Drought-Mediated Changes in Tree Physiological Processes Weaken Tree Defenses to Bark Beetle Attack

Drought increases Norway spruce susceptibility to the Eurasian spruce bark beetle and its associated fungi

Drought affects the complex interactions between Norway spruce, the bark beetle Ips typographus and associated microorganisms. We investigated the interplay of tree water status, defense and carbohydrate reserves with the incidence of bark beetle attack and infection of associated fungi in mature spruce trees. We installed roofs to induce a 2-yr moderate drought in a managed spruce stand to examine a maximum of 10 roof and 10 control trees for resin flow (RF), predawn twig water potentials, terpene, phenolic and carbohydrate bark concentrations, and bark beetle borings in field bioassays before and after inoculation with Endoconidiophora polonica and Grosmannia penicillata. Drought-stressed trees showed more attacks and significantly longer fungal lesions than controls, but maintained terpene resin defenses at predrought levels. Reduced RF and lower mono- and diterpene, but not phenolic concentrations were linked with increased host selection. Bark beetle attack and fungi stimulated chemical defenses, yet G. penicillata reduced phenolic and carbohydrate contents. Chemical defenses did not decrease under mild, prolonged drought in our simulated small-scale biotic infestations. However, during natural mass attacks, reductions in carbon fixation under drought, in combination with fungal consumption of carbohydrates, may deplete tree defenses and facilitate colonization by I. typographus.

Early warning signals indicate climate change-induced stress in Norway spruce in the Eastern Carpathians

Climate change is affecting forest ecosystems globally, in particular through warming as well as increases in the frequency and intensity of extreme events. Norway spruce (Picea abies (L.) Karst.) is one of the most important coniferous tree species in Europe. In recent extremely dry years in Central Europe, spruce suffered and large dieback has been observed. In parts of Eastern Europe, however, no large-scale decline in spruce has been reported so far, though anticipated changes in climate pose the question how the future of these forests may look like. To assess the current state of spruce forests in Eastern Europe, we established a tree-ring network consisting of 157 Norway spruce chronologies (from >3000 trees) of different ages distributed along elevational transects in the Eastern Carpathians, Romania. We evaluated early warning signals of climate change-induced stress, i.e. (1) growth decline, (2) increased sensitivity of tree growth (assessed over the statistics first-order autocorrelation and standard deviation), and (3) increased growth synchrony. A pronounced growth decline was observed over the last two decades, which was strongest in younger stands and at lower elevations. However, growth sensitivity and synchrony did not show consistent patterns, suggesting that forest decline may not be immediately imminent. Overall, our findings highlight an increased vulnerability of spruce in the Eastern Carpathians. With ongoing climate change, spruce dieback may be expected in this part of Europe as well.

Declining tree growth resilience mediates subsequent forest mortality in the US Mountain West

Climate change-triggered forest die-off is an increasing threat to global forests and carbon sequestration but remains extremely challenging to predict. Tree growth resilience metrics have been proposed as measurable proxies of tree susceptibility to mortality. However, it remains unclear whether tree growth resilience can improve predictions of stand-level mortality. Here, we use an extensive tree-ring dataset collected at ~3000 permanent forest inventory plots, spanning 13 dominant species across the US Mountain West, where forests have experienced strong drought and extensive die-off has been observed in the past two decades, to test the hypothesis that tree growth resilience to drought can explain and improve predictions of observed stand-level mortality. We found substantial increases in growth variability and temporal autocorrelation as well declining drought resistance and resilience for a number of species over the second half of the 20th century. Declining resilience and low tree growth were strongly associated with cross- and within-species patterns of mortality. Resilience metrics had similar explicative power compared to climate and stand structure, but the covariance structure among predictors implied that the effect of tree resilience on mortality could partially be explained by stand and climate variables. We conclude that tree growth resilience offers highly valuable insights on tree physiology by integrating the effect of stressors on forest mortality but may have only moderate potential to improve large-scale projections of forest die-off under climate change.

The role of height-driven constraints and compensations on tree vulnerability to drought

Frequent observations of higher mortality in larger trees than in smaller ones during droughts have sparked an increasing interest in size-dependent drought-induced mortality. However, the underlying physiological mechanisms are not well understood, with height-associated hydraulic constraints often being implied as the potential mechanism driving increased drought vulnerability. We performed a quantitative synthesis on how key traits that drive plant water and carbon economy change with tree height within species and assessed the implications that the different constraints and compensations may have on the interacting mechanisms (hydraulic failure, carbon starvation and/or biotic-agent attacks) affecting tree vulnerability to drought. While xylem tension increases with tree height, taller trees present a range of structural and functional adjustments, including more efficient water use and transport and greater water uptake and storage capacity, that mitigate the path-length-associated drop in water potential. These adaptations allow taller trees to withstand episodic water stress. Conclusive evidence for height-dependent increased vulnerability to hydraulic failure and carbon starvation, and their coupling to defence mechanisms and pest and pathogen dynamics, is still lacking. Further research is needed, particularly at the intraspecific level, to ascertain the specific conditions and thresholds above which height hinders tree survival under drought.

Abundance of Dendroctonus frontalis and D. mexicanus (Coleoptera: Scolytinae) along altitudinal transects in Mexico: Implications of climatic change for forest conservation

Bark beetle infestations have historically been primary drivers of stand thinning in Mexican pine forests. However, bark beetle impacts have become increasingly extensive and intense, apparently associated with climate change. Our objective was to describe the possible association between abundance of bark beetle flying populations and the occurrence of given value intervals of temperature, precipitation and their balance, in order to have a better comprehension of the climatic space that might trigger larger insect abundances, an issue relevant in the context of the ongoing climatic change. Here, we monitored the abundance of two of the most important bark beetle species in Mexico, Dendroctonus frontalis and D. mexicanus. We sampled 147 sites using pheromone-baited funnel traps along 24 altitudinal transects in 11 Mexican states, from northwestern Chihuahua to southeastern Chiapas, from 2015 to 2017. Through mixed model analysis, we found that the optimum Mean Annual Temperatures were 17°C-20°C for D. frontalis in low-elevation pine-oak forest, while D. mexicanus had two optimal intervals: 11-13°C and 15-18°C. Higher atmospheric Vapor Pressure Deficit (≥ 1.0) was correlated with higher D. frontalis abundances, indicating that warming-amplified drought stress intensifies trees' vulnerability to beetle attack. As temperatures and drought stress increase further with projected future climatic changes, it is likely that these Dendroctonus species will increase tree damage at higher elevations. Pine forests in Mexico are an important source of livelihood for communities inhabiting those areas, so providing tools to tackle obstacles to forest growth and health posed by changing climate is imperative.

Drought impairs herbivore-induced volatile terpene emissions by ponderosa pine but not through constraints on newly assimilated carbon

Volatile terpenes serve multiple biological roles including tree resistance against herbivores. The increased frequency and severity of drought stress observed in forests across the globe may hinder trees from producing defense-related volatiles in response to biotic stress. To assess how drought-induced physiological stress alters volatile emissions alone and in combination with a biotic challenge, we monitored pre-dawn water potential, gas-exchange, needle terpene concentrations and terpene volatile emissions of ponderosa pine (Pinus ponderosa) saplings during three periods of drought and in response to simulated herbivory via methyl jasmonate application. Although 3-, 6- and 7-week drought treatments reduced net photosynthetic rates by 20, 89 and 105%, respectively, the magnitude of volatile fluxes remained generally resistant to drought. Herbivore-induced emissions, however, exhibited threshold-like behavior; saplings were unable to induce emissions above constitutive levels when pre-dawn water potentials were below the approximate zero-assimilation point. By comparing compositional shifts in emissions to needle terpene concentrations, we found evidence that drought effects on constitutive and herbivore-induced volatile flux and composition are primarily via constraints on the de novo fraction, suggesting that reduced photosynthesis during drought limits the carbon substrate available for de novo volatile synthesis. However, results from a subsequent 13CO2 pulse-chase labeling experiment then confirmed that both constitutive (<3% labeled) and herbivore-induced (<8% labeled) de novo emissions from ponderosa pine are synthesized predominantly from older carbon sources with little contribution from new photosynthates. Taken together, we provide evidence that in ponderosa pine, drought does not constrain herbivore-induced de novo emissions through substrate limitation via reduced photosynthesis, but rather through more sophisticated molecular and/or biophysical mechanisms that manifest as saplings reach the zero-assimilation point. These results highlight the importance of considering drought severity when assessing impacts on the herbivore-induced response and suggest that drought-altered volatile metabolism constrains induced emissions once a physiological threshold is surpassed.

Acetophenone and Green Leaf Volatiles Do Not Enhance the Efficacy of Verbenone for Inhibiting Attraction of Ips pini (Coleoptera: Curculionidae) to Pheromone-baited Traps in Northern Arizona

We assessed attraction of pine engraver, Ips pini (Say) (Coleoptera: Curculionidae; Scolytinae), to pheromone-baited funnel traps treated with repellent semiochemicals in ponderosa pine, Pinus ponderosa var. scopulorum Engelm., forests in northern Arizona. Treatments included: 1) baited control (B, ipsdienol + lanierone), 2) 70 g of SPLAT Verb (a flowable, biodegradable formulation containing 10% verbenone, ISCA Technologies Inc., Riverside, CA, USA) + B, 3) 70 g of SPLAT Verb + (E)-2-hexen-1-ol+(Z)-2-hexen-1-ol + acetophenone + B, 4) 7.84-g verbenone pouch (Product #3413, Synergy Semiochemicals Corp., Delta, British Columbia, Canada) + B, and 5) 7.84-g verbenone pouch + (E)-2-hexen-1-ol+(Z)-2-hexen-1-ol + acetophenone + B. In total, 472 I. pini were collected. Trap catches were highest in baited traps and declined significantly with the addition of both formulations of verbenone. Traps treated with SPLAT Verb caught significantly fewer I. pini and male I. pini than those treated with verbenone pouches. The addition of (E)-2-hexen-1-ol+(Z)-2-hexen-1-ol + acetophenone to SPLAT Verb and the verbenone pouch had no effect on trap catch. Verbenone has potential as an effective tool for protecting P. ponderosa trees and slash from I. pini in northern Arizona, but the addition of (E)-2-hexen-1-ol+(Z)-2-hexen-1-ol + acetophenone to verbenone is unwarranted.

Spatiotemporal outbreak dynamics of bark and wood-boring insects

Bark and wood-boring insects (BWI) can cause important disturbances in forest ecosystems, and their impact depends on their spatiotemporal dynamics. Populations are usually at stable, low densities but can be disrupted by stochastic perturbations that trigger a transition to an epidemic phase. For less aggressive species, outbreaks die out quickly once the perturbation is removed, while aggressive species exhibit density-dependent feedback mechanisms facilitating persistent landscape-scale outbreaks. The interactions of attributes of tree, stand, landscape- and regional-scale stressors, and insect life history and behavior determine system-specific dynamics. However, most of our knowledge is based on a few species of mainly Holarctic bark beetles. With global change, it is becoming increasingly important to improve our understanding of the frequency and severity of BWI outbreaks.

The Life Cycle of the Xylophagous Beetle, Steraspis speciosa (Coleoptera, Buprestidae), Feeding on Acacia Trees in Saudi Arabia

The xylophagous beetle, Steraspis speciosa, has infected Acacia forests in Saudi Arabia, causing significant damage and even leading to the death of several trees. In the Ha'il region, in the north of Saudi Arabia, an investigation of 13 study sites shows that the Acacia population is mainly composed of three species: A. gerrardii (90.3%), A. ehrenbergiana (7.5%), and A. raddiana (2.2%) and that 21.7% of this population was infected by S. speciosa. The age of the tree (young, adult, old) and environment habitat (Dam, Wadi, Plateau) effects, and insect life-cycle were studied in the protected Machar National Park. Infection in the park, estimated at 25.4%, mainly affects the oldest trees (20.1%) more than the youngest ones (2.3%), while the driest environments (Plateau, 38.8%) are more vulnerable to infection than humid habitats (Dam, 9.4%). The life cycle of S. speciosa lasts about two years, with four stages to complete metamorphosis: mating and eggs (≈3 months), larvae (≈16 months), pupae (≈3 months), and emergency and adults (≈3 months). The larvae stage with many metamorphic instars was the most harmful for tree and takes the longest. The female beetle lays its eggs on weak stem parts. Steraspis speciosa larvae feed on the stems of Acacia trees, and the instar larvae were gathered under the bark of infected stems, harming most of the phloem and a large portion of the xylem. When combined with a prolonged period of drought, S. speciosa causes the withering of numerous branches and, in extreme cases, kills the entire tree.

Pathophysiology and transcriptomic analysis of Picea koraiensis inoculated by bark beetle-vectored fungus Ophiostoma bicolor

Ophiostomatoid fungi exhibit a complex relationship with bark beetles; exhausting of host tree defenses is traditionally regarded as one of the key benefits provided to beetle vectors. Ophiostoma bicolor is one of the dominant species of the mycobiota associated with Ips genus bark beetles which infect the spruce trees across the Eurasian continent. Host spruce trees resist fungal invasion through structural and inducible defenses, but the underlying mechanisms at the molecular level, particularly with respect to the interaction between bark beetle-associated fungi and host trees, remain unclear. The aim of this study was to observe the pathological physiology and molecular changes in Picea koraiensis seedlings after artificial inoculation with O. bicolor strains (TS, BH, QH, MX, and LWQ). This study showed that O. bicolor was a weakly virulent pathogen of spruce, and that the virulent of the five O. bicolor strains showed differentiation. All O. bicolor strains could induce monoterpenoid release. A positive correlation between fungal virulence and release of monoterpenoids was observed. Furthermore, the release rate of monoterpenoids peaked at 4 days post-inoculation (dpi) and then decreased from 4 to 90 dpi. Transcriptomic analysis at 4 dpi showed that many plant-pathogen interaction processes and mitogen-activated protein kinase (MAPK) metabolic processes were activated. The expression of monoterpenoid precursor synthesis genes and diterpenoid synthesis genes was upregulated, indicating that gene expression regulated the release rate of monoterpenoids at 4 dpi. The enriched pathways may reveal the immune response mechanism of spruce to ophiostomatoid fungi. The dominant O. bicolor possibly induces the host defense rather than defense depletion, which is likely the pattern conducted by the pioneers of beetle-associated mycobiota, such as Endoconidiophora spp.. Overall, these results facilitate a better understanding of the interaction mechanism between the dominant association of beetles and the host at the molecular level.

Climatic and stand drivers of forest resistance to recent bark beetle disturbance in European coniferous forests

Bark beetle infestation is a major driver of tree mortality that may be critical for forest persistence under climate change and the forecasted increase of extreme heat and drought episodes. Under this context, the environmental position of host tree populations within the species' climatic niche (central vs. marginal populations) is expected to be a determinant in the dynamics of insect-host systems. Here, we analyzed the recent patterns of bark beetle disturbance and forest resistance across European coniferous forests during the 2010-2018 period. We obtained bark beetle attack and tree mortality data from successive continental-scale forest condition surveys on 130 plots including five host trees and five bark beetle species, and characterized the climatic niche of each species. Then, we analyzed the overall forest resistance and species-specific responses, in terms of bark beetle attack and induced tree mortality, in relation to the distance to the niche optimum of both host tree and beetle species, previous drought events, and plot characteristics. Regional patterns of recent disturbance revealed that forests in central, north, and east of Europe could be at risk under the attack of multivoltine bark beetle species. We found that overall forest resistance to beetle attack was determined by several driving factors, which varied among species responses. Particularly, the environmental position of the affected forest within the host and beetle species' climatic niche and plot characteristics mediated the influence of drought on the resistance to beetle attack. In turn, forest resistance to induced tree mortality was determined exclusively by the maximum intensity and duration of drought events. Our findings highlight the importance of disturbance interactions and suggest that the joint influence of drought events and bark beetle disturbance will threaten the persistence of European coniferous forests, even in those tree populations close to their species' climatic optimum.

Changes in tree drought sensitivity provided early warning signals to the California drought and forest mortality event

Climate warming in recent decades has negatively impacted forest health in the western United States. Here, we report on potential early warning signals (EWS) for drought-related mortality derived from measurements of tree-ring growth (ring width index; RWI) and carbon isotope discrimination (∆¹³ C), primarily focused on ponderosa pine (Pinus ponderosa). Sampling was conducted in the southern Sierra Nevada Mountains, near the epicenter of drought severity and mortality associated with the 2012-2015 California drought and concurrent outbreak of western pine beetle (Dendroctonus brevicomis). At this site, we found that widespread mortality was presaged by five decades of increasing sensitivity (i.e., increased explained variation) of both tree growth and ∆¹³ C to Palmer Drought Severity Index (PDSI). We hypothesized that increasing sensitivity of tree growth and ∆¹³ C to hydroclimate constitute EWS that indicate an increased likelihood of widespread forest mortality caused by direct and indirect effects of drought. We then tested these EWS in additional ponderosa pine-dominated forests that experienced varying mortality rates associated with the same California drought event. In general, drier sites showed increasing sensitivity of RWI to PDSI over the last century, as well as higher mortality following the California drought event compared to wetter sites. Two sites displayed evidence that thinning or fire events that reduced stand basal area effectively reversed the trend of increasing hydroclimate sensitivity. These comparisons indicate that reducing competition for soil water and/or decreasing bark beetle host tree density via forest management-particularly in drier regions-may buffer these forests against drought stress and associated mortality risk. EWS such as these could provide land managers more time to mitigate the extent or severity of forest mortality in advance of droughts. Substantial efforts at deploying additional dendrochronological research in concert with remote sensing and forest modeling will aid in forecasting of forest responses to continued climate warming.

Combined drought and bark beetle attacks deplete non-structural carbohydrates and promote death of mature pine trees

How carbohydrate reserves in conifers respond to drought and bark beetle attacks are poorly understood. We investigated changes in carbohydrate reserves and carbon-dependent diterpene defences in ponderosa pine trees that were experimentally subjected to two levels of drought stress (via root trenching) and two types of biotic challenge treatments (pheromone-induced bark beetle attacks or inoculations with crushed beetles that include beetle-associated fungi) for two consecutive years. Our results showed that trenching did not influence carbohydrates, whereas both biotic challenges reduced amounts of starch and sugars of trees. However, only the combined trenched-bark beetle attacked trees depleted carbohydrates and died during the first year of attacks. While live trees contained higher carbohydrates than dying trees, amounts of constitutive and induced diterpenes produced did not vary between live and beetle-attacked dying trees, respectively. Based on these results we propose that reallocation of carbohydrates to diterpenes during the early stages of beetle attacks is limited in drought-stricken trees, and that the combination of biotic and abiotic stress leads to tree death. The process of tree death is subsequently aggravated by beetle girdling of phloem, occlusion of vascular tissue by bark beetle-vectored fungi, and potential exploitation of host carbohydrates by bark beetle symbionts as nutrients.

Production of complementary defense metabolites reflects a co-evolutionary arms race between a host plant and a mutualistic bark beetle-fungal complex

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.

Ecotrons: Powerful and versatile ecosystem analysers for ecology, agronomy and environmental science

Ecosystems integrity and services are threatened by anthropogenic global changes. Mitigating and adapting to these changes require knowledge of ecosystem functioning in the expected novel environments, informed in large part through experimentation and modelling. This paper describes 13 advanced controlled environment facilities for experimental ecosystem studies, herein termed ecotrons, open to the international community. Ecotrons enable simulation of a wide range of natural environmental conditions in replicated and independent experimental units while measuring various ecosystem processes. This capacity to realistically control ecosystem environments is used to emulate a variety of climatic scenarios and soil conditions, in natural sunlight or through broad-spectrum lighting. The use of large ecosystem samples, intact or reconstructed, minimizes border effects and increases biological and physical complexity. Measurements of concentrations of greenhouse trace gases as well as their net exchange between the ecosystem and the atmosphere are performed in most ecotrons, often quasi continuously. The flow of matter is often tracked with the use of stable isotope tracers of carbon and other elements. Equipment is available for measurements of soil water status as well as root and canopy growth. The experiments ran so far emphasize the diversity of the hosted research. Half of them concern global changes, often with a manipulation of more than one driver. About a quarter deal with the impact of biodiversity loss on ecosystem functioning and one quarter with ecosystem or plant physiology. We discuss how the methodology for environmental simulation and process measurements, especially in soil, can be improved and stress the need to establish stronger links with modelling in future projects. These developments will enable further improvements in mechanistic understanding and predictive capacity of ecotron research which will play, in complementarity with field experimentation and monitoring, a crucial role in exploring the ecosystem consequences of environmental changes.

Interactions among Norway spruce, the bark beetle Ips typographus and its fungal symbionts in times of drought

Resilience and functionality of European Norway spruce forests are increasingly threatened by mass outbreaks of the bark beetle Ips typographus promoted by heat, wind throw and drought. Here, we review current knowledge on Norway spruce and I. typographus interactions from the perspective of drought-stressed trees, host selection, colonisation behaviour of beetles, with multi-level effects of symbiotic ophiostomatoid fungi. By including chemo-ecological, molecular and behavioural perspectives, we provide a comprehensive picture on this complex, multitrophic system in the light of climate change. Trees invest carbon into specialised metabolism to produce defence compounds against biotic invaders; processes that are strongly affected by physiological stress such as drought. Spruce bark contains numerous terpenoid and phenolic substances, which are important for bark beetle aggregation and attack success. Abiotic stressors such as increased temperatures and drought affect composition, amounts and emission rates of volatile compounds. Thus, drought events may influence olfactory responses of I. typographus, and further the pheromone communication enabling mass attack. In addition, I. typographus is associated with numerous ophiostomatoid fungal symbionts with multiple effects on beetle life history. Symbiotic fungi degrade spruce toxins, help to exhaust tree defences, produce beetle semiochemicals, and possibly provide nutrition. As the various fungal associates have different temperature optima, they can influence the performance of I. typographus differently under changing environmental conditions. Finally, we discuss why effects of drought on tree-killing by bark beetles are still poorly understood and provide an outlook on future research on this eruptive species using both, field and laboratory experiments.

Few generalizable patterns of tree-level mortality during extreme drought and concurrent bark beetle outbreaks

Tree mortality associated with drought and concurrent bark beetle outbreaks is expected to increase with further climate change. When these two types of disturbance occur in concert it complicates our ability to accurately predict future forest mortality. The recent extreme California USA drought and bark beetle outbreaks resulted in extensive tree mortality and provides a unique opportunity to examine questions of why some trees die while others survive these co-occurring disturbances. We use plot-level data combined with a three-proxy tree-level approach using radial growth, carbon isotopes, and resin duct metrics to evaluate 1) whether variability in stand structure, tree growth or size, carbon isotope discrimination, or defenses precede mortality, 2) how relationships between these proxies differ for surviving and now-dead trees, and 3) whether generalizable risk factors for tree mortality exist across pinyon pine (Pinus monophylla), ponderosa pine (P. ponderosa), white fir (Abies concolor), and incense cedar (Calocedrus decurrens) affected by the combination of drought and beetle outbreaks. We find that risk factors associated with mortality differ between species, and that few generalizable patterns exist when bark beetle outbreaks occur in concert with a particularly long, hot drought. We see evidence that both long-term differences in physiology and shorter-term beetle-related selection and variability in defenses influence mortality susceptibility for ponderosa pine, whereas beetle dynamics may play a more prominent role in mortality patterns for white fir and pinyon pine. In contrast, incense cedar mortality appears to be attributable to long-term effects of growth suppression. Risk factors that predispose some trees to drought and beetle-related mortality likely reflect species-specific strategies for dealing with these particular disturbance types. The combined influence of beetles and drought necessitates the consideration of multiple, species-specific risk factors to more accurately model forest mortality in the face of similar extreme events more likely under future climates.

Differences in Gut Bacterial Communities of Ips typographus (Coleoptera: Curculionidae) Induced by Enantiomer-Specific α-Pinene

The spruce bark beetle (Ips typographus L.) is a destructive pest of Eurasian spruce forests. Although the gut bacteria of this insect are considered to play important roles in its lifecycle, the relationship between I. typographus and its gut bacterial community is poorly characterized. In this study, 16S rRNA gene sequencing was used to determine gut bacterial community composition across successive I. typographus life stages. Responses of the gut bacteria to α-pinene enantiomers were also explored. Ips typographus gut bacterial populations were dominated by the phyla Proteobacteria, Firmicutes, Bacteroidetes, and Actinobacteria, and the relative abundance of these phyla varied across different developmental stages of the beetle. Bacterial species diversity and richness indices increased with developmental stage progression. Relative abundances of the dominant genera, Erwinia (Enterobacteriales: Enterobacteriaceae), Pseudoxanthomonas (Xanthomonadales: Xanthomonadaceae), Serratia (Enterobacteriales: Enterobacteriaceae), and Romboutsia (Clostridiales: Peptostreptococcaceae), also varied across successive I. typographus life stages. Large disparities in the gut bacterial community of male adults were observed when the beetles were treated with S-(-)-α-pinene and R-(+)-α-pinene. The relative abundances of Lactococcus (Lactobacillales: Streptococcaceae) and Lelliottia (Enterobacteriales: Enterobacteriaceae) increased drastically with R-(+)-α-pinene and S-(-)-α-pinene treatment, respectively. This indicated a distinct enantiomer-specific effect of α-pinene on the I. typographus gut bacteria. This study demonstrated the plasticity of gut bacteria during I. typographus development, when α-pinene host monoterpenes are encountered. This study provides new insights into the relationship between 'I. typographus-gut bacteria' symbionts and host trees.

Spruce beetle outbreak was not driven by drought stress: Evidence from a tree-ring iso-demographic approach indicates temperatures were more important

Climate change has amplified eruptive bark beetle outbreaks over recent decades, including spruce beetle (Dendroctonus rufipennis). However, for projecting future bark beetle dynamics there is a critical lack of evidence to differentiate how outbreaks have been promoted by direct effects of warmer temperatures on beetle life cycles versus indirect effects of drought on host susceptibility. To diagnose whether drought-induced host-weakening was important to beetle attack success we used an iso-demographic approach in Engelmann spruce (Picea engelmannii) forests that experienced widespread mortality caused by spruce beetle outbreaks in the 1990s, during a prolonged drought across the central and southern Rocky Mountain region. We determined tree death date demography during this outbreak to differentiate early- and late-dying trees in stands distributed across a landscape within this larger regional mortality event. To directly test for a role of drought stress during outbreak initiation we determined whether early-dying trees had greater sensitivity of tree-ring carbon isotope discrimination (∆¹³ C) to drought compared to late-dying trees. Rather, evidence indicated the abundance and size of host trees may have modified ∆¹³ C responses to drought. ∆¹³ C sensitivity to drought did not differ among early- versus late-dying trees, which runs contrary to previously proposed links between spruce beetle outbreaks and drought. Overall, our results provide strong support for the view that irruptive spruce beetle outbreaks across North America have primarily been driven by warming-amplified beetle life cycles whereas drought-weakened host defenses appear to have been a distant secondary driver of these major disturbance events.