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Members of the Pinus genus are well known for their medicinal properties, which can be attributed to their essential oils. In this work, we have examined the leaf essential oils of five understudied Pinus species collected from various locations in western North America. The essential oils were obtained by hydrodistillation and analyzed by gas chromatographic methods, including enantioselective gas chromatography. Pinus albicaulis was dominated by (+)-δ-3-carene; Pinus flexilis was dominated by α-pinene (mostly (+)-α-pinene) and (−)-β-pinene; Pinus lambertiana was dominated by (−)-β-pinene; Pinus monticola was dominated by (−)-β-pinene, (+)-δ-3-carene, and (−)-α-pinene; and Pinus sabiniana was rich in (−)-α-pinene and limonene. While this work adds to our knowledge of Pinus essential oils, additional research is needed to more fully appreciate the geographic and altitudinal variations in the volatile compositions of these Pinus species.

Whitebark pine (WBP; Pinus albicaulis) is a five-needle conifer tree species native to the western United States (US) and southwestern Canada. In the US, this ecologically important species is listed as ‘threatened’ under the Endangered Species Act because of population declines from compound stressors, including climate change, introduced white pine blister rust (WPBR), widespread outbreaks of mountain pine beetle, and altered fire regimes. Species recovery depends primarily on planting WBP seedlings that are resistant to WPBR, but suitable planting locations are likely to shift under climate warming. We modeled and mapped WBP climatic suitability in the US using forest inventory data and fine-scale (∼220 m) climate datasets under reference period (1961–1990) and mid-21st century climatic conditions. We projected an 80% reduction in the area climatically suitable for WBP by the mid-21st century. Moreover, 75% of the climatically suitable area for WBP under mid-21st century climate is located in designated wilderness areas and national parks. This could challenge WBP recovery efforts, as these protected areas strive to reduce human manipulation of ecological systems. WBP climate suitability maps resulting from our models can be used to identify priority or ‘target areas’ where planting will have the highest likelihood of success and to identify areas within the WPB’s current range that are most vulnerable to rapid change should high-severity wildfires or mountain pine beetle outbreaks occur. Mapping WBP reference period and mid-21st century climate suitability is a fundamental step to efficiently prioritize restoration areas and develop a successful recovery program.

• Parenchyma cells in the xylem store nonstructural carbohydrates (NSC), providing reserves of energy that fuel woody perennials through periods of stress and/or limitations to photosynthesis. If the capacity for storage is subject to selection, then the fraction of wood occupied by living parenchyma should increase towards stressful environments. • Ray parenchyma fraction (RPF) and seasonal NSC dynamics were quantified for 12 conifers and three oaks along a transect spanning warm dry foothills (500 m above sea level) to cold wet treeline (3250 m asl) in California’s central Sierra Nevada. • Mean RPF was lower for both conifer and oak species with warmer dryer ranges. RPF variability increased with elevation or in relation to associated climatic variables in conifers – tree-line-dominant Pinus albicaulis had the lowest mean RPF measured (c. 3.7%), but the highest environmentally standardized variability index. Conifer RPF variability was explained by environment, increasing predominantly towards cooler wetter range edges. In oaks, NSC was explained by environment – values increasing for evergreen and decreasing for deciduous oaks with elevation. Lastly, all species surveyed appear to prioritize filling available RPF with sugar to achieve molarities that balance reasonable tensions over starch to maximize stored carbon. • RPF responds to environment but is unlikely to spatially constrain NSC storage.

Widespread outbreaks of mountain pine beetles (MPB) are occurring throughout the range of this native insect. Episodic outbreaks are a common occurrence in the beetles' primary host, lodgepole pine. Current outbreaks, however, are occurring in habitats where outbreaks either did not previously occur or were limited in scale. Herein, we address widespread, ongoing outbreaks in high-elevation, whitebark pine forests of the Greater Yellowstone Ecosystem, where, due to an inhospitable climate, past outbreaks were infrequent and short lived. We address the basic question: are these outbreaks truly unprecedented and a threat to ecosystem continuity? In order to evaluate this question we (1) present evidence that the current outbreak is outside the historic range of variability; (2) examine system resiliency to MPB disturbance based on adaptation to disturbance and host defenses to MPB attack; and (3) investigate the potential domain of attraction to large-scale MPB disturbance based on thermal developmental thresholds, spatial structure of forest types, and the confounding influence of an introduced pathogen. We conclude that the loss of dominant whitebark pine forests, and the ecological services they provide, is likely under continuing climate warming and that new research and strategies are needed to respond to the crisis facing whitebark pine.

Warming-induced mountain pine beetle (Dendroctonus ponderosae; MPB) outbreaks have caused extensive mortality of whitebark pine (Pinus albicaulis; WBP) throughout the species’ range. In the highest mountains where WBP occur, they cross alpine treeline ecotones (ATEs) where growth forms transition from trees to shrub-like krummholz, some of which survived recent MPB outbreaks. This observation motivated the hypothesis that ATEs are refugia for WBP because krummholz growth forms escape MPB attack and have the potential to produce viable seed. To test this hypothesis, we surveyed WBP mortality along transects from the ATE edge (locally highest krummholz WBP) downslope into the forest and, to distinguish if survival mechanisms are unique to ATEs, across other forest ecotones (OFEs) from the edge of WBP occurrence into the forest. We replicated this design at 10 randomly selected sites in the U.S. Northern Rocky Mountains. We also surveyed reproduction in a subset of ATE sites. Mortality was nearly absent in upper ATEs (mean ± SE percent dead across all sites of 0.03% ± 0.03% 0–100 m from the edge and 14.1% ± 1.7% 100–500 m from the edge) but was above 20% along OFEs (21.4 ± 5.2% 0–100 m and 32.4 ± 2.7% 100–500 m from the edge). We observed lower reproduction in upper ATEs (16 ± 9.9 cones/ha and 12.9 ± 5.3 viable seeds/cone 0–100 m from the edge) compared to forests below (317.1 ± 64.4 cones/ha and 32.5 ± 2.5 viable seeds/cone 100–500 m from the edge). Uniquely high WBP survival supports the hypothesis that ATEs serve as refugia because krummholz growth forms escape MPB attack. However, low reproduction suggests ATE refugia function over longer time periods. Beyond our WBP system, we propose that plant populations in marginal environments are candidate refugia if distinct phenotypes result in reduced disturbance impacts.

Increased mortality of forest trees, driven directly or indirectly by climate change, is occurring around the world. In western North America, whitebark pine, a high elevation keystone species, and lodgepole pine, a widespread ecologically and economically important tree, have experienced extensive mortality in recent climate-driven outbreaks of the mountain pine beetle. However, even in stands experiencing high levels of mortality, some mature trees have survived. We hypothesized that the outbreak acted as a natural selection event, removing trees most susceptible to the beetle and least adapted to warmer drier conditions. If this was the case, genetic change would be expected at loci underlying beetle resistance. Given we did not know the basis for resistance, we used inter-simple sequence repeats to compare the genetic profiles of two sets of trees, survivors (mature, living trees) and general population (trees just under the diameter preferred by the beetles and expected to approximate the genetic structure of each tree species at the site without beetle selection). This method detects high levels of polymorphism and has often been able to detect patterns associated with phenotypic traits. For both whitebark and lodgepole pine, survivors and general population trees mostly segregated independently indicating a genetic basis for survivorship. Exceptions were a few general population trees that segregated with survivors in proportions roughly reflecting the proportion of survivors versus beetle-killed trees. Our results indicate that during outbreaks, beetle choice may result in strong selection for trees with greater resistance to attack. Our findings suggest that survivorship is genetically based and, thus, heritable. Therefore, retaining survivors after outbreaks to act as primary seed sources could act to promote adaptation. Further research will be needed to characterize the actual mechanism(s) of resistance.

Whitebark pine (WBP, Pinus albicaulis) is a white pine of subalpine regions in the Western contiguous United States and Canada. WBP has become critically threatened throughout a significant part of its natural range due to mortality from the introduced fungal pathogen white pine blister rust (WPBR, Cronartium ribicola) and additional threats from mountain pine beetle (Dendroctonus ponderosae), wildfire, and maladaptation due to changing climate. Vast acreages of WBP have suffered nearly complete mortality. Genomic technologies can contribute to a faster, more cost-effective approach to the traditional practices of identifying disease-resistant, climate-adapted seed sources for restoration. With deep-coverage Illumina short reads of haploid megagametophyte tissue and Oxford Nanopore long reads of diploid needle tissue, followed by a hybrid, multistep assembly approach, we produced a final assembly containing 27.6 Gb of sequence in 92,740 contigs (N50 537,007 bp) and 34,716 scaffolds (N50 2.0 Gb). Approximately 87.2% (24.0 Gb) of total sequence was placed on the 12 WBP chromosomes. Annotation yielded 25,362 protein-coding genes, and over 77% of the genome was characterized as repeats. WBP has demonstrated the greatest variation in resistance to WPBR among the North American white pines. Candidate genes for quantitative resistance include disease resistance genes known as nucleotide-binding leucine-rich repeat receptors (NLRs). A combination of protein domain alignments and direct genome scanning was employed to fully describe the 3 subclasses of NLRs. Our high-quality reference sequence and annotation provide a marked improvement in NLR identification compared to previous assessments that leveraged de novo-assembled transcriptomes.

Clark's nutcrackers (Nucifraga columbiana) are obligate seed dispersers for whitebark pine (Pinus albicaulis), but they frequently use other conifer seed resources because of annual variability in cone production or geographic variation in whitebark pine availability. Whitebark pine is declining from several threats including white pine blister rust, leading to potential population declines in the nutcracker and the pine. We hypothesize that where there are few additional seed resources, whitebark pine becomes the key and limiting resource supporting nutcracker populations. We investigated how nutcrackers use coniferous forest community types within Yellowstone National Park to determine potential seed resources and the importance of whitebark pine. We established sites representing five forest community types, including whitebark pine, lodgepole pine (P. contorta), Engelmann spruce (Picea engelmannii), limber pine (P. flexilis), and Douglas‐fir (Pseudotsuga menziesii). Each transect annually generated nutcracker point counts, conifer cone production indices, community composition data, and seed resource use observations. We compared hierarchical distance sampling models, estimating nutcracker density and its relationship to forest community type, seed harvesting time‐period, year, study site, and cone seed energy. We found cone production varied across years indicating annual variability in energy availability. Nutcracker density was best predicted by forest community type and survey time‐period and was highest in whitebark pine stands during the mid‐harvesting season. Nutcracker density was comparatively low for all other forest community types. This finding underscores the importance of whitebark pine as a key seed resource for Clark's nutcracker in Yellowstone National Park. The decline of whitebark pine potentially leads to a downward spiral in nutcrackers and whitebark pine, arguing for continued monitoring of nutcrackers and implementation of restoration treatments for whitebark pine. The coevolved mutualism between whitebark pine (Pinus albicaulis) and Clark's nutcracker (Nucifraga columbiana) is well documented, but our understanding of nutcracker use of additional conifer seed resources is lacking. Data collected over 3 years in Yellowstone National Park suggest that whitebark pine was the only seed resource that nutcrackers visited regularly and that bird density was highest during mid‐September when whitebark pine seeds ripen. In areas without additional seed resources to sustain populations of nutcrackers, we hypothesize that Clark's nutcracker populations may be more directly linked to whitebark pine seed production.

Global declines in tree populations have led to dramatic shifts in forest ecosystem composition, biodiversity, and functioning. These changes have consequences for both forest plant and wildlife communities, particularly when declining species are involved in coevolved mutualisms. Whitebark pine (Pinus albicaulis) is a declining keystone species in western North American high‐elevation ecosystems and an obligate mutualist of Clark's nutcracker (Nucifraga columbiana), an avian seed predator and disperser. By leveraging traditional point count surveys and passive acoustic monitoring, we investigated how stand characteristics of whitebark pine in a protected area (Glacier National Park, Montana, USA) influenced occupancy and vocal activity patterns in Clark's nutcracker. Using Bayesian spatial occupancy models and generalized linear mixed models, we found that habitat use of Clark's nutcracker was primarily supported by greater cone density and increasing diameter of live whitebark pine. Additionally, we demonstrated the value of performing parallel analyses with traditional point count surveys and passive acoustic monitoring to provide multiple lines of evidence for relationships between Clark's nutcracker and whitebark pine forest characteristics. Our findings allow managers to gauge the whitebark pine conditions important for retaining high nutcracker visitation and prioritize management efforts in whitebark pine ecosystems with low nutcracker visitation. Анотація Глобальні зменшення популяцій дерев призвело до різких змін у складі лісових екосистем, біорізноманіття та функціонування. Ці зміни мають наслідки як для лісових рослин, також для диких тварин, особливо коли види, що скорочуються, залучені до коеволюційних мутуалізмів. Кількість білокорої сосни (Pinus albicaulis) скорочується у високогірних екосистемах західної Північної Америки і є обов'язковим мутуалістом горіхівки Кларка (Nucifraga columbiana), пташиного хижака, який розповсюджує насіння. Доповнюючи традиційний метод підрахування з результатами пасивного акустичного моніторингу, ми досліджували як характеристики ділянок білокорої сосни на охоронюваній території (Національний парк Глейшер, штат Монтана, США) впливають на розміщення та голосової активності горіхівки Кларка. Використовуючи Байєсівські моделі просторового розміщення та узагальнені лінійні змішані моделі, ми виявили, що середовище існування горіхівки Кларка в першу чергу підтримується більшою кількістю шишок а також більшим діаметром живої білокорої сосни. Наші висновки дозволяють менеджерам ресурсів оцінювати стан білокорої сосни, необхідний для збереження високого рівня знаходження горіхівки і визначити пріоритетність зусиль для відновлення їх в районах, де вона низька. Наша структура також надає менеджерам протокол відбору проб, який полегшує довгостроковий моніторинг екосистеми білокорої сосни. Whitebark pine (Pinus albicaulis) is a rapidly declining keystone species in high‐elevation ecosystems and an obligate mutualist of Clark's Nutcracker (Nucifraga columbiana), an avian seed predator and disperser. Using passive acoustic monitoring, point count surveys, and Bayesian occupancy models, we found that habitat use of Clark's Nutcracker is primarily influenced by cone production and increasing diameter of live whitebark pine, emphasizing the importance of restoration strategies that work to protect existing large cone‐bearing whitebark pines.

A multiyear study of forest-to-alpine ecotones across extensive krummholz zones in whitebark pine (WBP; Pinus albicaulis), Sierra Nevada, California, resolved mean treeline growing season temperature (GST) of 9.3°C, 2.6°C warmer than global thresholds previously described, and mean growing season length of 143 days. Temperatures declined with increasing elevation; GST at the upper krummholz line (8.9°C), however, was 2.2°C warmer than the mean global treeline threshold, suggesting that by thermal criteria these environments should support tree growth. Possible explanations for the warm conditions and persistence of krummholz rather than treeline advance include a role for moisture limitations, disequilibrium with Little Ice Age temperatures, and the influence of krummholz as a buffer to treeline dynamics. Radial growth in treeline WBP trees was negatively correlated with maximum annual temperature and positively correlated to water year precipitation. Krummholz stems had low correlations to one another, to treeline trees, and to climate, suggesting nonclimatic controls on growth. These findings underscore the variable nature of treeline response to climate change, suggest that krummholz ecotones behave differently from diffuse treelines, and add to examples of mountain conifers that may be exhibiting lag effects and have not shifted with contemporary warming.