David C. Shaw

Western oak mistletoe (Phoradendron villosum subsp. villosum; WOM) occurs on native oaks (Quercus spp., Fagaceae) from Baja California, Mexico to the northern Willamette Valley, Oregon, USA. A variety of other native and non-native hardwoods within its native range are also hosts. We used the diverse urban forests of western Oregon (Salem, Corvallis, Eugene, Medford, Central Point, Rogue River, and Ashland) as a natural laboratory to investigate the host range of WOM and to assess the effects of WOM on tree condition. During the leaf-off period of deciduous hardwood trees, we surveyed these urban forests for the occurrence of WOM in the crowns of non-native trees. We collected data on tree species, size, height, and condition, and mistletoe abundance. We observed WOM in 227 non-native trees, of which 85% were pin oak (Q. palustris) and northern red oak (Q. rubra) (both in the red oak group), 12% were other oak species, and only 3% were individuals of non-oak species, despite the fact that non-oak genera represented 81–96% of the urban trees in four cities with tree inventories. WOM tended to occur in large trees, which is consistent with the literature. We found no observable effects of mistletoe infestation level on tree condition. The decision to manage WOM should involve oaks only and depend on whether tree owners are concerned about potential negative effects of heavy infestations on tree health or desire increased bird biodiversity via mistletoe presence. Urban forest managers could also consider alternatives to red oak group species if WOM is abundant in the area. Key points We surveyed the occurrence of western oak mistletoe on non-native urban trees to provide insight into major hosts and management. Western oak mistletoe occurred almost exclusively on Quercus species, especially pin, northern red, and scarlet oaks. Mistletoe plants tended to occur in larger trees, and we found no observable relationship between mistletoe infestation level and tree condition.

As we contemplate the future of forest landscapes under changing climate conditions and land‐use demands, there is increasing value in studying historic forest conditions and how these landscapes have changed following past disturbances. Historic landscape paintings are a potential source of data on preindustrial forests with highly detailed, full‐color depictions of overstory and understory environments. They display key details about forest community composition, microhabitat features, and structural complexity from a time well before the advent of color photography. Despite these paintings' potential, their scientific applications have been impeded by questions of validity.How truly accurate are the images portrayed in these paintings? How much of an image is an artist's manipulation of a scene to best illustrate an allegory or romanticized view of nature?Following an established assessment model from historical ecology for evaluating resource validity, we demonstrate how scholarship on art history can be integrated with ecological understanding of forest landscapes to follow this model and address these questions of image veracity in 19th century American art. Further, to illustrate the potential use of these historic images in ecological studies, we present in a case study assessing microhabitat features of 10 different paintings. While this paper explores 19th century landscape art broadly, we focus our art historical review in particular on Asher Durand, a prolific and influential artist associated with the so‐called “Hudson River School” in the mid‐1800s. Durand left clear records about his perspectives on accurately depicting nature, and from a review of images and writings of Durand, we find support for the potential use of many of his paintings and sketches in historic forest ecology research. However, we also identify important caveats regarding potential ecological interpretations from these images. More broadly, because 19th century landscape paintings are not always directly transcriptive, and because regional art cultures differed in the 1800s, we cannot within this paper speak about landscape image veracity across all 19th century landscape art. However, in following established methods in historical ecology and integrating tools from art history research, we show that one can identify accurate historic landscape paintings for application in scientific studies.

Nothophaeocryptopus gaeumannii is a common native, endophytic fungus of Douglas-fir foliage, which causes Swiss needle cast, an important foliage disease that is considered a threat to Douglas-fir plantations in Oregon. Disease expression is influenced by fungal fruiting bodies (pseudothecia), which plug the stomata and inhibit gas exchange. Trees are impacted when pseudothecia plug stomates on 1-year-old and older needles resulting in early needle abscission. Mature (100 years+) trees appear to be less impacted from disease, and we hypothesize this is due to the greater emergence of pseudothecia on older than younger needles, which allows for more needle retention. We measured the density of pseudothecia occluding stomates across 2- to 5-year-old needles from upper, middle, and lower canopy positions of mature trees at three sites in the Oregon Coast Range and two sites in the western Oregon Cascade Mountains. Binomial generalized linear mixed model (GLMM) was used to test for the effects of canopy position (upper, middle, and lower), sites, needle age (2–5 years old), and years (2016 and 2017), and their interactions on the pseudothecia density. Pseudothecia density varied annually depending on sites, needle age and canopy positions. Pseudothecia density peaked on 3-, and 4-year-old needles, however, needles emerging from the same year, like 2-year-old needles in 2016 and 3-year-old needles in 2017 both emerged in 2014, had consistently similar patterns of pseudothecia density for both years, across site and canopy positions. Canopy position was important for 3-, and 4-year-old needles, showing less pseudothecia in the lower canopy. This research confirms that N. gaeumannii pseudothecia density is greatest in 3- and 4-year old needles in mature trees in contrast to plantations where pseudothecia density usually peaks on 2-year-old needles, and that pseudothecia density (disease severity) is generally lower in mature trees. Something about mature forest canopies and foliage appears to increase the time it takes for pseudothecia to emerge from the needles, in contrast to younger plantations, thus allowing the mature trees to have greater needle retention.

Swiss needle cast (SNC), caused by Nothophaeocryptopus gaeumannii, is a foliage disease of Douglas-fir (Pseudotsuga menziesii), that reduces growth in native stands and exotic plantations worldwide. An outbreak of SNC began in coastal Oregon in the mid-1990s and has persisted since that time. Here we review the current state of knowledge after 24 years of research and monitoring, with a focus on Oregon, although the disease is significant in coastal Washington and has recently emerged in southwestern British Columbia. We present new insights into SNC distribution, landscape patterns, disease epidemiology and ecology, host-pathogen interactions, trophic and hydrologic influences, and the challenges of Douglas-fir plantation management in the presence of the disease. In Oregon, the SNC outbreak has remained geographically contained but has intensified. Finally, we consider the implications of climate change and other recently emerged foliage diseases on the future of Douglas-fir plantation management.

Dwarf mistletoes (Arceuthobiumspecies) are arboreal, hemiparasitic plants of conifers that can change the structure and function of the tree crown. Hemlock dwarf mistletoe (Arceuthobium tsugensesubsp.tsugense)principally parasitizes western hemlock (Tsuga heterophylla) and effects 10.8% of all western hemlock trees in Oregon, USA. In this study, we climbed 16 western hemlock trees (age 97–321 years, height 33–54.7 m) across a gradient of infection (0%–100% of branches infected) and measured occurrence of all dwarf mistletoe infections, dwarf mistletoe caused deformities, foliage, branch and crown metrics, and sapwood area. We then modelled over 25 different response variables using linear and generalized linear models with three metrics of severity as explanatory variables: total infection incidence, proportion of all live branches infected, and proportion of all live, infected branches with 33 per cent or more foliage distal to infection. A strong effect of dwarf mistletoe intensification was the reduction of branch foliage and an increase in the proportional amount of foliage distal to infections, with severely infected trees having the majority of foliage distal to infections. Increasing severity led to an apparent crown compaction as crown volumes decreased and became increasingly comprised of deformities. Sapwood area was unrelated to infection severity. Branch length and diameters were unrelated to increasing infection severity despite severely infected branches supporting 1–70 infections. The most severely infected tree had 3,615 individual plants in the crown. Our results suggested that shifts in crown structure and branch deformation, foliage amount, and foliage distal to infection, reflected a likely reduction of capacity for tree growth that coincided with a hypothesized increase in resource demand by dwarf mistletoe plants as infection severity intensified.