The classification of dwarf mistletoes (Arceuthobium spp., Viscaceae) in section Campylopoda, series Campylopoda

The taxonomic classification of dwarf mistletoes (Arceuthobium spp., Viscaceae) is complicated due to their reduced morphology, requiring the integration of not only morphology but also phenology, geography, and host relationships. This has been particularly true for the classification of taxa in subgenus Vaginata, section Campylopoda, series Campylopoda. Most of the species in this group have been recently circumscribed in synonymy with or reduced to subspecies of Arceuthobium campylopodum Engelm; however, we contend they deserve separate species recognition. To address this question, we have conducted morphological analyses of the taxa in ser. Campylopoda using univariate and multivariate statistical analyses. Our results have demonstrated that these taxa can be determined to species using morphological data without consideration of geographic location or host specificity; however, the host specialization and geographic distribution exhibited by these taxa also supports their classification as species. Here, we discuss the evidence supporting the specific classification of ser. Campylopoda taxa. This taxonomic framework permits the treatment of several dwarf mistletoe populations with geographically restricted distributions, fewer morphological differences, and specialized host affinities as subspecies of Arceuthobium abietinum (Engelm.) Abrams, Arceuthobium microcarpum (Engelm.) Hawksw. & Wiens, and Arceuthobium tsugense (Rosend.) G.N.Jones. It is also the most practical classification for the management of these economically and ecologically important parasitic plants.

Dwarf Mistletoe and Drought Contribute to Growth Decline, Dieback and Mortality of Junipers

Rising temperatures and aridification, combined with the stressing effect of some hemiparasitic plants such as mistletoes, may contribute to reduce vigour and growth of trees and shrubs leading to dieback and increasing mortality. This has been rarely explored in pioneer shrubs such as junipers, which are assumed to be more drought tolerant than coexisting trees. To test these ideas, we reconstructed radial growth patterns of common junipers (Juniperus communis L.) with different crown cover and infestation degree by dwarf mistletoe (Arceuthobium oxycedri (DC.) M. Bieb.) in two sites with contrasting aspect and water availability located in north-eastern Spain. We used dendrochronology to study the response of junipers’ radial growth to climatic factors (temperature, precipitation, and soil moisture), an index of drought severity, and mistletoe infestation. Juniper growth was constrained by elevated temperatures and low precipitation leading to drought during the growing season. Infestation by dwarf mistletoe contributed to a short-term growth decline in junipers. The interaction between low summer precipitation and high dwarf mistletoe infestation constrained juniper growth, particularly in the north-oriented wetter site, where hosts presented higher growth rates during wet periods. The negative impact of low summer precipitation on juniper growth overrides the effects due to dwarf mistletoe infestation. Aridification and mistletoe infestation could trigger dieback and mortality of shrubs slowing down successional dynamics and delaying shrub encroachment into former croplands and grasslands.

The Difficulty of Predicting Eastern Spruce Dwarf Mistletoe in Lowland Black Spruce: Model Benchmarking in Northern Minnesota, USA

Insects, fungi, and diseases play an important role in forest stand development and subsequently, forest management decisions and treatments. As these disturbance agents commonly occur within and across landscapes, modeling has often been used to inform forest planning and management decisions. However, models are rarely benchmarked, leaving questions about their utility. Here, we assessed the predictive performance of a Bayesian hierarchical model through on–the-ground sampling to explore what features of stand structure or composition may be important factors related to eastern spruce dwarf mistletoe (Arceuthobium pusillum Peck) presence in lowland black spruce (Picea mariana (Mill.) B. S. P.). Twenty-five state-owned stands included in the predictive model were sampled during the 2019 and 2020 growing seasons. Within each stand, data related to the presence of eastern spruce dwarf mistletoe, stand structure, and species composition were collected. The model accurately predicted eastern spruce dwarf mistletoe occurrence for 13 of the 25 stands. The amount of living and dead black spruce basal area differed significantly based on model prediction and observed infestation, but trees per hectare, total living basal area, diameter at breast height, stand age, and species richness were not significantly different. Our results highlight the benefits of model benchmarking to improve model interpretation as well as to inform our understanding of forest health problems across diverse stand conditions.

Pre-Emptive Detection of Mature Pine Drought Stress Using Multispectral Aerial Imagery

Drought, ozone (O3), and nitrogen deposition (N) alter foliar pigments and tree crown structure that may be remotely detectable. Remote sensing tools are needed that pre-emptively identify trees susceptible to environmental stresses could inform forest managers in advance of tree mortality risk. Jeffrey pine, a component of the economically important and widespread western yellow pine in North America was investigated in the southern Sierra Nevada. Transpiration of mature trees differed by 20% between microsites with adequate (mesic (M)) vs. limited (xeric (X)) water availability as described in a previous study. In this study, in-the-crown morphological traits (needle chlorosis, branchlet diameter, and frequency of needle defoliators and dwarf mistletoe) were significantly correlated with aerially detected, sub-crown spectral traits (upper crown NDVI, high resolution (R), near-infrared (NIR) Scalar (inverse of NDVI) and THERM Δ, and the difference between upper and mid crown temperature). A classification tree model sorted trees into X and M microsites with THERM Δ alone (20% error), which was partially validated at a second site with only mesic trees (2% error). Random forest separated M and X site trees with additional spectra (17% error). Imagery taken once, from an aerial platform with sub-crown resolution, under the challenge of drought stress, was effective in identifying droughted trees within the context of other environmental stresses.