Chapter 7—Ecological Relationships


Stand history is the most significant factor affecting the distribution of populations of Arceuthobium. Stand disturbances such as fire (Alexander and Hawksworth 1975), volcanic activity (Hawksworth 1960a), long-term climatic change, and silvicultural practice can profoundly affect dwarf mistletoe distribution. Long-term climatic change has eliminated dwarf mistletoe species from some areas. For example, Arceuthobium divaricatum occurred in the Chisos Mountains, Texas, during the Pleistocene Epoch (Van Devender and Hawksworth 1986), but the nearest modern populations are now about 200 km north in the Davis Mountains. Climatic warming in the southwestern United States over the last few thousand years has greatly fragmented the surviving populations of various dwarf mistletoes. Some populations of A. vaginatum subsp. cryptopodum that were apparently once contiguous are now separated by distances as large as 80 km (Hawksworth 1969). In addition to these long-term changes, climatic, topographic, and site factors also influence the current distribution of dwarf mistletoe populations. Weir 1916(d) commented that "these parasites follow very distinct predilections as to type of stand, topography, and to a certain extent, climate, from which the zones of greatest mistletoe infection may be quite readily determined."


Climatic Factors

Quantitative effects of climate on the distribution of dwarf mistletoe species are little studied. Climatic factors, however, are probably responsible for the limited distributions of several species that do not occur throughout the range of their host.

Working in British Columbia, Smith and Wass (1979) inoculated the principal hosts of Arceuthobium americanum and A. douglasii at sites beyond the natural range of these species. Their results indicated that minimum temperature was probably the factor limiting mistletoe distribution. Temperatures less than -39°C were lethal to A. americanum, and temperatures less than -29°C were lethal to A. douglasii. Correspondingly, the latitudinal distribution of A. americanum is greater than A. douglasii (see chapter 16 for additional information).

The northern limits of Arceuthobium vaginatum subsp. cryptopodum are in northern Colorado, but its host, Pinus ponderosa var. scopulorum, extends more than 700 km farther north to central Montana. The northern limit of distribution for this mistletoe species is likely related to climatic factors. Inoculation tests of P. ponderosa seedlings from the Black Hills of South Dakota (350 km north of present mistletoe distribution) showed that these seedlings are as susceptible to infection as those from within the present distribution of the parasite (Hawksworth 1963). Mark and Hawksworth (unpublished data) compared temperatures for 93 weather stations at sites in P. ponderosa forests that were either within or outside the range of A. vaginatum subsp. cryptopodum. The mistletoe was absent at all sites where mean January temperature was less than 6°C.

Arceuthobium douglasii is distributed throughout most of the range of Pseudotsuga menziesii, with the following notable exceptions: (1) the mistletoe is absent from the northeastern range of the host species in Colorado, Wyoming, and Montana; (2) it generally does not occur west of the Cascade Crest in British Columbia, Washington, and Oregon; and (3) its northern limits in British Columbia are about 500 km south of the northern distribution of its host.

The absence of Arceuthobium douglasii in western Oregon and Washington has been the topic of several studies. Wicker (1969) successfully inoculated native Pseudotsuga menziesii with A. douglasii at Diamond Point and Wind River in western Washington. Wicker (1974b) suggested that physiographic processes, forest succession, and fire have prevented the immigration and establishment of this mistletoe in western Washington. Tinnin and Knutson (1973) found A. douglasii at 7 sites west of the Cascade Crest (Clackamas, Linn, and Lane Counties in northern Oregon), but these sites were all within 15 km of the Crest. Tinnin and others (1976) and Tinnin (1978) later reported 2 additional sites with A. douglasii in the same general area. They agreed with Wicker (1974b) that fire and other natural disturbances probably account for the species’ limited distribution on the western slope of the Cascades. The Columbia River Gorge, with its continuous stands of P. menziesii through the Cascades, would seem to provide a natural corridor for the migration of A. douglasii. The mistletoe, however, does not occur in these low-elevation stands (<700 m), which may lie below the lower elevational limit of the parasite.


Topographic Factors

The most detailed information on distribution of dwarf mistletoe populations in relation to elevation, topographic position, steepness of slope, and aspect is based on research in Arizona and New Mexico.



Hawksworth (1959a) surveyed Arceuthobium vaginatum subsp. cryptopodum on the Mescalero Apache Indian Reservation in southern New Mexico and collected information for 2,464 plots distributed over 560,000 ha. In that forest, the distribution of dwarf mistletoe was strongly related to elevation (fig. 7.1) and was most abundant within the mid-elevational range of Pinus ponderosa (2,350 to 2,400 m). In a broader survey of forests throughout Arizona and New Mexico, Andrews and Daniels (1960) concurred that mistletoe abundance was greater above 2,010 m (table 7.1). Gottfried and Embry (1977) studied mistletoe distribution in a high-elevation, mixed conifer watershed in eastern Arizona. They found that the incidence of mistletoe (percentage of host trees infected) was highest (79%) at elevations below 2,650 m, moderate (45%) within 2,650 to 2,750 m, and least (30%) at elevations above 2,750 m. Studies in southern New Mexico (Hawksworth 1961a) and along the Front Range in Colorado (Williams and others 1972) show that dwarf mistletoe is found to the upper limits of P. ponderosa but not to the lowest limits. In northern Colorado, the approximate upper limits of both the host and mistletoe are 2,800 m, but pine populations are found as low as 1,600 m, and mistletoe populations descend to 1,900 m. The absence of the mistletoe at the low elevations may be due to high summer temperatures (Williams and others 1972), moisture stress (Fisher 1975), or both these factors.

On the Mescalero Apache Indian Reservation, Arceuthobium douglasii was distributed about equally throughout the elevational range (2,315 to 2,530 m) of its host, Pseudotsuga menziesii (Hawksworth 1959a). Gottfried and Embry (1977) found a similar relationship in their high-elevation watershed (2,560 to 2,835 m). Throughout the southwestern United States, A. douglasii has an upper elevational limit at least 60 m below that of its host (unpublished data). In the Capitan Mountains, New Mexico the upper limit of the mistletoe is about 2,750 m, approximately 300 m below the upper limits of its host. In the Magdalena Mountains, New Mexico the upper limits for parasite and host are 2,900 m and 3,600 m, respectively.

Acciavatti and Weiss (1974) reported on the distribution of Arceuthobium microcarpum for the Fort Apache Indian Reservation, Arizona. Although Picea engelmannii was found at elevations from 2,750 m to over 3,350 m, A. microcarpum occurred only as high as 3,170 m and was abundant only below 2,900 m. On the San Francisco Peaks in central Arizona, the dwarf mistletoe’s upper elevation limit is 3,140 m—even though the host reaches an elevation of 3,600 m (Mathiasen and Hawksworth 1980).

In the central Rocky Mountains, the upper elevational limit of Arceuthobium americanum is at least 185 m below that of its principal host, Pinus contorta (Hawksworth 1956b, Hawksworth and Johnson 1989a, Williams and others 1972). The elevation of this upper limit for mistletoe varies with latitude, and it ranges from 2,800 m in northern Wyoming to 3,350 m in central Colorado (fig. 7.2). The factors controlling for this limit are not known, but some information is available from an unpublished study conducted by Hawksworth and Laut in northern Colorado. They transplanted mistletoe-infected seedlings to a site about 120 m above the natural limits of the parasite. The mistletoe has survived for at least 20 years. Each year, the plants flowered, were pollinated, and initiated fruit development, but fruits failed to mature before the first killing frosts of autumn. A similar phenomenon may limit the northern distribution of A. douglasii (Smith 1972).

Arceuthobium tsugense has an upper distributional limit well below the upper limits of its host (Tsuga heterophylla) in southeast Alaska (Drummond and Hawksworth 1979). Although the host attains elevations in excess of 610 m, the dwarf mistletoe occurs to only 365 m and is rare above 150 m.

Observations along the Granite Cairn road and the Brunton trail northeast of Augustine, Belize, indicate a lower elevational limit of Arceuthobium hawksworthii on Pinus caribaea var. hondurensis near 700 m, but earlier collections place it as low as 520 m. Regardless of its actual lower altitudinal limit, A. hawksworthii is clearly absent from the lower elevational range of P. caribaea var. hondurensis in the Mountain Pine Ridge region! Given the tropical distribution of this dwarf mistletoe (latitude 17°N), its absence from the lower 300 m of the elevational distribution of A. caribaea var. hondurensis is an interesting ecological question.


Topographic Position

Topographic position may affect the distribution of some dwarf mistletoe species. For example, several studies show that Arceuthobium vaginatum subsp. cryptopodum on Pinus ponderosa is most abundant on ridge tops, intermediate on slopes, and least common on bottom sites (table 7.2). Various other studies have also recorded high abundances of dwarf mistletoe species on ridge sites—A. americanum on P. contorta in the central Rocky Mountains (Hawksworth 1958), A. americanum on P. banksiana in Alberta (Dowding 1929), and A. oxycedri on Juniperus in Pakistan (Zakaullah 1977). An exception is A. douglasii on Pseudotsuga menziesii in New Mexico, which only showed weak correlation with topographic position (Hawksworth 1959a).

Some studies suggest that dwarf mistletoes are more abundant on middle slopes than on upper or lower slopes—Arceuthobium douglasii on Pseudotsuga menziesii and A. vaginatum subsp. cryptopodum on Pinus ponderosa in Arizona (Gottfried and Embry 1977). Other studies, however, report greater abundance on upper slopes for A. vaginatum subsp. cryptopodum on P. ponderosa in Colorado (Merrill and others 1987) and Arizona (Larson and others 1970). Obviously, the relationship of dwarf mistletoe distribution to topographic position is variable and requires further study.


Steepness of Slope

The incidence of dwarf mistletoe varies with respect to slope angle, categorized as gentle for slopes <10%, moderate for slopes 10 to 30%, and steep for slopes >30%. Hawksworth (1959a) found that incidence of Arceuthobium vaginatum subsp. cryptopodum on Pinus ponderosa in southern New Mexico was greater (57%) on gentle slopes than on steep slopes (45%). Incidence of A. douglasii on Pseudotsuga menziesii in the same area showed a similar but lower trend of 27% on gentle slopes and 3% on steep slopes. In a survey of A. vaginatum subsp. cryptopodum on Pinus ponderosa conducted throughout Arizona and New Mexico, Andrews and Daniels 1960 found that differences in mistletoe incidence were not statistically significant between moderate and steep slopes but were significantly different between gentle slopes and either moderate or steep slopes. In central Colorado, Hawksworth (1968b) observed a strong negative correlation between slope and incidence of A. vaginatum subsp. cryptopodum—gentle slopes, 87%; moderate slopes, 51%; and steep slopes, 34%. In contrast, Gottfried and Embry (1977) observed a positive correlation between slope and incidence on their high-elevation watershed—gentle slopes, 1%; moderate slopes, 25%; and steep slopes, 69%. In the same study area, the highest incidence of A. douglasii occurred on moderate slopes. Larson and others (1970) reported no correlation between slope and incidence of A. vaginatum subsp. cryptopodum near Flagstaff, Arizona. Working on the Pringle Falls Experimental Forest in central Oregon, Roth (1954) found the incidence of A. campylopodum on P. ponderosa was positively correlated with slope angle.



Hawksworth (1959a) observed that Arceuthobium vaginatum subsp. cryptopodum on Pinus ponderosa in southern New Mexico was more abundant on west and southwest slopes and was less abundant on north and northeast slopes, but A. douglasii was most abundant on north aspects. Gottfried and Embry (1977), in contrast, reported that A. vaginatum subsp. cryptopodum was more abundant on east and south slopes and that A. douglasii was more abundant on south aspects. As with other site factors, the relation between aspect and mistletoe incidence is not definitive.


Soil Types

The effect of soil type on dwarf mistletoe abundance is poorly understood, and most available information is only anecdotal. Douglas (1914) in the journals of his 1826 expedition through eastern Washington, noted that dwarf mistletoe (presumably Arceuthobium americanum on Pinus contorta) was most common on dry, sandy soils. Loret (1859) observed that A. oxycedri in southern France was confined to Juniperus growing on sterile clay soils. Korstian (1924b) reported that A. campylopodum on P. ponderosa in the Payette National Forest, Idaho, was most abundant on basaltic soils.

Several quantitative studies involving soils and dwarf mistletoe distribution have been conducted for Arceuthobium vaginatum subsp. cryptopodum on Pinus ponderosa. On the Manitou Experimental Forest in central Colorado, Hawksworth (1968a) evaluated mistletoe abundance on 3 soil types: sandstone, granitic, and arkose. Although there were differences in mistletoe incidence by soil type, soil effects were confounded with steepness of slope—mistletoe was more abundant (59%) on sites with gentle slopes and arkose soils than on sites with steep slopes and granitic soils (32%). On the Beaver Creek Watershed south of Flagstaff, Arizona, Larson and others (1970) found mistletoe on 19% of plots in the Siesta–Sponseller soils group (derived from volcanic cinders and basalt) and 12% of plots in the less fertile Brolliar soils group (derived from basalt alone).


Habitat Types

Classification of potential vegetation by habitat types based on climax overstory and understory species is commonly used throughout the western United States (Daubenmire and Daubenmire 1968). The relationships between dwarf mistletoes and habitat types in western forests were summarized by Mathiasen and Blake (1984).

Most of the literature on dwarf mistletoe–habitat relationships is observational; few quantitative studies are available. One of the earliest reports was by Dowding (1929); she commented that Arceuthobium americanum on Pinus banksiana in Alberta was more common in the dry "pine–moss" habitat than in the mesic "pine–heath" habitats. The first detailed report of a relationship between dwarf mistletoe occurrence and habitat type was by Daubenmire (1961) for A. campylopodum on P. ponderosa in eastern Washington and northern Idaho. He recognized 7 habitat types in the P. ponderosa series but found dwarf mistletoe only in the driest types—P. ponderosa / Agropyron spicatum and P. ponderosa / Purshia tridentata. Later, Daubenmire and Daubenmire (1968) reported 2 other dry habitat types were also infested—Pinus ponderosa / Festuca idahoensis and P. ponderosa / Stipa comata.

Roe and Amman (1970) studied Arceuthobium americanum on Pinus contorta in southeastern Idaho and western Wyoming. They reported the parasite was most abundant in the Abies lasiocarpa / Vaccinium scoparium habitat type, intermediate in the A. lasiocarpa / Pachystima myrsinites habitat type, and least common in the Pseudotsuga menziesii / Calamagrostis rubescens habitat type.

Merrill and others (1987) studied abundance of Arceuthobium vaginatum subsp. cryptopodum on 3 national forests in Colorado and 8 habitat types in the Pinus ponderosa series. The greatest incidence and severity of dwarf mistletoe was found in the dry P. ponderosa / Muhlenbergia montana habitat type, and the lowest was on the more mesic P. ponderosa / Quercus gambelii habitat type in southwestern Colorado.

Mathiasen and Blake (1984) studied the effects of Arceuthobium douglasii on growth of Pseudotsuga menziesii in mixed conifer habitat types of Arizona and New Mexico. Dwarf mistletoe and its host occurred on 13 habitat types, and impacts on growth varied by habitat type. For example, reduction in 10-year radial increment for severely infested trees ranged from 20% in the Abies concolor–P. menziesii / Quercus gambelii habitat type to 75% in A. concolor–P. menziesii / Berberis repens habitat type.


Site Quality Factors

The relationship between site quality for host growth and dwarf mistletoe abundance is complex and has long been debated (Hawksworth 1969). Some mistletoe species are more abundant on poorer sites, whereas others show little relation to site quality (Parmeter 1978). Although the incidence of a particular dwarf mistletoe species in a stand may not be related to site quality, the effects of parasite on the growth and mortality of the host are strongly related to site quality (Hawksworth and Johnson 1989a, van der Kamp 1987). There is, however, a tendency to underestimate the importance of site quality as a factor influencing the abundance of dwarf mistletoes because of poor host growth and low tree densities typical of infested stands (Hawksworth 1961a, Childs and Wilcox 1966).

Korstian and Long (1922) suggested that Arceuthobium vaginatum subsp. cryptopodum is more abundant on "poorer" sites in the Southwest, but a region-wide survey by Andrews and Daniels (1960) found greater incidence on "better" sites. In a detailed inventory of the Beaver Creek Watershed with 1,412 plots, Larson and others (1970) observed the highest incidence on sites of intermediate quality (measured by potential dominant tree height, Meyer 1961):

Site index
(m at age 100)
Percent of
trees infected

I >22 5
II 17-22 28
III <17 14


Merrill and others (1987) studied the relationship of dwarf mistletoe and site index in 3 national forests in Colorado and also found incidence levels were higher on intermediate sites than poor or good sites.

Daubenmire (1961) observed that Arceuthobium campylopodum was found only on the 2 "poorest" of the 7 Pinus ponderosa habitat types he recognized (see discussion above on habitat type). In California, Offord (1961) noted that dwarf mistletoe infestation was more serious on the "poorer" P. ponderosa sites in eastern and southern California than on the "better" sites along the western Sierra Nevada. However, Childs and Edgren (1967) found no correlation between dwarf mistletoe abundance and site quality near Chemault, Oregon.

Hadfield (1977) intensively surveyed stands in eastern Oregon and eastern Washington; he found incidence of Arceuthobium americanum varied with site index and was greatest in stands of intermediate site quality. Alexander (1975) reported that A. americanum in the Rocky Mountains is more abundant on "poor" than on "good" sites .

There are conflicting reports on the site relationships of Arceuthobium americanum on Pinus banksiana. Jameson (1961) reported that the dwarf mistletoe was most common on "poor" sites in Saskatchewan, but Muir and Robbins (1973) noted that the mistletoe is common on both "poor" and "good" sites in northern Alberta.

Buckland and Marples (1952) noted that in British Columbia Arceuthobium tsugense appears to be less abundant on "good" sites where mature stands of Tsuga heterophylla are open. However, in even-aged or climax stands, dwarf mistletoe infection is frequently severe, irrespective of the quality of the site.

Many reports agree that Arceuthobium pusillum is most abundant on "poor," boggy sites. For example, in Newfoundland, Singh (1982) indicated that the parasite was usually found on low-lying, moist to wet sites and did not occur on drier or upland sites. Magasi (1984) surveyed dwarf mistletoe on Picea mariana in the Maritime Provinces and found mistletoe on 20% of all sites but 44% of wet, boggy sites. The data for all sites and wet, boggy sites, respectively, were: 14% and 29% in New Brunswick, 25% and 72% in Nova Scotia, and 8% and 17% on Prince Edward Island.

Offord (1961) suggested that Arceuthobium abietinum on Abies concolor and A. magnificae in California showed weak correlation with site quality and that Arceuthobium californicum may be more common on the best sites. Arceuthobium minutissimum on Pinus wallichiana in Pakistan occurs in the dry zone but not in the mesic zone (Hawksworth and Zakaullah 1985).


Relationships With Fire

Historically, wildfires have been the most important single factor governing the distribution and abundance of dwarf mistletoes (Alexander and Hawksworth 1975, Wicker and Leaphart 1974). Most of the literature on dwarf mistletoe–fire relationships has been observational, but there have been some quantitative studies. Wildfires are frequently effective in limiting dwarf mistletoe populations because trees usually return to burned sites much faster than the parasite returns. In several situations, however, fire may ultimately increase mistletoe abundance and distribution. Spotty fires can leave scattered, live, infected trees that not only regenerate the stand but also re-infest it. Fire can also increase dwarf mistletoe populations by maintaining seral forest types composed of mistletoe-susceptible host species rather than permitting succession to climax species that are immune to mistletoe infection. A good example of this phenomena is found in the Rocky Mountains, where wildfires tend to re-establish seral Pinus contorta stands, which are highly susceptible to Arceuthobium americanum, and to limit development of climax species (Picea and Abies), which are not principle hosts of indigenous mistletoe species (Hawksworth 1975).

Roth (1966) suggested that fires may tend to eliminate any mistletoe-resistant Pinus ponderosa populations that happen to evolve. Infested stands typically contain large amounts of fuels due to the accumulation of dead witches’ brooms, fallen trees, and live brooms in the lower crown. Because of these fuels, normally non-destructive fires become conflagrations that destroy not only infected trees but also any mistletoe-resistant individuals that may have become established.

Koonce and Roth (1980) studied the effects of prescribed burning on Arceuthobium campylopodum on Pinus ponderosa in central Oregon. Their results indicate that dwarf mistletoe can be partially sanitized from thinned or unthinned stands by prescribed understory fires. Scorch heights 30 to 60% of the crown length are required to reduce significantly dwarf mistletoe infestations. Koonce and Roth (1985) also studied the effects of mistletoe on fuel characteristics in sapling and pole-sized stands. Surface fuel loadings were correlated with stand density, and fuels in the lower crown were correlated with stand height and dwarf mistletoe intensity. Branches infected by dwarf mistletoe were larger, more resinous, and persisted longer than healthy branches. Infested stands had higher total fuel loadings.

A series of studies on the interrelationships of wildfires and prescribed burning in central Colorado was conducted in Pinus contorta stands infested with Arceuthobium americanum (Zimmerman 1990). In even-aged stands, 100 to 125 years old, the abundance of dwarf mistletoe was inversely proportional to fire frequencies during the period from the late 1800’s to the 1980’s (Zimmerman and Laven 1984). Non-merchantable and unproductive infested stands could be eliminated (75 to 100% tree mortality) with prescribed burning. Although the method is cost-effective, it requires detailed planning and careful implementation under precise burning conditions (Zimmerman and others 1990).

The effects of fire and Arceuthobium vaginatum subsp. cryptopodum on mortality of old-growth Pinus ponderosa at Grand Canyon National Park were studied by Harrington and Hawksworth (1990). Dwarf mistletoe-infested trees were influenced by fire in several ways. Because infested trees have highly flammable witches’ brooms and lower live crowns, a larger proportion of the crown of an infested tree was likely to be scorched than the crown of a healthy tree. With equal amounts of crown scorch in the 40 to 90% range, the probability of survival of heavily infested trees was less than half that of healthy trees.

The effects of smoke and heat on viability of dwarf mistletoe seeds was studied by Zimmerman and Laven (1987). Seeds of Arceuthobium americanum, A. cyanocarpum, and A. vaginatum subsp. cryptopodum were exposed to smoke from burning conifer needles and branch wood. Seed germination was inhibited for all mistletoe species exposed to smoke for 60 minutes or longer. Seeds of A. americanum were unaffected by 40-minute exposures to smoke from fuels with high moisture contents, but germination was enhanced after exposure for 30 minutes to smoke from dry fuels. The germination of seeds of A. cyanocarpum and A. vaginatum subsp. cryptopodum was little affected by exposure to smoke for 30 minutes.

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