequently infected, particularly when associated with the above hosts, are P. coulteri and P. attenuata. The three races of variety ponderosa recognized by Conkle and Critchfield (1988)—North Plateau, Pacific, and Southern California—appear to be about equally susceptible. In the Spring (Charleston) Mountains of southern Nevada, P. ponderosa var. scopulorum is a common host, but this is the only known area where Arceuthobium campylopodum occurs naturally within the range of variety scopulorum. In Oregon, Roth (1967) inoculated varieties scopulorum and ponderosa and found them to be about equally susceptible to A. campylopodum. Occasional hosts for A. campylopodum are P. contorta vars. latifolia and murrayana. T. W. Childs (personal communication) informed us that P. lambertiana is very rarely infected by A. campylopodum in the vicinity of Hammer Butte in northern Klamath County, Oregon.

In certain areas in the southern Sierra Nevada where Pinus sabiniana occurs above the elevational range of Arceuthobium occidentale, A. campylopodum occasionally parasitizes P. sabiniana associated with infected P. ponderosa or P. jeffreyi. However, little or no cross-infection occurs in areas where the two dwarf mistletoes occur sympatrically (chapter 6 and 14).

Weir (1918a) successfully inoculated seedlings of Pinus sylvestris, P. mugo (as P. montana), P. resinosa, Picea abies, Abies concolor, A. grandis, and Larix occidentalis with Arceuthobium campylopodum. The latter three species are commonly associated with Pinus ponderosa when it is infected by A. campylopodum, but we have never observed the parasite on any of them. Kuijt (1960b) found the European Pinus pinaster infected naturally at the Institute of Forest Genetics Arboretum at Placerville, California, and J. R. Weir collected A. campylopodum on planted P. sylvestris near Hayden Lake, Idaho.

Pinus washoensis is probably susceptible to Arceuthobium campylopodum, but we know of no collections on this host. We found no infections on the extensive populations of this tree in the vicinity of Patterson Guard Station in the southern end of the Warner Mountains, Modoc County, California. Similarly, R. S. Peterson (personal communication) found no dwarf mistletoe in the type locality for P. washoensis on Mt. Rose, near Reno, Nevada.

Distribution: United States (Washington, Idaho, Oregon, California, and Nevada) and Mexico (Baja California Norte) (fig. 16.34). Arceuthobium campylopodum occurs from northern Washington and eastern Idaho, south through Oregon and California (but it is not known in the southern Coast Range) to the Sierra Juárez and Sierra de San Pedro Mártir in Baja California, Mexico. In Nevada, it occurs in the vicinity of Lake Tahoe and in the Spring (Charleston) Mountains of Clark County. Arceuthobium campylopodum is not known from British Columbia, but it occurs within about 30 km of the international boundary near Kettle Falls, Washington. Gill’s (1935) report of this species in western Montana is based on occasional parasitism of Pinus ponderosa by the larch dwarf mistletoe, A. laricis.

Arceuthobium campylopodum is distributed by elevation from 30 m along the Columbia River, near Hood River, Oregon, to 2,500 m in the Spring (Charleston) Mountains of southern Nevada.

Discussion: The type locality is listed as "Oregon," although Piper (1906) comments that the specimen must have come from northern Idaho or northeastern Washington, because Geyer did not collect in the area that is now Oregon.

G. G. Hedgcock (unpublished manuscript dated 1914) noted differences between Arceuthobium campylopodum and A. occidentale and suggested that they warranted separate taxonomic status. Gill’s (1935) A. campylopodum f. typicum is here considered to comprise four species—A. campylopodum (sensu stricto), A. occidentale, A. siskiyouense, and A. littorum (Hawksworth and others 1992b). These are separable on the basis of phenology, morphology, geographic distribution, and host preference.

Arceuthobium campylopodum is a serious pathogen of Pinus jeffreyi and P. ponderosa (fig. 16.35). Our observations suggest that host damage is more severe in the southern parts of the distribution; the most severely infested stands occur in the Laguna Mountains in San Diego County, California. In the northern Sierra Nevada, pathogenic effects are more severe on the drier east-side forests than along the wetter west-side forests. Roth (1966) reports that drooping-needled races of P. ponderosa in Oregon are less frequently infected by the dwarf mistletoe than are populations with typical needles, but grafts from these trees showed no evidence of resistance (Roth 1974a and chapter 13). Recently, Scharpf (1987) has shown that some populations of P. jeffreyi show high resistance to infection by A. campylopodum.

Arceuthobium cyanocarpum
Limber Pine Dwarf Mistletoe

10. A. cyanocarpum (A. Nelson ex Rydberg) Coulter & Nelson, New Manual of Botany of the Central Rocky Mountains, p. 146, 1909. TYPE COLLECTION: WYOMING: Carbon County: Ferris Mountains, on Pinus flexilis, Nelson 4959 in 1898 (Lectotype RM! Isotypes: NY, MO). Razoumofskya cyanocarpa A. Nelson in Rydberg, Colorado Agricultural Experiment Station Bulletin 100: 101, 1906. A. cyanocarpum (A. Nels.) Abrams, Illustrated Flora Pacific Coast States I: 531, 1923. A. campylopodum Engelm. forma cyanocarpum (A. Nels.) Gill, Transactions Connecticut Academy Arts and Sciences 32: 204, 1935.

Description: Mean shoot height ca. 3 (max. 7) cm, yellow green, flabellately branched, densely clustered (fig. 16.36). Basal diameter of dominant shoots 1–2 (mean 1.4) mm. Third internode 2–14 (mean 5.2 ± 2.0) mm long, 1.0–1.5 (mean 1.1) mm wide (15 collections); length/width ratio 4.7:1. Staminate flowers 3.0 mm across; perianth 3-merous (rarely 4-merous), same color as shoots; segments ca. 1.4 mm long, 1.0 mm wide, apex acute. Mean anther diameter 0.4 mm, centered 0.7 mm from tip of segment. Pollen polar diameter 15–21 (mean 19) µm; equatorial diameter 20–25 (mean 22) µm; polar/equatorial diameter ratio 1:1.19; spine height (ca. 1.5 µm) approximately 2 x wall thickness (ca. 0.8 µm) (6 collections). Mature fruit 3.5 x 2.0 mm; proximal portion ca. 2.0 mm long. Seeds 2.0 x 0.9 mm. n = 14.

Phenology: Meiosis in July. Peak anthesis from mid-July to early September, with extremes from early July to mid-September (fig. 16.37). Fruits mature from mid-August to late September; maturation averages ca. 12 months. Seed germination mostly in June.

Hosts: Pinus flexilis is the most common host of this dwarf mistletoe throughout its extensive geographical distribution. However, P. aristata, P. longaeva, and P. albicaulis are also classed as principal hosts because, even though they are not as common within the range of Arceuthobium cyanocarpum, they appear to be about as susceptible as P. flexilis. Infection of P. aristata is known from only an area near La Veta Pass in southern Colorado where this mistletoe is also common on associated P. flexilis. Pinus longaeva is parasitized in many areas in Utah and Nevada, and P. albicaulis is infected in western Wyoming, northern Nevada, central Oregon, and northern California.

Pinus monticola in northern California and Tsuga mertensiana in central Oregon are secondary hosts of this dwarf mistletoe (Mathiasen and Hawksworth 1988). Pinus balfouriana subsp. balfouriana is an infrequently but severely infected host in northern California (Miller and Bynum 1965, Mathiasen and Hawksworth 1988). No dwarf mistletoe has been found on the southern subspecies of foxtail pine, P. balfouriana subsp. austrina, in the southern Sierra Nevada in California. Pinus ponderosa var. scopulorum and P. contorta var. latifolia are rare hosts in the Rocky Mountains (Hawksworth and Peterson 1959). Von Tubeuf (1919) mentions a single collection of dwarf mistletoe on Picea engelmannii from Pikes Peak, Colorado, which Hawksworth and Graham (1963) surmised was Arceuthobium cyanocarpum.

Greenhouse inoculations of the seedlings of both Pinus strobus and P. strobiformis show that they are also susceptible to infection by Arceuthobium cyanocarpum (Hawksworth and Wiens 1972).

Distribution: United States (Idaho, Montana, Oregon, California, Nevada, Utah, Wyoming, and Colorado) (fig. 16.38). This dwarf mistletoe occurs from southern Montana (Brown 1970) and northern Wyoming south to southern Colorado and west to Oregon and California where it occurs on the east side of the Sierra Nevada, in the Panamint Mountains in Death Valley National Monument (Johnson 1976) and in the San Bernardino and San Jacinto Mountains of southern California.

Arceuthobium cyanocarpum occurs on many mountain ranges in Wyoming and Utah, including an isolated locality in the La Sal Mountains, near the Utah–Colorado border. Arceuthobium cyanocarpum is common in northern Colorado from near Idaho Springs to the Wyoming border, and it occurs in three other isolated localities in Colorado—Pikes Peak, Sangre de Cristo Mountains near Coaldale, and Silver Mountain near La Veta Pass. Arceuthobium cyanocarpum is presently known from only four localities in Idaho. The species is widely distributed in the mountain ranges of Nevada on Pinus flexilis, P. longaeva, and P. albicaulis (Mathiasen and Hawksworth 1990). The specimens listed by Gill (1935) as A. campylopodum f. cyanocarpum on P. albicaulis from the Crater Lake region of Oregon are referable to A. tsugense subsp. mertensianae. The only verified collection of A. cyanocarpum in Oregon is at Three Creek Lake, southwest of Sisters, on P. albicaulis and Tsuga mertensiana (Knutson and Tinnin 1981). We have tentatively listed a collection on P. albicaulis from "Obsidian Cliff," Oregon, as this species, but we have not been able to locate the site. We previously identified the dwarf mistletoe on P. aristata on the San Francisco Peaks, Arizona, as A. cyanocarpum (Hawksworth and Wiens 1972). That population, however, has since been transferred to A. microcarpum (Crawford and Hawksworth 1979, Mathiasen and Hawksworth 1980). Arceuthobium cyanocarpum apparently does not occur in Arizona.

Weir collected Arceuthobium cyanocarpum in 1913 and 1917 on Pinus flexilis in the vicinity of Anaconda, Montana. Several attempts to locate these populations were unsuccessful. Possibly sulfur dioxide fumes have resulted in its extirpation in this area. Scheffer and Hedgcock (1955), who studied the area in the late 1920’s, noted that P. flexilis and A. cyanocarpum were relatively rare in the fume-damaged area.

Distribution maps for Arceuthobium cyanocarpum are available for Colorado (Hawksworth 1987c) and Nevada (Mathiasen and Hawksworth 1990). Elevational range is 1,600 m in southern Montana to nearly 3,050 m in central Colorado.

Discussion: Although Rydberg (1906) stated that he described no new species in that publication, Tiehm (1989) concluded that he did, in fact, validly describe several new species, including Razoumofskya cyanocarpa. We concur with Tiehm and accept Rydberg (1906) as having provided the first valid description of the species.

This dwarf mistletoe, which characteristically infects Pinus flexilis and associated high-altitude white pines, is easily recognized by its small, densely clustered shoots, especially the staminate plants (fig. 16.36). In many cases shoot mortality is very high (the cause is unknown but is probably associated with a secondary fungal pathogen).

Arceuthobium cyanocarpum causes heavy mortality in Pinus flexilis in many areas in the Rocky Mountains and in P. albicaulis on the north slopes of Mt. Shasta, California. In the latter locality, the high mortality caused by the dwarf mistletoe has resulted in large areas of "ghost forests" (Cooke 1955, Mathiasen and Hawksworth 1988).

Witches’ brooms induced by Arceuthobium cyanocarpum are typically small and compact (fig. 16.39). In many instances, however, infection is general throughout the crown of the tree.

11. A. divaricatum Engelmann in U.S. Geographical Survey West of 100th Meridian (Wheeler Report) 6: 253, 1878. TYPE COLLECTION: ARIZONA: County unknown, Salt River Valley, on Pinus edulis, Gilbert 116 in 1873 (Lectotype MO! Isotype US). A. gracile Engelmann in Gray, Memoires American Academy N.S. 4: 59, 1849, nomen nudum. Razoumofskya divaricata Coville, Contributions U.S. National Herbarium 4: 192, 1892. A. campylopodum Engelmann forma divaricatum (Engelm.) Gill, Transactions Connecticut Academy Arts and Sciences 32: 193, 1935.

Description: Mean shoot height ca. 8 (max. 13) cm, olive green to brown, flabellately branched (fig. 16.40). Basal diameter of dominant shoots 1.5–4.0 (mean 2) mm. Third internode 6–15 (mean 9.8 ± 2.4) mm long, 1–2 (mean 1.6) mm wide (19 collections), length/width ratio 6.1:1. Staminate flowers 2.5 mm across; perianth 3-merous; segments ca. 1.1 mm long, 0.9 mm wide. Mean anther diameter 0.5 mm, centered 0.5 mm from tip of segment. Pollen polar diameter 18–26 (mean 20) µm; equatorial diameter 21–30 mean 24 µm; polar/equatorial diameter ratio 1:1.17; spine height (1.6 µm) ca. 1.5 x the wall thickness (ca. 1.0 µm) (8 collections). Mature fruit 3.5 x 2.0 mm; proximal portion ca. 2.0 mm long. Seeds 2.0 x 0.9 mm. n = 14.

Phenology: Meiosis in July. Peak anthesis usually from early August to late September (fig. 16.41). Fruits usually mature from early September to late October in the year following pollination; maturation period averages ca. 13 months.

Hosts: Arceuthobium divaricatum is restricted to pinyons. It is most common on Pinus edulis and P. monophylla. Other pinyons are also classed as principal hosts because they appear to be about equally susceptible to infection when the dwarf mistletoe is present—P. quadrifolia, P. cembroides, P. discolor, and P. californiarum subspecies californiarum and fallax (Bailey and Hawksworth 1988). Arceuthobium divaricatum, however, does not commonly occur within the distribution of the latter group of pinyons.

The principal host in California and Nevada is Pinus monophylla, but in Arizona, Colorado, New Mexico, Texas, and Utah the primary host is P. edulis. This dwarf mistletoe has been collected on P. quadrifolia only in the Sierra Juárez, Baja California (Hawksworth and others 1968), but it has been reported on this tree on the east slope of the Laguna Mountains in San Diego County, California (Beauchamp 1986), and in the Sierra San Pedro Mártir, Baja California (Hawksworth and others 1968). Pinus cembroides is parasitized only at the northern limits of its distribution in the Davis Mountains, Jeff Davis County, in west Texas. Pinus discolor is also parasitized only at its northern limits near Fort Bayard, and in the Mule Mountains, New Mexico, where this tree is associated with infected P. edulis. Infection is local on the two subspecies of P. californiarum it parasitizes—subsp. californiarum in several Mojave Desert ranges (New York Mountains, Providence Mountains, and in Joshua Tree National Monument) and subsp. fallax in southwestern Utah (Zion National Park), northwestern Arizona (Black Hole Mountains), and central Arizona near Prescott, Sedona, and Strawberry. Infected pinyons are not often associated with other pines, but where they are sympatric with P. ponderosa and P. jeffreyi, these trees were not parasitized by Arceuthobium divaricatum. No other dwarf mistletoe infects pinyon in the United States. Arceuthobium pendens, however, also parasitizes pinyons in Mexico.

Distribution: United States (California, Nevada, Utah, Colorado, Arizona, New Mexico, and Texas) and Mexico (Baja California) (fig. 16.42). Arceuthobium divaricatum occurs in eastern and southern California (the White and Inyo Mountains, the Mount Pinos area, the San Bernardino Mountains and the Mojave Desert Ranges), the southern three-fourths of Nevada and Utah, western Colorado, Arizona (except southwest), New Mexico (except northeast), and south to the Davis Mountains of western Texas. In Mexico it is known only in northern Baja California. J. R. Weir (unpublished manuscript, 1920) reported A. divaricatum on Pinus monophylla in Cassia County in southern Idaho, but we have not confirmed its presence there, nor do we know of any collections of this dwarf mistletoe from Idaho. The northernmost population of which we are aware is in the Pilot Range, western Box Elder County in northwestern Utah (latitude 44°30'N) (Albee and others 1988). Arceuthobium divaricatum has been reported on P. discolor (as P. cembroides) in the Santa Catalina Mountains in southern Arizona (G. G. Hedgcock, unpublished manuscript, 1914), but this has not been confirmed. We are aware of no collections from Arizona south of the Gila River.

Distribution maps have been published for Arceuthobium divaricatum in Colorado (Hawksworth 1987c), Utah (Albee and others 1988), and New Mexico (Martin and Hutchins 1980). Elevational range is from 1,200 m near Sedona, Arizona, to 3,000 m in the San Mateo Mountains of New Mexico.

Discussion: The type locality for Arceuthobium divaricatum is given by Gill (1935) as Salt "Run" Valley. The original label, however, is handwritten and scarcely legible, but it appears that Salt "River" Valley was the intended locality. Kuijt (1960a) points out the scattered distributions of this parasite in California. We suggest that A. divaricatum is probably more common than indicated by collections to date. The witches’ brooms induced by this dwarf mistletoe are often poorly developed and not conspicuous because of the stunted habit of even healthy trees (fig. 16.43). Our observations suggest that witches’ brooms are more consistent in Pinus edulis than in P. monophylla. Shoots of the mistletoe are often long, slender, and spreading, especially the staminate plants which also tend to have relatively few flowers per shoot.

Arceuthobium divaricatum is often associated with another mistletoe, Phoradendron juniperinum, a parasite of junipers, in the juniper–pinyon communities where both mistletoes may co-occur. The extent of their co-occurrence and the factors underlying their distribution could be a subject of considerable interest.

12. A. douglasii Engelmann in U.S. Geographical Survey West of 100th Meridian (Wheeler Report) 6: 253, 1878. TYPE COLLECTION: NEW MEXICO: Santa Fe County: Santa Fe River, on Pseudotsuga menziesii , Rothrock 69 in 1874 (Lectotype MO! Isotypes: F, US). Razoumofskya douglasii (Engelm.) Kuntze, Revision of Genera of Plants 2: 587, 1891.

Description: Mean shoot height ca. 2 (max. 8) cm, olive green, flabellately branched (fig. 16.44 and 16.45). Basal diameter of dominant shoots 1.0–1.5 (mean 1) mm. Third internode 2–6 (mean 3.6 ± 1.2) mm long, ca. 1.0 mm wide (29 collections), length/width ratio 3.6:1. Flowers usually axillary in pairs, occasionally borne on pedicel-like segments as in Arceuthobium americanum. Staminate flowers 2.0 mm long, 2.3 mm across; perianth mostly 3-merous (occasionally 4- or 2-merous); segments rounded at the apex, without a keel, inner surface reddish to purple, lower surface same color as shoots, ca. 1.0 mm long, 1.0 mm wide. Mean anther diameter 0.4 mm, centered 0.6 mm from tip of segment. Pollen polar diameter 16–25 (mean 21) µm; equatorial diameter 20–28 (mean 24) µm; polar/equatorial diameter ratio 1:1.16; spine height approximately equal to wall thickness (1.0–1.2 µm) (8 collections). Pistillate flowers ca. 1.5 mm long, 1.5 mm across. Mature fruit olive green 3.5–4.5 (mean 4) mm long, 1.5–2.0 mm wide, obovate; proximal portion ca. 2.5 mm long. Seeds 2.4 x 1.1 mm. n = 14.

Phenology: Staminate meiosis in September, pistillate meiosis in April (Jones and Gordon 1965). Peak anthesis is usually in April or May (fig. 16.46). However, this species exhibits a marked latitudinal variation in flowering—March in Mexico, late April to early May in Arizona and New Mexico, late May in Colorado, Utah, Oregon, and early to mid-June in Washington, north Idaho, and Montana. In contrast, fruit maturity is more uniform and usually occurs from late August to late September throughout the distribution; maturation period averages 17–18 months. The seeds germinate in March (Wicker 1967a).

Hosts: The principal and only commonly infected host is Pseudotsuga menziesii. Both var. menziesii (Washington, Oregon, and California) and var. glauca (from British Columbia through the Rocky Mountains to central Mexico) are parasitized, although it is much more common on the inland variety. The following trees are occasional hosts when they are associated with infected Pseudotsuga menziesii: Abies amabilis in Oregon (Mathiasen and Loftis 1987) and A. lasiocarpa var. arizonica in Arizona and New Mexico (Mathiasen 1984). Rare hosts are A. grandis (Kuijt 1954a), A. concolor (Hawksworth 1952), A. lasiocarpa var. lasiocarpa, Picea pungens, and P. engelmannii. We concur with Kuijt (1960a) that reports of this dwarf mistletoe parasitizing Pseudotsuga macrocarpa (Jepson 1925, Munz 1935) are erroneous.

Distribution: Canada (British Columbia), United States (Washington, Idaho, Montana, Oregon, Wyoming, California, Nevada, Utah, Colorado, Arizona, New Mexico, and Texas) and Mexico (Chihuahua, Durango, Coahuila, and Nuevo León) (figs. 16.47, 16.48, and 16.49). Arceuthobium douglasii has the greatest latitudinal range (3,000 km) of any species in the genus (fig. 16.47). Its northernmost distribution is near Lytton and Sicamous in southern British Columbia, about 210 km north of the international boundary (Smith 1974, Wood 1986). This is somewhat north of the distribution given by Kuijt (1963). In Washington and Oregon, it occurs east of the Cascade Crest but is common on the Pacific slope from the Umpqua River drainage south to about latitude 40°N in northern California (California Forest Pest Council 1968). The report by Palmer and others (1983) of A. douglasii from the headwaters of the North Fork of the American River in Placer County, California, has not been confirmed and is probably in error. This dwarf mistletoe is common in eastern Washington, eastern Oregon, Idaho, western Montana, Idaho, Utah, Colorado, and New Mexico. It is rare in Nevada (Wheeler Peak, Hawksworth 1965c), Wyoming (Teton County, Hawksworth 1965c), and Texas (Guadalupe Mountains, Warnock 1974). The distribution of the dwarf mistletoe in Mexico is poorly known, and it is probably more widespread than the few available records in Chihuahua, Coahuila, Durango, and Nuevo León suggest (fig. 16.49).

The distribution of this species exhibits some interesting peculiarities. For further details refer to chapter 5.

Distribution maps of Arceuthobium douglasii have been published for British Columbia (Wood 1986), Montana (Dooling and Eder 1981), Utah (Albee and others 1988), Colorado (Hawksworth 1987c), New Mexico (Martin and Hutchins 1980), and California (Kuijt 1960a).

The altitudinal range of this dwarf mistletoe is correlated with latitude, and it occurs as low as 300 m near Lytton in southern British Columbia and as high as 3,250 m on Cerro Potosí in Nuevo León, Mexico.

Discussion: Arceuthobium douglasii is one of the most distinctive and stable species in the genus. In spite of its extensive latitudinal distribution, we find no characteristics that suggest subspecific separation. Much of the confusion in the literature on this species is due to its supposed affinities with various members of the A. campylopodum complex (Gill 1935). Weir (1918a) discussed the occurrence of "a small purple-flowered form on Picea" in the Inland Empire that he (and we) consider to be merely A. douglasii on an unusual host.

This dwarf mistletoe is the smallest in western North America, but its typically systemic mode of infection produces large witches’ brooms (fig. 16.50) and causes severe growth loss and mortality in Pseudotsuga menziesii.

13. A. durangense (Hawksworth & Wiens) Hawksworth & Wiens, Phytologia 66: 7, 1989. TYPE COLLECTION: MEXICO: Durango: 58 km west of El Salto on Route 40, on Pinus durangensis, Hawksworth & Wiens 3507 in 1963 (Holotype COLO! Isotypes: DS, F, FPF, INIF, MEXU, MO, US). A. vaginatum subsp. durangense Hawksworth & Wiens, Brittonia 17: 230, 1965.

Description: Mean shoot height ca. 20–30 (max. 50) cm, bright orange; older shoots becoming pendulous (fig. 16.51). Basal diameter of dominant shoots 4–8 (mean 6) mm. Third internode 9–22 (mean 17.9 ± 4.1) mm long, 3.5–6.0 (mean 4.5) mm wide (3 collections), length/width ratio 3.3:1. Internodes often slightly swollen at base. Staminate flowers ca. 2.5 mm long, 2.5 mm across, segments ca. 1 mm long, 1 mm wide. Pollen polar diameter 18–19 (mean 18) µm; equatorial diameter 20–21 (mean 21) µm; polar/equatorial diameter ratio 1:1.15; spine height (1.5 µm) about 1.5 x wall thickness (1.0 µm) (1 collection). Mature fruit 7 x 3.5 mm; bluish; proximal portion ca. 4 mm long. Seeds 4 x 1.5 mm. n = ?

Phenology: Time of meiosis unknown (probably in February). Anthesis usually in April. Fruits mature from mid-July to September of the year following pollination; maturation period averages 15–18 months.

Hosts: Pinus douglasiana, P. durangensis, P. montezumae, P. michoacana, and P. pseudostrobus are the principal hosts. Pinus herrerai is secondarily parasitized when it occurs near infected principal hosts. Pinus oocarpa is reputed to be a host in Jalisco and southeastern Durango, but this requires confirmation. We have seen no collections from this tree. In the Sierra de Quilla, Jalisco, P. oocarpa growing with infected P. montezumae and P. pseudostrobus was not infected by Arceuthobium durangensis. The following pines have been observed growing near infected trees but were not themselves parasitized: P. leiophylla var. leiophylla, P. lumholtzii, and P. teocote.

Distribution: Mexico (Durango, Sinaloa, and Jalisco) (fig. 16.52). This rather local dwarf mistletoe occurs in extreme western Durango and adjacent Sinaloa on the western edge of the escarpment near the crest of the Sierra Madre Occidental and in the Sierra de Quilla in Jalisco. It probably occurs in other areas along the western slopes of the Sierra Madre Occidental, e.g., in Nayarit, but this region is difficult of access and little collected. Elevational range is 1,450–2,750 m.

Discussion: We originally classified this taxon as a subspecies in the Arceuthobium vaginatum complex, but our subsequent field studies have convinced us that it is a distinct species (Hawksworth and Wiens 1989). This dwarf mistletoe is not sympatric with either subspecies of A. vaginatum, and it differs from A. vaginatum subsp. vaginatum by its bright orange shoots and from A. vaginatum subsp. cryptopodum by its larger size, brighter color, branching pattern of the staminate spikes, larger fruit, and geographical distribution.

14. A. gillii Hawksworth & Wiens, Brittonia 16: 55, 1964. TYPE COLLECTION: ARIZONA: Cochise County: Huachuca Mountains, 0.8 km east of Reef, on Pinus leiophylla var. chihuahuana, Hawksworth & Lightle 236, in 1962 (Holotype COLO!, Isotypes: FPF, MO, ARIZ, DS).

Description: Mean shoot height ca. 8–15 (max. 25) cm, greenish brown, flabellately branched (fig. 16.53). Basal diameter of dominant shoots 2.5–8.0 (mean 4) mm. Third internode 5–18 (mean 10.7 ± 3.4) mm long, 2.0–4.5 (mean 2.8) mm wide (9 collections), length/width ratio 3.8:1. Staminate flowers 3.5 mm long, 2.5–4.0 (mean 3.2) mm across. Pollen polar diameter 19–25 (mean 22) µm; equatorial diameter 24–28 (mean 26) µm; polar/equatorial diameter ratio 1:1.19; spine height (1.7 µm) slightly greater than wall thickness (1.3 µm) (4 collections). Pistillate flowers ca. 1.5 mm long, 1 mm across. Mature fruit 4–5 mm long, 2–3 mm wide, the proximal portion of fruit conspicuously glaucous. Seeds 3.1 x 1.4 mm. n = 14.

Phenology: Meiosis in September. Anthesis usually in March and April. Fruits mature in October of the year following pollination; maturation period averages ca. 19 months, the longest in the genus. Seed germination begins in April (Chihuahua).

Hosts: The principal and only commonly infected hosts are Pinus leiophylla var. chihuahuana, P. lumholtzii, and P. herrerai. However, P. leiophylla var. leiophylla is about equally susceptible to the parasite but not common within the range of Arceuthobium gillii. In western Chihuahua, this dwarf mistletoe rarely parasitizes P. cooperi and P. arizonica var. arizonica. Pinus ponderosa var. scopulorum and P. engelmannii frequently occur in association with infected Pinus leiophylla var. chihuahuana in Arizona, but these two species are not parasitized.

Distribution: United States (Arizona and New Mexico) and Mexico (Chihuahua, Durango, Sinaloa, and Sonora) (fig. 16.54). This dwarf mistletoe occurs in southeastern Arizona (Santa Catalina, Rincon, Santa Rita, Huachuca, and Chiricahua Mountains) (Hawksworth and Wiens 1964) and the Animas Mountains in southwestern New Mexico (Hawksworth and Weiss 1975). It is most common in western Chihuahua but is also distributed in adjacent northern and eastern Sonora, northern Durango, and northeastern Sinaloa.

We have examined several additional populations of Pinus leiophylla var. chihuahuana in New Mexico (Bear Mountain near Silver City and in the Mule and Peloncillo Mountains) but Arceuthobium gillii was not found. Several localities in Arizona where P. leiophylla var. chihuahuana occurs have also been examined, but no A. gillii were observed in the Pinaleno (Mount Graham) and Pinal Mountains nor below the Mogollon Rim near Colcord Mountain, Gila County. Elevational range is from 1,700 m in southern Arizona to 2,650 m in southern Chihuahua.

Discussion: This dwarf mistletoe is named for Lake S. Gill. It had long been confused with Arceuthobium vaginatum subsp. cryptopodum, but it differs in host preference, phenology, and its conspicuously glaucous fruits (Hawksworth and Wiens 1964). These two dwarf mistletoes are usually separated by at least 300 m of elevation in Arizona and New Mexico. They co-occur, however, in several areas in central Chihuahua, but there was no evidence of hybridization.

A characteristic feature of Arceuthobium gillii is its strong sexual dimorphism—staminate plants are tall and openly branched and the pistillate plants are small and densely branched.

This dwarf mistletoe causes open, non-systemic witches’ brooms similar to those induced by Arceuthobium nigrum. It causes serious mortality in Pinus leiophylla var. chihuahuana and P. lumholtzii in Chihuahua, particularly on poor, rocky sites.

15. A. globosum Hawksworth & Wiens, Brittonia 17: 223, 1965. TYPE COLLECTION: MEXICO: Durango: 2.4 km east of El Salto on Route 40, on Pinus cooperi, Hawksworth & Wiens 3414 in 1963 (Holotype COLO! Isotypes: DS, F, FPF, INIF, MEXU, MO, US).

Description: Shoot height ca. 20–50 (max. 70) cm, yellow to greenish, flabellately branched. Basal diameter of dominant shoots 3–48 mm. Third internode 4–37 mm long, 2–24 mm wide. Staminate flowers ca. 3.5–5.0 mm long, 3.0–3.5 mm across; perianth 3- or 4-merous; same color as shoots; segments ca. 1.3 mm long, 1.0 mm wide. Mean anther diameter 0.6 mm, centered 0.9 mm from tip of segment. Pollen polar diameter 19–23 (mean 22) µm; equatorial diameter 24–28 (mean 26) µm; polar/equatorial diameter ratio 1:1.18; spine height (ca. 1.9 µm), ca. 2 x wall thickness (ca. 1.0 µm) (4 collections). Pistillate flowers ca. 1.5 mm long, 1.5 mm across. Mature fruit ca. 5–7 mm long, ca. 3–4 mm wide; proximal portion ca. 3.5 mm long, with pedicels 4.0–5.0 mm long. Seeds 5 x 2 mm. n = 14.

Hosts: Common on pines of subgenus Diploxylon.

Discussion: Previously, we (Hawksworth and Wiens 1972) noted that there was considerable variation within this dwarf mistletoe and that further field research was needed to determine whether the variation was sufficiently consistent to warrant the segregation of additional taxa. Subsequent studies in Mexico, Guatemala, and Belize have shown that the former Arceuthobium globosum (sensu lato) consists of five taxa (Hawksworth and Wiens 1977, Wiens and Shaw 1994):

• Arceuthobium globosum subsp. globosum (northwest Mexico)
• Arceuthobium globosum subsp. grandicaule (central Mexico and Guatemala)
• Arceuthobium aureum subsp. aureum (Guatemala)
• Arceuthobium aureum subsp. petersonii (southern Mexico)
• Arceuthobium hawksworthii (Belize)

Key to the subspecies:

15a. A. globosum Hawksworth & Wiens subsp. globosum

Description: Shoots 15–20 (max. 50) cm high, bright yellow, flabellately branched (fig. 16.55). Basal diameter of dominant shoots 3–10 (mean 7) mm. Third internode 19 mm long, 4 mm wide. Staminate flowers ca. 4 mm wide. Mature fruit 5 x 2.5 mm. Seeds 4 x 2 mm. n = ?

Phenology: Anthesis usually March–April. Fruits mature June–July; maturation period averages 15–16 months.

Hosts: The principal hosts are Pinus cooperi, P. engelmannii, and P. durangensis. Pinus arizonica is occasionally parasitized, but it appears to be less susceptible to infection than the three principal hosts. Pinus teocote is a rare host. We have classified the host near the summit of Pico Candelaria (2,650 m) south of Chuchuichupa, Chihuahua, as P. cooperi, but the trees exhibit some characteristics of P. rudis.

Distribution: Mexico (Sonora, Chihuahua, Durango, and Jalisco) (fig. 16.56). This subspecies is widely distributed in the pine forests of the Sierra Madre Occidental from northwestern Chihuahua and adjacent Sonora, through Durango to northern Jalisco. Elevational range is 2,300–2,800 m.

Discussion: This dwarf mistletoe is characterized by its bright yellow, globose clusters and absence of witches’ broom formation.

15b. A. globosum Hawksworth & Wiens subsp. grandicaule Hawksworth & Wiens, Brittonia 29: 413, 1977. TYPE COLLECTION: MEXICO: Mexico: km-59 on Highway 59, 35 km east of Zitácuaro, on Pinus montezumae, Hawksworth & Player 1607 in 1975 (Holotype US! Isotypes: ENCB, FPF, INIF, MO, UNAM, UT).

Description: Shoots 18–50 (max. 70, mean 25) cm tall, yellow green, typically dark at the base of older shoots, flabellately branched. Basal diameter of dominant shoots 10–48 (mean 17) mm. Third internode 14–37 (mean 27) mm long, 3–20 (mean 7) mm wide. Staminate flowers ca. 5 mm wide, 4-merous. Mature fruits 6–7 mm long, 3.5 mm wide. Seeds 5 x 3 mm. n = 14.

Phenology: Meiosis in December. Anthesis from January–May, with peak March–April. Fruits maturing July–October. Detailed studies by Escudero and Cibrián (1985) in the Sierra Nevada between the states of Mexico and Puebla showed that seed dispersal lasted for 95 days from early July to early November, with a peak from mid-August to mid-September; maturation period averages 16–18 months.

Hosts: This subspecies has one of the broadest host ranges of any dwarf mistletoe. It infects at least 12 species of Mexican pines, all of which appear to be about equally susceptible: Pinus douglasiana, P. durangensis, P. hartwegii, P. lawsonii, P. maximinoi, P. michoacana, P. montezumae, P. patula, P. pringlei, P. pseudostrobus, P. teocote, and P. rudis. Valdivia (1964) studied the hosts of Arceuthobium globosum subsp. grandicaule in northeastern Michoacán, listing them in decreasing order of susceptibility: P. maximinoi (as P. tenuifolia), P. pseudostrobus, P. pseudostrobus f. protuberans, P. montezumae, P. michoacana, P. durangensis, P. douglasiana, P. lawsonii, P. pringlei, P. teocote, and P. rudis. Pinus leiophylla and Abies religiosa were recorded as immune.

The hosts of Arceuthobium globosum subsp. grandicaule in Guatemala are poorly known. However, Pinus pseudostrobus, P. maximinoi, P. rudis, and Cupressus sp. are reported to be infected there (Standley and Steyermark 1946). This report on Cupressus sp. is the only report of Arceuthobium on a host other than the Pinaceae in the Western Hemisphere and requires confirmation. The putative collection on Cupressus (Steyermark 4904 in 1942 from the Sierra Cuchumatanes) has no host material. If the report were correct, the occurrence of A. globosum subsp. grandicaule on Cupressus would be rare. We did not observe any infections on Cupressus during our field studies in this region, even though the tree was common in many stands where P. rudis was heavily parasitized by A. globosum subsp. grandicaule.

Distribution: Mexico (Jalisco, Michoacán, Mexico, Hidalgo, Distrito Federal, Guerrero, Puebla, Tlaxcala, Veracruz, and Oaxaca) and Guatemala (Huehuetenango) (fig. 16.56). Arceuthobium globosum subsp. grandicaule is the most abundant dwarf mistletoe in central Mexico, and it is also locally common in the highlands of western Guatemala. Elevational range is 2,450–4,000 m (Hernandez and others 1992).

Discussion: This subspecies has the largest shoots in the genus, reaching a height of 70 cm and with a basal diameter of 5 cm.

Seed dispersal was studied by Escudero and Cibrián (1985). They showed that the maximum distance of seed dispersal was 15 m but that 91% of the seeds traveled less than 8.7 m. More than 7.3 million seeds per hectare landed on seed traps on the ground in stands of Pinus hartwegii. This number does not include the large proportion of seeds that were undoubtedly intercepted by the trees.

Valdivia (1964), who surveyed 400,000 ha of pine forest in northeastern Michoacán, reported that Arceuthobium globosum was present on nearly 40% of the forest area.

16. A. guatemalense Hawksworth & Wiens, Brittonia 22: 267, 1970. TYPE COLLECTION: GUATEMALA: Department Huehuetenango, Sierra Cuchumatanes, on road from Huehuetenango to Santa Eulalia; 54 km north of Huehuetenango (10 km south of San Juan Ixcoy), on Pinus ayacahuite, Hawksworth & Wiens 1221, September 11, 1969. (Holotype US! Isotypes: COLO, DS, EAP, FPF, MO, UT).

Description: Mean shoot height ca. 1–3 cm on systemic witches’ brooms, but shoots on non-systemic infections up to ca. 7 cm high; living shoots greenish to purple, yellow to brown when dried, flabellately branched (fig. 16.57). Basal diameter of dominant shoots 2.0–2.5 mm. Third internode 8–15 (mean 11.4 ± 2.8) mm long, 1.5–2.0 (mean 1.7) mm wide; length/ width ratio 6.7:1 (3 collections). Staminate flowers ca. 2 mm across; perianth 2- or 3-merous, segments 0.9 mm long, 0.7 mm wide. Mean anther diameter 0.5 mm. Pollen polar diameter 21 µm and mean equatorial diameter 24 µm; polar/equatorial diameter ratio 1:1.14; spine height (ca. 1.5 µm), wall thickness (ca. 1.0 µm) (1 collection, few grains available). Mature fruit ca. 3.5–4.0 mm long, 1.5–2.0 mm wide; distal portion ca. 1.2 mm long; dark green, glabrous, with a slightly swollen ring at the base of the fruit where it joins the pedicel. Seeds 2.0 x 0.8 mm. n = ?

Phenology: Time of meiosis unknown. Anthesis apparently in August and early September. Fruits mature in September; maturation period ca. 12–13 months. Seed germination in September.

Hosts: Known only on Pinus ayacahuite var. ayacahuite. The only other pine in subgenus Haploxylon that occurs within the range of Arceuthobium guatemalense is P. chiapensis. This tree, however, usually grows at lower elevations than P. ayacahuite (Martínez 1948). These two species, however, co-occur from about 2,700–2,800 m just north of the Continental Divide on Highway 175 north of Ixtlán, Oaxaca (Hawksworth and Wiens 1977), but Arceuthobium guatemalense was not present in these mixed stands.

Distribution: Mexico (Oaxaca and Chiapas) and Guatemala (Huehuetenango and Totonicapán) (fig. 16.58). This distinctive species is known only from the high mountains of western Guatemala and southern Mexico. It is common at the type locality in the vicinity of Santa Eulalia, in the Sierra Cuchumatanes in Guatemala. Elevational range is poorly known. Our collections were made between 2,450–3,100 m.

Discussion: The consistent formation of systemic witches’ brooms is a distinctive characteristic of this species, and brooms sometimes measure 3–5 m across (fig. 16.59). An unusual feature of these witches’ brooms is that the shoots of the dwarf mistletoe are consistently formed on 1-year-old host shoots and, in some cases, we found buds on the current year’s growth (September). This species causes extensive damage and considerable mortality to Pinus ayacahuite.

17. A. hawksworthii Wiens & C. G. Shaw III, Journal of Idaho Academy of Science 30 (1): 25-32. 1994. TYPE COLLECTION: BELIZE: Cayo District, Mountain Pine Ridge Region, 2.4 km south of Cooma Cairn Lookout, elevation 940 m, on Pinus caribaea var. hondurensis, Wiens and Shaw 7768, February 16, 1994 (Holotype US! Isotypes FPF, MO, UC).

Description: Shoot height 10–20 (max. 30) cm, pale yellow to light green, flabellately branched (fig. 16.60 and 16.61). Basal diameter of dominant shoots 3–5 mm. Third internode 10–19 (mean 14) mm long, 2–4 mm wide. Staminate flowers ca. 2.5 mm long, 3 mm across; perianth mostly 3-merous or occasionally 4-merous, adaxial surface dark red or purple, 1.2–1.5 mm long, 1.0–1.5 mm wide; anther diameter ca. 0.75 mm, sepals, stigma, and subtending bracts often dark brown to gray when flowering, giving the pistillate flowers a dark appearance. Mature (?) fruit 4.0 mm long, 1.5 mm wide, equatorial line approximately medial. n = 14.

Phenology: Meiosis probably in December and/or early January. Anthesis probably from mid-late January to early March, with a peak from mid-January to mid-February. Fruit dispersal was complete by late February, but a few fruits were present on previous collections (Carty 3) made sometime in June (exact date not specified). Fruit maturation therefore presumably requires about 3–4 months, the shortest period known for any dwarf mistletoe. If this is correct, then Arceuthobium hawksworthii likely has at least two flower and fruit crops annually and possibly a third (depending on whether it flowers during the wet season). This type of multiple, periodic pattern of flowering also occurs in other tropical species, e.g., A. juniperi-procera, A. nigrum, and A. abietis-religiosae (see discussion under those species). Arceuthobium aureum subsp. aureum produces new flowers continuously, at least during the dry season.

Hosts: The only known hosts are Pinus caribaea var. hondurensis and what appears to be P. oocarpa at higher elevations. Previous collections of Arceuthobium hawksworthii (Carty 3A) indicate these two pines may hybridize at higher elevations.

Distribution: Belize, known only from the Mountain Pine Ridge Region of Cayo District (fig. 16.62). Arceuthobium hawksworthii is not present in the coastal stands of P. caribaea south of Belize City. Pinus caribaea var. hondurensis also occurs at higher elevations in eastern Guatemala in the Poptún area, which is only about 75 km from the population in western Belize, and this area should be examined for the presence of A. hawksworthii. Elevational range is about 700–900 m. Arceuthobium hawksworthii does not occur in the lower 300 m of the elevational distribution of P. caribaea var. hondurensis in the Mountain Pine Ridge Region.

Discussion: This dwarf mistletoe commemorates Frank G. Hawksworth. The presence of a dwarf mistletoe in Belize has been known since 1957, but it was previously confused with Arceuthobium aureum subsp. aureum (table 16.3). The presence of red staminate flowers and its geographic distribution suggest affinities with the section Campylopoda series Rubra; but the lack of red pigment in the shoots, general stature, and presumed multiple flowering periods indicate a relationship with section Vaginata where it is placed provisionally (see tables 14.1 and 15.2).

18. A. hondurense Hawksworth & Wiens, Brittonia 22: 267, 1970. TYPE COLLECTION: HONDURAS: Department Francisco Morazán, Piedra Herrada, 22 km southeast of Tegucigalpa on road to Escuela Agricola Panamericana, on Pinus oocarpa, Hawksworth, Wiens, & Molina 1203, September 4, 1969 (Holotype US! Isotypes: EAP, COLO, DS, F, FPF, MO, UT).

Description: Mean shoot height ca. 14 (max. 21) cm, olive brown to grayish green, markedly glaucous; flabellately branched (fig. 16.63). Basal diameter of dominant shoots 3–9 (mean 5) mm; nodes of older shoots swollen; lateral branches of staminate plants at nearly right angles to the axis of the main shoot; third internode 7–12 (mean 9.1 ± 1.5) mm long, 2.5–4.0 (mean 3.2) mm wide; length/width ratio 6.1:1 (2 collections). Staminate flowers ca. 2.5 mm across; inner surface reddish, lower surface the same color as the shoots; perianth usually 3-merous (sometimes 2- or 4-merous), segments ca. 1.2 mm long, 0.8 mm wide; nectary with 2 large and 1 small lobe. Pollen polar diameter 18–23 (mean 20) µm, equatorial diameter 21–25 (mean 24) µm; polar/equatorial diameter ratio 1:1.21; spine height (ca. 2.5 µm) ca. 2 x wall thickness (1.1 µm) (2 collections). Pistillate flowers with stigmas exserted ca. 0.5 mm, with copious stigmatic exudate at anthesis. Mature fruit 5.5 x 3.0 mm, greenish glaucous; proximal portion ca. 4.0 mm long; stigma exserted. Seeds ca. 3.1 x 1.5 mm. n = 14.

Phenology: Meiosis in August or early September. Anthesis and fruit maturity in September; maturation period averages ca. 12 months.

Hosts: The only known hosts are Pinus oocarpa and P. maximinoi.

Distribution: Honduras and possibly El Salvador (fig. 16.62). The distribution of this species is poorly known, but it is presently one of the rarest dwarf mistletoes in the New World with the possible exception of Arceuthobium hawksworthii . Only four collections are known, all from Honduras. Three are from the same general area near Piedra Herrada, southeast of Tegucigalpa on the road to Escuela Agricola Panamericana. The fourth population occurs 200 km to the northwest near Choloma. A roadside reconnaissance of 225 km through forests of Pinus oocarpa in the Departments of Francisco Morazán, El Paraiso, and Comayagua failed to uncover additional populations of A. hondurense. Recently, Jerry Beatty (personal communication) traveled extensively in the pine forests of Honduras and noted that this dwarf mistletoe was exceedingly rare and limited to the localities noted above. The indiscriminate cutting of the original forests has apparently reduced the populations of A. hondurense in these areas. These lands now have been either converted to agriculture or consist of second-growth forests. Unless A. hondurense has a broader distribution and occurs in areas less subject to deforestation, the species is in danger of extinction.

James Davis of the New Mexico State Forest Service (personal communication.) observed a dwarf mistletoe on pines in the Monte Cristo area (Santa Ana Province) of northeastern El Salvador near the Honduran border. This could be Arceuthobium hondurense, but unfortunately no collections of this dwarf mistletoe were obtained. Elevational range is poorly known. Our collections were made between 1,200 and 1650 m.

Discussion: Originally, we believed Arceuthobium hondurense might be conspecific with A. bicarinatum, a species endemic to Hispaniola. Observations of living plants of both dwarf mistletoes, however, demonstrated discontinuities in many characters. The species nonetheless appear closely related. Arceuthobium hondurense also exhibits some similarities with A. rubrum and A. oaxacanum. Arceuthobium bicarinatum and A. hondurense occupy the southern distribution limits of Arceuthobium in the New World.

19. A. laricis (Piper) St. John, Flora of Southeast Washington and Adjacent Idaho: 115, 1937. TYPE COLLECTION: WASHINGTON: Kittitas County: North of Ellensburg, on Larix occidentalis, Brandegee 1071 in 1883 (Lectotype US! Isotypes: GH, PH, UC). Razoumofskya douglasii (Engelm.) Kuntze subsp. laricis Piper, Contributions U.S. National Herbarium 11: 223, 1906, nomen nudum. A. douglasii Engelm. var. laricis M. E. Jones, University of Montana Bulletin 61 (Biological Series 15): 25, 1910, nomen nudum. Razoumofskya laricis Piper, in Piper and Beattie, Flora Southeast Washington and Adjacent Idaho: 80, 1917. A. campylopodum Engelm. forma laricis (Piper) Gill, Transactions Connecticut Academy Arts and Sciences. 32: 202, 1935.

Description: Mean shoot height ca. 4 (max. 6) cm, mostly dark purple, flabellately branched (fig. 16.64). Basal diameter of dominant shoots 1.5–3.0 (mean 2) mm. Third internode 5–14 (mean 8.0 ± 2.0) mm long, 1.0–2.5 (mean 1.3) mm wide (12 collections), length/ width ratio 6.1:1. Staminate flowers 2.7 mm across; perianth mostly 3-merous (sometimes 4-merous); segments ca. 1.4 mm long, 1.1 mm wide. Mean anther diameter 0.5 mm, centered 0.5 mm from tip of segment. Pollen polar diameter 18–25 (mean 20) µm; equatorial diameter 21–27 (mean 23) µm; polar/equatorial diameter ratio 1:1.16; spine height (ca. 2 µm) about 2 x wall thickness (ca. 1.0 µm) (3 collections). Pistillate flowers ca. 1 mm long, 1 mm across. Mature fruit 4.5 x 2.5 mm; proximal portion ca. 2.5 mm long. n = 14.

Phenology: Meiosis in June. Peak anthesis from mid-July to late August, with extremes from early July to early September (fig. 16.65). Fruits usually mature in September, with extremes from early August to early October; maturation period averages 13–14 months.

Hosts: The principal and most commonly infected host is Larix occidentalis, but Tsuga mertensiana is also a principal host. Pinus contorta var. latifolia is a secondary host, and occasional hosts include Abies lasiocarpa and Pinus ponderosa var. ponderosa. Rare hosts are Abies grandis (Smith and others 1972), Picea engelmannii (Kuijt 1945b), Pinus albicaulis and P. monticola (Kuijt 1953, 1954b). Larix lyallii is no longer considered to be a natural host of Arceuthobium laricis. J. R. Weir’s two fragmented collections from the Bitterroot and Cabinet Mountains of Montana are clearly A. laricis, but the host was misidentified as L. lyallii. Recent field studies in these areas have discovered numerous infected L. occidentalis at lower elevations but no infected L. lyallii at higher elevations (R. M. Mathiasen, personal communication).

Exotic trees infected naturally by Arceuthobium laricis include Pinus banksiana (Graham 1959a), P. resinosa (Hawksworth and Wiens 1972), P. sylvestris (Graham and Leaphart 1961), and Picea abies (Hawksworth and Wiens 1972). Weir (1918a) has shown that, on the basis of seedling inoculations, Larix decidua (as L. europaea) and L. kaempferi (as L. leptolepis), are susceptible to infection by this dwarf mistletoe.

The occurrence of Arceuthobium laricis on Tsuga mertensiana in Idaho and Montana is of interest because this dwarf mistletoe will not infect T. heterophylla. Our observations in many mixed Larix–Tsuga stands in Oregon, Washington, Idaho, and Montana confirm that T. heterophylla is not infected by A. laricis. In mixed T. heterophylla–T. mertensiana stands in the Coeur d’Alene National Forest, Idaho, only the latter was infected by A. laricis. It is unknown if A. laricis is associated with T. mertensiana in the Cascade Mountains of Oregon and Washington.

Distribution: Canada (British Columbia) and the United States (Washington, Oregon, Idaho, and Montana) (fig. 16.66). Arceuthobium laricis occurs generally throughout the range of its principal host, Larix occidentalis, in southern British Columbia, east of the Cascade Mountains in Washington and northern Oregon, north and central Idaho, and western Montana.

Distribution maps have been published for Arceuthobium laricis from British Columbia (Wood 1986) and Montana (Dooling and Eder 1981). Elevational range is 650–2,250 m.

Discussion: Arceuthobium laricis is a serious pathogen of Larix occidentalis (Weir 1916a). Infection usually results in the formation of heavy but compact brooms; because larch branches are brittle, larger brooms are readily broken off (fig. 16.67). Surveys show that about two-thirds of L. occidentalis stands are infested on the Coeur d’Alene National Forest, Idaho, and the Kootenai National Forest, Montana (Graham 1959b, 1959c). On the Colville National Forest and adjacent private lands in northeastern Washington, infestation was 86% (Graham and Frazier 1962).

20. A. littorum Hawksworth, Wiens & Nickrent, Novon 2: 206, 1992. TYPE COLLECTION: CALIFORNIA: Mendocino County: 4.8 km miles east of Novo on State Highway 20, on Pinus muricata, elevation 30 m, R. L. Mathiasen 8940, December 12, 1989. Holotype US! Isotypes: CAS, FPF, HSC, MO, UC).

Description: Shoots 8–20 (mean 12) cm, brown to yellow-brown, flabellately branched (fig. 16.68). Basal diameter of dominant shoots 2–5 (mean 3.5) mm. Third internode 10–20 (mean 15) mm long, 2–2.5 (mean 2.2) mm wide, mature fruits 4–5 mm long; staminate flowers mostly 4-merous. n = 14.

Phenology: The phenology is poorly known. Meiosis occurs in July, flowering begins in August, with peak anthesis probably occurring in September. Seed dispersal probably peaks in September or October.

Hosts: Pinus radiata and P. muricata are the primary hosts. It occasionally infects P. contorta var. bolanderi where this tree is associated with infected P. muricata.

Distribution: United States: (California: Mendocino, Sonoma, Marin, Monterey, and San Luis Obispo Counties) (fig. 16.69). Arceuthobium littorum is restricted to within about 10 km of the Pacific Ocean from Fort Bragg south to the Point Reyes area on Pinus muricata and along the central coast at Monterey and Cambria on P. radiata. It also parasitizes the small population of P. muricata surrounded by infected P. radiata on Huckleberry Hill near Monterey.

This dwarf mistletoe has been established in at least three locations in the Bay Area, presumably by transplantation of infected Pinus radiata: Monterey-Stanford Arboretum (Peirce 1905), North Berkeley (Offord 1964a), and Hillsborough (Butler 1992). Elevational range is from sea level to ca. 250 m.

Discussion: Previously, we included Arceuthobium littorum in A. occidentale (Hawksworth and Wiens 1972, 1984). Arceuthobium occidentale is primarily a parasite of Pinus sabiniana in the foothills surrounding the Central Valley of California and is not sympatric with A. littorum near the coast. Electrophoretic studies support the segregation of A. littorum as a species distinct from other members of the A. campylopodum–A. occidentale complex (Nickrent and Butler 1989, 1990). A primary feature for distinguishing A. littorum from A. occidentale is the production of large, non-systemic witches’ brooms by A. littorum (fig. 16.70); other differences between A. littorum and A. occidentale are presented in table 16.4.

21. A. microcarpum (Engelmann) Hawksworth & Wiens, Brittonia 22: 268, 1970. TYPE COLLECTION: ARIZONA: Apache County: Sierra Blanca, on Picea engelmannii, Gilbert 112 in 1873 (Lectotype MO! Isotypes: ILL, US). A. douglasii Engelm. var.? microcarpum Engelm. in U.S. Geographical Survey West of 100th Meridian (Wheeler Report) 6: 253, 1878. Razoumofskya microcarpa (Engelm.) Wooton & Standley, Contributions U.S. National Herbarium 19: 179, 1915. Razoumofskya douglasii (Engelm.) Kuntze var. microcarpa (Engelm.) Tubeuf, Naturwissen-schaftliche Zeitschrift für Forstund Landwirtschaft 17: 216, 1919. A. campylopodum Engelm. forma microcarpum (Engelm.) Gill, Transactions Connecticut Academy Arts and Sciences. 32: 209, 1935.

Description: Mean shoot height ca. 5 (max. 11) cm, green to purple, flabellately branched (fig. 16.71). Basal diameter of dominant shoots 1.5–3.0 (mean 2) mm. Third internode 5–16 (mean 9.3 ± 2.2) mm long, 1–2 (mean 1.5) mm wide (9 collections), length/width ratio 6.2:1. Staminate flowers 2.3 mm across; perianth mostly 3-merous (rarely 4-merous); segments ca. 1.2 mm long, 1.0 mm wide. Mean anther diameter 0.4 mm, centered 0.6 mm from tip of segment. Pollen polar diameter 18–21 (mean 19) µm; equatorial diameter 21–23 (mean 22) µm; polar/equatorial diameter ratio 1:1.6; spine height (1.2 µm) approximately equal to the wall thickness (1.4 µm) (1 collection). Pistillate flowers ca. 1 mm long, 1 mm across. Mature fruit 3.5 x 2.0 mm; proximal portion ca. 2.5 mm long. Seeds 2.4 x 1.0 mm. n = 14.

Phenology: Meiosis in July. Anthesis in mid-August to early September, with extremes from late July to late September (fig. 16.72). Fruits mature in September, with extremes from late August to early October; maturation period averages 12–13 months.

Hosts: This dwarf mistletoe is a common and serious pathogen only on Picea engelmannii and P. pungens, but on the San Francisco Peaks (northern Arizona) it also parasitizes Pinus aristata (Mathiasen and Hawksworth 1980). Pinus strobiformis and Abies lasiocarpa var. arizonica are rarely infected, even where they are associated with infected Picea or Pinus.

Distribution: United States (Arizona and New Mexico) (fig. 16.73). Arceuthobium microcarpum has one of the more restricted distributions in the genus. In Arizona, the parasite occurs on the North Rim of the Grand Canyon, the San Francisco Peaks and nearby Kendrick Peak, White Mountains, and in the Pinaleno (Graham) Mountains, but not in the Chiricahua Mountains. In New Mexico, this dwarf mistletoe is present at several locations in the Mogollon Mountains, and it was discovered in the Sacramento Mountains, an outlier some 275 km from the closest known populations in the Mogollon Mountains (Mathiasen and Jones 1983).

A distribution map of Arceuthobium microcarpum in New Mexico was published by Martin and Hutchins (1980). Elevational range is 2,400–3,150 m (Acciavatti and Weiss 1974).

Discussion: The original specimens cited by Engelmann are Gilbert 100 and 102. The only specimen now at the Missouri Botanic Garden is Gilbert 112, which is labeled in Engelmann’s handwriting as the type. The original label for specimen 100 is in the same packet. Presumably, Gilbert 102 in the original citation should have been 112. The collections are now combined, so Gilbert 112 is our designate for a lectotype.

This localized species in Arizona and New Mexico is characterized by its near exclusive occurrence on Picea. Gill (1935) gave a much larger distribution for Arceuthobium microcarpum (to California, Idaho, and Montana). Hawksworth and Graham (1963) have shown, however, that Gill’s records are based on rare parasitism of spruce by other dwarf mistletoes (A. laricis, A. douglasii, A. americanum, A. tsugense, and, possibly, A. cyanocarpum).

This species induces small, dense witches’ brooms. Heavily infected trees may bear hundreds of such witches’ brooms. This dwarf mistletoe causes heavy mortality in stands of Picea pungens and, to a lesser extent, of P. engelmannii. This dwarf mistletoe is not found in the principal range of P. engelmannii in the central or northern Rocky Mountains.

22. A. monticola Hawksworth, Wiens & Nickrent, Novon 2: 205, 1992. TYPE COLLECTION: OREGON: Josephine County, 11 km south of O’Brien on Old Gasquet Toll Road (Oregon Mountain Road); parasite of Pinus monticola, T. 13 S., R. 9 W., Section 9; latitude 42°1' N, longitude 123°46' W, elevation 650 m, D. Wiens 6575, 20 July 1987 (Holotype US! Isotypes: FPF, MO, ORE, OSC, UC).

Description: Shoots 5–10 (mean 7) cm tall, dark brown, flabellately branched (fig. 16.74). Basal diameter of dominant shoots 2–4 (mean 3) mm; third internode 8–15 (mean 12) mm long, 1.5–2.0 mm wide. Staminate flowers mostly 3-merous. Mature fruits 4.0–4.5 mm long, 2.0–2.5 mm wide. n = 14.

Phenology: The period of anthesis is poorly known but apparently occurs in late July–August. Fruits mature October–November; maturation period averages ca. 15 months.

Hosts: The principal and only commonly infected host is Pinus monticola. Pinus lambertiana is a secondary host, Picea breweriana an occasional host, and Pinus jeffreyi a rare host (only one case known).

Distribution: United States (Oregon and California) (fig. 16.75). Arceuthobium monticola is a local endemic in the Klamath Mountains of southwestern Oregon (Coos, Curry, and Josephine Counties) a