Arceuthobium is widely distributed in the Northern Hemisphere (fig. 5.1). The 8 species in the Old World range from Spain and Morocco to the Himalayas of southwestern China, with outlying species on the Azores and in East Africa. In the New World, the genus is represented by 39 taxa concentrated primarily in Mexico and the western United States (fig. 5.2). The following two regions of North America show the greatest concentration of taxa:
Of the 21 taxa in the United States, all but 1 are western (8 are endemic to the western states); 14 occur in California (3 of which are endemic), 11 in Oregon, and 8 in Arizona (table 5.1). A single species, Arceuthobium pusillum, occurs in the north-central and northeastern United States. Six taxa found in the United States also occur in Canada:
There are 22 known taxa of Arceuthobium in Mexico (table 5.2). Although dwarf mistletoes are reported for 24 of the 32 states, actual distributions are poorly understood. Potential hosts occur in 5 of the states not reporting the occurrence of dwarf mistletoe (Aguascalientes, Baja California Sur, Colima, Morelos, and Quintana Roo); there are no indigenous Pinaceae in the remaining 3 states (Campeche, Tabasco, and Yucatán). Eight taxa are found in both Mexico and the United States (A. abietinum f. sp. concoloris, A. apachecum, A. blumeri, A. campylopodum, A. divaricatum, A. douglasii, A. gillii, and A. vaginatum subsp. cryptopodum). Guatemala has 4 known taxa; and Honduras, Belize, and Hispaniola each has a single endemic species. Arceuthobium has been reported from El Salvador on the basis of visual observations of witches’ brooms, but no specimens are available.
In the Old World, Arceuthobium is represented by 8 species—Arceuthobium azoricum, A. chinense, A. juniperi-procerae, A. minutissimum, A. oxycedri, A. pini, A. sichuanense, and A. tibetense. The widely distributed A. oxycedri is found from China (Xizang), throughout the Mediterranean region, to Spain. This extensive east–west distribution (nearly 10,000 km) can be explained by its presence (and persistence) in latitudes relatively unaffected by Pleistocene glaciation. Distributions of the 6 species from central and eastern Asia are listed in table 5.3. Arceuthobium minutissimum is a markedly reduced species that parasitizes Pinus wallichiana in the Himalayas of Pakistan, India, Nepal, and Bhutan. Although it resembles A. pusillum and A. douglasii of the New World in diminutive size, systemic infection, and host specificity, we now believe the similarity to be the result of convergence rather than close phyletic relationship. Previously, we listed only 2 species in China—A. chinense on Abies and Keteleeria, and A. pini on Pinus (Hawksworth and Wiens 1972). Recent studies in China have revealed 3 additional species—A. oxycedri on Juniperus, A. sichuanense on Picea, and A. tibetense on Abies (Kiu 1984a).
The distribution of a dwarf mistletoe species is generally centered within the range of its principal host or hosts. There are only four exceptions to this "principle of central distribution." The explanation probably lies in the particular paleogeographic history of each species.
The geographic co-occurrence of species, sympatry, is an important consideration in taxonomy. The rule of sympatry is that two genetically compatible taxa will hybridize if they have overlapping flowering periods, are in close proximity, and are pollinated by mutual agents.
Dwarf mistletoe plants within 30 m of each other are sufficiently close to be considered sympatric because that distance is well within the maximum range of pollen dispersal. In Manitoba, Canada, Gilbert and Punter (1984) trapped airborne pollen of Arceuthobium americanum 400 m from the closest pollen source; in Colorado, Coppola (1989) found A. americanum pollen 500 m from the closest pollen source. In New Mexico, Potter and Rowley (1960) discovered a single grain of Arceuthobium pollen 5 km from the nearest pollen source. In California, Leopold (1967) discovered dwarf mistletoe pollen in 8 of 11 pollen traps in a transect across the dry Searles Lake. The only dwarf mistletoe in the area was A. divaricatum and its nearest population was 16 km north on Argus Peak.
Most North American taxa of dwarf mistletoes are sympatric in some part of their distributions with at least one other species (fig. 5.3, tables 5.4 and 5.5). The allopatric taxa are Arceuthobium aureum subsp. aureum, A. bicarinatum, A. guatemalense, A. hawksworthii, A. hondurense, A. littorum, A. oaxacanum, and A. pendens.
Distributions of Old World species are too poorly known for determining which species are sympatric. Arceuthobium minutissimum and A. oxycedri are probably sympatric in the Indian Himalayas at elevations where host species overlap (Brandis 1907). The Chinese species, however, are probably separated by elevation and thus not sympatric (Kiu 1984b).
Sympatry between 2 species of Arceuthobium is not unusual (fig. 5.3), but we have observed only 10 instances (table 5.4) of sympatry among 3 species and 2 cases (table 5.5) of sympatry involving 4 species.
An interesting but rare aspect of dwarf mistletoe parasitism is the occurrence of 2 different dwarf mistletoe species on a single host tree, "dual parasitism" (table 5.6 and fig. 5.4). The phenomenon is probably rare because of host competitive exclusion (chapter 6). Dwarf mistletoes also rarely parasitize trees already infected by other mistletoe genera; we know of only 5 such associations (table 5.7).
The oldest known fossil pollen of Arceuthobium is from the middle Eocene Epoch in eastern Germany. Identified as Spinulaepollis arceuthobiodes (Krutzsch 1962), it was found at more than 60 sites. Most pollen grains were found in strata from the upper Eocene to Miocene Epochs (extremes from middle Eocene to Pliocene Epochs). Stuchlik (1964) studied Miocene deposits in Poland and considered the pollen similar to, if not identical with, modern A. oxycedri. The northern limits of A. oxycedri are now about 800 km south of the German and Polish fossil pollen sites (table 5.8). Stuchlik also found a rare large-grained pollen, putatively belonging to a new subspecies Spinulaepollis arceuthobiodes subsp. major. Pollen grains of this subspecies ranged 24 to 30 µm in diameter, whereas other collections of the species ranged 20 to 24 µm. Although swelling caused by fossilization or laboratory preparation could have affected pollen size, our studies indicate that a range as great as 20 to 30 µm is unusual within a species (pollen of A. oxycedri is only 18 to 20 µm). Therefore, the large-grained collection may well represent an extinct taxon.
The fossil genus Patzea (Caspary 1872), which was considered by Engler and Krause (1935) and Nemejc (1975) to be synonymous with Arceuthobium, was described from Oligocene amber formations along the Baltic Coast. However, drawings published by Conwentz (1886) suggest to us and to Lancucka-Srodoniowa (1980) that Patzea is not congeneric with Arceuthobium.
Only 3 North American sites of the Miocene Epoch have yielded Arceuthobium pollen: (1) north flank of the Alaska Range, associated with Pinus and Picea pollen (Wahrhaftig and others 1969); (2) Troublesome Formation in north-central Colorado, associated predominantly with the pollen of Picea, Pinus, and Abies (Weber 1965); and (3) Split Rock, Carbon County, Wyoming (Leopold and Denton 1987).
Several reports of Arceuthobium pollen from the Quaternary Period in western North America and Mexico are listed in table 5.9. Pollen ages range from the present to about 30,000 years BP. A reasonable guess regarding the species of pollen can usually be made from the associated host’s pollen and location of the site. Adam (1967) reported the irregular occurrence of Arceuthobium pollen over the last 10,000 years in the central Sierra Nevada of California. He noted that Arceuthobium pollen abundance peaked when pine populations were decreasing and suggested that Arceuthobium was more abundant during drier periods. Certainly, dwarf mistletoe on Pinus ponderosa in California is now more abundant in drier regions. Janssen (1968) also noted that A. pusillum in Minnesota was more abundant when spruce populations were decreasing.
Arceuthobium pusillum has the most complete fossil pollen record within the genus. A number of pollen-bearing sites have been located both within and outside the modern distribution of the species (table 5.10). Pollen that is presumably A. pusillum has been found in the southeastern United States (North Carolina, Georgia, and Tennessee), 500 km south of its present southern limit in northern New Jersey. Recorded pollen ages range from 10,000 to nearly 40,000 years BP (table 5.10). Whitehead and Barghoorn (1962) suggest that this pollen may represent a species of dwarf mistletoe from the western United States or an extinct species. Most fossil pollen, however, is associated with spruce pollen and needles, and A. pusillum is the only species of dwarf mistletoe in eastern North America. It rarely parasitizes any host other than spruce, and no other species of dwarf mistletoe presently occurs within 2,000 km of the fossil sites. Therefore, it is likely that the pollen is either A. pusillum or a close extinct relative.
Spaulding and Martin (1979) identified pollen of Arceuthobium in 11,000-year-old ground sloth dung from the Guadalupe Mountains, Texas. Because pine pollen was also common in the dung, the species would likely be either A. divaricatum or A. vaginatum subsp. cryptopodum, both of which presently occur in the Guadalupe Mountains.
The oldest macrofossils of Arceuthobium are known from the upper Miocene Epoch in Lower Silesia, Poland (Lancucka-Srodoniowa 1980). These collections include many well preserved plants with shoots, flowers, and fruits. Morphologically, the fossil plants resemble modern A. oxycedri except for the number of perianth segments. The frequency of 3- or 4-merous flowers in fossil plants is approximately equal; flowers of modern plants are mostly 3-merous (95%) (Hawksworth and Wiens 1976). Lancucka-Srodoniowa (1980) states that some fossil dwarf mistletoe plants are still attached to the host branches. Although the host is not identified, Juniperus is the only modern Old World host found west of the Himalayas.
Chaney and Mason (1930, 1933) and Mason (1934) discovered fossils of Arceuthobium in Pleistocene formations of coastal California. Axelrod (1966) dated the sites as late Pleistocene Epoch and obtained radiocarbon dates of 30,000 to 40,000 years BP. The sites were Carpinteria, Santa Cruz Island, and Tomales Bay (fossils were abundant at Santa Cruz and Tomales). The dwarf mistletoe was associated with Pinus radiata and P. muricata and was indistinguishable from modern A. littorum (reported as A. campylopodum), which presently parasitizes these pines. This species does not now occur at the Carpinteria or Santa Cruz sites, although Pinus muricata is still present on Santa Cruz Island. The closest populations of A. littorum are on P. radiata near Cambria, about 60 km north of Carpinteria, and on P. muricata at Inverness Ridge adjacent to Tomales Bay. Chaney and Mason (1934) note that the fossil plant had primarily 4-merous staminate flowers; although this character is variable, 4-merous flowers are most common in modern populations of A. littorum.
Fragments of Arceuthobium have been identified in packrat (Neotoma spp.) middens from the southwestern United States and adjacent Mexico (Van Devender and Hawksworth 1986). If a site is well protected under a rock ledge, material can remain identifiable for thousands of years (occasionally as long as 40,000 years). Because packrats collect vegetation only within 50 m of a den, these accumulations provide a natural herbarium specific to a small, local site (in contrast to pollen collections, which tend to be more regional). By radiocarbon dating various layers of the midden, it is possible to construct a nearly complete floristic history of the site (Van Devender and others 1977). At least two species (A. divaricatum and A. cyanocarpum) and possibly a third species (A. abietinum) have been detected in packrat middens. Arceuthobium divaricatum, a parasite of pinyon pines, has been found in middens from the following areas:
In all cases except the last, the host species is still present near the fossil site. The Chisos Mountains site is of particular interest because the dwarf mistletoe was associated with Pinus remota, a pinyon not now known to be parasitized by Arceuthobium. No dwarf mistletoe now occurs in the Chisos Mountains, but P. cembroides is present. Pinus cembroides is parasitized by A. divaricatum in the Davis Mountains, 190 km north of the Chisos.
Arceuthobium cyanocarpum, a parasite of Pinus flexilis, P. longaeva, and related pines, has been found in middens at 3 locations:
In the latter area, the dwarf mistletoe was associated with P. flexilis, a species that no longer occurs in the Grand Canyon area, but P. flexilis is now parasitized by A. cyanocarpum near Navajo Lake in southern Utah, 190 km north.
Arceuthobium abietinum is perhaps the species reported from a 30,400-year-old midden in the Sheep Range, Nevada (Spaulding 1977). Abies concolor was the predominant associated conifer, and Arceuthobium abietinum still occurs on this host in the Sheep Range.
Because dwarf mistletoes have not colonized eastern and southern pines, the occurrence of Arceuthobium pusillum on eastern spruces is perplexing. Arceuthobium pusillum could have evolved somewhere outside its present distribution, perhaps in northwestern North America. According to Mirov (1967), the pines in eastern and western North America have been essentially isolated since the Pliocene Epoch; hence, the dwarf mistletoes may not have had an opportunity to colonize the eastern pines.
Perhaps Pleistocene glaciations played a role in eliminating Arceuthobium pusillum, or its immediate ancestors, from the northern and western portions of the present distribution of spruces. The pollen of Arceuthobium occurred in the southern spruce refugia in the Appalachian Mountains of North Carolina, South Carolina, and Georgia and possibly also on the Gaspé Peninsula, Québec. During the Holocene Epoch, A. pusillum again moved northward but not as far as its spruce hosts had migrated. Both Picea mariana and P. glauca now occur as far northwestward as Alaska, but A. pusillum just enters eastern Saskatchewan.
Picea apparently migrated to North America from Asia in two or more waves (Gordon 1968, Wright 1955). The available information on dwarf mistletoe parasitism of Picea tends to confirm this. The species that arrived in the Cretaceous Period or earlier (relatives of Picea breweriana, P. chihuahuana, and possibly P. sitchensis) are not principal hosts for any dwarf mistletoes. Spruces that evolved later, in the early Tertiary Period (P. pungens, P. engelmannii, P. mariana, P. rubens, and P. glauca) are heavily parasitized (by Arceuthobium microcarpum and A. pusillum).
The three dwarf mistletoe taxa that presently have far northern distributions are Arceuthobium americanum, A. pusillum, and A. tsugense subsp. tsugense. Their presumed northward migrations of 500 to 1,500 km since the last Wisconsin glaciation are difficult to explain unless there were refugia within the glaciated areas where the host and parasite persisted, or there was significant long-distance dispersal by birds. Current rates of spread by explosive fruits average only 0.3 to 0.6 m per year through infested stands (Hawksworth 1958, 1961a); these rates are insufficient to account for the observed migrations.
The history of Pinus contorta in the late Quaternary Period was reviewed by Critchfield (1985). There is limited agreement that a refugium for P. contorta existed in the Yukon, but the evidence for an ice-free corridor along the eastern slope of the Rocky Mountains in Alberta is more debatable. Pollen evidence does not indicate that Arceuthobium americanum survived in the Yukon refugium, but there are records of 7,000- to 10,000-year-old pollen of this dwarf mistletoe as far as latitude 56°30´ N in the Banff-Jasper and Peace River areas (White and Mathewes 1986, Macdonald 1989). This is within about 400 km of the present northern limits of A. americanum, which requires a 40-m/year rate of spread (2 orders of magnitude greater than spread by explosive fruits).
The higher rates of migration than explained by explosive fruits suggest that a supplemental means of dispersal, possibly by birds, may have been involved (Nicholls and others 1984). Several studies document the dispersal of dwarf mistletoes for distances up to 0.5 km (chapter 8). Definitive studies are needed on the possible role of birds in dispersal for longer distances. Because dwarf mistletoes are dioecious, establishment of a new population requires that least 2 seeds of different sex are transported to distant safe-sites near each other, and that these seeds germinate, cause infection, flower, and produce mature seeds. Although di-embryonic seeds occur in Arceuthobium (chapter 3), their frequency is so low it seems unlikely that polyembryony would play any role in long-distance dispersal. Because seeds mature in the autumn, long-distance bird migration would tend to disseminate seeds southward. Although the responsible dispersal mechanisms are unknown, at least 6 species of mistletoe have become established 10 to 200 km from the closest naturally infested stands (table. 5.11).
The widespread occurrence of Arceuthobium tsugense in the forests of southeastern Alaska, the Queen Charlotte Islands, and coastal British Columbia suggests that there were refugia where Tsuga heterophylla and its dwarf mistletoe survived well north of the southern limits of glaciation in British Columbia (Dillon 1956). This is approximately 1,600 km south of the present northern limits of distribution (latitude 60°N) of this dwarf mistletoe.
The disjunct distributions of two species on oceanic islands (Arceuthobium bicarinatum and A. azoricum) have special biogeographical interest. Arceuthobium bicarinatum likely reached Hispaniola during the Miocene Epoch when land connections may have existed between Caribbean islands and Central America (Mirov 1967). Although separated by 1,600 km, A. bicarinatum and A. hondurense (an endemic of Honduras) are morphologically similar and may be vicariants that were widespread during the Miocene Epoch (but see chapter 15). The distinctiveness of A. azoricum and its host Juniperus brevifolia suggests a long and isolated evolutionary history rather than recent introduction by long-distance dispersal as proposed by Ridley (1930) (also see chapter 16). These species are probably survivors from the early Tertiary Period as continental genera became restricted to recurrent volcanic islands along the mid-Atlantic ridge (McKenna 1972).
The lack of dwarf mistletoes on low-elevation Caribbean pines is difficult to explain. These trees were presumably exposed to parasitism by dwarf mistletoes during the evolution of Arceuthobium bicarinatum when there was a land bridge between Central America and Hispaniola. Pinus occidentalis, the host of A. bicarinatum is, however, a relatively high-elevation species. Although A. hawksworthii infests P. caribaea var. hondurensis in the uplands of Belize, no dwarf mistletoes occur on the extensive low-elevation stands of P. caribaea elsewhere in the Caribbean Basin. In fact, there are no species of Arceuthobium in any lowland, moist, tropical climate. Arceuthobium tsugense is widespread on Tsuga heterophylla in the temperate rain forests along the northwestern coast of North America, and A. juniperi-procerae occurs in moist, high-elevation forests in the Kenya highlands near the Equator. It appears that dwarf mistletoes, which presumably originated in temperate regions, are unable to adapt to warm, moist, low-elevation tropical environments (see chapters 15 and 16).