Arceuthobium Vaginatum Research Articles

Bioactive compounds from the parasitic plant Arceuthobium vaginatum inhibit Haemonchus contortus egg hatching.

The aim of this study was to assess the in vitro ovicidal activity of an ethyl acetate extract from Arceuthobium vaginatum (EtOAc-E) and their subfractions (AvR5-AvR14) against Haemonchus contortus using the egg hatching inhibition (EHI) test. The EtOAc-E and subfractions were tested at 0.12-2.00 and at 0.015-2.0 mg/mL, respectively. Distilled water and methanol (2%) were used as negative controls and Thiabendazole (0.10 mg/mL) as a positive control. Treatments with a dependent effect on concentration were subjected to regression analysis to determine the effective concentrations (EC50 and EC90). The major secondary compounds present in the extract and subfractions were identified by high performance liquid chromatography (HPLC). The EtOAc-E and AvR9 exhibited the best ovicidal effect recording 97.5 and 100% of EHI at 0.25 mg/mL, respectively. The EtOAc-E and AvR9 displayed an EC50= 0.12 and 0.08 mg/mL, respectively. The HPLC analysis in the EtOAc-E and bioactive fractions indicated the presence of a polyphenol, glycosylated flavanones, quercetin glucoside, cinnamates, coumarin, cinnamic acid derivative, ferulic acid, coumarate, naringenin, protocatechuic acid and naringin. Results demonstrated that A. vaginatum extract and fraction is able to inhibit the egg hatch process of H. contortus and could be a viable option for the control of small ruminant haemonchosis.

Arceuthobium vaginatum (southwestern dwarf mistletoe)

Arceuthobium vaginatum (southwestern dwarf mistletoe)

Recent evidence of Mexican temperate forest decline and the need for ex situ conservation, assisted migration, and translocation of species ensembles as adaptive management to face projected climatic change impacts in a megadiverse country

Symptoms of forest decline, apparently due to climate change, have become evident in the last 10 years on the Trans-Mexican Volcanic Belt and northwestern temperate forest of Mexico, particularly at the xeric (low elevational) limit of several forest tree species. We review and provide recent evidence of massive infestation of timberline Pinus hartwegii Lindl. by the mistletoes Arceuthobium globosum Hawksw. & Wiens and Arceuthobium vaginatum (Humb. & Bonpl. ex Willd.) J.Presl; insufficient Abies religiosa (Kunth) Schltdl. & Cham. seedling recruitment at the Monarch Butterfly Biosphere Reserve; indications of inbreeding and defoliation in endangered Picea chihuahuana Martínez, Picea martinezii T.F. Patt., Picea mexicana Martínez, and extreme southern populations of Pseudotsuga menziesii (Mirb.) Franco; and the incidence of unusual pest and disease outbreaks (e.g., Dendroctonus Erichson, 1836 spp., Neodiprion autumnalis Smith, and Phytophthora cinnamomi Rands) in several conifer and oak species. We also discuss a difficult question: Is natural genetic variation sufficient to provide populations with the adaptive variation necessary to survive the natural selection imposed by projected climate change scenarios, or will phenotypic plasticity be exhausted and populations decline? Controversial ex situ conservation within natural protected areas, assisted migration, and translocation of species ensembles are discussed as options by which to accommodate projected climatic change impacts on the management and conservation practices of the megadiverse Mexican temperate forest.

Effects of Mexican dwarf mistletoe (Arceuthobium vaginatumsubsp.vaginatum) on the growth ofPinus cooperiin Durango, Mexico—A case study

AbstractMexican dwarf mistletoe (Arceuthobium vaginatumsubsp.vaginatum, Viscaceae) is the most widespread and damaging parasitic plant in Mexico. It parasitizes 10 species of pines (Pinusspp., Pinaceae) as principal hosts, includingPinus cooperi, one of the economically most important pines in the state of Durango. As a case study, we used stem analysis to estimate the effects of Mexican dwarf mistletoe on volume and height growth of dwarf mistletoe‐infectedP. cooperiin western Durango (Ejido El Brillante, Municipality Pueblo Nuevo). This case study sampled a total of 48 trees, 12 in each of four infection classes estimated using the 6‐class dwarf mistletoe rating system (DMR): uninfected (DMR0), lightly infected (DMR1–2), moderately infected (DMR3–4) and severely infected (DMR5–6). Significant reductions in both volume and height growth were found for moderately and severely infected trees when compared to uninfected trees. On average, reductions in volume growth and height growth were as high as 50% and 17%, respectively. The largest growth reductions were for moderately infected trees, but large growth reductions also occurred for severely infected trees. Because of the reduced growth associated with moderate to severe infection, Mexican dwarf mistletoe‐infested pine forests in Durango, where timber production is a high priority, should be managed using harvesting practices that reduce dwarf mistletoe infection, and thereby, increase forest productivity.

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.

Competition and facilitation determine dwarf mistletoe infection dynamics

Summary Interspecific interactions have a fundamental role in plant population dynamics, as they may set the conditions for species coexistence. Parasitic plants, like dwarf mistletoes, offer the opportunity to study competition for resources that are different from those consumed by most plants, allowing for a better understanding of the interaction. We explored how interspecific interactions between two dwarf mistletoe species (Arceuthobium), co‐infecting the same host species (even sharing the same individual tree of Pinus hartwegii) affect their infection dynamics at two different stages of population development (colonization of new hosts and subsequent growth), and if heterogeneity in resource availability (host density and size structure) affects these interactions. For that purpose, we integrated these processes into a spatially explicit model of density‐dependent population growth. We found that self‐regulation (density‐dependence) was strong for both species; however, the intensity and sign of interspecific interactions changed depending on host size and demographic process. Population growth in Arceuthobium globosum was reduced by competition, except for smaller hosts where A. globosum growth was facilitated by Arceuthobium vaginatum. Arceuthobium vaginatum was facilitated by A. globosum regardless of host size. Colonization of new hosts by A. globosum was enhanced by previous infection by the other species, showing intraguild facilitation. Demographic importance of interactions depended on stand structure: in homogeneous, low‐density forests, facilitation predominates, increasing the population sizes of both species, whereas the opposite occurs in heterogeneous and dense forests. Both species achieved stable coexistence, fulfilling the invasibility criterion because each mistletoe species can invade a forest that is already infected by the other species. Synthesis. Despite the fundamentally different mechanisms underlying the interactions between mistletoes compared with non‐parasitic plants, our results reveal that their behaviour at the population level is similar. Stabilizing mechanisms, like strong self‐limiting population growth, allow dwarf mistletoe coexistence. Interactions shift as populations develop, and they depend largely on environmental factors such as forest structure. Intraguild mutualism is shown as a relevant process for colonization of new spaces, highlighting the complexity of competitive/facilitative interactions between parasitic plants, a formerly unexplored subject. Interactions can only be fully understood when integrating all their components at the population level. Analysing these interactions may contribute to the understanding of plant–plant interactions in general, and convey interesting implications for forest management.

Plantas medicinales utilizadas para las afecciones respiratorias en Loma Alta, Nevado de Toluca, México

Se documenta el conocimiento sobre las especies vegetales para el tratamiento de afecciones respiratorias entre los habitantes de la comunidad de Loma Alta en el Área Natural Protegida del Nevado de Toluca, México. Se identificaron las plantas medicinales, formas de uso, partes utilizadas, métodos de preparación, vías de administración y sitios de obtención. La representatividad de la muestra se determinó con la curva de acumulación de especies propuesta por Balick y O’Brien y las de mayor importancia cultural a través del Consenso de Informantes y el Índice de Friedman (FL). Se registran trece taxa: Arceuthobium vaginatum, Barkleyanthus salicifolius, Calendula officinalis, Cosmos parviflorus, Didymaea alsinoides, Eucalyptus globulus, Gnaphalium canescens, Matricaria recutita, Pelargonium x domesticum, Pinus hartwegii, Plantago australis, Simsia amplexicaulis y Symphoricarpos microphyllus. La familia mejor representada es Asteraceae (6 spp.). Las principales afecciones respiratorias tratadas son la tos, gripe, catarro, anginas y acumulación de flemas. Las plantas con mayor trascendencia de acuerdo con el índice de importancia cultural son G. canescens (FL = 82), E. globulus (FL = 80) y M. recutita (FL = 50). Tales especies son altamente accesibles a los usuarios y están disponibles con relativa frecuencia en las zonas perturbadas aledañas al área de estudio así como en mercados locales. Se concluye que la utilización de plantas combinadas con productos farmacéuticos es uno de los principales factores que potencialmente afecta el conocimiento sobre la flora medicinal.

Pine Bark Beetle and Dwarf Mistletoe Infestation in a Remnant Old-Growth Stand

Abstract. We examined the incidence of bark beetle (Coleoptera: Curculionidae: Scolytinae) colonization and subsequent mortality, and the presence of dwarf mistletoe (Arceuthobium vaginatum subsp. crytopodum) within a rare remnant old-growth stand of ponderosa pine. In 2011, 96% of the old growth trees were remaining with additional dense ingrowth of ponderosa pine (Pinus ponderosa Douglas ex. Lawson) within the understory of the oldest trees. We compared the old-growth stand to abutting stands of ponderosa pine: a low-density seed-tree cut and a high-density post-1900 second growth. The old-growth stand had levels of bark beetle colonization similar to the seed-tree stand, and both of these stands had significantly more bark beetles than the high-density stand. Across all sites, bark beetles were found in higher numbers in the 51–60 cm diameter class. We found a trend between the proportion of trees infected per site and the percentage of trees with bark beetle activity, although a direct relationship be...

Dinámica temporal de la infestación por muérdago enano (Arceuthobium globosum y A. vaginatum) en Zoquiapan (Parque Nacional Iztaccíhuatl Popocatépetl), México

Los muérdagos enanos se encuentran entre los parási­tos forestales que tienen mayor impacto biológico y económico en los bosques templados; sin embargo, se sabe poco acerca de la dinámica temporal de la infestación. El objetivo de este trabajo fue conocer cómo cambia en el tiempo la incidencia de Arceuthobium vaginatum y A. glo­bosum sobre Pinus hartwegii en la región de Zoquiapan, dentro del Parque Nacional Iztaccíhuatl Popocatépetl, México. Durante dos años y medio (noviembre 2008 a mayo 2011) se registró la inciden­cia de muérdago (proporción de árboles infestados), la abundancia de hospederos y la abundancia de pinos menores a 2 m en 24 parce­las de 3 300 m2. Se encontró que la infestación es altamente variable entre fechas, especies y sitios, sin embargo, A. vaginatum mostró una mayor incidencia que A. globosum (Z = 44.09, P < 0.0001). Asimismo, la ocurrencia de incendios y la tala parecen haber afectado la inciden­cia de los muérdagos, e incluso, en algunos casos, se observaron rebrotes severos. La incidencia de A. vaginatum tuvo una correlación negativa con la abundancia de pinos menores a 2 m (rs = - 0.414, P < 0.05), por lo que esta especie pudo estar afectando el recluta­miento de individuos juveniles, mientras A. globosum no mostró una correlación significativa. Es necesario tomar en cuenta los sucesos estocásticos (como incen­dios) en los planes de manejo del muérdago enano, para mejorar la conservación de los recursos forestales. Archivo XML (SciELO)

Nuclear intergenic DNA sequence divergence in a Texas dwarf mistletoe (Arceuthobium divaricatum) population

Arceuthobium divaricatum is a species of dwarf mistletoe that parasitizes six species of pinyon pine (Pinus subsec. Cembroides). Previous phylogeographic research discovered a population of A. divaricatum in the Guadalupe Mountains of Texas that had significant sequence divergence in intergenic regions of chloroplast DNA compared to all other populations. In this study, conserved regions of nuclear intergenic DNA from the same Guadalupe Mountains population and its two neighboring populations were amplified using PCR and then sequenced. The DNA sequences were edited and aligned to determine if there was a significant amount of nuclear sequence divergence. There is 3.8% nuclear sequence divergence between the Guadalupe Mountain population and its two neighboring populations, which is more than the 3.5% nuclear sequence divergence in two recognized subspecies of Arceuthobium vaginatum obtained from GenBank. Based on genetic information the divergent population may warrant further investigation to determine whether it might be better classified as its own subspecies.

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.

Incidence, severity, and aggregation patterns of two sympatric dwarf mistletoe species (Arceuthobium spp.) in Central Mexico

Dwarf mistletoes are widely studied because of their significant impact on host populations; however, few studies have focused on sympatric species. The understanding of their coexistence is important because it can provide useful knowledge for future management plans. We investigated the incidence, severity, and aggregation patterns of Arceuthobium vaginatum and A. globosum coexisting in Central Mexico. We estimated the correlation between both species incidences (N = 75 plots), the incidence–severity correlation (N = 47 plots) for each species, and the effect of biotic (host and non-host species abundance) and abiotic (altitude and slope) factors on the mistletoe incidence. In addition, we compared the hierarchical aggregation among and within plots of the two mistletoe species with a χ 2 test. There is a clear dominance of A. vaginatum in the area, and both species incidences are negatively correlated with each other (r s = − 0.54, P < 0.05) and host abundance (r = − 0.26, P < 0.05). The remaining factors were non-significant. Both species have a linear relationship between incidence and severity, i.e., they show a uniform increase in severity with incidence, which could help diagnose the degree of tree infection from incidence measurements. The species are aggregated within plots, but only A. globosum shows an aggregation among plots (χ 2 = 82.25, P < 0.001); aggregation has not been shown previously for two sympatric dwarf mistletoe species. Our results can improve the scientific basis for forest management planning to control dwarf mistletoe and maintain biodiversity.

Distribución diferencial de dos especies de muérdago enano sobre Pinus hartwegii en el área natural protegida “Zoquiapan y Anexas”, Estado de México

Los muérdagos enanos (Arceuthobium spp.) son plantas importantes en los bosques templados de México debido a su alta incidencia. Al sur del cerro Papayo en el Área Natural Protegida “Zoquiapan y Anexas”, Estado de México, coexisten Arceuthobium vaginatum subsp. vaginatum y A. globosum subsp. grandicaule incluso en el mismo árbol hospedero de la especie Pinus hartwegii. Al estudiar su disposición vertical en el tronco se registra que A. vaginatum se distribuye a menor altura en los pinos que A. globosum. Esto sugiere que la eficiencia de obtención de recursos nutricionales del hospedero podría ser diferencial, por lo que es posible que haya una relación de competencia entre ambas especies.

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. &amp; 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.

First Report of Durangan Dwarf Mistletoe, Arceuthobium vaginatum subsp. durangense, on Pinus cooperi and P. engelmannii in Mexico.

The Durangan dwarf mistletoe (Arceuthobium vaginatum subsp. durangense Hawksw. & Wiens, Viscaceae) parasitizes several species of pines (Pinus spp., Pinaceae) in the Mexican states of Durango, Sinaloa, and Jalisco (1,3). This mistletoe has primarily been reported from the western edge of the Sierra Madre Occidental in eastern Sinaloa and western Durango, but its distribution there is not well documented (3). In March 2007, I found Durangan dwarf mistletoe parasitizing Pinus engelmannii Carr., approximately 25 km north of Mexico Route 40 (24°00'51″N, 105°26'48″W, elevation 2,200 m), and on P. cooperi Blanco approximately 90 km north of Route 40 (24°24'40″N, 105°35'26″W, elevation 2,710 m) along the road to San Miguel de las Cruces, Durango. These populations are approximately 50 and 80 km northeast of the closest known population of Durangan dwarf mistletoe west of Buenos Aires along Route 40 in extreme western Durango. Infection of P. engelmannii was severe on 25 trees, but only severe on two trees of P. cooperi. No mortality associated with infection by Durangan dwarf mistletoe was observed at either location. Infection caused large, nonsystemic witches' brooms on P. engelmannii, but no brooms were observed on infected P. cooperi. To my knowledge, this is the first report of Durangan dwarf mistletoe on P. cooperi and P. engelmannii, and the first report of this mistletoe from the central Sierra Madre Occidental (3). Although Hawksworth and Wiens (2,3) treated Durangan dwarf mistletoe as a species (A. durangense Hawksw. & Wiens), I use the earlier classification of Durangan dwarf mistletoe as a subspecies of Mexican dwarf mistletoe (A. vaginatum (Willd.) Presl subsp. vaginatum) (1) because of recent molecular evidence (4) and morphological similarities with Mexican dwarf mistletoe. The principal difference between these mistletoes is that plants of Durangan dwarf mistletoe are bright orange while those of Mexican dwarf mistletoe are dark brown to black (1-3). Specimens of Durangan dwarf mistletoe on Pinus engelmannii and P. cooperi have been deposited at the Deaver Herbarium, Northern Arizona University, Flagstaff (Accession Nos. 76455 and 76456, respectively).

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 &lt; 0.05) tree density and higher snag density, but higher (P &lt; 0.05) total surface fuel loadings and total fuel loadings &gt;7.62 cm and &lt;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.

Contrasting cascades: insectivorous birds increase pine but not parasitic mistletoe growth

1. Intraguild predation occurs when top predators feed upon both intermediate predators and herbivores. Intraguild predators may thus have little net impact on herbivore abundance. Variation among communities in the strength of trophic cascades (the indirect effects of predators on plants) may be due to differing frequencies of intraguild predation. Less is known about the influence of variation within communities in predator-predator interactions upon trophic cascade strength. 2. We compared the effects of a single predator community between two sympatric plants and two herbivore guilds. We excluded insectivorous birds with cages from ponderosa pine Pinus ponderosa trees parasitized by dwarf mistletoe Arceuthobium vaginatum. For 3 years we monitored caged and control trees for predatory arthropods that moved between the two plants, foliage-feeding caterpillars and sap-feeding hemipterans that were host-specific, and plant damage and growth. 3. Excluding birds increased the abundance of ant-tended aphids on pine and resulted in an 11% reduction in pine woody growth. Mutualist ants protected pine-feeding aphids from predatory arthropods, allowing aphid populations to burgeon in cages even though predatory arthropods also increased in cages. By protecting pine-feeding aphids from predatory arthropods but not birds, mutualist ants created a three-tiered linear food chain where bird effects cascaded to pine growth via aphids. 4. In contrast to the results for tended aphids on pine, bird exclusion had no net effects on untended pine herbivores, the proportion of pine foliage damaged by pine-feeding caterpillars, or the proportion of mistletoe plants damaged by mistletoe-feeding caterpillars. These results suggest that arthropod predators, which were more abundant in cages as compared with control trees, compensated for bird predation of untended pine and mistletoe herbivores. 5. These contrasting effects of bird exclusion support food web theory: where birds were connected to pine by a linear food chain, a trophic cascade occurred. Where birds fed as intraguild predators, the reticulate food webs linking birds to pine and mistletoe resulted in no net effects on herbivores or plant biomass. Our study shows that this variation in food web structure occurred between sympatric plants and within plants between differing herbivore guilds.