Native Bark Beetle Research Articles

Seven mitochondrial genomes of tribe Hylurgini (Coleoptera: Curculionidae: Scolytinae) in Eurasia and their phylogenetic analysis.

The Hylurgini tribe (Coleoptera: Curculionidae: Scolytinae) comprises commercially significant bark beetles, including invasive species within the genera Dendroctonus and Hylurgus. These invasive species coexist with native Tomicus species of Hylurgini and cooperatively infest host trees in China. However, we lack sufficient mitochondrial genome data of Hylurgini to conduct phylogenetic studies, clarify the phylogenetic relationships of the above species, and improve the understanding of niche divergence and common hazards. Here, we sequenced and analyzed the mitochondrial genomes of seven Hylurgini species, including Dendroctonus valens, Hylurgus ligniperda, Hylurgus micklitzi, Tomicus piniperda, Tomicus brevipilosus, Tomicus minor and Tomicus yunnanensis. All sequenced mitochondrial genomes ranged from 15,339 bp to 17,545 bp in length, and their AT contents ranged from 73.24% to 78.81%. The structure of the seven mitochondrial genomes was consistent with that of ancestral insects. Based on 13 protein-coding genes from the reported mitochondrial genomes of 29 species of bark beetles, we constructed phylogenetic trees using maximum likelihood and Bayesian inference methods. The topology of the two phylogenetic trees was almost consistent. The findings elucidated the taxonomy classification of Hylurgini and the evolutionary connections of its sister taxa within the Scolytinae. This study offers insights for examining the evolutionary connections between invasive and native bark beetles, as well as the molecular identification and detection of newly invading species.

Quantification of heritable variation in multiple lodgepole pine chemical and physical traits that contribute to defense against mountain pine beetle (Dendroctonus ponderosae)

Bark beetles are major causes of conifer mortality worldwide. We quantified genetic contributions to key chemical and physical traits in lodgepole pine that contribute to defense against mountain pine beetle, a native tree-killing herbivore. Assays were conducted with mature trees in replicated provenance trials. Among chemical defenses, monoterpene composition (relative proportions of various compounds) had higher genetic variability than monoterpene concentration (absolute quantities). Additionally, constitutive concentrations had higher genetic variabilities than induced concentrations elicited by simulated attack. Resin flow elicited by mechanical wounding likewise showed very low genetic variability. In previous studies with this and related systems, induced monoterpene concentration and resin flow are the two best predictors respectively of which trees will survive outbreaks under natural conditions. This means that expression of the most efficacious defense traits is strongly influenced by environmental factors. These low heritabilities despite their survival values likely arise from specific life history and ecological aspects of this system, such as the lengthy period between when lodgepole pines begin cone production and become large enough for brood development, the extended viability of serotinous cones post-mortem, the high heterogeneity of both resources and stressors that trees experience across landscapes, and a strong proximate-neighbor effect on tree mortality arising from beetle aggregation pheromones. However, low heritability values do not preclude long-term evolvability and coadaptation of defenses, for which there is extensive mechanistic and associational evidence in this system. Among other traits we evaluated, sesquiterpenes had higher genetic variability than monoterpenes. Sesquiterpenes are not known to influence conifer-bark beetle interactions but have anti-herbivore properties in some other plant-insect systems. The lengths of lesions formed in response to simulated attack showed no heritable component and likewise do not predict tree survival in nature. Bark texture was highly heritable and is known to influence beetle post-landing orientation and colonization patterns, but whether it influences survival patterns at the whole-tree level is unknown. Relationships between tree defenses and growth were generally positive, suggesting that environmental heterogeneity in resource quality among trees supersedes potential underlying substrate allocation processes within trees. Similarly, relationships between various defense traits were generally positive rather than showing trade-offs, suggesting conifer resistance to native bark beetles entails coordinated defense syndromes. Implications to tree breeding, forest management, and coevolution between long-lived plants and rapidly reproducing herbivores are discussed. The extent to which these trends may apply to other bark beetle systems is evaluated, along with specific priorities for future research.

Arthropod Recolonization of Soil Surface Habitat in Post-Fire Mulch Treatments

Increasing size, severity, and human proximity to fires in the western US are driving a need for more effective ecosystem restoration in the immediate post-fire period. Surface treatments, such as mastication of logging slash, reduce erosion and improve soil nutrient and water retention on steep slopes. However, few studies have investigated the impact of these treatments on arthropod communities over time. Our objective was to determine which insect communities return to these treated areas and if the mulch changes the community structure over time. We surveyed arthropod abundance using pitfall traps in mulch treatments in a landscape-scale fire near Flagstaff, Arizona, and a controlled split-plot experiment outside of the larger fire footprint. Predatory beetles were more abundant in mulch in the large landscape treatment, with no differences in abundance in the split plots. Fungivores had no significant mulch preference, and several native bark beetles were more abundant in the untreated sites. We found that the size of the fire footprint and distance to the intact forest matrix likely impact arthropod community composition over time. We were unable to fully evaluate vegetation recovery, but further work will allow us to understand how surface treatments impact the interaction of arthropods and vegetation.

Growth, drought response, and climate-associated genomic structure in whitebark pine in the Sierra Nevada of California.

Whitebark pine (Pinus albicaulis Engelm.) has experienced rapid population declines and is listed as threatened under the Endangered Species Act in the United States. Whitebark pine in the Sierra Nevada of California represents the southernmost end of the species' distribution and, like other portions of its range, faces threats from an introduced pathogen, native bark beetles, and a rapidly warming climate. Beyond these chronic stressors, there is also concern about how this species will respond to acute stressors, such as drought. We present patterns of stem growth from 766 large (average diameter at breast height >25 cm), disease-free whitebark pine across the Sierra Nevada before and during a recent period of drought. We contextualize growth patterns using population genomic diversity and structure from a subset of 327 trees. Sampled whitebark pine generally had positive to neutral stem growth trends from 1970 to 2011, which was positively correlated with minimum temperature and precipitation. Indices of stem growth during drought years (2012 to 2015) relative to a predrought interval were mostly positive to neutral at our sampled sites. Individual tree growth response phenotypes appeared to be linked to genotypic variation in climate-associated loci, suggesting that some genotypes can take better advantage of local climatic conditions than others. We speculate that reduced snowpack during the 2012 to 2015 drought years may have lengthened the growing season while retaining sufficient moisture to maintain growth at most study sites. Growth responses may differ under future warming, however, particularly if drought severity increases and modifies interactions with pests and pathogens.

Interactions between white pine blister rust, bark beetles, and climate over time indicate vulnerabilities to limber pine health

IntroductionLimber pine is a keystone species in the Rocky Mountains that grows on harsh, high elevation sites where few other tree species can. Recent studies suggest the species is threatened by the combined impacts of the exotic, invasive disease, white pine blister rust (WPBR), native bark beetles, and climate change. Information on changes in the health of limber pine populations and long-term impacts posed by these threats is needed to inform management efforts.MethodsWe established 106 long-term monitoring plots in 10 study areas that were surveyed three times between 2004 and 2017. We assessed site and stand factors, tree health, and regeneration over time to detect changes in limber pine abundance and health, cumulative impacts of WPBR and bark beetles, and to evaluate the drivers of WPBR occurrence and severity.ResultsLimber pine health declined significantly over the study with more than 20% of initially live limber pine trees dead by the last measurement cycle, primarily due to WPBR and bark beetles. While some recruitment occurred, mortality rates greatly outpaced recruitment of ingrowth. Disease incidence and how it changed over time was variable, but disease severity increased substantially overall and in all study areas. Limber pine regeneration was low or absent in most sites and mortality caused by WPBR increased significantly. We found strong relationships between WPBR and aridity. Trees in habitats with high vapor pressure deficit were less likely to be infected with WPBR, but trees that were already infected were more likely to develop severe symptoms and die. Longer growing seasons increased the likelihood of WPBR presence and mortality. Growing season length and vapor pressure deficit tended to increase over the study, suggesting that climate change may exacerbate disease impacts.DiscussionDeclining health of limber pine coupled with high mortality rates, increasing disease severity, and low levels of natural regeneration indicate successful recovery may not occur in some locations without management intervention. Proactive management strategies to reduce insect and disease impacts and promote stand recovery and resilience should be pursued in remaining, healthy limber pine ecosystems.

Rocky Mountain forests are poised to recover following bark beetle outbreaks but with altered composition

AbstractAmplified by warming temperatures and drought, recent outbreaks of native bark beetles (Curculionidae: Scolytinae) have caused extensive tree mortality throughout Europe and North America. Despite their ubiquitous nature and important effects on ecosystems, forest recovery following such disturbances is poorly understood, particularly across regions with varying abiotic conditions and outbreak effects.To better understand post‐outbreak recovery across a topographically complex region, we synthesized data from 16 field studies spanning subalpine forests in the Southern Rocky Mountains, USA. From 1997 to 2019, these forests were heavily affected by outbreaks of three native bark beetle species (Dendroctonus ponderosae,Dendroctonus rufipennisandDryocoetes confusus). We compared pre‐ and post‐outbreak forest conditions and developed region‐wide predictive maps of post‐outbreak (1) live basal areas, (2) juvenile densities and (3) height growth rates for the most abundant tree species – aspen (Populus tremuloides), Engelmann spruce (Picea engelmannii), lodgepole pine (Pinus contorta) and subalpine fir (Abies lasiocarpa).Beetle‐caused tree mortality reduced the average diameter of live trees by 28.4% (5.6 cm), and species dominance was altered on 27.8% of field plots with shifts away from pine and spruce. However, most plots (82.1%) were likely to recover towards pre‐outbreak tree densities without additional regeneration. Region‐wide maps indicated that fir and aspen, non‐host species for bark beetle species with the most severe effects (i.e.Dendroctonusspp.), will benefit from outbreaks through increased compositional dominance. After accounting for individual size, height growth for all conifer species was more rapid in sites with low winter precipitation, high winter temperatures and severe outbreaks.Synthesis. In subalpine forests of the US Rocky Mountains, recent bark beetle outbreaks have reduced tree size and altered species composition. While eventual recovery of the pre‐outbreakforest structureis likely in most places, changes inspecies compositionmay persist for decades. Still, forest communities following bark beetle outbreaks are widely variable due to differences in pre‐outbreak conditions, outbreak severity and abiotic gradients. This regional variability has critical implications for ecosystem services and susceptibility to future disturbances.

Both native and invasive bark beetles threaten exotic conifers within the spa towns in the Czech part of “The Great Spas of Europe”

Exotic coniferous trees in forests adjacent to spa towns and in parks within spa towns are increasingly being attacked by outbreak populations of native bark beetles in western Bohemia (Central Europe). In the 12 localities in western Czech Republic the current study, we detected a total of 19 native bark beetle species infesting more than 10 % of exotic tree species in the genera Abies, Picea, Pinus, and Pseudotsuga in 1 year. Several exotic tree species were identified as new hosts for native European bark beetles. We also identified three non-native bark beetles, including Phloeosinus aubei, a species that transmits the pathogens that cause cypress canker disease.The numbers of infested trees in the localities were not correlated with the distance from surrounding bark beetle outbreak sites in commercial forests, because all forests adjacent to spas and all forests in spa towns (i.e., “urban forests”) are close to commercial forests with infested trees. Infestations were greater in spa towns than in the forests adjacent to spa towns. We therefore assume that trees within spa towns will be further attacked due to the increasing abundance of native bark beetles.

Mongolian pine forest decline by the combinatory effect of European woodwasp and plant pathogenic fungi

Interactions between the decline of Mongolian pine woodlands and fungal communities and invasive pests in northeastern China are poorly understood. In this study, we investigated the fungal communities occurring in three tree samples: the woodwasp Sirex noctilio infested, healthy uninfested and unhealthy uninfested Mongolian pine trees. We analyzed the relationships of the Mongolian pine decline with fungal infection and woodwasp infestation. Twenty-six fungal species were identified from the sampled trees. Each tree sample harbored a fungal endophyte community with a unique structure. Pathogenic fungi richness was four times higher in infested and unhealthy un-infested trees compared to that in healthy uninfested trees. Sphaeropsis sapinea was the most dominant pathogenic fungus in the sampled Mongolian pine trees. The number of S. noctilio was higher than native bark beetles in the declining Mongolian pine trees. The invasion of the woodwasp appeared to be promoted by the fungal infection in the Mongolian pine trees. The incidence of S. noctilio infestation was higher in the fungi infected trees (83.22%) than those without infection (38.72%). S. sapinea population exhibited positive associations with within-tree colonization of S. noctilio and bark beetle. Collectively, these data indicate that the fungal disease may have caused as the initial reason the decline of the Mongolian pine trees, and also provided convenient conditions for the successful colonization of the woodwasp. The woodwasps attack the Mongolian pine trees infected by fungi and accelerated its decline.

Predicting multi-species bark beetle (Coleoptera: Curculionidae: Scolytinae) occurrence in Alaska: First use of open access big data mining and open source GIS to provide robust inference and a role model for progress in forest conservation

Native bark beetles (Coleoptera: Curculionidae: Scolytinae) are a multi-species complex that rank among the key disturbances of coniferous forests of western North America. Many landscape-level variables are known to influence beetle outbreaks, such as suitable climatic conditions, spatial arrangement of incipient populations, topography, abundance of mature host trees, and disturbance history that include former outbreaks and fire. We assembled the first open access data, which can be used in open source GIS platforms, for understanding the ecology of the bark beetle organism in Alaska. We used boosted classification and regression tree as a machine learning data mining algorithm to model-predict the relationship between 14 environmental variables, as model predictors, and 838 occurrence records of 68 bark beetle species compared to pseudo-absence locations across the state of Alaska. The model predictors include topography- and climate-related predictors as well as feature proximities and anthropogenic factors. We were able to model, predict, and map the multi-species bark beetle occurrences across the state of Alaska on a 1-km spatial resolution in addition to providing a good quality environmental dataset freely accessible for the public. About 16% of the mixed forest and 59% of evergreen forest are expected to be occupied by the bark beetles based on current climatic conditions and biophysical attributes of the landscape. The open access dataset that we prepared, and the machine learning modeling approach that we used, can provide a foundation for future research not only on scolytines but for other multi-species questions of concern, such as forest defoliators, and small and big game wildlife species worldwide.

Rocky Mountain Bristlecone Pine (Pinus aristata) is a Confirmed Host to Mountain Pine Beetle (Dendroctonus ponderosae)

Mountain pine beetle (MPB) (Dendroctonus ponderosae Hopkins) is a native bark beetle that reproduces in pine (Pinus) species across western North America. High population levels can result in widespread host tree mortality. Over the past 2 decades, MPB has been responsible for pine mortality across millions of forested hectares in the western United States. Although a majority of the pine species found in the western United States are considered hosts to MPB, the host status of Rocky Mountain (RM) bristlecone pine (Pinus aristata) is unclear. We surveyed stands across the range of RM bristlecone pine in Colorado, USA, and quantified MPB-caused mortality within the past 10 years in stands where RM bristlecone and at least one other pine species co-occurred. We also evaluated in the field whether successful MPB brood production occurred in RM bristlecone pine. Our results confirm that RM bristlecone pine is susceptible to MPB attack and suitable for MPB reproduction. In mixed-species stands, pine species availability influenced MPB attack occurrence. The proportion of trees experiencing fatal beetle attack within a particular Pinus species was best predicted by the basal area proportion of that species in the stand prior to the most recent 10 years of mortality. These results indicate that RM bristlecone pine is vulnerable to ongoing climate change–induced contact with MPB.

Effects of Bark Beetle Outbreaks on Forest Landscape Pattern in the Southern Rocky Mountains, U.S.A.

Since the late 1990s, extensive outbreaks of native bark beetles (Curculionidae: Scolytinae) have affected coniferous forests throughout Europe and North America, driving changes in carbon storage, wildlife habitat, nutrient cycling, and water resource provisioning. Remote sensing is a crucial tool for quantifying the effects of these disturbances across broad landscapes. In particular, Landsat time series (LTS) are increasingly used to characterize outbreak dynamics, including the presence and severity of bark beetle-caused tree mortality, though broad-scale LTS-based maps are rarely informed by detailed field validation. Here we used spatial and temporal information from LTS products, in combination with extensive field data and Random Forest (RF) models, to develop 30-m maps of the presence (i.e., any occurrence) and severity (i.e., cumulative percent basal area mortality) of beetle-caused tree mortality 1997–2019 in subalpine forests throughout the Southern Rocky Mountains, USA. Using resultant maps, we also quantified spatial patterns of cumulative tree mortality throughout the region, an important yet poorly understood concept in beetle-affected forests. RF models using LTS products to predict presence and severity performed well, with 80.3% correctly classified (Kappa = 0.61) and R2 = 0.68 (RMSE = 17.3), respectively. We found that ≥10,256 km2 of subalpine forest area (39.5% of the study area) was affected by bark beetles and 19.3% of the study area experienced ≥70% tree mortality over the twenty-three year period. Variograms indicated that severity was autocorrelated at scales < 250 km. Interestingly, cumulative patch-size distributions showed that areas with a near-total loss of the overstory canopy (i.e., ≥90% mortality) were relatively small (<0.24 km2) and isolated throughout the study area. Our findings help to inform an understanding of the variable effects of bark beetle outbreaks across complex forested regions and provide insight into patterns of disturbance legacies, landscape connectivity, and susceptibility to future disturbance.

Responses of Native and Non-native Bark and Ambrosia Beetles (Coleoptera: Curculionidae: Scolytinae) to Different Chemical Attractants: Insights From the USDA Forest Service Early Detection and Rapid Response Program Data Analysis.

More than 60 non-native bark and ambrosia beetle species (Coleoptera: Curculionidae: Scolytinae) are established in North America and several have had severe negative impacts on ecosystems. Non-native scolytines can introduce fungi which may cause vascular wilts and compete with native fungi and lead to reductions in native species through host reduction. The Early Detection Rapid Response (EDRR) program was created by the USDA Forest Service in 2007 to detect non-native bark and ambrosia beetles and provide a baseline for tracking populations over time. This program has led to new collection records and increased communication among agencies to delimit non-native scolytine populations and perform appropriate management. Although insect responses to different lure types vary, it is unknown how different lures compare in attracting bark and ambrosia beetles. Our goal was to examine how lure combinations used in the EDRR program affect captures of bark and ambrosia beetle communities and to determine the most effective combination of lures for targeting non-native scolytines. The highest proportion of non-native scolytines was captured with ethanol, as was the greatest total number of species, and the most diverse beetle community. Traps with Ips (Coleoptera: Curculionidae) lures captured the highest proportion of native scolytines but the lowest total number of total species and was also the least diverse. Communities of scolytines differed significantly among lures, states, and years. While ethanol is an appropriate lure for generalist trapping and targeting a wide range of non-native bark and ambrosia beetles, more targeted lures are needed for monitoring certain species of non-natives.

Black-Backed Woodpecker Nest Density in the Sierra Nevada, California

In the western U.S., the black-backed woodpecker has been found to be associated with dense montane conifer forests with high snag densities, typically resulting from moderate- to high-severity wildland fires. However, black-backed woodpeckers are occasionally also detected nesting in unburned forests, raising questions about the type of habitat in which they nest and the potential abundance of such habitat. We conducted intensive black-backed woodpecker nest density surveys in large plots within the middle/upper-montane conifer forests of the Sierra Nevada, California, within general (undisturbed) forests, snag forest habitat from moderate/high-severity wildland fire, and unburned snag forest habitat from drought and native bark beetles. We found black-backed woodpeckers nesting only in the two snag forest conditions, mostly in burned snag forest, and their preferential selection of burned snag forest was statistically significant. No nest was found in general forests. Our spatial analysis indicates that snag forest is rare in the forests of the Sierra Nevada due to fire suppression and logging, raising concerns regarding small population size, which we estimate to be only 461 to 772 pairs in the Sierra Nevada.

Neighborhood context mediates probability of host tree mortality in a severe bark beetle outbreak

AbstractUnderstanding drivers of disturbances across scales is critical as environmental constraints change in a warming climate. Outbreaks of native bark beetles (Curculionidae: Scolytinae) are key natural disturbances that shape the structure and function of conifer forests across the northern hemisphere. While drivers of bark beetle outbreaks have been studied extensively at spatial scales ranging from stands to continents, within‐stand processes governing individual tree mortality in an outbreak are less well understood. Here, we use a spatially explicit long‐term monitoring dataset of a lodgepole pine (Pinus contorta var. latifolia) forest (>9000 individually mapped trees in three 2‐ha plots) impacted by a severe mountain pine beetle (Dendroctonus ponderosae) outbreak to explore interactions among fine scale drivers of beetle‐caused tree mortality. Using a Bayesian spatial modeling approach, we evaluated how tree scale and tree neighborhood scale characteristics interact with tree size to mediate host tree susceptibility to mountain pine beetle outbreak in the Southern Rocky Mountains (USA). We found evidence that both tree growth rate preceding the outbreak and neighborhood structure (within a 10 meter radius of the host tree) mediate the effect of tree size, and that the direction and magnitude of these mediating effects vary with tree size. Tree scale mortality probability increased with pre‐outbreak growth rate for small to medium sized host trees (~10–25 cm diameter), but that same effect was not detected for large trees. Conversely, tree scale mortality probability increased with greater neighborhood density, with the most pronounced effects for medium to large sized host trees (~15–30 cm diameter). Within‐stand topographic variability was not an important predictor of mortality probability; among stands, however, the stand in the driest topographic position experienced the greatest overall mortality. By explicitly considering how within‐stand heterogeneity mediates individual tree scale susceptibility to bark beetle outbreak, our findings bridge an important gap in understanding multi‐scale drivers of disturbance dynamics.

Biodiversity Conservation Targets: How to Allocate Resources

In this issue of One Earth, Chauvenet et al. identify strategies for the near-term allocation of protected areas on an ecoregion basis. However, the biodiversity challenge extends beyond ecoregions. More resources are needed to maximize the potential co-benefits between conservation, climate-change mitigation, indigenous communities, and sustainable land-management practices.

Records of seven species of native and exotic bark beetles new to Pu’u Wa’awa’a Dry Forest Unit, Hawai‘i Island (Coleoptera: Curculionidae, Scolytinae)

As part of ongoing surveys for native bark beetles (Coleoptera: Curculionidae: Scolytinae) across the Hawaiian Islands, we undertook targeted sampling at Pu’u Wa’awa’a Experimental Forest Unit, North Kona, on the northwestern part of Hawai‘i Island during February to April of 2018 and 2019. This is one of the few areas containing remaining native dry forest on the leeward, dry side of the island. Our sampling revealed the presence of seven species of bark beetles not previously recorded from Pu’u Wa’awa’a. These included two native and endemic Hawaiian species belonging to the genus Xyleborus Eichhoff (tribe Xyleborini). The other five species are the exotic Hypothenemus hampei (Ferrari, 1867), or the coffee berry borer, belonging to the tribe Cryphalini, which is a serious pest of coffee in the Hawaiian Islands, and four widespread adventive species belonging to the tribe Xyleborini, including Xyleborus ferrugineus (Fabricius, 1801), whose frass has been demonstrated to be able to contain a fungus that is a causative agent of the plant disease Rapid ʻŌhiʻa Death which currently poses a threat to native ʻōhiʻa lehua trees. These records are presented and discussed in detail, and the newly recorded species are illustrated in colour photographs.

A duplex ddPCR assay for simultaneously detectingIps sexdentatusandIps typographus(Coleoptera: Curculionidae) in bulk trap samples

Bark beetles in the family Curculionidae present a growing hazard to forests worldwide. Like native bark beetles, introduced exotic species can pose a serious threat to North American forests. Ips typographus (Boerner) and Ips sexdentatus (Linnaeus), both native to Europe, are two such pests that have caused widespread forest loss in their native ranges. International trade has led to increased interceptions of Scolytine beetles at ports of entry to the United States. Most intercepted individuals are not identified to species due to lack of expert identifiers, poor specimen quality, or incomplete taxonomy. These same problems affect identification for domestic surveys. Therefore, development of molecular methods for identification of potentially invasive Ips species is essential. Because of the need to scrutinize large numbers of beetles in an efficient manner, we describe a duplex droplet digital PCR (ddPCR) assay to identify I. typographus and I. sexdentatus simultaneously in bulk trap samples containing 500 Scolytinae specimens using a scalable, two-step DNA extraction. This ddPCR method is highly effective for processing the entire contents of beetle traps and identifying these potentially invasive species in a timely and definitive manner. We also describe a nondestructive DNA extraction technique that preserves specimens for morphological identification.

Which trees die during drought? The key role of insect host‐tree selection

Abstract During drought, the tree subpopulations (such as size or vigour classes) that suffer disproportionate mortality can be conceptually arrayed along a continuum defined by the actions of biotic agents, particularly insects. At one extreme, stress dominates: insects are absent or simply kill the most physiologically stressed trees. At the opposite extreme, host selection dominates: outbreaking insects kill trees independently of their stress, instead selecting trees based on size or other traits. Intermediate responses are also possible. Yet for mixed‐species forests, we lack a broad understanding of the relative importance of insects in determining exactly which subpopulations of trees suffer disproportionate mortality during drought, and whether these subpopulations differ among co‐occurring tree species. During an extreme drought, we documented the roles of native bark beetles in the mortality of five tree species in California’s Sierra Nevada. We analysed the patterns and agents of tree mortality in 12 permanent plots and the patterns of mortality in 89 temporary plots. Most tree mortality was associated with bark beetles. However, the growth rates (an indicator of chronic stress) and sizes of trees that suffered greatest bark beetle‐related mortality differed sharply among tree taxa, variously conforming with domination by stress (Abies concolor), domination by host selection (Pinus lambertiana and P. ponderosa) or a mix of the two (Calocedrus decurrens). Quercus kelloggii mortality remained relatively low. Thus, even during extreme drought substantial proportions of stressed trees survived because they were of sizes that mostly avoided fatal insect attack. Conversely, substantial proportions of comparatively unstressed trees died because they were of sizes that were selectively killed by outbreaking insects. Synthesis. Native bark beetles were primarily responsible for determining which subpopulations of trees suffered greatest mortality during drought. However, idiosyncratic host‐tree selection by the different bark beetle taxa meant that the tree subpopulations suffering greatest mortality differed strikingly among tree taxa—for example, high mortality of small trees of one species, but of large trees of another. If idiosyncratic host‐tree selection by biotic mortality agents proves to be a generally common phenomenon, it could help explain weak broadscale correlations between tree traits and tree mortality during drought.

Anomalous outbreaks of an invasive defoliator and native bark beetle facilitated by warm temperatures, changes in precipitation and interspecific interactions

Biotic disturbance agents such as insects can be highly responsive to climatic change and have widespread ecological and economic impacts on forests. Quantifying the responses of introduced and native insects to climate, including how dynamics of one agent may mediate those of another, is important for forecasting disturbance and associated impacts on forest structure and function. We investigated drivers of outbreaks by larch casebearer Coleophora laricella, an invasive defoliator, and eastern larch beetle Dendroctonus simplex, a native, tree‐killing bark beetle, on tamarack Larix laricina from 2000 to in Minnesota, USA. We evaluated the utility of temporal, spatial and climatic variables in predicting the presence/absence of outbreaks of each insect in cells of rasterized aerial survey data. The role of defoliation by larch casebearer in outbreaks of eastern larch beetle was also investigated. For both species, the most important predictors of outbreak occurrence were proximity of conspecific outbreaks in space and time. For larch casebearer, outbreak occurrence was positively associated with spring precipitation and warmer growing seasons. Outbreak occurrence of eastern larch beetle was positively associated with warmer and dryer years and was more likely in cells with prior defoliation by larch casebearer. Our results demonstrate that climate can drive large scale outbreaks of introduced and non‐native disturbance agents on a single host species, and that interactions at the tree level between such agents may scale up to manifest across large temporal and spatial scales.

Stand dynamics and the mountain pine beetle — 30 years of forest change in Waterton Lakes National Park, Alberta, Canada

The mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) is a native bark beetle and a major disturbance agent in western North American forests. In the 1970s and 1980s, a MPB outbreak occurred in Waterton Lakes National Park (WLNP) in southwestern Alberta. The MPB outbreak resulted in variable levels of mortality of mature lodgepole pine (Pinus contorta var. latifolia Engelm. ex S. Watson), reducing density, volume, and basal area of overstory trees. By 2010, lodgepole pine was proportionally no longer the dominant overstory species, with increases in non-pine conifer and broadleaf species. The MPB susceptibility index decreased in most stands over time, especially in stands with the highest MPB-caused mortality. Downed woody material was characterized by fine and coarse fuel mass and volume, which both increased from 2002 to 2010, and the abundance of coarse fuels was highest in 2010, nearly 30 years after peak MPB activity. Density of understory saplings and small regeneration increased from 2002 to 2010 and was dominated by non-pine conifer and broadleaf species; lodgepole pine was nearly absent. Hierarchical clustering using 2010 MPB susceptibility and composition data characterized biological legacies remaining after the MPB outbreak. These legacies suggest multiple successional trajectories in WLNP dominated by species other than lodgepole pine. The MPB outbreak resulted in greater heterogeneity in composition and structure and suggests that stands have been resilient to this disturbance.