Southwestern Dwarf Mistletoe Research Articles

Arceuthobium vaginatum (southwestern dwarf mistletoe)

Arceuthobium vaginatum (southwestern dwarf mistletoe)

Dwarf Mistletoe and Breeding Bird Abundance in Ponderosa Pine Forests

Southwestern dwarf mistletoe (Arceuthobium vaginatum subsp. cryptopodum) parasitizes ponderosa pine (Pinus ponderosa). It can kill severely infected trees and induce the growth of dense masses of branches that can affect foraging and nesting habitat for wildlife. We tested the hypothesis that higher densities of breeding birds would be correlated with higher levels of southwestern dwarf mistletoe in ponderosa pine forests of Arizona. We estimated densities of 15 species of breeding birds and measured 26 habitat elements in 19 stands. Average dwarf mistletoe ratings (DMR) ranged from 0.0 (uninfested) to 3.7 (severely infested). Although we observed higher densities with higher infestation of dwarf mistletoe for 2 bird species and lower densities for 3 bird species, the effect of dwarf mistletoe on these was minor. Instead, higher snag density, an indirect measure of past mistletoe infestation, was a more important predictor of bird density. With greater snag size or density, the species richness and densities of 3 bird species increased. Because dwarf mistletoe infection can create snags, retaining groups of dwarf mistletoe-infected trees in ponderosa pine stands will provide a continued source of snags, which are important habitat for many wildlife species.

Bark beetles and dwarf mistletoe interact to alter downed woody material, canopy structure, and stand characteristics in northern Colorado ponderosa pine

Due to the recent outbreaks of bark beetles in western U.S.A., research has focused on the effects of tree mortality on forest conditions, such as fuel complexes and stand structure. However, most studies have addressed outbreak populations of bark beetles only and there is a lack of information on the effect of multiple endemic, low level populations of biotic disturbance agents that could influence forest dynamics, fuel heterogeneity, and species composition. Downed woody material, fuel parameters and stand characteristics were assessed in areas of ponderosa pine (Pinus ponderosa) infested with southwestern dwarf mistletoe (Arceuthobium vaginatum subsp. cryptopodum), and in areas with endemic populations of mountain pine beetle (Dendroctonus ponderosae) and Ips spp. (Coleoptera: Curculionidae, Scolytinae) in northern Colorado. Both endemic bark beetles and dwarf mistletoe were associated with more dense stands than uninfested plots and resulted in reducing basal area of live trees. The amount of downed woody material was positively related to time since tree mortality and basal area of bark beetle-attacked trees. There was an increase of up to 10% for fine and coarse downed woody material for every increase of 1m2ha−1 basal area of mortality from mountain pine beetle that was 4–15years after death. The average intensity of dwarf mistletoe infections on live trees was positively related to amount of 10-h downed woody material and negatively associated with percent live ponderosa pine crown. Fuel bed and canopy characteristics were dependent on the presence of both dwarf mistletoe and mountain pine beetle-caused mortality. Together these endemic biotic disturbances have an effect on downed woody material biomass accumulation, fuel arrangements, stand densities and species composition.

Mortality Associated with a Bark Beetle Outbreak in Dwarf Mistletoe-Infested Ponderosa Pine Stands in Arizona

Abstract Ponderosa pine (Pinus ponderosa Douglas ex C. Lawson var. scopulorum Engelm.) mortality was evaluated from a 2002 bark beetle outbreak in areas infested with southwestern dwarf mistletoe (Arceuthobium vaginatum [Willd.] Presl subsp. cryptopodum [Engelm.] Hawksw. & Wiens) in a total of nine study sites in northern Arizona. Ponderosa pine mortality attributable to bark beetles (Ips and Dendroctonus spp., Scolytidae) was systematically sampled, and stand attributes, such as basal area, tree density, dwarf mistletoe severity, and site indices were recorded. Ponderosa pine mortality was predominately attributed to Ips spp. Although the prolonged drought likely was the inciting factor responsible for the Ips spp. outbreak, results suggested a strong relationship between ponderosa pine mortality and the interaction between crown class and dwarf mistletoe rating class. Ponderosa pines severely infected with dwarf mistletoe and in the intermediate crown class are at the greatest risk of Ips spp. attack during outbreak years in northern Arizona.

Dwarf mistletoe effects on fuel loadings in ponderosa pine forests in northern Arizona

Southwestern dwarf mistletoe ( Arceuthobium vaginatum (Willd.) J. Presl ssp. cryptopodum ) infests about 0.9 million ha in the southwestern United States. Several studies suggest that dwarf mistletoes affect forest fuels and fire behavior; however, few studies have quantified these effects. We compared surface fuel loadings and predicted fire behavior among four levels of dwarf mistletoe infestation (none, light, moderate, and severe) in a total of 239 plots on 11 sites on basaltic soils in northern Arizona. In each plot we measured tree attributes, dwarf mistletoe rating and surface fuel loading. Stands severely infested by dwarf mistletoe had lower (P < 0.05) tree density and higher snag density, but higher (P < 0.05) total surface fuel loadings and total fuel loadings >7.62 cm and <7.62 cm, than non-infested stands. However, there were no statistical differences in any canopy fuel variables among infestation classes. Predicted fire behavior indicated that the wind speed required to promote the spread of a surface fire into the canopy was lower in severely infested stands than in non-infested stands. These results suggest that stands in northern Arizona that are severely infested with dwarf mistletoe should be priority areas for fuels treatments.

Linking Parasitic Plant-Induced Host Morphology to Tritrophic Interactions

We investigated the tritrophic interactions among southwestern dwarf mistletoe [Arceuthobium vaginatum (Willd.) Presl subsp. cryptopodum], mistletoe herbivores, and host pine (Pinus ponderosa Dougl. ex Laws. and C. Laws. variety scopulorum Engelm.)-associated predators. In an observational study, we characterized differences in pine-associated arthropods and pine branch morphology between branches either parasitized by mistletoe (brooms) or not visibly infected. Compared with noninfected branches, brooms had a more reticulate branching structure, collected 36 times more dead needles and supported 1.7 times more arthropod predators. In a manipulative field experiment, we investigated whether pine-associated predators fed upon lepidopteran herbivores of mistletoe and thereby reduced herbivore damage to the parasite. Over a 30-d trial, herbivores fed upon approximately two-thirds of available mistletoe shoots. Predator removal increased herbivore survival by 56% but had no detectable effect on the level of herbivory damage. We speculate that herbivores compete for mistletoe shoots and that increased per-capita feeding compensated for predator reduction of herbivore abundance. In summary, our results demonstrate that mistletoe parasitism altered the pine arthropod community, including an increase in the density of predators that likely feed upon mistletoe herbivores.

SELECTION OF DWARF MISTLETOE–INFECTED PONDEROSA PINES BY IPS SPECIES (COLEOPTERA: SCOLYTIDAE) IN NORTHERN ARIZONA

In 2002 a bark beetle epidemic severely impacted ponderosa pine (Pinus ponderosa Douglas ex C. Lawson.) forests in northern Arizona. Ponderosa pine mortality occurred over approximately 211,000 ha statewide (USDA 2002). Ips spp. (Ips pini Lecontei and I. lecontei Swaine, Coleoptera: Scolytidae) predominated within infestations and accounted for approximately 99% or about 208,000 infested hectares statewide (McMillin 2003, S.C. Kenaley personal communication). High rates of infestation and tree mortality occurred within the Coconino (24,400 ha), Prescott (30,600 ha), and Tonto National Forests (27,000 ha). In the Tonto National Forest alone, 47% of the ponderosa pine forest type was infested, and many stands had mortality rates of 80%‐90%. Western pine beetle (Dendroctonus brevicomis Leconte), roundheaded pine beetle (Dendroctonus adjunctunus Blandford), and mountain pine beetle (Dendroctonus ponderosae Hopkins) have also been detected within infested areas in northern Arizona, but these bark beetle populations have not yet reached epidemic levels. In many areas where bark beetle infestations occurred, ponderosa pine mortality was greatest in areas infested with southwestern dwarf mistletoe (Arceuthobium vaginatum

Use of Witches' Brooms by Abert Squirrels in Ponderosa Pine Forests

Abert squirrels (Sciurus aberti) are ecologically dependent on ponderosa pine forests. Southwestern dwarf mistletoe (Arceuthobium vaginatum) is a common parasite of ponderosa pine (Pinus ponderosa) that can result in the formation of an abnormal growth structure or branching pattern in its host known as a witches' broom. Management of this parasite has generally included intensive removal of infected trees. Previous studies have identified witches' brooms as important for wildlife; however, little is known about the relationship between it and Abert squirrels. We examined the contents of 226 dwarf mistletoe-induced witches' brooms in 144 ponderosa pine trees for evidence of Abert squirrel use between September 2000 and November 2001 and documented use in 39 brooms (31 caching and foraging sites and 8 nest sites). We compared the physical characteristics of brooms with evidence of use to those with no evidence of use to predict the probability of a broom being used as a caching and foraging site by Abert squirrels. As number of branches within a broom and tree height increased so did the probability of Abert squirrel use. We recommend managers retain ponderosa pine trees ≥18 m in height having brooms with >7 branches to provide usable caching and foraging sites for Abert squirrels.

Southwestern Dwarf Mistletoe, Arceuthobium vaginatum subsp. cryptopodum, Found Parasitizing Picea pungens in Colorado

Southwestern dwarf mistletoe (Arceuthobium vaginatum (Willd.) Presl subsp. cryptopodum (Engelm.) Hawksw. & Wiens, Viscaceae) severely parasitizes several species of pines (Pinus spp., family Pinaceae) in Colorado, Utah, Arizona, New Mexico, and northern Mexico, but it has not been reported to parasitize any species of spruce (Picea, family Pinaceae) (1). However, in June 2004, this dwarf mistletoe was observed parasitizing blue spruce (Picea pungens Engelm.) in the Black Forest north of Colorado Springs, CO (39°02.118'N, 104°36.028'W, elevation 2,250 m). The infected blue spruce was planted as an ornamental approximately 4 m from a 16-m-high ponderosa pine (Pinus ponderosa Douglas ex Lawson & C. Lawson) severely infected with southwestern dwarf mistletoe. Mature dwarf mistletoe shoots were produced on five infected branches of the blue spruce. These shoots were compared with a morphological description of southwestern dwarf mistletoe (1) and this was sufficient for a positive identification of the dwarf mistletoe. The other dwarf mistletoes reported to infect blue spruce are Arceuthobium microcarpum (Engelm.) Hawksw. & Wiens, A. americanum Nutt. ex Engelm., and A. douglasii Engelm.; these are all morphologically distinct from southwestern dwarf mistletoe (1). Three of the infected branches formed small (less than 0.3 m in diameter), nonsystemic witches' brooms. All of the infections on the 6-m-high blue spruce were higher than 1 m on the tree. Thus, it is likely that the spruce was infected after it was transplanted. Three other blue spruces were also located within 4 m of the infected ponderosa pine, but these trees were not infected. To our knowledge, this is the first report of southwestern dwarf mistletoe parasitizing blue spruce and the first report of this dwarf mistletoe on Picea spp. Voucher specimens have been deposited in the Deaver Herbarium, Northern Arizona University, Flagstaff (Accession No. 73959).

A Comparison of Rating Systems for Dwarf Mistletoe-Induced Witches' Brooms in Ponderosa Pine

Abstract We compared four methods of assessing dwarf mistletoe infection and estimating volume of dwarf mistletoe-induced witches' brooms in ponderosa pine (Pinus ponderosa). Witches' brooms can provide important wildlife habitat and can influence fire behavior. We used Hawksworth's dwarf mistletoe rating system (DMR), Tinnin's broom volume rating system (BVR), an estimate of the percentage of live crown occupied by witches' brooms (PCB), and a new system, total broom volume (TBV). We rated 12,536 trees for southwestern dwarf mistletoe (Arceuthobium vaginatum subsp. cryptopodum) infection and broom volume. Using Hawksworth's DMR and Tinnin's BVR systems, we were not able to distinguish trees with different witches' broom volumes at all sample sizes. At all sample sizes, TBV ratings accurately and precisely identified amount of witches' broom volume. We recommend that TBV be used to quantify witches' broom volume in ponderosa pine where wildlife habitat and fire ecology are primary concerns. West. J. Appl. For. 19(1):54–59.

First Report of Arceuthobium vaginatum subsp. cryptopodum on Pinus mugo.

Southwestern dwarf mistletoe (Arceuthobium vaginatum (Willd.) Presl subsp. cryptopodum (Engelm.) Hawksw. & Wiens, family Viscaceae) is a serious and common pathogen of ponderosa pine (Pinus ponderosa Douglas ex Lawson & C. Lawson) in Colorado, Utah, Arizona, New Mexico, and northern Mexico (1). In July 2002, this dwarf mistletoe was observed parasitizing a 1.4-m tall mugo pine (P. mugo Turra) in the Black Forest north of Colorado Springs, CO (39°02.118'N, 104°36.028'W, elevation 2,250 m). The infected mugo pine was planted as an ornamental approximately 6 m from a ponderosa pine infected with A vaginatum subsp. cryptopodum. Dwarf mistletoe shoots were produced on the only infected branch observed but this was sufficient for a positive identification of the dwarf mistletoe. Although J. Weir successfully inoculated mugo pine with western dwarf mistletoe (A. campylopodum Engelm.) and lodgepole pine dwarf mistletoe (A. americanum Nutt. ex Engelm.) (2), to our knowledge, this is the first report of a dwarf mistletoe occurring naturally on P. mugo, as well as the first report of A vaginatum subsp. cryptopodum on P. mugo (1). Specimens of A vaginatum subsp. cryptopodum from P. mugo have been deposited in the Deaver Herbarium, Northern Arizona University, Flagstaff (Accession No. 73761).

Scots Pine Is Susceptible to Southwestern Dwarf Mistletoe

Scots Pine Is Susceptible to Southwestern Dwarf Mistletoe