Mistletoe Abundance Research Articles

The effects of European mistletoe (Viscum album L.) on urban tree health

Background. Urban forests play a crucial role in making the urban environment and provide for sustainable development of urban areas. At the same time, urban habitats are stressful for trees therefore the latter become more susceptive to a range of disturbances, in particular to biotic agents such as the European mistletoe. The paper aims to evaluate the detrimental impacts of this hemiparasitic plant on urban trees. Materials and Methods. The study was conducted in an urban watershed of central and northwestern parts of the city of Kharkiv, Ukraine. Urban tree health survey took place in different stand types along 4 transects of the total length of 6.54 km. Totally, 956 individual trees were surveyed during the vegetation season of 2019. The health status of individual trees was assessed applying the 7-grade scale after O. D. Maslov, modified by the Ukrainian Forestry Research Institute. The mistletoe presence on individual trees was quantified using a semi-logarithmic index. The statistical analysis of the data collected involved the comparison of two empirical distributions, the normality test of the initial data, and the determination of the relationship between two independent samples. Results and Discussion. The analysis of the collected data revealed that the studied trees divided into two groups – “non-infested” and “mistletoe-infested” – grew in quite similar conditions and showed quite similar distribution by stand types, classes of soil conditions and age classes. We found differences in the mean health status between non-infested (transient from “weakened” to “dry-top”), and mistletoe-infested trees (“dry-top”): the tree health index of 2.43±0.03 for the former vs. 2.98±0.08 for the latter. Empirical distributions of the number of trees by the tree health classes in both groups were different too (the Kolmogorov–Smirnov criterion λ = 3.14 at Р >0.999). We found a significant positive correlation between both mistletoe abundance and the age of individual infested trees (Spearman’s R = 0.36 at Р >0.999), as well as the health index of individual infested trees (Spearman’s R = 0.23 at Р >0.95). Conclusions. The study has proven a negative effect of mistletoe’s infestation on the tree crown foliage assessed with the tree health index. Such an effect increases with the age of infested trees. The findings of this study suggest that in urban forests the mistletoe acts as a trigger towards acceleration of tree stand health deterioration.

How wrong are we in estimating the abundance of mistletoe occurring on Scots pine? – a case study from Central Europe

This paper presents data on the occurrence of the semi-parasitic mistletoe plant (Viscum album L.) in a pine stand in central Poland. The number of mistletoe bushes, the percentage of mistletoe in the crown volume, and the location of its occurrence were determined from the ground level on 193 pine trees growing in a 212c subcompartment in the Głuchów Forestry, Rogów Forest Experimental Station. In addition, 15 trees were analyzed in detail after felling to verify the previously obtained results. The age, diameter, and location (top, middle, and bottom of the tree crown) of the shrubs were measured. The study showed that the pine stand analyzed was strongly infested by mistletoe. The number of mistletoe bushes recorded on standing trees from the ground level was significantly lower than that found on trees after felling. An in-depth visibility analysis showed that in some cases, an operator assessing mistletoe bush abundance from the ground level may miss a very large number of juvenile individuals, as well as mistletoe shrubs with large diameters. This is probably related to the clumping of perennial individuals growing in proximity to a single shrub that is visible from the ground. V. album mainly colonizes the apical zone (observations from the ground) and middle zone of the host crown (detailed analysis of mistletoe abundance on felled trees). Older shrubs were most abundant at the apex of the crown, while younger shrubs were most likely to colonize the middle and bottom of the host crown. In a study of felled trees, 289 mistletoe individuals were found on the most infested trees. It was also found that the most numerous shrubs, V. album were young, aged between three and six years. The increase in mistletoe infestations in forests observed in recent years may pose a serious threat to pine stands weakened by abiotic factors.

The culturable leaf mycobiome of Viscum album subsp. austriacum

AbstractIn Poland's pine forests, mistletoe (Viscum album subsp. austriacum) abundance has increased over the past 20 years. This mistletoe infestation has decreased the growth and vigour of Scots pine trees. In this study, we surveyed the culturable fungi from healthy and diseased V. album subsp. austriacum leaves from two stands in Poland. In total, 63 distinct species were identified, 99.8% of which belonged to the phylum Ascomycota. The community compositions of fungi in mistletoe leaves were similar in healthy and diseased leaves as Alternaria alternata, Aureobasidium pullulans, Botryosphaeria visci, Fusarium paeoniae and Microsphaeropsis olivacea were consistently found in leaves of all symptom types. The most frequently isolated fungus from asymptomatic leaves was M. olivacea, followed by A. alternata, A. pullulans and Hypoxylon rubiginosum. In comparison, the most frequently isolated fungi from leaves with rusty‐brown necrotic spots were (in decreasing order) M. olivacea, B. visci, F. paeoniae and A. alternata, while the most frequently isolated fungi from leaves with black or dark brown spots were M. olivacea, A. alternata, A. pullulans, Epicoccum layuense and F. paeoniae. This study was the first comprehensive report showing that certain fungal species may be pathogens of V. album subsp. austriacum in Poland. This study was also the first report of F. paeoniae, F. juglandicola, Diaporthe vacuae and Heterotruncatella spartii from V. album, and the first report of D. vacuae and H. spartii in Poland.

Mistletoes could moderate drought impacts on birds, but are themselves susceptible to drought-induced dieback.

Mistletoes are hemiparasitic plants and keystone species in many ecosystems globally. Given predicted increases in drought frequency and intensity, mistletoes may be crucial for moderating drought impacts on community structure. Dependent on host vascular flows, mistletoes can succumb to stress when water availability falls, making them susceptible to mortality during drought. We counted mistletoe across greater than 350 000 km2 of southeastern Australia and conducted standardized bird surveys between 2016 and 2021, spanning a major drought event in 2018–2019. We aimed to identify predictors of mistletoe abundance and mortality and determine whether mistletoes might moderate drought impacts on woodland birds. Live mistletoe abundance varied with tree species composition, land use and presence of mistletoebirds. Mistletoe mortality was widespread, consistent with high 2018/2019 summer temperatures, low 2019/2020 summer rainfall and the interaction between summer temperatures and rainfall in 2019/2020. The positive association between surviving mistletoes and woodland birds was greatest in the peak drought breeding seasons of 2018/2019 and 2019/2020, particularly for small residents and insectivores. Paradoxically, mistletoes could moderate drought impacts on birds, but are themselves vulnerable to drought-induced mortality. An improved understanding of the drivers and dynamics of mistletoe mortality is needed to address potential cascading trophic impacts associated with mistletoe die-off.

Dissecting the contributions of dispersal and host properties to the local abundance of a tropical mistletoe

Summary The interplay between dispersal and adaptation to local environments ultimately determines the distribution of plant species, but their relative contribution remains little understood. Tropical mistletoes provide the opportunity to dissect these contributions of dispersal and adaptation, because as hemiparasitic plants, they are typically adapted to grow on a handful of species within diverse tropical communities and are non‐randomly dispersed by mutualistic frugivorous birds. Here we hypothesized that the primary determinant of the abundance of a tropical mistletoe (Dendropemon caribaeus, Loranthaceae) in Puerto Rico will be the compatibility between the mistletoe and plant species in a community. Alternatively, the mistletoe's abundance could be primarily shaped by other factors such as the availability of potential host plants, or factors that determine how mistletoe seeds are dispersed by avian frugivores. We conducted surveys and experiments to assess the capacity of this mistletoe to grow on trees available in the local community, and measured the monthly phenology and seed dispersal patterns of the mistletoe and other bird‐dispersed plants in the community over a period of 4 years. A path model was used to evaluate how the abundance of the mistletoe was shaped by host abundance, fruiting phenology, bird dispersal and compatibilities with host plants. Our analyses show that the compatibility between mistletoe and host tree species, measured by mistletoe survival and growth rate, was the most important factor for mistletoe abundance. The next most important factor was the phenological characteristics of the hosts; this outcome likely arose because frugivory and seed dispersal services for mistletoes and hosts are performed by the same birds. Synthesis. Mistletoes often parasitize only a subset of the suitable plant species that are available in a given community. Our results indicate that such patterns are not only determined by host quality and abundance but also by the phenological patterns of trees that influence the probabilities of mistletoe seeds being deposited on them by shared avian seed dispersers.

Fire and dwarf mistletoe (Viscaceae: Arceuthobium species) in western North America: contrasting Arceuthobium tsugense and Arceuthobium americanum

Dwarf mistletoes (Viscaceae: Arceuthobium spp.) and fire interact in important ways in the coniferous forests of western North America. Fire directly affects dwarf mistletoes by killing the host, host branch, or heating/smoking the aerial shoots and fruits. Fire is a primary determinant of dwarf mistletoe distribution on the landscape, and time since fire controls many aspects of dwarf mistletoe epidemiology. Conversely, dwarf mistletoes can influence fire by causing changes in forest composition, structure, and fuels. Prescribed fire is important for management of dwarf mistletoes, while fire suppression is thought to have increased dwarf mistletoe abundance in western forests. Two dwarf mistletoes are compared in order to illustrate fire interactions in Oregon and Washington, USA: Arceuthobium americanum Nutt. ex Engelm. (lodgepole pine dwarf mistletoe) and Arceuthobium tsugense (Rosendahl)(western hemlock dwarf mistletoe). Arceuthobium persists on the landscape where the host is not killed by fire. Arceuthobium americanum spreads directly into the regenerating Pinus contorta Dougl. ex Loud., while A. tsugense persists in refugia for 200 years or more following fire. Host successional status is a driver of fire – dwarf mistletoe interactions, but forest disturbance agents also play a role. Given the importance of these interactions to the ecology of fire-prone forests, dwarf mistletoes warrant inclusion in disturbance ecology research.

Análisis de la abundancia e infección por muérdago en Sierra Fría, Aguascalientes, México

Los muérdagos, plantas hemiparasíticas, son el segundo agente de destrucción de los bosques mexicanos después del insecto descortezador, por lo que en el presente estudio se evaluó su infectación en poblaciones forestales de Sierra Fría, Aguascalientes, esto con la finalidad de: a) conocer los factores que influyen en la abundancia de muérdago, b) determinar el grado de infectación que presentan las especies hospederas, c) identificar si existe especificidad del muérdago por especie y tamaño del hospedero y/o por hábitat. En enero de 2007 se seleccionaron 20 sitios de muestreo dentro de los cuales se establecieron dos unidades de muestreo (UM) de 2500 m2. En cada UM se midió la altura y diámetro de todos los individuos (infestados y no infestados) >1 m de altura y se registró la presencia de muérdago. Se registraron, además, la altitud, inclinación y orientación de la pendiente, nivel de perturbación y grado de infección. Los resultados indican que el grado de infección por muérdago muestra asociación positiva con la altitud y orientación y una asociación negativa con la perturbación. Se encontró que al aumentar en tamaño los atributos dasométricos (altura, diámetro) se incrementa el grado de infección. Phoradendron bolleanum fue la especie de muérdago más abundante, siendo su hospedero más común Juniperus deppeana. En contraste con lo mencionado en otros trabajos, las poblaciones forestales de Sierra Fría no se encuentran severamente afectadas por muérdago; sin embargo, se recomienda realizar estudios futuros que permitan comprender los factores que dan forma a la distribución e interacción hemiparásito-hospedero, dentro y entre poblaciones, con la finalidad de evaluar los daños reales o futuros, así como evaluar la selectividad hacia ciertas especies hospederas y los cambios químicos que sufren los árboles a través de la infección.

Use of mistletoes by the Grey Go‐away‐bird (Corythaixoides concolor, Musophagidae) in a semi‐arid savannah, south‐west Zimbabwe

AbstractMistletoes are preferred nesting sites for many bird species in a range of habitats. However, no studies have examined the use of mistletoes by nesting birds in the semi‐arid savannah. We studied nesting in mistletoe and its role in determining nesting success in the Grey Go‐away‐bird in south‐west Zimbabwe. We modelled the effects of mistletoe, mistletoe abundance, nest microclimate, concealment and nest height on daily survival rates (DSR) using program MARK. A constant survival model was best fitted for the Grey Go‐away‐bird suggesting a constant nest survival rate across the nesting period. Mistletoe nests had lower DSR than nests placed elsewhere in the canopy. Mistletoe abundance and nest height had a positive association with DSR whereas visibility distance, microclimate and concealment were negatively associated with DSR. Overall, survival for nests in mistletoe was 22.1% compared with 90.5% for nests in other substrates over the 50‐day nesting period. In conclusion, the low nest survival in mistletoe suggests either that the factors used to select mistletoe as nest sites by these birds are poor predictors of nest success or that nesting in mistletoe may be maladaptive.

Biotic factors associated with the spatial distribution of the mistletoe Psittacanthus calyculatus in a tropical deciduous forest of central Mexico

Psittacanthus calyculatus (DC.) G. Don (Loranthaceae) is a neotropical hemiparasitic mistletoe and one of the most abundant and widely distributed species of the genus. Like most mistletoes, the interaction with its hosts and dispersers are directly related with its distribution and community dynamics. For the last decade, the infested areas in central México have increased, becoming a potential plant health problem. The aim of this study was to investigate three main biotic factors associated with the distribution of P. calyculatus in a tropical deciduous forest: (1) the plant community structure, (2) the host spatial distribution and level of association with P. calyculatus , and (3) bird consumers. Modified Whittaker plots were used to analyze the structure and composition of the plant community, comparing a highly infected mistletoe sites with non-infected sites. The spatial distribution and association of P. calyculatus with its most frequent hosts were calculated through a spatial analysis by distance indices. Study sites differed in plant diversity and species composition, showing a positive relation of mistletoe abundance with low diversity sites. The distance indices analysis presented a clumped distribution of P. calyculatus and Acacia hosts, as well as a high association with leguminous species such as A. farnesiana (X = 0.31, P = 0.0041) and A. schaffneri (X = 0.3977, P = 0.001). Three species consumed P. calyculatus’ berries: the Northern mockingbird, the Silky-Flycatcher and the Scott’s Oriole.

Divergent effects of forest edges on host distribution and seed disperser activity influence mistletoe distribution and recruitment

Summary Species interactions define functional diversity and community stability across ecosystems, and depend on the spatial distribution, the habitat requirements and the sensitivity to disturbances of all interacting partners. Hence, assessing the effects of such anthropogenic disturbances on multi‐species interactions may be essential to improve adaptation and mitigation measures for biodiversity conservation. We determined the importance of edge effects on the interaction and distribution of three keystone species in South American temperate rain forests: the hemiparasitic mistletoe Tristerix corymbosus, its main host (the liana Campsidium valdivianum) and its only seed disperser (the marsupial Dromiciops gliroides). The discordant impacts of forest edges on host (positive) and seed disperser (negative) affected mistletoe distribution at large spatial scales, owing to the combined effects of increased dispersal limitation and decreased host availability. More importantly, marsupial abundance had contrasting effects on mistletoe abundance at small and large spatial scales – suggesting a potential trade‐off between local and long‐distance dispersal. We found the number of adult mistletoes per host increased with host size, which likely indicates that mistletoe colonization accumulated over the host's lifespan. However, the number of juveniles found per host peaked at medium‐sized hosts, increased with marsupial abundance and host availability and showed a negligible response to edges. Synthesis. The lack of spatial congruence between host and seed disperser probably explains the scarcity of mistletoes in the study area, although the discordant drivers of juvenile and adult distributions suggest that there is a trade‐off between recruitment patterns but also potential dispersal limitation at small scales. In essence, the interdependence amongst species linked by (mutualistic and antagonistic) interactions makes them more sensitive to anthropogenic disturbance, exacerbating its impact on the diversity and functioning of forest ecosystems.

Hemiparasitism effect on Baccharis dracunculifolia DC. and consequences to its major galling herbivore

Mistletoes obtain nutrients and water from their hosts, with varying effects among those hosts. We assessed the factors that influence the colonization of the mistletoe Struthanthus flexicaulis on Baccharis dracunculifolia and the subsequent effects on host performance. We evaluated the incidence of S. flexicaulis according to size (height classes) and architecture of the host as well as its effects on various physiological parameters of the host. Furthermore, we assessed the occurrence of insect galls induced by Baccharopelma dracunculifoliae(Psyllidae), including the number of leaves infected, and the mortality of infected and non-infected branches. Taller hosts had a higher abundance of mistletoes (60%, p> 0.05). Physiological parameters of hosts were not affected by parasitism, although galling occurred more often (p 0.05) on infected branches. Taller individuals are more colonized by mistletoes and more architecturally complex hosts support a greater number of mistletoes. Mistletoe causes a top-down effect on host-associated organisms on parasitized branches. Mistletoes had a strong top-downeffect on B. dracunculifolia due to a reduction in the number of leaves on parasitized branches and the replacement of the bush crown, as well as an increased incidence of insect galls. Furthermore, the occurrence of a heavy parasite load increased the mortality rate of the host branches.

Repeated spring precipitation shortage alters individual growth patterns in Scots pine forests in the Western Alps

In the inner-Alpine Aosta Valley, severe growth reductions occurred in Scots pine stands during 1987–1993 and 2003–2005. Trees more sensitive to May precipitation exhibited stronger growth decline during these periods. Over the last decades, Scots pine (Pinus sylvestris L.) decline has involved large areas in the European Alps. Although the species is supposed to be drought resistant, increased temperatures and droughts are often indicated as predisposing causes of the decline. Nevertheless, the exact climate conditions that initiate the decline, and the reasons why they differentially affect individual trees, are largely unknown. Our aims were to identify climate constraints on Scots pine growth, and elucidate the effect of individual characteristics, such as tree age, size, crown condition, mistletoe occurrence, competition, and sensitivity to climate, on tree growth decline and recovery after repeated climatic stress. We analysed 232 trees in four second-growth stands located at 985–1350 m a.s.l. in the inner-Alpine Aosta Valley, NW Italy. Multi-year growth declines, occurred in 1987–1993 and 2003–2005, were related to repeated May precipitation shortage, while temperatures and summer precipitations played a minor role. Growth decline was stronger in the stand at lower elevation and with higher competition intensity. At the individual scale, trees more sensitive to May precipitation exhibited lower growth rates during both dry periods, but not a faster recovery. In the lower elevation stand, tree growth decline was significantly related to both crown transparency and mistletoe abundance, which was almost absent in the other stands. We conclude that future variations in the spring precipitation regime could threaten Scots pine more than warming per se, in inner-Alpine valleys around 1000–1400 m a.s.l. Still, different individual sensitivity to precipitation will likely result in patchy patterns of healthy and declining trees within the same stand.

Mistletoe infection in an oak forest is influenced by competition and host size.

Host size and distance from an infected plant have been previously found to affect mistletoe occurrence in woody vegetation but the effect of host plant competition on mistletoe infection has not been empirically tested. For an individual tree, increasing competition from neighbouring trees decreases its resource availability, and resource availability is also known to affect the establishment of mistletoes on host trees. Therefore, competition is likely to affect mistletoe infection but evidence for such a mechanism is lacking. Based on this, we hypothesised that the probability of occurrence as well as the abundance of mistletoes on a tree would increase not only with increasing host size and decreasing distance from an infected tree but also with decreasing competition by neighbouring trees. Our hypothesis was tested using generalized linear models (GLMs) with data on Loranthus europaeus Jacq., one of the two most common mistletoes in Europe, on 1015 potential host stems collected in a large fully mapped plot in the Czech Republic. Because many trees were multi-stemmed, we ran the analyses for both individual stems and whole trees. We found that the probability of mistletoe occurrence on individual stems was affected mostly by stem size, whereas competition had the most important effects on the probability of mistletoe occurrence on whole trees as well as on mistletoe abundance. Therefore, we confirmed our hypothesis that competition among trees has a negative effect on mistletoe occurrence.

Spatial distribution of oak mistletoe as it relates to habits of oak woodland frugivores.

This study addresses the underlying spatial distribution of oak mistletoe, Phoradendron villosum, a hemi-parasitic plant that provides a continuous supply of berries for frugivorous birds overwintering the oak savanna habitat of California's outer coast range. As the winter community of birds consuming oak mistletoe varies from group-living territorial species to birds that roam in flocks, we asked if mistletoe volume was spatially autocorrelated at the scale of persistent territories or whether the patterns predicted by long-term territory use by western bluebirds are overcome by seed dispersal by more mobile bird species. The abundance of mistletoe was mapped on trees within a 700 ha study site in Carmel Valley, California. Spatial autocorrelation of mistletoe volume was analyzed using the variogram method and spatial distribution of oak mistletoe trees was analyzed using Ripley's K and O-ring statistics. On a separate set of 45 trees, mistletoe volume was highly correlated with the volume of female, fruit-bearing plants, indicating that overall mistletoe volume is a good predictor of fruit availability. Variogram analysis showed that mistletoe volume was spatially autocorrelated up to approximately 250 m, a distance consistent with persistent territoriality of western bluebirds and philopatry of sons, which often breed next door to their parents and are more likely to remain home when their parents have abundant mistletoe. Using Ripley's K and O-ring analyses, we showed that mistletoe trees were aggregated for distances up to 558 m, but for distances between 558 to 724 m the O-ring analysis deviated from Ripley's K in showing repulsion rather than aggregation. While trees with mistletoe were aggregated at larger distances, mistletoe was spatially correlated at a smaller distance, consistent with what is expected based on persistent group territoriality of western bluebirds in winter and the extreme philopatry of their sons.

The influence of mistletoes on birds in an agricultural landscape of central Mexico

Mistletoes are hemiparasitic flowering plants that function as keystone resources in forests and woodlands of temperate regions, where a positive relationship between mistletoe density and avian species richness has been observed. Mistletoes have been less studied in tropical regions and the relationship between birds and mistletoes has seldom been explored in tropical agricultural systems. Therefore, we studied the presence of infected trees and infection prevalence (i.e., number of parasitized trees/total number of trees) by Psittacanthus (Loranthaceae) mistletoes in 23 hedgerows located in an agricultural landscape of central Mexico during the dry and rainy seasons, and investigated the relationship between bird species richness and abundance and the abundance of mistletoes. We found a mean of 74 mistletoe plants per 100-m transect of only one species, Psittacanthus calyculatus. Thirty-one percent of the trees surveyed were infected and tree species differed in infection prevalence, mesquite (Prosopis laevigata) being the most infected species with 86% of the surveyed trees infected. For both seasons, we found a positive and significant association between bird species richness and number of mistletoe plants. The same pattern was observed for total bird abundance. Many resident and Neotropical migratory birds were observed foraging on mistletoes. Our results show that mistletoes are important in promoting a higher bird species richness and abundance in tropical agricultural landscapes.

Incidence of competitors and landscape structure as predictors of woodland-dependent birds

Globally, modification of landscapes for agriculture has had a strong influence on the distribution and abundance of biota. In particular, woodland-dependent birds are under threat across agricultural landscapes in Britain, North America and Australia, with their decline and extirpation attributed to the loss and fragmentation of habitat. Other native species have become over-abundant in response to anthropogenic landscape change and have strong interactive effects on avian assemblage structure. In eastern Australia, the hyper-aggressive noisy miner (Manorina melanocephala) often dominates woodlands in agricultural landscapes through interspecific competition, resulting in declines of species richness of woodland-dependent birds. We aimed to determine the relative influence and importance of interspecific competition, in situ habitat structure and landscape structure for woodland-dependent bird species at the landscape level. We recorded species-specific landscape incidence of woodland-dependent birds in 24 agricultural-woodland mosaics (25 km2) in southern Queensland, Australia. We selected extensively cleared landscapes (10–23 % woodland cover) where fragmentation effects are expected to be greatest. We applied generalised linear models and hierarchical partitioning to quantify the relative importance of the landscape-level incidence of the noisy miner, mistletoe abundance, shrub cover, woodland extent, woodland subdivision and land-use intensity for the incidence of 46 species of woodland birds at the landscape-scale. The landscape-level incidence of the noisy miner was the most important explanatory variable across the assemblage. Both in situ habitat structure and landscape structure were of secondary importance to interspecific aggression, although previous research suggests that the increasing incidence of the noisy miner in fragmented agricultural landscapes is itself a consequence of anthropogenic changes to landscape structure. Species’ responses to fragmentation varied from positive to negative, but complex habitat structure had a consistently positive effect, suggesting in situ restoration of degraded habitats could be a conservation priority. Landscape wide conservation of woodland-dependent bird populations in agricultural landscapes may be more effective if direct management of noisy miner populations is employed, given the strong negative influence of this species on the incidence of woodland-dependent birds among landscapes.

Spatiotemporal mutualistic model of mistletoes and birds

A mathematical model which incorporates the spatial dispersal and interaction dynamics of mistletoes and birds is derived and studied to gain insights of the spatial heterogeneity in abundance of mistletoes. Fickian diffusion and chemotaxis are used to model the random movement of birds and the aggregation of birds due to the attraction of mistletoes, respectively. The spread of mistletoes by birds is expressed by a dispersal operator, which is typically a convolution integral with a dispersal kernel. Two different types of kernel functions are used to study the model, one is a Dirac delta function which reflects the special case that the spread behavior is local, and the other one is a general non-negative symmetric function which describes the nonlocal spread of mistletoes. When the kernel function is taken as the Dirac delta function, the threshold condition for the existence of mistletoes is given and explored in terms of parameters. For the general non-negative symmetric kernel case, we prove the existence and stability of spatially nonhomogeneous equilibria. Numerical simulations are conducted by taking specific forms of kernel functions. Our study shows that the spatial heterogeneous patterns of mistletoes are related to the specific dispersal pattern of birds which carry mistletoe seeds.

Mortality and growth of dwarf mistletoe‐infected red and white fir and the efficacy of thinning for reducing associated losses

SummaryIn managed forests dominated by true fir (Abies) species, stands are often restocked using understory trees retained during timber harvest, making the effects of dwarf mistletoe (Arceuthobium spp.) infestation on small true fir a concern. This study examined the response of small red (A. magnifica) and white (A. concolor) fir and their dwarf mistletoes (A. abietinum f.sp. magnificae and A. abietinum f.sp. concoloris, respectively) to precommercial thinning treatments in fir‐dominated stands in the Sierra Nevada Mountains of California. Tree diameters and dwarf mistletoe ratings were monitored from 1981 to 2001, and mortality was observed from 1981 to 2006. Red and white fir survival and radial growth decreased significantly with greater mistletoe ratings and increased with larger diameters and live crown ratios. Thinning significantly increased survival and growth of white, but not red fir. Over the course of the study, mistletoe ratings for both fir species did not change significantly in unthinned stands, but increased in thinned stands. However, while statistically significant, increases in mistletoe ratings in thinned stands were gradual and mistletoe ratings did not statistically differ between treatments 20 years post‐thinning. Additionally, thinning did not significantly influence mistletoe spread to uninfected trees, indicating that precommercial thinning in lightly infested red and white fir stands will not likely result in substantial increases in dwarf mistletoe abundance over typical harvesting intervals. Overall, while severe dwarf mistletoe infection ratings reduced tree survival and growth, because ratings remained low, actual losses resulting from mistletoes during this study were minor and will not likely result in substantial economic losses over normal harvesting intervals. This finding indicates that precommercial thinning treatments designed specifically to compensate for mistletoe‐associated losses may not be necessary when managing red and white fir for timber production.

Habitat influences on nest-site selection by the Painted Honeyeater (Grantiella picta): do food resources matter?

Understanding the habitat requirements of threatened species is crucial for their effective management. This paper examines the role of mistletoe food resources and vegetative habitat structure in shaping nest-site selection by the Painted Honeyeater. Painted Honeyeaters responded to the abundance of mistletoes and the proximity of mistletoe clumps near the nest tree as well as habitat structure when selecting nest-trees. Compared with randomly selected trees, nest-trees were on average less than half the distance to the nearest mistletoe-infected host and mistletoe abundance was 2.8 times greater in the surrounding habitat. Vegetation surrounding nest-trees was more open than that surrounding randomly selected trees, but the canopy was more closed within 25 m of nest-trees. Nest-site selection by Painted Honeyeaters was best explained by a combination of mistletoe abundance, proximity of mistletoe clumps to the nest-tree and structural qualities of the surrounding habitat. Hierarchical partitioning further supported the analysis and suggests that Painted Honeyeaters respond primarily to mistletoe abundance when selecting nest-sites. This study supports a link between mistletoe resources and habitat structure and Painted Honeyeater nest-site selection. Conservation management strategies for this declining honeyeater should include effective management promoting an abundance of mistletoe populations as well as a diverse woodland community structure.

Pine and mistletoes: how to live with a leak in the water flow and storage system?

The mistletoe, Viscum album, living on Scots pine (Pinus sylvestris) has been reported barely to regulate its transpiration and thus heavily to affect the gas exchange of its host. The extent of this mistletoe effect and its underlying mechanism has, so far, only been partially analysed. In this study, pine branches with different mistletoe infestation levels were investigated by sap flow gauges and analysed with a modelling approach to identify the mistletoe-induced stomatal regulation of pine and its consequences for the water and carbon balances of the tree. It was found that Viscum album barely regulates its stomata and that pines consequently compensate for the additional water loss of mistletoes by closing their own stomata. Despite the reduced stomatal aperture of the needles, the total water loss of branches with mistletoes increased. Furthermore, the increasingly closed stomata reduced carbon assimilation for the pine. Such a negative effect of the mistletoes on pine's stomatal conductance and carbon gain was particularly strong during dry periods. Our study therefore suggests that mistletoe-induced stomatal closure is a successful mechanism against dying from hydraulic failure in the short term but increases the risk of carbon starvation in the long term. With the current conditions in Valais, Switzerland, a tree with more than about 10-20% of its total leaf area attributable to mistletoes is at the threshold of keeping a positive carbon balance. The currently increasing mistletoe abundance, due to increasing mean annual temperatures, is therefore accelerating the ongoing pine decline in many dry inner-Alpine valleys.