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Plant invasions are a widespread and recurring phenomenon that cause significant economic and environmental damage. Invasive alien conifers are weeds that are not only costly to manage, but consistently reinvade after management efforts. Understanding how many seeds survive to germinate is a key part in understanding the weed life cycle puzzle. Here we investigated the contribution that seed predators have on reducing invasive alien conifer seed survival across both invaded and uninvaded habitats in Aotearoa New Zealand. We combined quantitative and qualitative experiments to measure seed predation across invaded and uninvaded habitats, as well as to identify which fauna are the most prolific seed predators. We utilised ex-situ empirical evidence with in-situ observations to provide realistic impacts from different seed predator species. We found that introduced mammals, particularly rodents, were the primary seed predators of invasive conifers. Seed predation pressure was highest in herbicide treated invasive alien conifer forests, indigenous beech forests, and managed pasture containing grazing livestock. Indigenous tussock areas support fewer vertebrate seed predators and as a result are particularly vulnerable to conifer invasion. The majority of seed predation occurs within the first two weeks post-dispersal. These results suggest that introduced mammal control operations, which are essential to protect endemic New Zealand species, will likely result in increasing invasive conifer populations by reducing seed predation pressure. Seed predation varies greatly between habitats, suggesting invasion and reinvasion rates are higher in ungrazed areas of lower forest density that support fewer introduced mammals. [ABSTRACT FROM AUTHOR]

Understanding the factors influencing species range limits is increasingly crucial in anticipating migrations due to human‐caused climate change. In the boreal biome, ongoing climate change and the associated increases in the rate, size, and severity of disturbances may alter the distributions of boreal tree species. Notably, Interior Alaska lacks native pine, a biogeographical anomaly that carries implications for ecosystem structure and function. The current range of lodgepole pine (Pinus contorta var. latifolia) in the adjacent Yukon Territory may expand into Interior Alaska, particularly with human assistance. Evaluating the potential for pine expansion in Alaska requires testing constraints on range limits such as dispersal limitations, environmental tolerance limits, and positive or negative biotic interactions. In this study, we used field experiments with pine seeds and transplanted seedlings, complemented by model simulations, to assess the abiotic and biotic factors influencing lodgepole pine seedling establishment and growth after fire in Interior Alaska. We found that pine could successfully recruit, survive, grow, and reproduce across our broadly distributed network of experimental sites. Our results show that both mammalian herbivory and competition from native tree species are unlikely to constrain pine growth and that environmental conditions commonly found in Interior Alaska fall well within the tolerance limits for pine. If dispersal constraints are released, lodgepole pine could have a geographically expansive range in Alaska, and once established, its growth is sufficient to support pine‐dominated stands. Given the impacts of lodgepole pine on ecosystem processes such as increases in timber production, carbon sequestration, landscape flammability, and reduced forage quality, natural or human‐assisted migration of this species is likely to substantially alter responses of Alaskan forest ecosystems to climate change. [ABSTRACT FROM AUTHOR]

Mountain pine beetles (MPBs) pose a substantial threat to North American pine forests, causing extensive tree mortality over large areas. Their tree‐killing ability is closely linked to mass aggregation on host trees triggered via pheromones and dependence on their symbiotic fungi. However, the influence of a changing climate on the biology of MPBs and their co‐evolved interactions with their fungal symbionts remains uncertain. To investigate this, male and female pairs of beetles were introduced into freshly cut logs from lodgepole pine trees and placed in controlled climate chambers with manipulated environmental conditions, including two levels of CO2 (ambient vs. 1000 ppm), O3 (ambient vs. 100 ppb) and humidity (33% vs. 65%). The beetle‐infested logs were left in these chambers for 1 month and then returned to ambient conditions until brood emergence. Emerging broods were collected for further analysis. Additionally, three species of fungal symbionts (Grosmannia clavigera, Ophiostoma montium and Leptographium longiclavatum) were subjected to the same CO2, O3 and humidity conditions for 5 days. Lower humidity promoted MPB reproduction and fungal growth. Elevated CO2 accelerated larval growth and emergence while improving brood pheromone production. Elevated O3 had a negative impact on MPB reproduction and brood fitness while improving its immune responses to an entomopathogenic fungus (Beauveria bassiana). It also inhibited fungal growth and reproduction, whereas elevated CO2 had varied (positive or negative) effects on fungal growth and ergosterol (proxy to fungal mass) production depending on the fungal species. Together, these findings suggest that climate change can potentially alter the interactions between MPBs and their fungal symbionts, highlighting the importance of understanding how climate change affects forest pests and their symbiotic relationships to develop effective management strategies in the future. [ABSTRACT FROM AUTHOR]

The recent expansion of mountain pine beetle (MPB, Dendroctonus ponderosae) from its native range in western North America into northern boreal pine forests in Alberta, Canada has resulted in conditions for tree regeneration that are dramatically different from those after wildfire, the predominant natural disturbance in these forests. We assessed natural regeneration post-MPB for northern boreal lodgepole pine sites in Alberta, Canada via intensive surveys of small plots at 33 severely attacked pine stands and using data from 205 permanent sample plots representing various site types and levels of MPB mortality. We used model selection to identify factors explaining regeneration. Overall, pine regeneration was very poor 6–9 years post-MPB; only 42% of the 33 intensively surveyed plots and only 9% of the 205 permanent plots had pine seedlings. This poor regeneration is attributed to high levels of cone serotiny in these populations, unsuitable regeneration microsites due to undisturbed litter or feathermoss layers, and competition from the residual canopy and understory vegetation. Other species (aspen, birch, poplar, and black and white spruce) were found on most sites, either as post-attack regeneration or regeneration established in advance. Without intervention, many of these stands will likely transition away from pine, to broadleaf and other conifer species. [ABSTRACT FROM AUTHOR]

In North America, lodgepole pine is frequently subjected to attacks by various biotic agents that compromise its ability to defend against subsequent attacks by insect herbivores. We investigated whether infections of lodgepole pine by different pathogenic fungal species have varying effects on its defense chemistry. We selected two common pathogens, Atropellis canker, Atropellis piniphila, and western gall rust, Endocronartium harknessii, affecting mature lodgepole pine trees in western Canada. We also included three ophiostomatoid fungi Grosmannia clavigera, Ophiostoma montium, and Leptographium longiclavatum associated with the mountain pine beetle (Dendroctonus ponderosae), because they are commonly used to investigate induced defenses of host trees of bark beetles. We collected phloem samples from lodgepole pines infected with the rust or the canker and healthy lodgepole pines in the same stand. We also inoculated mature lodgepole pines with the three fungal symbionts and collected phloem samples 2 weeks later when the defense chemistry was at its highest level. Different fungal species differentially altered the terpene chemistry of lodgepole pine trees. E. harknessii and the fungal symbionts altered the terpene chemistry in a similar pattern while trees responded to the infection by the A. piniphila differently. Our study highlights the importance of considering specific biotic stress agents in tree susceptibility or resistance to the subsequent attacks by insect herbivores, such as mountain pine beetle. [ABSTRACT FROM AUTHOR]

Anticipating consequences of disturbance interactions on ecosystem structure and function is a critical management priority as disturbance activity increases with warming climate. Across the Northern Hemisphere, extensive tree mortality from recent bark beetle outbreaks raises concerns about potential fire behavior and post-fire forest function. Silvicultural treatments (that is, partial or complete cutting of forest stands) may reduce outbreak severity and subsequent fuel loads, but longevity of pre-outbreak treatment effects on outbreak severity and post-outbreak fuel profiles remains underexplored. Further, treatments may present tradeoffs for other management objectives focused on ecosystem services (for example, carbon storage). We measured structure in old-growth subalpine forests following a recent (early 2000s) severe mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak to examine effects of historical (1940s) cutting intensity on gray stage (~10 years after peak of outbreak) post-outbreak (1) fuel profiles and (2) aboveground biomass carbon. Compared to control (uncut) stands, historically cut stands subjected to the same MPB outbreak had approximately half the post-outbreak surface fuel loads, about 2–3x greater live canopy fuel loads, and greater within-stand spatial heterogeneity of dead canopy cover and available canopy fuel load. Post-outbreak total aboveground biomass carbon was similar across all stands, though historically cut stands had about 2x greater carbon in live biomass compared to uncut stands. These findings suggest tradeoffs with altered post-outbreak potential fire behavior and carbon storage in cut stands. Additional implications of historical silvicultural treatments for wildlife habitat, firefighting operations, and long-term carbon trajectories highlight temporal legacies of management on directing forest response to interacting disturbances. [ABSTRACT FROM AUTHOR]

Insect herbivores must contend with constitutive and induced plant defenses. The mountain pine beetle, Dendroctonus ponderosae Hopkins (Coleoptera: Curculionidae, Scolytinae) has expanded its range east of the Rocky Mountains into the western boreal forest and is encountering evolutionarily naïve lodgepole pines (Pinus contorta) and jack pines (Pinus banksiana). Pinus contorta and P. banksiana in the expanded range have different constitutive and induced defenses in response to wounding and inoculation with fungal associates of D. ponderosae. In the historic range, previous studies have examined phloem terpene content prior to and just after D. ponderosae mass attack, but the terpene profile of attacked trees post-overwintering is unknown. We examined the response of mature P. contorta and P. banksiana trees to experimentally-applied mass attack by D. ponderosae and quantified phloem terpenes at three time points, pre-attack, post-attack (same season), and the following spring, post-overwintering. Phloem content of total terpenes as well as many individual terpenes increased after D. ponderosae attack but were only significantly higher than pre-attack levels at the post-overwintering time point in both P. contorta and P. banksiana. The absence of a significant increase in phloem terpenes in the month following attack in naïve pines is a potential cause for increased D. ponderosae offspring production reported in naïve P. contorta. Beetle attack density did not influence the phloem terpene profiles of either species and there was no significant interaction between attack density and sampling time on terpene content. High phloem terpenes in trees that are attacked at low densities could prime these trees for defense against attacks in the following season but it could also make these trees more apparent to early-foraging beetles and facilitate efficient mass attack at low D. ponderosae population densities in the expanded range. [ABSTRACT FROM AUTHOR]

Endophytes have been defined as microorganisms living inside plant tissues without causing negative effects on their hosts. Endophytic microbes have been extensively studied for their plant growth-promoting traits. However, analyses of endophytes require complete removal of epiphytic microorganisms. We found that the established tests to evaluate surface sterility, polymerase chain reaction, and leaf imprints, are unreliable. Therefore, we used scanning electron microscopy (SEM) as an additional assessment of epiphyte removal. We used a diverse suite of sterilization protocols to remove epiphytic microorganisms from the leaves of a gymnosperm and an angiosperm tree to test the influence of leaf morphology on the efficacy of these methods. Additionally, leaf tissue damage was also evaluated by SEM, as damaging the leaves might have an impact on endophytes and could lead to inaccurate assessment of endophytic communities. Our study indicates, that complete removal of the leaf cuticle by the sterilization technique assures loss of epiphytic microbes, and that leaves of different tree species may require different sterilization protocols. Furthermore, our study demonstrates the importance of choosing the appropriate sterilization protocol to prevent erroneous interpretation of host-endophyte interactions. Moreover, it shows the utility of SEM for evaluating the effectiveness of surface sterilization methods and their impact on leaf tissue integrity.

Endophytes have been defined as microorganisms living inside plant tissues without causing negative effects on their hosts. Endophytic microbes have been extensively studied for their plant growth-promoting traits. However, analyses of endophytes require complete removal of epiphytic microorganisms. We found that the established tests to evaluate surface sterility, polymerase chain reaction, and leaf imprints, are unreliable. Therefore, we used scanning electron microscopy (SEM) as an additional assessment of epiphyte removal. We used a diverse suite of sterilization protocols to remove epiphytic microorganisms from the leaves of a gymnosperm and an angiosperm tree to test the influence of leaf morphology on the efficacy of these methods. Additionally, leaf tissue damage was also evaluated by SEM, as damaging the leaves might have an impact on endophytes and could lead to inaccurate assessment of endophytic communities. Our study indicates, that complete removal of the leaf cuticle by the sterilization technique assures loss of epiphytic microbes, and that leaves of different tree species may require different sterilization protocols. Furthermore, our study demonstrates the importance of choosing the appropriate sterilization protocol to prevent erroneous interpretation of host-endophyte interactions. Moreover, it shows the utility of SEM for evaluating the effectiveness of surface sterilization methods and their impact on leaf tissue integrity.

Abstract Allometric equations predict organism attributes from simple measurements and underlie many global‐scale estimates, from plant productivity to ecosystem carbon stocks. In forests, destructive harvesting of trees in common groups (e.g. plant functional type) or at the species level is necessary to develop allometry but, since sampling is extremely difficult, predictions from these equations have high uncertainty due to low sample size, spatial bias and unrepresentative sampling of tree size. Terrestrial laser scanning (TLS) is a promising remote sensing technology that enables efficient and nondestructive estimates of tree‐level structure for developing allometric equations. Here, we nondestructively estimated component biomass of three coniferous tree species (Pinus ponderosa, Pinus contorta and Pseudotsuga menziesii) in Colorado, USA using TLS. We evaluated the suitability for this nondestructive data to be supplanted for destructive data in the development of species‐specific allometric equations and compared the prediction accuracy against a commonly used national‐scale allometry. We found TLS biomass estimates were consistently more precise across species (RMSE = ~19%) than nation‐scale allometry (RMSE = ~39%). Nondestructive biomass estimates from TLS are a suitable addition to or replacement for traditional sampling methods, with indistinguishable biomass predictions across most of the tested diameter range. We further show how TLS can be used to develop allometric equations compatible with airborne LiDAR and other remote sensing variables (e.g. height and crown area), developing generalized biomass predictions from crown area and tree height (R2 = 0.87). The ability for TLS to support the development of nondestructive allometry at a global scale will enable a more nuanced understanding of the drivers of individual tree architecture, while supporting the next generation of biomass remote sensing.

The environment could alter growth and resistance tradeoffs in plants by affecting the ratio of resource allocation to various competing traits. Yet, how and why functional tradeoffs change over time and space is poorly understood particularly in long-lived conifer species. By establishing four common-garden test sites for five lodgepole pine populations in western Canada, combined with genomic sequencing, we revealed the decoupling pattern and genetic underpinnings of tradeoffs between height growth, drought resistance based on δ13C and dendrochronology, and metrics of pest resistance based on pest suitability ratings. Height and δ13C correlation displayed a gradient change in magnitude and/or direction along warm-to-cold test sites. All cold test sites across populations showed a positive height and δ13C relationship. However, we did not observe such a clinal correlation pattern between height or δ13C and pest suitability. Further, we found that the study populations exhibiting functional tradeoffs or synergies to various degrees in test sites were driven by non-adaptive evolutionary processes rather than adaptive evolution or plasticity. Finally, we found positive genetic relationships between height and drought or pest resistance metrics and probed five loci showing potential genetic tradeoffs between northernmost and the other populations. Our findings have implications for deciphering the ecological, evolutionary, and genetic bases of the decoupling of functional tradeoffs due to environmental change. [ABSTRACT FROM AUTHOR]

Previous research on the effects of neighborhood crowding and soil moisture on tree height growth have been limited by time-consuming and sometimes inaccurate ground-based measurements of tree height. Recent developments in unmanned aerial vehicles (UAVs) allow detailed 3D point clouds of the canopy surface to be generated at relatively low cost. Using UAV-derived point clouds, we obtained height measurements of 4386 trees for the years 2019 and 2021. We also calculated four neighborhood crowding indices and a topography-based moisture index (depth-to-water) for these trees. Using initial tree height, neighborhood crowding indices and the depth-to-water index, we developed Bayesian hierarchical models to predict height growth for three tree species (Picea glauca (white spruce), Populus tremoluides (trembling aspen) and Pinus contorta (lodgepole pine)) across different stands. Bayes-R 2 values of the final models were highest for white spruce (35%) followed by trembling aspen (28%) and lodgepole pine (25%). Model outputs showed that the effect of crowding and depth-to-water on height growth are limited and species-dependent, adding a maximum of 7% to the Bayes-R 2 metric. Comparing different neighborhood crowding indices revealed that no index is clearly superior to others across all three species, as different neighborhood crowding indices resulted in only minor differences in model performance. While height growth can be partially explained by aerially derived neighborhood crowding indices and the depth-to-water index, future studies should focus on identifying relevant site characteristics to predict tree growth with greater accuracy. [ABSTRACT FROM AUTHOR]

Bark beetles maintain symbiotic associations with a diversity of microbial organisms, including ophiostomatoid fungi. Studies have frequently reported the role of ophiostomatoid fungi in bark beetle biology, but how fungal symbionts interact with host chemical defenses over time is needed. We first investigated how inoculations by three fungal symbionts of mountain pine beetle affect the terpene chemistry of live lodgepole pine trees. We then conducted a complimentary laboratory experiment specifically measuring the host metabolite degradation by fungi and collected the fungal organic volatiles following inoculations with the same fungal species on lodgepole pine logs. In both experiments, we analyzed the infected tissues for their terpene chemistry. Additionally, we conducted an olfactometer assay to determine whether adult beetles respond to the volatile organic chemicals emitted from each of the three fungal species. We found that all fungi upregulated terpenes as early as two weeks after inoculations. Similarly, oxygenated monoterpene concentrations also increased by several folds (only in logs). A large majority of beetles tested showed a strong attraction to two fungal species, whereas the other fungus repelled the beetles. Together this study shows that fungal symbionts can alter host defense chemistry, assist beetles in overcoming metabolite toxicity, and provide possible chemical cues for bark beetle attraction. [ABSTRACT FROM AUTHOR]

Allometric equations predict organism attributes from simple measurements and underlie many global‐scale estimates, from plant productivity to ecosystem carbon stocks. In forests, destructive harvesting of trees in common groups (e.g. plant functional type) or at the species level is necessary to develop allometry but, since sampling is extremely difficult, predictions from these equations have high uncertainty due to low sample size, spatial bias and unrepresentative sampling of tree size. Terrestrial laser scanning (TLS) is a promising remote sensing technology that enables efficient and nondestructive estimates of tree‐level structure for developing allometric equations.Here, we nondestructively estimated component biomass of three coniferous tree species (Pinus ponderosa, Pinus contorta and Pseudotsuga menziesii) in Colorado, USA using TLS. We evaluated the suitability for this nondestructive data to be supplanted for destructive data in the development of species‐specific allometric equations and compared the prediction accuracy against a commonly used national‐scale allometry.We found TLS biomass estimates were consistently more precise across species (RMSE = ~19%) than nation‐scale allometry (RMSE = ~39%). Nondestructive biomass estimates from TLS are a suitable addition to or replacement for traditional sampling methods, with indistinguishable biomass predictions across most of the tested diameter range. We further show how TLS can be used to develop allometric equations compatible with airborne LiDAR and other remote sensing variables (e.g. height and crown area), developing generalized biomass predictions from crown area and tree height (R2 = 0.87).The ability for TLS to support the development of nondestructive allometry at a global scale will enable a more nuanced understanding of the drivers of individual tree architecture, while supporting the next generation of biomass remote sensing. [ABSTRACT FROM AUTHOR]

Climate change can enhance or hinder the spread of invasive plant species, as climatic conditions influence the population dynamics of invaders in their new range. Pines are one of the most widely distributed and problematic invasive tree species globally. Here we experimentally evaluated how two invasive pine species (Pinus ponderosa and Pinus contorta) respond to simulated climate change conditions in northwestern Patagonia. We conducted a full factorial manipulative field experiment with 3-year-old pine saplings growing in warmed (Open Top Chamber), watered, warmed + watered, and control treatments. In the warmed treatments P. contorta saplings showed lower survival and tended to grow taller while maintaining their root growth. In contrast, P. ponderosa saplings, which did not show differences in survival among treatments, did not vary their aerial growth under warmed conditions, but tended to increase their root-to-shoot ratio. Both species showed similar survival and growth in the watered treatments compared to the control, indicating that the implemented difference in water (ca. 30% during the growing season) had no significant effect. Our findings suggest that a decrease in the invasive capacity (initial stages) of P. contorta may occur under climate change but no significant changes are expected for P. ponderosa. This observed differential response may modify the current relative abundance of invading pine species in northwestern Patagonia and has implications for the response of invasive species to climate change elsewhere. [ABSTRACT FROM AUTHOR]

Understanding how climate warming will affect the demographic rates of different ecotypes is critical to predicting shifts in species distributions. Here, we present results from a common garden, climate change experiment in which we measured seedling recruitment of lodgepole pine, a widespread North American conifer that is also planted globally. Seeds from a low-elevation provenance had more than three-fold greater recruitment to their third year than seeds from a high-elevation provenance across sites within and above its native elevation range and across climate manipulations. Heating halved recruitment to the third year of both low- and high-elevation seed sources across the elevation gradient, while watering more than doubled recruitment, alleviating some of the negative effects of heating. Demographic models based on recruitment data from the climate manipulations and long-term observations of adult populations revealed that heating could effectively halt modeled upslope range expansion except when combined with watering. Simulating fire and rapid postfire forest recovery at lower elevations accelerated lodgepole pine expansion into the alpine, but did not alter final abundance rankings among climate scenarios. Regardless of climate scenario, greater recruitment of low-elevation seeds compensated for longer dispersal distances to treeline, assuming colonization was allowed to proceed over multiple centuries. Our results show that ecotypes from lower elevations within a species' range could enhance recruitment and facilitate upslope range shifts with climate change.

Successful host plant colonization by tree-killing bark beetle-symbiotic fungal complexes depends on host suitability, which is largely determined by host defense metabolites such as monoterpenes. Studies have shown the ability of specific blends of host monoterpenes to influence bark beetles or their fungal symbionts, but how biologically relevant blends of host monoterpenes influence bark beetle-symbiotic fungal interaction is unknown. We tested how interactions between two host species (lodgepole pine or jack pine) and two fungal symbionts of mountain pine beetle (Grosmannia clavigera or Ophiostoma montium) affect the performance of adult female beetles in vitro. Beetles treated with the propagules of G. clavigera or O. montium or not treated (natural fungal load) were introduced into media amended with a blend of the entire monoterpene profile of either host species and beetle performance was compared. Overall, host blends altered beetle performance depending on the fungal species used in the beetle amendment. When beetles were amended with G. clavigera, their performance was superior over beetles amended with O. montium in either host blend. Furthermore, G. clavigera-amended beetles performed better in media amended with host blends than without a host blend; in contrast, O. montium-amended beetles performed better in media without a host blend than with a host blend. Overall, this study showed that host defense metabolites affect host suitability to bark beetles through influencing their fungal symbionts and that different species of fungal symbionts respond differentlly to host defense metabolites. [ABSTRACT FROM AUTHOR]

While droughts, intensified by climate change, have been affecting forests worldwide, pest epidemics are a major source of uncertainty for assessing drought impacts on forest trees. Thus far, little information has documented the adaptability and evolvability of traits related to drought and pests simultaneously. We conducted common-garden experiments to investigate how several phenotypic traits (i.e. height growth, drought avoidance based on water-use efficiency inferred from δ13C and pest resistance based on defence traits) interact in five mature lodgepole pine populations established in four progeny trials in western Canada. The relevance of interpopulation variation in climate sensitivity highlighted that seed-source warm populations had greater adaptive capability than cold populations. In test sites, warming generated taller trees with higher δ13C and increased the evolutionary potential of height growth and δ13C across populations. We found, however, no pronounced gradient in defences and their evolutionary potential along populations or test sites. Response to selection was weak in defences across test sites, but high for height growth particularly at warm test sites. Response to the selection of δ13C varied depending on its selective strength relative to height growth. We conclude that warming could promote the adaptability and evolvability of growth response and drought avoidance with a limited evolutionary influence from pest (biotic) pressures. [ABSTRACT FROM AUTHOR]

Forest disturbances can alter mycorrhizal fungal communities with consequences for subsequent plant establishment and growth. One possible mitigation strategy to promote forest recovery following disturbances is inoculation with soil from late‐successional forests. However, whether these soil transfers change the composition of resident fungal communities and how seedling growth responds to such transfers is unknown.To test whether soil transfers can mitigate disturbance‐mediated impacts on ectomycorrhizal fungal (EMF) communities, we transferred soils containing fungal inoculum collected from conspecific late‐successional forests into stands disturbed by wildfire, mountain pine beetle outbreak, clear‐cut logging or salvage logging. Afterward, lodgepole pine seeds were sown to measure germination, establishment and growth of seedlings, and to characterize root‐associated and soil EMF communities by sequencing the ITS1 rDNA region.Despite evidence that soil inocula contained viable EMF communities, transfers did not impact the root‐associated or soil EMF community composition, or seed germination, seedling establishment and growth. Resident EMF community composition remained unchanged upon transfers of fungi from late‐stage successional forests, and disturbance type singularly explained the EMF community composition associated with roots and soils.Ectomycorrhizal fungal taxa typically associated with mature pine forests were particularly abundant on roots of seedlings growing in plots impacted by beetle outbreak, while 'early‐stage' EMF taxa were associated with seedlings growing in wildfire and logging disturbances.Synthesis and applications. Following disturbances, soil and root‐associated EMF exhibit high functional redundancy, as seedling performance did not vary with differences in EMF community composition across disturbance types. Our findings suggest that soil transfers are unlikely to be an effective strategy in these stands and may be better targeted to highly degraded forests. The presence and abundance of EMF in disturbed forests should be assessed before committing to soil transfers as part of restoration. [ABSTRACT FROM AUTHOR]

Mountain meadows have high biodiversity and help regulate stream water release following the snowmelt pulse. However, many meadows are experiencing woody plant encroachment, threatening these ecosystem services. While there have been field surveys of individual meadows and remote sensing-based landscape-scale studies of encroachment, what is missing is a broad-scale, ground-based study to understand common regional drivers, especially at high elevations, where land management has often played a less direct role. With this study, we ask: What are the climate and landscape conditions conducive to woody plant encroachment at the landscape scale, and how has historical climate variation affected tree recruitment in subalpine meadows over time? We measured density of encroaching trees across 340 subalpine meadows in the central Sierra Nevada, California, USA, and used generalized additive models (GAMs) to determine the relationship between landscape-scale patterns of encroachment and meadow environmental properties. We determined ages of trees in 30 survey meadows, used observed climate and GAMs to model the relationship between timing of recruitment and climate since the early 1900s, and extrapolated recruitment patterns into the future using downscaled climate scenarios. Encroachment was high among meadows with lodgepole pine (Pinus contorta Douglas ex Loudon var. murrayana (Balf.) Engelm.) in the immediate vicinity, at lower elevations, with physical conditions favoring strong soil drying, and with maximum temperatures above or below average. Climatic conditions during the year of germination were unimportant, with tree recruitment instead depending on a 3-yr seed production period prior to germination and a 6-yr seedling establishment period following germination. Recruitment was high when the seed production period had high snowpack, and when the seedling establishment period had warm summer maximum temperatures, high summer precipitation, and high snowpack. Applying our temporal model to downscaled output from four global climate models indicated that the average meadow will shift to forest by the end of the 21st century. Sierra Nevada meadow encroachment by conifers is ubiquitous and associated with climate conditions increasingly favorable for tree recruitment, which will lead to substantial changes in subalpine meadows and the ecosystem services they provide.

Bark beetles maintain symbiotic associations with a diversity of microbial organisms, including ophiostomatoid fungi. Studies have frequently reported the role of ophiostomatoid fungi in bark beetle biology, but how fungal symbionts interact with host chemical defenses over time is needed. We first investigated how inoculations by three fungal symbionts of mountain pine beetle affect the terpene chemistry of live lodgepole pine trees. We then conducted a complimentary laboratory experiment specifically measuring the host metabolite degradation by fungi and collected the fungal organic volatiles following inoculations with the same fungal species on lodgepole pine logs. In both experiments, we analyzed the infected tissues for their terpene chemistry. Additionally, we conducted an olfactometer assay to determine whether adult beetles respond to the volatile organic chemicals emitted from each of the three fungal species. We found that all fungi upregulated terpenes as early as two weeks after inoculations. Similarly, oxygenated monoterpene concentrations also increased by several folds (only in logs). A large majority of beetles tested showed a strong attraction to two fungal species, whereas the other fungus repelled the beetles. Together this study shows that fungal symbionts can alter host defense chemistry, assist beetles in overcoming metabolite toxicity, and provide possible chemical cues for bark beetle attraction.

Aims: The response of understory plants is an important, yet understudied element of forest dynamics associated with mountain pine beetle (MPB) disturbance. Here, we provide a detailed assessment of both the changes in understory species and communities over a 5‐year period following severe MPB‐induced lodgepole pine mortality. Specifically, we examine species turnover, rarity and mobility over time, and evaluate shifts in community composition, including the potential for post‐outbreak community homogenization or differentiation. Location: Western Rocky Mountain National Park in the southern Rocky Mountains, Colorado. Methods: We collected species occurrence data across lodgepole pine‐dominated forest understories using repeated sampling of the same study sites (during the peak of MPB activity and 5 years after). Results: More species appeared than disappeared 5 years after the peak of the outbreak. New species were comprised of both early‐ and late‐successional species, suggesting the presence of rather high microhabitat heterogeneity. Although most species remained relatively rare, the number of exceedingly rare species decreased, and the number of highly common species increased. Several species were able to take advantage of the new stand conditions and expand throughout the study area. Some species appeared to spread locally over short distances mainly via vegetative reproduction, others expanded primarily upwards in elevation, probably through long‐distance dispersal and some spread over both short and long distances. Non‐native species doubled in occurrence and tended to spread upslope, towards the interior of the park, yet remained a small component of the understory overall. Shifts in community composition were minimal, but a slight convergence of plant community groups suggests a trend towards community homogenization. Conclusions: Our results demonstrate that even relatively sparse and depauperate lodgepole pine forest understories may be altered in diverse ways following MPB‐induced overstory mortality, depending on the unique ability of species to respond to changes in site conditions and resource availability. [ABSTRACT FROM AUTHOR]

Premise of research. Leaf stomatal and mesophyll conductances limit photosynthesis and influence water use efficiency. Few studies have quantified the relative limitations imposed by these CO2 diffusion pathways on photosynthesis in mature conifer trees under natural conditions. Here, we report observations of stomatal and mesophyll conductance changes during seasonal drying across contrasting topographic positions in two Rocky Mountain conifers. We predicted that topographic controls on soil water availability and energy balance would determine limitations to photosynthesis by mesophyll conductance across conifer species with contrasting patterns of stomatal and hydraulic traits. Methodology. Concurrent measurements of leaf gas exchange and carbon isotope discrimination were used to estimate stomatal (g s) and mesophyll (g m) conductance in branches of an isohydric species, lodgepole pine (Pinus contorta), and an anisohydric species, Engelmann spruce (Picea engelmannii), in the central Rocky Mountains. Quantitative limitation analysis of photosynthesis (A) was then performed using data from CO2 response curves. Pivotal results. Stomatal conductance imposed greater limitations on photosynthesis (42%–67%) than mesophyll conductance (5%–17%), but no significant differences in g m were observed between the two conifer species. At the mesic lower hillslope position, A , g s, and g m increased during the growing season despite declines in soil moisture. In contrast, at the drier upper hillslope position, declines in soil moisture and increases in air temperature during the growing season are correlated with reductions in g s but not with A or g m. Conclusions. Adjustments in g m played a potentially important role in sustaining photosynthesis and improving plant water use efficiency when stomatal conductance decreased with water limitation during the growing season at the research site. Sustained g m with seasonal drought may be an important mechanism allowing conifers to survive and maintain competitive dominance in low-resource habitats. [ABSTRACT FROM AUTHOR]

Abstract Background Water-use efficiency (WUE) represents the coupling of forest carbon and water. Little is known about the responses of WUE to thinning at multiple spatial scales. The objective of this research was to use field measurements to understand short-term effects of two thinning treatments (T1: 4500 stems ha−1; and T2: 1100 stems ha−1) and the control (NT: 27,000 stems ha−1) on WUE at the three spatial scales (leaf level: the ratio of leaf photosynthesis to leaf transpiration; tree-level: tree growth to tree transpiration; and stand level: net primary production (NPP) to stand transpiration) and intrinsic WUEi (the ratio of leaf photosynthesis to stomatal conductance at leaf-level; and NPP to canopy conductance at stand-level) in a 16-year old natural lodgepole pine forest. Leaf-level measurements were conducted in 2017, while tree- and stand-level measurements were conducted in both 2016 (the normal precipitation year) and 2017 (the drought year). Results The thinning treatments did not significantly affect the tree- and stand-level WUE in the normal year of 2016. However, the thinning significantly affected WUE in the drought year of 2017: T2 exhibited significantly higher tree-level WUE (0.49 mm2 kg−1) than NT (0.08 mm2 kg−1), and compared to NT, the stand-level WUE values in the thinned stands (T1 and T2) were significantly higher, with means of 0.31, 0.56 and 0.70 kg m−3, respectively. However, the leaf-level and stand-level WUEi in the thinned stands in the drought year were significantly lower than those in the unthinned stands. No significant differences in the leaf-level WUE were found among the treatments in 2017. In addition, the thinning did not significantly change the WUE-VPD relationships at any studied spatial scale. Conclusions The thinning treatments did not cause significant changes in all studied WUE metrics in a normal year. However, their effects were significantly promoted under the drought conditions probably due to the decrease in soil water availability, demonstrating that thinning can improve WUE and consequently support forests to cope with the drought effects. The inconsistent results on the effects of the thinning on forest carbon and water coupling at the spatial scales and the lack of the consistent WUE metrics constraint across-scale comparison and transferring of WUE.

Promoting ecological resilience to increasing disturbance activity is a key management priority under warming climate. Across the Northern Hemisphere, tree mortality from widespread bark beetle outbreaks raises concerns for how forest management can foster resilience to future outbreaks. Density reduction (i.e., thinning) treatments can increase vigor of remaining trees, but the longevity of treatment efficacy for reducing susceptibility to future disturbance remains a key knowledge gap. Using one of the longest‐running replicated experiments in old‐growth subalpine forests, we measured stand structure following a recent (early 2000s) severe mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak to examine the legacy of historical (1940s) thinning treatments on two components of resilience. We asked: 'How did historical thinning intensity affect (1) tree‐scale survival probability and stand‐scale survival proportion (collectively "resistance" to outbreak) for susceptible trees (lodgepole pine [Pinus contorta] ≥ 12 cm diameter) and (2) post‐outbreak stand successional trajectories?' Overall outbreak severity was high (MPB killed 59% of susceptible individuals and 78% of susceptible basal area), and historical thinning had little effect on tree‐scale and stand‐scale resistance. Tree‐scale survival probability decreased sharply with increasing tree diameter and did not differ from the control (uncut stands) in the historical thinning treatments. Stand‐scale proportion of surviving susceptible trees and basal area did not differ from the control in historically thinned stands, except for treatments that removed nearly all susceptible trees, in which survival proportion approximately doubled. Despite limited effects on resistance to MPB outbreak, the legacy of historical treatments shifted dominance from large‐diameter to small‐diameter lodgepole pine by the time of outbreak, resulting in historically thinned stands with ~2× greater post‐outbreak live basal area than control stands. MPB‐driven mortality of large‐diameter lodgepole pine in control stands and density‐dependent mortality of small‐diameter trees in historically thinned stands led to convergence in post‐outbreak live tree stand structure. One exception was the heaviest historical thinning treatments (59–77% basal area removed), for which sapling dominance of shade‐tolerant, unsusceptible conifers was lower than control stands. After six decades, thinning treatments have had minimal effect on resistance to bark beetle outbreaks, but leave persistent legacies in shaping post‐outbreak successional trajectories. [ABSTRACT FROM AUTHOR]

Abstract Background Biomass maps are valuable tools for estimating forest carbon and forest planning. Individual-tree biomass estimates made using allometric equations are the foundation for these maps, yet the potentially-high uncertainty and bias associated with individual-tree estimates is commonly ignored in biomass map error. We developed allometric equations for lodgepole pine (Pinus contorta), ponderosa pine (P. ponderosa), and Douglas-fir (Pseudotsuga menziesii) in northern Colorado. Plot-level biomass estimates were combined with Landsat imagery and geomorphometric and climate layers to map aboveground tree biomass. We compared biomass estimates for individual trees, plots, and at the landscape-scale using our locally-developed allometric equations, nationwide equations applied across the U.S., and the Forest Inventory and Analysis Component Ratio Method (FIA-CRM). Total biomass map uncertainty was calculated by propagating errors from allometric equations and remote sensing model predictions. Two evaluation methods for the allometric equations were compared in the error propagation—errors calculated from the equation fit (equation-derived) and errors from an independent dataset of destructively-sampled trees (n = 285). Results Tree-scale error and bias of allometric equations varied dramatically between species, but local equations were generally most accurate. Depending on allometric equation and evaluation method, allometric uncertainty contributed 30–75% of total uncertainty, while remote sensing model prediction uncertainty contributed 25–70%. When using equation-derived allometric error, local equations had the lowest total uncertainty (root mean square error percent of the mean [% RMSE] = 50%). This is likely due to low-sample size (10–20 trees sampled per species) allometric equations and evaluation not representing true variability in tree growth forms. When independently evaluated, allometric uncertainty outsized remote sensing model prediction uncertainty. Biomass across the 1.56 million ha study area and uncertainties were similar for local (2.1 billion Mg; % RMSE = 97%) and nationwide (2.2 billion Mg; % RMSE = 94%) equations, while FIA-CRM estimates were lower and more uncertain (1.5 billion Mg; % RMSE = 165%). Conclusions Allometric equations should be selected carefully since they drive substantial differences in bias and uncertainty. Biomass quantification efforts should consider contributions of allometric uncertainty to total uncertainty, at a minimum, and independently evaluate allometric equations when suitable data are available.

Background and Aims: Pollen germination and tube growth are essential processes for successful fertilization. They are among the most temperature-vulnerable stages and subsequently affect seed production and determine population persistence and species distribution under climate change. Our study aims to investigate intra- and inter-specific variations in the temperature dependence of pollen germination and tube length growth and to explore how these variations differ for pollen from elevational gradients., Methods: We focused on three conifer species, Pinus contorta, Picea engelmannii, and Pinus ponderosa, with pollen collected from 350 to 2200m elevation in Washington State, USA. We conducted pollen viability tests at temperatures from 5 to 40°C in 5°C intervals. After testing for four days, we took images of these samples under a microscope to monitor pollen germination percentage (GP) and tube length (TL). We applied the Gamma function to describe the temperature dependence of GP and TL and estimated key parameters, including the optimal temperature for GP (Topt_GP) and TL (Topt_TL)., Key Results: Results showed that pollen from three species and different elevations within a species have different GP, TL, Topt_GP, and Topt_TL. The population with a higher Topt_GP would also have a higher Topt_TL, while Topt_TL was generally higher than Topt_GP, i.e., a positive but not one-to-one relationship. However, only Pinus contorta showed that populations from higher elevations have lower Topt_GP and Topt_TL and vice versa. The variability in GP increased at extreme temperatures, whereas the variability in TL was greatest near Topt_TL., Conclusions: Our study demonstrates the temperature dependences of three conifers across a wide range of temperatures. Pollen germination and tube growth are highly sensitive to temperature conditions and vary among species and elevations, affecting their reproduction success during warming. Our findings can provide valuable insights to advance our understanding of how conifer pollen responds to rising temperatures., (© The Author(s) 2024. Published by Oxford University Press on behalf of the Annals of Botany Company. All rights reserved. For commercial re-use, please contact reprints@oup.com for reprints and translation rights for reprints. All other permissions can be obtained through our RightsLink service via the Permissions link on the article page on our site—for further information please contact journals.permissions@oup.com.)

Mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) is a devastating forest insect pest that has killed millions of hectares of pines in western North America over the past two decades. Like other bark beetles, MPB vectors ophiostomatoid fungal species, some of which are pathogenic to host pine species. The phytopathogenicity of these fungal symbionts has sparked considerable debate regarding their role in facilitating MPB attack success. We tested the hypothesis that MPB ophiostomatoid fungal associates like Grosmannia clavigera (Robinson-Jeffrey and Davidson) Zipfel, de Beer and Wingfield contribute to overwhelming host defenses during MPB mass attack. We compared responses of mature lodgepole pine ( Pinus contorta Dougl. ex Loud. var. latifolia Engelm.) trees growing in natural stands that were mass attacked by MPB with those inoculated with G. clavigera by examining host defense hormones, secondary metabolites, and gene expression profiles. The jasmonate and ethylene signatures of necrotrophic pathogen-triggered response were identified in G. clavigera -inoculated trees, but only the jasmonate signature of a herbivore-triggered response was measured in MPB-attacked trees. Several G. clavigera -induced changes in pine phenolic metabolite profiles and phenolic biosynthesis gene expression patterns were absent in MPB-attacked pines. These findings indicate that ophiostomatoid fungi like G. clavigera are not a major factor in overwhelming host defenses during MPB mass attack. Instead, fungal pathogenicity likely is more important in aiding MPB colonization and development within the host tree. Phenolics appear to play a larger role in the host response to G. clavigera than to MPB, although phenolics may also influence MPB feeding and behavior. [Formula: see text] Copyright © 2024 The Author(s). This is an open access article distributed under the CC BY 4.0 International license., Competing Interests: The author(s) declare no conflict of interest.

Intra‐specific variation in conifers has been extensively studied with respect to defense against herbivores and pathogens. While studies have shown the ability of individual or specific mixtures of compounds to influence insects and microbes, research testing biologically relevant mixtures of defense compounds reflecting intra‐specific variation amongst tree populations to enemy complexes is needed. We characterized the variations in lodgepole pine monoterpenes from a progeny trial in western Canada and grouped trees in four clusters using their monoterpene profiles. We then selected 11 representative families across four clusters and amended their entire monoterpene profiles (with the exception of β‐phellandrene) in media to determine how representative families affect the performance of the mountain pine beetle or its fungal symbiont. We placed adult beetles or inoculated fungus on the amended media and measured beetle performance and fungal growth as a proxy to host suitability. We found that different clusters or families differentially influenced beetle or fungal responses. However, monoterpene profiles of trees suitable to the beetle or the fungus were dissimilar. These outcomes reflect a co‐evolutionary arms‐race between the host and the bark beetle‐fungus complex, which has resulted in the production of complementary defense metabolites among different pine populations to enhance tree survival. Lodgepole pine phenotypes having different monoterpene profiles differentially affected the mountain pine beetle and its fungal symbiont, suggesting the production of complementary defense compounds in the same monoterpene mixture to target multi‐species of enemy complexes, enhancing tree survival. [ABSTRACT FROM AUTHOR]

Tree‐species mixture effects (e.g. complementarity and facilitation) have been found to increase individual‐tree productivity, lessen mortality and improve recruitment in forests worldwide. By promoting more efficient and complete resource use, mixture effects may also lessen individual‐tree‐level water stress, thus improving drought resistance. We investigated the influence of mixture effects on tree productivity, mortality and recruitment across broad compositional and moisture gradients in high‐elevation Interior West US mixed‐conifer communities, where Populus tremuloides (trembling aspen) is the major contributor to functional diversity. Our goal was to provide a more complete scientific foundation for managing these drought‐prone, fire‐excluded systems under an uncertain climate.We used landscape‐scale national forest inventory data to examine mixture effects on P. tremuloides and the major associated conifer species, Pseudotsuga menziesii, Pinus contorta, Abies lasiocarpa and Picea engelmannii. Using generalized linear mixed modelling, we isolated the influences of P. tremuloides relative density and climate on tree‐level (stems ≥ 12.7 cm DBH) growth, mortality and stand‐level recruitment (presence/absence of new trees). Cold‐season precipitation (PPT) and warm‐season vapour pressure deficit (VPD) served to represent soil moisture supply and demand, respectively.Populus tremuloides growth declined as interspecific density increased. In contrast, Pinus contorta and A. lasiocarpa growth increased with P. tremuloides density. For all species except A. lasiocarpa and P. menziesii, growth increased under higher PPT and VPD. Populus tremuloides mortality increased under high VPD but not with interspecific relative density. We found limited evidence that A. lasiocarpa mortality decreased as P. tremuloides density increased. Populus tremuloides recruitment declined steeply above 25% interspecific relative density. We found a decline in conifer recruitment odds as P. tremuloides density increased, ranging from strong in P. contorta to insubstantial in P. engelmannii.Synthesis. Our findings have implications for sustaining mixed‐conifer communities impacted by climate change and historical fire exclusion. Mixtures of P. tremuloides and conifers may improve conifer growth while adversely impacting P. tremuloides growth relative to pure stands. Higher conifer productivity combined with lower P. tremuloides recruitment at high conifer relative density may accelerate succession. [ABSTRACT FROM AUTHOR]

Suillus luteus is a common ectomycorrhizal (EM) fungus associated with several Pinus species. It is distributed throughout the Northern Hemisphere and has been introduced into South America and New Zealand. We examined the genetic structure and population biology of S.luteus, which was introduced into Patagonian plantations with Pinus species in Argentina. Overall, 106 samples were collected at 11 geographically separated sites (i.e., Pinus plantations) along a latitudinal gradient in Patagonia (ca. 38°–46° south latitude). Phylogenetic analyses confirmed placement in S.luteus. Genetic analysis demonstrated moderate within-site genetic diversity, but low differentiation between sites. No clear clusters were detected geographically or in relation to host species of Pinus. Our results suggest that the weak genetic structure of the species reflects the short time that has elapsed since the introduction of S.luteus into Patagonia, and its expansion with exotic afforestation there. Moreover, the lack of structure is consistent with a founder effect, suggesting the introduction of a small number of genets that spread throughout all the plantations. Therefore, the high level of gene flow and weak genetic structure observed are probably related to the anthropogenic movement of inoculum associated with forestry practices. [ABSTRACT FROM AUTHOR]

Tree encroachment threatens the biodiversity and ecological functioning of grasslands worldwide. Reversing effects of encroachment requires not only removing trees, but limiting subsequent invasions, which can stall grassland reassembly. We quantified rates and patterns of conifer reinvasion of mountain grasslands (meadows) in western Oregon, 7 and 15 years after experimental tree removal with or without burning. We assessed frequency (percentage of 100 m2 subplots) and density of reinvading trees in six 1‐ha plots as a function of burn treatment, abundance in the adjacent forest (a proxy for seed availability), and distance to or orientation of forest edges. After 15 years, frequency and density of colonists varied widely among plots (24–57% of subplots and 60–250 trees/ha, respectively), but burning had no effect on either measure. Abies grandis, which dominated adjacent forests, was the principal invader. Colonization by less‐common Pinus contorta and A. procera was correlated with species' density and basal area in adjacent forests, suggesting that establishment was seed limited. Annualized rates of invasion were similar between early and late sampling intervals (9–10/trees/ha). Establishment was generally greater along the forested edges of plots (0–10 m) than in the core (10–50 m). Prominence of shade‐tolerant A. grandis along northerly‐facing edges suggests strong microclimatic controls on establishment. In sum, tree removal, with or without prescribed fire, is successful in reversing effects of conifer encroachment. Despite abundant seed rain, reinvasion is slow and spatially restricted. Periodic hand removal of colonizing trees, while they are small, can be done efficiently and economically. [ABSTRACT FROM AUTHOR]

Legacy effects from one disturbance may influence successional pathways by amplifying or buffering forest regeneration after the next disturbance. We assessed vegetation and tree regeneration in non-serotinous Sierra lodgepole pine (Pinus contorta var. murrayana) stands after a 1984 wildfire which burned with variable severity and again after a high-severity subsequent fire in 2012. The legacy effects of the 1984 fire were amplified; seedlings and saplings were abundant in areas initially burned at low severity (1267 stems ha−1) despite high reburn severity, but regeneration was low in areas twice burned at high severity (31 stems ha−1). Our results suggest that the severity of the 1984 fire may have influenced post-2012 tree regeneration by creating variable fuel loading, which may have affected soils, litter cover and shade after the 2012 fire and therefore affected seedling establishment and survival. A canopy seed bank of unburnt cones from trees killed by the 2012 fire potentially contributed to a strong effect of prior burn severity on regeneration after the 2012 fire despite a lack of serotinous or resprouting tree species, although the influence of this canopy seedbank was likely limited to the year following the fire. Our results suggest that a low- to moderate-severity fire increases forest resilience relative to a high-severity fire even when the next fire burns at high severity. [ABSTRACT FROM AUTHOR]

Increased climatic variability can impact tree physiological processes beyond what is predicted from changes in mean conditions. We assessed the sensitivity of conifer saplings to spatial and temporal variability in meteorological conditions, taking advantage of the end of California's historic drought and the exceedingly wet winter of 2017. We sought to understand how very dry and very wet conditions constrain photosynthesis and growth in four regionally dominant conifers and whether sensitivity in these processes changes across a 500 m gradient in elevation. All species demonstrated phenotypic plasticity in response to temporal differences in precipitation on both inter-annual and seasonal timescales. Net photosynthesis in Pinus contorta decreased from an early season 2016 average of 12.4 to 6.89 μmol CO2 m−2 s−1 later in the summer, but increased 14.1% between seasons in the wet year. By contrast, elevation had almost no effect on instantaneous photosynthetic gas exchange, CO2 response curve parameters, or stem water potential in any of the years for any of the species. Effects of the heavy snow year (2017) on needle growth differed between elevations. Pinus contorta showed a 38.9% increase in average needle length at the lower two elevations but a 31.6% decrease at the highest site compared to the height of the drought. Despite these differences, biological variation was dampened compared to the physical variation between years, suggesting these trees can effectively withstand substantial meteorological variability. Our results show that these species demonstrated considerable ability to tolerate and recover from an extreme drought event. [ABSTRACT FROM AUTHOR]

The effects of changing climate and disturbance on mountain forest carbon (C) stocks vary with tree species distributions and over elevational gradients. Warming can not only increase C uptake by stimulating productivity at high elevations but also enhance C release by increasing respiration and the frequency, intensity and size of wildfires.To understand the consequences of climate change for temperate mountain forests, we simulated interactions among climate, wildfire, tree species and their combined effects on regional C stocks in forests of the Greater Yellowstone Ecosystem, USA (GYE) with the LANDIS‐II landscape change model. Simulations used historical climate and future potential climate represented by downscaled projections from five general circulation models (GCMs) that bracket the range of variability under the representative concentration pathway (RCP) 8.5 emissions scenario.Total ecosystem C increased by 67% through 2100 in simulations with historical climate, and by 38%–69% with GCM climate. Differences in C uptake among GCMs resulted primarily from variation in area burned, not productivity. Warming increased productivity by extending the growing season, especially near upper tree line, but did not offset biomass losses to fire. By 2100, simulated area burned increased by 27%–215% under GCM climate, with the largest increases after 2050. With warming >3°C in mean annual temperature, the increased frequency of large fires reduced live C stocks by 4%–36% relative to the control, historical climate scenario. However, relative losses in total C were delayed under GCMs with large increases in summer precipitation and buffered by C retained in soils and the wood of fire‐killed trees. Increasing fire size limited seed dispersal, and reductions in soil moisture limited seedling establishment; both effects will likely constrain long‐term forest regeneration and C uptake.Synthesis. Forests in the GYE can maintain a C sink through the mid‐century in a warming climate but continued warming may cause the loss of forest area, live above‐ground biomass and, ultimately, ecosystem C. Future changes in C stocks in similar forests throughout western North America will depend on regional thresholds for extensive wildfire and forest regeneration and therefore, changes may occur earlier in drier regions. [ABSTRACT FROM AUTHOR]

Climate change has a large influence on plant functional and phenotypic traits including plant primary and secondary metabolites. One well-established approach to investigating the variation in plant metabolites involves studying plant populations along elevation and latitude gradients. We considered how two space-for-time climate change gradients (elevation and latitude) influence carbohydrate reserves (soluble sugars, starches) and secondary metabolites (monoterpenes, diterpene resin acids) of lodgepole pine trees in western Canada. We were particularly interested in the relationship of terpenes and carbohydrates with a wide range of tree, site, and climatic factors. We found that only elevation had a strong influence on the expression of both terpenes and carbohydrates of trees. Specifically, as elevation increased, concentrations of monoterpenes and diterpenes generally increased and soluble sugars (glucose, sucrose, total sugars) decreased. In contrast, latitude had no impact on either of terpenes or carbohydrates. Furthermore, we found a positive relationship between concentrations of starch and total terpenes and diterpenes in the elevation study; whereas neither starches nor sugars were correlated to terpenes in the latitude study. Similarly, both terpenes and carbohydrates had a much greater number of significant correlations to site characteristics such as slope, basal area index, and sand basal area, in the elevational than in the latitude study. Overall, these results support the conclusion that both biotic and abiotic factors likely drive the patterns of primary and secondary metabolite profiles of lodgepole pine along geographical gradients. Also, presence of a positive relationship between terpenes and starches suggests an interaction between primary ad secondary metabolites of lodgepole pine trees. [ABSTRACT FROM AUTHOR]

Aim: Climate change poses significant challenges for tree species, which are slow to adapt and migrate. Insight into genetic and phenotypic variation under current landscape conditions can be used to gauge persistence potential to future conditions and determine conservation priorities, but landscape effects have been minimally tested in trees. Here, we use Pinus contorta, one of the most widely distributed conifers in North America, to evaluate the influence of landscape heterogeneity on genetic structure as well as the magnitude of local adaptation versus phenotypic plasticity in a widespread tree species. Location: Western North America. Methods: We paired landscape genetics with fully reciprocal in situ common gardens to evaluate landscape influence on neutral and adaptive variation across all subspecies of P. contorta. Results: Landscape barriers alone play a minor role in limiting gene flow, creating marginal geographically‐based structure. Local climate determines population performance, with survival highest at home but growth greatest in mild climates (e.g., warm, wet). Survival of two of the three populations tested was consistent with patterns of local adaptation documented for P. contorta, while growth was indicative of plasticity for populations grown under novel conditions and suggesting that some populations are not currently occupying their climatic optimum. Main Conclusions: Our findings provide insight into the role of the landscape in shaping population genetic structure in a widespread tree species as well as the potential response of local populations to novel conditions, knowledge critical to understanding how widely distributed species may respond to climate change. Geographically based genetic structure and reduced survival under water‐limited conditions may make some populations of widespread tree species more vulnerable to local maladaptation and extirpation. However, genetically diverse and phenotypically plastic populations of widespread trees, such as many of the P. contorta populations sampled and tested here, likely possess high persistence potential. [ABSTRACT FROM AUTHOR]

Bark beetle outbreaks alter forests in many ways including stand structure, fuels and fire behavior, wildlife habitat, and aesthetic value. Less understood are the effects outbreaks have on understory vegetation, despite the importance for overstory succession, nutrient cycling, water quality, soil erosion, and wildlife. Beetle outbreaks also change forests in ways that could promote invasion by nonnative weeds, but this is rarely studied. We assessed changes in cover of understory vegetation and presence/abundance of weeds in lodgepole pine forests in Colorado, Idaho, Montana, Utah and Wyoming, USA following recent mountain pine beetle outbreaks. Incipient beetle populations began in 2004, peaked around 2008, and returned to endemic levels by 2012. Understory vegetation was sampled in 2010, 2012, 2014 and 2018. Total understory cover and cover of shrubs and graminoids remained unchanged, while cover of forbs generally increased. Forb cover was negatively correlated with shrub and canopy cover. Approximately 20% of plots contained weeds and weed abundance increased over time. Presence of weeds was negatively correlated with graminoid cover and positively correlated with tree mortality and snag fall. By 2018, weed abundance was only positively correlated with snag fall. Localized soil disturbance created when snags uproot and fall appears to act as sites of weed invasion and proliferation. Lastly, we provide a global review of the effects of bark beetle outbreaks on understory vegetation which reveals a prevailing short-term increase following outbreaks, but the magnitude of response and cover type affected varies with beetle species/forest type and time since outbreak.

Summary: Disturbances have altered community dynamics in boreal forests with unknown consequences for belowground ecological processes. Soil fungi are particularly sensitive to such disturbances; however, the individual response of fungal guilds to different disturbance types is poorly understood.Here, we profiled soil fungal communities in lodgepole pine forests following a bark beetle outbreak, wildfire, clear‐cut logging, and salvage‐logging. Using Illumina MiSeq to sequence ITS1 and SSU rDNA, we characterized communities of ectomycorrhizal, arbuscular mycorrhizal, saprotrophic, and pathogenic fungi in sites representing each disturbance type paired with intact forests. We also quantified soil fungal biomass by measuring ergosterol.Abiotic disturbances changed the community composition of ectomycorrhizal fungi and shifted the dominance from ectomycorrhizal to saprotrophic fungi compared to intact forests. The disruption of the soil organic layer with disturbances correlated with the decline of ectomycorrhizal and the increase of arbuscular mycorrhizal fungi. Wildfire changed the community composition of pathogenic fungi but did not affect their proportion and diversity. Fungal biomass declined with disturbances that disrupted the forest floor.Our results suggest that the disruption of the forest floor with disturbances, and the changes in C and nutrient dynamics it may promote, structure the fungal community with implications for fungal biomass–C. See also the Commentary on this article by Marín & Kohout, 229: 656–658. [ABSTRACT FROM AUTHOR]

The Craig–Gordon type (C–G) leaf water isotope enrichment models assume a homogeneous distribution of enriched water across the leaf surface, despite observations that Δ18O can become increasingly enriched from leaf base to tip. Datasets of this 'progressive isotope enrichment' are limited, precluding a comprehensive understanding of (a) the magnitude and variability of progressive isotope enrichment, and (b) how progressive enrichment impacts the accuracy of C–G leaf water model predictions. Here, we present observations of progressive enrichment in two conifer species that capture seasonal and diurnal variability in environmental conditions. We further examine which leaf water isotope models best capture the influence of progressive enrichment on bulk needle water Δ18O. Observed progressive enrichment was large and equal in magnitude across both species. The magnitude of this effect fluctuated seasonally in concert with vapour pressure deficit, but was static in the face of diurnal cycles in meteorological conditions. Despite large progressive enrichment, three variants of the C–G model reasonably successfully predicted bulk needle Δ18O. Our results thus suggest that the presence of progressive enrichment does not impact the predictive success of C–G models, and instead yields new insight regarding the physiological and anatomical mechanisms that cause progressive isotope enrichment. Δ18O of needle water in two conifer species was drastically enriched down the length of the needle, the magnitude of which was driven by seasonal increases in VPD. Despite this progressive enrichment, traditional Craig‐Gordon model variants successfully predicted bulk needle water Δ18O. [ABSTRACT FROM AUTHOR]

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Understory plants are an important element of forests, having a considerable influence on ecosystem functioning and canopy-tree development following disturbance. Recent bark beetle outbreaks across western North American forests have caused extensive canopy mortality, creating new growing conditions that provide the opportunity for changes within the intact understory. Over a five-year period following peak mountain pine beetle (MPB) activity across lodgepole pine-dominated forests in Rocky Mountain National Park, Colorado, we measured the changes in plant diversity, cover, and dominance by lifeform and quantified tree regeneration rates. Average species richness and diversity increased, but overall plant cover did not change. Graminoids appeared to benefit the most, increasing in average cover, richness, and relative dominance. The rise in graminoid dominance was largely at the expense of shrubs, which showed little ability to benefit from overstory mortality within the first years following attack. Most plant responses were positively related to the total tree basal area lost since the peak of the outbreak, suggesting that increased resource availability following tree death may benefit understory plants. However, a negative relationship between several understory variables and tree sapling density provides evidence that understory plants compete with saplings for the newly available resources. Tree seedling density nearly doubled over the duration of the study, indicating a strong regeneration pulse. Among species, lodgepole pine displayed the greatest tree seedling establishment. This is one of the first studies to use repeated measurements to describe this often-overlooked component of forest change associated with MPB disturbance. [ABSTRACT FROM AUTHOR]

Significant research has been devoted to the development of semiochemical repellents for mountain pine beetle Dendroctonus ponderosae Hopkins. Several formulations of the beetle's antiaggregation pheromone verbenone (4,6,6‐trimethylbicyclo[3.1.1]hept‐3‐en‐2‐one) are registered to protect pine trees and stands.We compared the efficacy of verbenone and Verbenone Plus for protecting individual lodgepole pine Pinus contorta Dougl. ex Loud. and small stands of P. contorta from mortality attributed to D. ponderosae. Verbenone Plus is a blend of semiochemicals [(−)‐verbenone, acetophenone, and (E)‐2‐hexen‐1‐ol + (Z)‐2‐hexen‐1‐ol] originally developed to inhibit aggregation of western pine beetle D. brevicomis LeConte. It is the only effective semiochemical repellent that protects ponderosa pine P. ponderosa Dougl. ex Laws. from mortality attributed to D. brevicomis, but has yet to be registered or commercialized.Both verbenone and Verbenone Plus were effective for protecting individual P. contorta and small stands of P. contorta from mortality attributed to D. ponderosae. Verbenone alone was as effective as Verbenone Plus.An obstacle to the broader use of semiochemical repellents for the management of bark beetles is the relatively high cost and specificity of the semiochemicals involved. A semiochemical repellent with efficacy across multiple bark beetle species and hosts, such as Verbenone Plus, would be desirable, assuming the additional components are not cost‐prohibitive or limit registration. [ABSTRACT FROM AUTHOR]

The large-scale introduction of lodgepole pine (Pinus contorta ssp. latifolia, LP) into Swedish forests was initiated around 1970, and currently 520,000 hectares of the forest land are dominated by the species. Even though the tree has mostly serotine cones, adapted to open after forest fires, it has proved able to self-regenerate in Sweden. This study is the first to present, scientifically, the extent of self-regeneration covering the whole current range of LP-forests in Sweden. LP-saplings were recorded for three years, 2015–2017, in 8194 subplots distributed over 214 randomly selected LP-stands from latitude 59.6–66.9°N and altitude 88–710 m asl. Of all subplots, 3% contained LP-saplings, and regeneration was found in 53% of all stands. The probability of finding LP-saplings was significantly dependent on the plots' distance from the edge of the LP-stand, and 78% of all saplings were found within and up to 15 m beyond the stand edge. Most, 63%, of the plots with LP were found on disturbed ground such as wheel tracks, roadsides and where there had been site preparation. The results show that LP can naturally spread under a range of conditions in the studied region. The regeneration is, however, concentrated in particular stands and should be possible to control with monitoring programmes and measures to eradicate self-dispersed trees in unwanted areas. [ABSTRACT FROM AUTHOR]

Disturbances are frequent events across the Canadian boreal forest and can affect both below‐ and above‐ground ecosystem processes. How disturbances change below‐ground soil fungal communities and in‐turn affect pine establishment and performance is poorly understood. Such understanding has become increasingly important in light of observed changes in disturbance regimes in recent years due to climate change.We used a greenhouse experiment to determine how soil inoculum collected from lodgepole pine stands undisturbed (control) or disturbed by fire, mountain pine beetle outbreak, logging and salvage logging affect pine seedling performance in western Canada. We first characterized whether fungal communities of seedling roots change as a function of inoculum source, and then determined whether changes in fungal community composition impact pine seedling performance (biomass and height).Root fungal communities of pine seedlings from logged and salvage logged disturbances differed from their respective paired controls, while soils from natural disturbances (fire and beetle outbreak) did not. Among disturbances, the pine root fungal communities of fire and salvage logged disturbances differed.In parallel to the root fungal communities, seedling performance also decreased when comparing logging and salvage logging disturbances to paired controls. Among disturbance treatments, seedlings from the salvage logged disturbance did not grow as big as seedlings inoculated with soils from burned forests.Synthesis and application. Our findings indicate that anthropogenic disturbances (logging and salvage logging) can have cross‐generational impacts on pine seedling performance, through functional shifts in seedling root fungal community structure. Furthermore, the impacts of soil fungi on pine seedlings appear to be pronounced following salvage logging, stressing the importance of compound disturbance events. These findings may be important to land managers considering clear‐cut logging or salvage logging in pine forests, particularly where soil biotic communities are likely to be one of the predominate factors in pine establishment. [ABSTRACT FROM AUTHOR]

Silicification is the most important process of fossilization resulting in the preservation of internal tissues in plants, thereby providing essential information on the anatomy, life history, and evolution of land plants. However, fundamental knowledge of silica uptake, precipitation, and contribution to in situ plant fossilization is limited. To identify the cellular pathway of aqueous silica and subsequent fluid–wood interaction processes, we investigated upright standing young trees of lodgepole pine (Pinus contorta) in the hot‐spring environment of Cistern Spring, Yellowstone National Park, USA. Our multi‐method analytical approach using x‐ray diffraction, Raman spectroscopy, scanning electron microscopy, and electron probe microanalyses shows that the surficial and internally co‐precipitated silica–halite–gypsum assemblage traces the flow of silica‐rich fluid and documents fluid retention after short‐term hot‐spring water immersion and evaporation. Element distribution maps reveal systematic 2D and 3D differences between silica quantities deposited in earlywood and latewood. The distribution of inorganic impurities in cell walls traces the anatomical structure of the wood and indicates rapid migration of homogeneous fluid into the waterlogged organic substrate. Our results show that the preferential pathway of silica‐rich fluid into the above‐ground wood was through the decorticated periphery which took place during a short‐term flooding, ranging from days to weeks, of hot‐spring fluid. In conclusion, the fluid retention capability in cellular pore space controls the in situ silicification process. The silicification of trees in growth position is a rapid process in which the in vivo transport of silica‐rich fluid upward through the secondary xylem plays an insignificant role. [ABSTRACT FROM AUTHOR]

Background: Biomass maps are valuable tools for estimating forest carbon and forest planning. Individual-tree biomass estimates made using allometric equations are the foundation for these maps, yet the potentially-high uncertainty and bias associated with individual-tree estimates is commonly ignored in biomass map error. We developed allometric equations for lodgepole pine (Pinus contorta), ponderosa pine (P. ponderosa), and Douglas-fir (Pseudotsuga menziesii) in northern Colorado. Plot-level biomass estimates were combined with Landsat imagery and geomorphometric and climate layers to map aboveground tree biomass. We compared biomass estimates for individual trees, plots, and at the landscape-scale using our locally-developed allometric equations, nationwide equations applied across the U.S., and the Forest Inventory and Analysis Component Ratio Method (FIA-CRM). Total biomass map uncertainty was calculated by propagating errors from allometric equations and remote sensing model predictions. Two evaluation methods for the allometric equations were compared in the error propagation—errors calculated from the equation fit (equation-derived) and errors from an independent dataset of destructively-sampled trees (n = 285). Results: Tree-scale error and bias of allometric equations varied dramatically between species, but local equations were generally most accurate. Depending on allometric equation and evaluation method, allometric uncertainty contributed 30–75% of total uncertainty, while remote sensing model prediction uncertainty contributed 25–70%. When using equation-derived allometric error, local equations had the lowest total uncertainty (root mean square error percent of the mean [% RMSE] = 50%). This is likely due to low-sample size (10–20 trees sampled per species) allometric equations and evaluation not representing true variability in tree growth forms. When independently evaluated, allometric uncertainty outsized remote sensing model prediction uncertainty. Biomass across the 1.56 million ha study area and uncertainties were similar for local (2.1 billion Mg; % RMSE = 97%) and nationwide (2.2 billion Mg; % RMSE = 94%) equations, while FIA-CRM estimates were lower and more uncertain (1.5 billion Mg; % RMSE = 165%). Conclusions: Allometric equations should be selected carefully since they drive substantial differences in bias and uncertainty. Biomass quantification efforts should consider contributions of allometric uncertainty to total uncertainty, at a minimum, and independently evaluate allometric equations when suitable data are available. [ABSTRACT FROM AUTHOR]

For tree seedlings in boreal forests, ectomycorrhizal (EM) fungal networks may promote, while root competition may impede establishment. Thus, disruption to EM fungal networks may decrease seedling establishment owing to the loss of positive interactions among neighbors. Widespread tree mortality can disrupt EM networks, but it is not clear whether seedling establishment will be limited by the loss of positive interactions or increased by the loss of negative interactions with surrounding roots. Depending upon the relative influence of these mechanisms, widespread tree mortality may have complicated consequences on seedling establishment, and in turn, the composition of future forests. To discern between these possible outcomes and the drivers of seedling establishment, we determined the relative importance of EM fungal networks, root presence, and the bulk soil on the establishment of lodgepole pine and white spruce seedlings along a gradient of beetle-induced tree mortality. We manipulated seedling contact with EM fungal networks and roots through the use of mesh-fabric cylinders installed in soils of lodgepole pine forests experiencing a range of overstorey tree mortality caused by mountain pine beetle. Lodgepole pine seedling survival was higher with access to EM fungal networks in undisturbed pine forests in comparison with that in beetle-killed stands. That is, overstorey tree mortality shifted fungal networks from being a benefit to a cost on seedling survival. In contrast, overstorey tree mortality did not change the relative strength of EM fungal networks, root presence and the bulk soil on survival and biomass of white spruce seedlings. Furthermore, the relative influence of EM fungal networks, root presence, and bulk soils on foliar N and P concentrations was highly contingent on seedling species and overstorey tree mortality. Our results highlight that following large-scale insect outbreak, soil-mediated processes can enable differential population growth of two common conifer species, which may result in species replacement in the future. [ABSTRACT FROM AUTHOR]