Native Bark Beetle Research Articles

Bark beetles as agents of change in social–ecological systems

Due to recent outbreaks of native bark beetles, forest ecosystems have experienced substantial changes in landscape structure and function, which also affect nearby human populations. As a result, land managers have been tasked with sustaining ecosystem services in impacted areas by considering the best available science, public perceptions, and monitoring data to develop strategies to suppress bark beetle epidemics, and in some cases to restore affected lands and ecosystem services. The effects of bark beetle outbreaks are often detrimental to the provision of ecosystem services, including degraded landscape aesthetics and diminished air and water quality. However, there have been instances where bark beetle outbreaks have benefited communities by, for example, improving habitat for grazing animals and enhancing real‐estate values. As a consequence of the interaction of a warming climate and susceptible forest stand conditions, the frequency, severity, and extent of bark beetle outbreaks are expected to increase and therefore will continue to challenge many social–ecological systems. We synthesize experiences from recent outbreaks to encourage knowledge transfer from previously impacted communities to potentially vulnerable locations that may be at risk from future bark beetle epidemics.

More ways than one: Mixed-severity disturbance regimes foster structural complexity via multiple developmental pathways

Abstract Mixed-severity disturbance regimes are prevalent in temperate forests worldwide, but key uncertainties remain regarding the variability of disturbance-mediated structural development pathways. This study investigates the influence of disturbance history on current structure in primary, unmanaged Norway spruce (Picea abies) forests throughout the Carpathian Mountains of central and eastern Europe, where windstorms and native bark beetle outbreaks are the dominant natural disturbances. We inventoried forest structure on 453 plots (0.1 ha) spanning a large geographical gradient (>1,000 km), coring 15–25 canopy trees per plot for disturbance history reconstruction (tree core total n = 11,309). Our specific objectives were to: (1) classify sub-hectare-scale disturbance history based on disturbance timing and severity; (2) classify current forest structure based on tree size distributions (live, dead, standing, downed); (3) characterize structural development pathways as revealed by the association between disturbance history and current forest structural complexity. We used hierarchical cluster analysis for the first two objectives and indicator analysis for the third. The disturbance-based cluster analysis yielded six groups associated with three levels of disturbance severity (low, moderate, and high canopy loss) and two levels of timing (old, recent) over the past 200 years. The structure-based cluster analysis yielded three groups along a gradient of increasing structural complexity. A large majority of plots exhibited relatively high (53%) or very high (26%) structural complexity, indicated by abundant large live trees, standing and downed dead trees, and spruce regeneration. Consistent with conventional models of structural development, some disturbance history groups were associated with specific structural complexity groups, particularly low-severity/recent (very high complexity) and high-severity/recent (moderate complexity) disturbances. In other cases, however, the distribution of plots among disturbance history and structural complexity groups indicated either divergent or convergent pathways. For example, multiple disturbance history groups were significantly associated with the high complexity group, demonstrating structural convergence. These results illustrate that complex forest structure – including features nominally associated with old-growth – can be associated as much with disturbance severity as it is with conventional notions of forest age. Because wind and bark beetles are natural disturbance processes that can induce relatively high levels of tree mortality while simultaneously contributing to structural complexity and heterogeneity, we suggest that forest management plans allow for the stochastic occurrence of disturbance and variable post-disturbance development trajectories. Such applications are especially appropriate in conservation areas where biodiversity and forest resilience are management objectives, particularly given projections of increasing disturbance activity under global change.

Time dependency of eddy covariance site energy balance

Energy flow through ecosystems plays a critical role in processes at multiple spatial and temporal scales, from phonologically driven growing season or monthly temporal scales of landscapes to sub-diurnal responses of soil respiration to temperature, photosynthesis and water inputs. The interaction of short and longwave radiation and their partitioning through ecosystems is complex with terrestrial canopies and aquatic structure both connecting above- and below-ground processes via energy fluxes. Previous work has shown that at 30-min timescales, only 8% of eddy covariance sites in the La Thuile dataset observe energy closure and when averaged to 24-h timescales, this goes up to 45%. This work examines the effect of temporal lags in energy storage in both terrestrial and aquatic ecosystems. Analyses show energy storage terms have unique temporal lags that vary between ecosystem and time of year, from having zero lag to several hour timelags within terrestrial ecosystems, depending primarily on water content. Large differences between ecosystem types are also highlighted as aquatic ecosystems have lags that range between daily and monthly timescales. Furthermore, ecosystem disturbance can alter time-lags as well and results from a native bark beetle disturbance show both vegetation and soil lag increasing following changes to ecosystem processes from tree mortality. Considering energy storage lags can improve site energy closure in 20% of site-days in the FLUXNET2015 dataset and these results will lead to a better understanding of surface energy budget closure as well as highlighting the importance of time-dependency of ecosystem energy fluxes as a unique method to infer ecosystem processes.

Fire severity unaffected by spruce beetle outbreak in spruce-fir forests in southwestern Colorado.

Recent large and severe outbreaks of native bark beetles have raised concern among the general public and land managers about potential for amplified fire activity in western North America. To date, the majority of studies examining bark beetle outbreaks and subsequent fire severity in the U.S. Rocky Mountains have focused on outbreaks of mountain pine beetle (MPB; Dendroctonus ponderosae) in lodgepole pine (Pinus contorta) forests, but few studies, particularly field studies, have addressed the effects of the severity of spruce beetle (Dendroctonus rufipennis Kirby) infestation on subsequent fire severity in subalpine Engelmann spruce (Picea engelmannii) and subalpine fir (Abies lasiocarpa) forests. In Colorado, the annual area infested by spruce beetle outbreaks is rapidly rising, while MPB outbreaks are subsiding; therefore understanding this relationship is of growing importance. We collected extensive field data in subalpine forests in the eastern San Juan Mountains, southwestern Colorado, USA, to investigate whether a gray-stage (< 5 yr from outbreak to time of fire) spruce beetle infestation affected fire severity. Contrary to the expectation that bark beetle infestation alters subsequent fire severity, correlation and multivariate generalized linear regression analysis revealed no influence of pre-fire spruce beetle severity on nearly all field or remotely sensed measurements of fire severity. Findings were consistent across moderate and extreme burning conditions. In comparison to severity of the pre-fire beetle outbreak, we found that topography, pre-outbreak basal area, and weather conditions exerted a stronger effect on fire severity. Our finding that beetle infestation did not alter fire severity is consistent with previous retrospective studies examining fire activity following other bark beetle outbreaks and reiterates the overriding influence of climate that creates conditions conducive to large, high-severity fires in the subalpine zone of Colorado. Both bark beetle outbreaks and wildfires have increased autonomously due to recent climate variability, but this study does not support the expectation that post-beetle outbreak forests will alter fire severity, a result that has important implications for management and policy decisions.

Evolving symbioses between insects and fungi that kill trees in Canada: new threats associated with invasive organisms

AbstractSymbiotic relationships between insects and fungi are known to cause tree mortality either through direct damage by larval feeding that can be facilitated by symbiotic fungi, or through insects vectoring pathogens directly to healthy trees. Within their native ranges, the impacts of many insect-fungus symbioses are restricted to weakened and declining trees; however, within the last century tree mortality caused by globally invasive insect–fungus associations has had a devastating impact on trees in both urban and natural forest ecosystems. Unfortunately, Canadian forests have been seriously affected by invasive organisms and an emerging threat is the expansion of a native bark beetle into the boreal forest of Alberta. This paper reviews the symbiotic relationships between selected invasive insects and pathogens that cause tree mortality within the urban and forested landscapes of Canada; it uses these case studies to illustrate potentially damaging new evolutionary trajectories.

Long-term landscape changes in a subalpine spruce-fir forest in central Utah, USA

In Western North America, increasing wildfire and outbreaks of native bark beetles have been mediated by warming climate conditions. Bioclimatic models forecast the loss of key high elevation species throughout the region. This study uses retrospective vegetation and fire history data to reconstruct the drivers of past disturbance and environmental change. Understanding the relationship among climate, antecedent disturbances, and the legacy effects of settlement-era logging can help identify the patterns and processes that create landscapes susceptible to bark beetle epidemics. Our analysis uses data from lake sediment cores, stand inventories, and historical records. Sediment cores were dated with radiometric techniques (14C and 210Pb/137Cs) and subsampled for pollen and charcoal to maximize the temporal resolution during the historical period (1800 CE to present) and to provide environmental baseline data (last 10,500 years). Pollen data for spruce were calibrated to carbon biomass (C t/ha) using standard allometric equations and a transfer function. Charcoal samples were analyzed with statistical models to facilitate peak detection and determine fire recurrence intervals. The Wasatch Plateau has been dominated by Engelmann spruce forests for the last ~10,500 years, with subalpine fir becoming more prominent since 6000 years ago. This landscape has experienced a dynamic fire regime, where burning events are more frequent and of higher magnitude during the last 3000 years. Two important disturbances have impacted Engelmann spruce in the historical period: 1) high-grade logging during the late 19th century; and (2) a high severity spruce beetle outbreak in the late 20th century that killed >90 % of mature spruce (>10 cm dbh). Our study shows that spruce-dominated forests in this region are resilient to a range of climate and disturbance regimes. Several lines of evidence suggest that 19th century logging promoted a legacy of simplified stand structure and composition such that, when climate became favorable for accelerated beetle population growth, the result was a landscape-scale spruce beetle outbreak. The lasting impacts of settlement-era landscape history from the Wasatch Plateau, UT may be relevant for other areas of western North America and Europe where sufficient host carrying capacity is important in managing for resistance and resilience to outbreaks.

Recent mountain pine beetle outbreaks, wildfire severity, and postfire tree regeneration in the US Northern Rockies.

Widespread tree mortality caused by outbreaks of native bark beetles (Circulionidae: Scolytinae) in recent decades has raised concern among scientists and forest managers about whether beetle outbreaks fuel more ecologically severe forest fires and impair postfire resilience. To investigate this question, we collected extensive field data following multiple fires that burned subalpine forests in 2011 throughout the Northern Rocky Mountains across a spectrum of prefire beetle outbreak severity, primarily from mountain pine beetle (Dendroctonus ponderosae). We found that recent (2001-2010) beetle outbreak severity was unrelated to most field measures of subsequent fire severity, which was instead driven primarily by extreme burning conditions (weather) and topography. In the red stage (0-2 y following beetle outbreak), fire severity was largely unaffected by prefire outbreak severity with few effects detected only under extreme burning conditions. In the gray stage (3-10 y following beetle outbreak), fire severity was largely unaffected by prefire outbreak severity under moderate conditions, but several measures related to surface fire severity increased with outbreak severity under extreme conditions. Initial postfire tree regeneration of the primary beetle host tree [lodgepole pine (Pinus contorta var. latifolia)] was not directly affected by prefire outbreak severity but was instead driven by the presence of a canopy seedbank and by fire severity. Recent beetle outbreaks in subalpine forests affected few measures of wildfire severity and did not hinder the ability of lodgepole pine forests to regenerate after fire, suggesting that resilience in subalpine forests is not necessarily impaired by recent mountain pine beetle outbreaks.

Do bark beetle remains in lake sediments correspond to severe outbreaks? A review of published and ongoing research

The recent continental-scale outbreak of native bark beetles in western North America is unprecedented at least since Euro-American settlement. Observational and modeling evidence suggest that warm temperatures observed during the late 20th century altered beetle population dynamics by accelerating beetle reproductive cycles leading to exponential population growth. The linkage between beetle outbreaks and climate warming has motivated efforts to reconstruct these disturbances using long-term environmental records using lake sediments. Here, we present data from across western North America in an effort to understand how beetle remains retrieved from lake sediments may be used as a proxy for reconstructing severe outbreaks and ecosystem response over centennial to millennial timescales. We (1) review existing literature related to beetle taphonomy; (2) present previously unpublished data of beetle remains in lake sediments; (3) comment on the development of a methodology to retrieve terrestrial beetle remains from lake sediments; (4) discuss potential controls on beetle carcass taphonomy into the sediment matrix; and lastly (5) speculate on the use of primary and secondary attack beetle remains as indicators of past outbreak episodes. Our synthesis suggests that the remains of primary attack beetles are rarely preserved in lake sediments, at least using small-diameter piston devices common in multi-proxy studies. Alternatively, remains of secondary attach beetles may be common but further work is required to understand how these insects can be used to aid in interpreting past forest disturbances, including bark beetle outbreaks and wildfire. A number of factors may influence whether or not bark beetle remains become entrained in the area of sediment focusing including lake water chemistry, fish predation and scavenging, and weather conditions during peak beetle emergence.

Using lake sediment records to reconstruct bark beetle disturbances in western North America

The recent outbreak of native bark beetles in western North America is unprecedented in severity and scale, at least during the historical period. The aim of this work is to develop a proxy-based methodology to understand how bark beetle disturbances are recorded in lake sediments. Three hypotheses are tested to determine how the ecological impacts of severe spruce beetle (Dendroctonus rufipennis) disturbances are recorded following mortality of Engelmann spruce (Picea engelmannii). Outbreaks are hypothesized to: (1) decrease the ratio of spruce to fir pollen; (2) increase soil erosion and mobilize terrestrial C; and (3) leach foliar N, enhancing algal productivity. To test these hypotheses, sediment cores from spruce beetle-affected basins were analyzed for pollen, insect remains, organic and minerogenic content, and isotopic and elemental concentrations. The dataset was tested statistically using generalized linear mixed models (GLMMs) to determine if the response variables differed significantly between outbreak and non-outbreak periods.

Salvage harvest effects on advance tree regeneration, soil nitrogen, and fuels following mountain pine beetle outbreak in lodgepole pine

The extent and severity of recent native bark beetle (Dendroctonae) outbreaks in western North America have created a pressing need for forest managers to understand potential consequences of post-disturbance management. For example, post-outbreak timber harvest (i.e., salvage harvest) could alter future forest development, productivity and susceptibility to subsequent disturbance. To assess the potential for such consequences, we measured first-year effects of post-outbreak timber harvest on tree regeneration, soil nitrogen (N) availability and fuels by using a paired and replicated before–after-control-impact (BACI) experimental design with eight pairs of 0.25-ha plots in beetle-killed lodgepole pine (Pinus contorta var. latifolia) in Greater Yellowstone (Wyoming, USA). Post-outbreak timber harvest reduced total (live +dead) lodgepole pine basal area by 90%. Total sapling density (advance regeneration) declined by about 50% following harvest, with tall (30–140cm) saplings declining most, but mean post-harvest sapling density still exceeded 1600 stems ha−1. Relative species density was unaffected and remained dominated by lodgepole pine. Soil temperature at the litter–soil interface was warmer during summer in harvested stands, and soil NO3- concentration increased with harvest relative to untreated plots. Soil NH4+ concentration and resin bag N accumulation increased through time in all beetle-killed plots and were not affected by harvest. Following harvest, dead woody surface fuels in all size categories doubled, and canopy fuel load and canopy bulk density both were reduced; dead fuel depth, duff depth, and canopy base height did not differ between untreated and harvested plots. Harvest did reduce canopy fuels, but the natural progression of needle shedding after beetle-kill accounted for 25–40% of this total canopy fuel reduction. Salvage harvest seems unlikely to alter post-outbreak successional trajectories in these lodgepole pine forests. However, the altered fuel complex (immediate increase in dead woody surface fuels and expected long-term reduction in large-diameter fuels) in harvested plots could cause subsequent fire behavior and effects to differ between harvested and untreated stands.

Temperature-driven range expansion of an irruptive insect heightened by weakly coevolved plant defenses

Warming climate has increased access of native bark beetles to high-elevation pines that historically received only intermittent exposure to these tree-killing herbivores. Here we show that a dominant, relatively naïve, high-elevation species, whitebark pine, has inferior defenses against mountain pine beetle compared with its historical lower-elevation host, lodgepole pine. Lodgepole pines respond by exuding more resin and accumulating higher concentrations of toxic monoterpenes than whitebark pine, where they co-occur. Furthermore, the chemical composition of whitebark pine appears less able to inhibit the pheromonal communication beetles use to jointly overcome tree defenses. Despite whitebark pine's inferior defenses, beetles were more likely to attack their historical host in mixed stands. This finding suggests there has been insufficient sustained contact for beetles to alter their complex behavioral mechanisms driving host preference. In no-choice assays, however, beetles readily entered and tunneled in both hosts equally, and in stands containing less lodgepole pine, attacks on whitebark pines increased. High-elevation trees in pure stands may thus be particularly vulnerable to temperature-driven range expansions. Predators and competitors were more attracted to volatiles from herbivores attacking their historical host, further increasing risk in less coevolved systems. Our results suggest cold temperatures provided a sufficient barrier against herbivores for high-elevation trees to allocate resources to other physiological processes besides defense. Changing climate may reduce the viability of that evolutionary strategy, and the life histories of high-elevation trees seem unlikely to foster rapid counter adaptation. Consequences extend from reduced food supplies for endangered grizzly bears to altered landscape and hydrological processes.

Organic, elemental, and geochemical contributions to lake sediment deposits during severe spruce beetle (Dendroctonus rufipennis) disturbances

Recent outbreaks of native bark beetles are unprecedented during the historical period. The aim of this manuscript is to develop a proxy-based methodology to infer past bark beetle outbreaks using lake sediments to provide long-term context for recent outbreaks. We test three hypotheses to determine how the ecological impacts of severe spruce beetle (Dendroctonus rufipennis Kirby) disturbances are recorded in lake sediment deposits. The resulting mortality and defoliation of Engelmann spruce is hypothesized to: (1) decrease the ratio of spruce to fir pollen; (2) reduce canopy interception of precipitation leading to an increase in soil erosion and/or enhanced mobilization of terrestrial carbon; and (3) leach foliar nitrogen and enhance algal productivity resulting in increased nitrogen values in lake sediments. To test these hypotheses, we analyzed sediment cores from six spruce beetle-affected basins in Utah for spruce/fir pollen ratio (hypothesis 1), loss-on-ignition, magnetic susceptibility of sediments (hypothesis 2), and δ13CBOM, δ15NTN, elemental C and N, and the C/N ratio of bulk organic material (hypotheses 2 and 3). The dataset was statistically tested using general linear mixed models (GLMMs) to determine if the response variables differed significantly between outbreak and non-outbreak period. The spruce/fir pollen ratio responded significantly to outbreaks at all sites suggesting that this metric may be the most suitable for identifying past spruce beetle outbreaks. For our second hypothesis we found little support for an influx of terrestrial C due to strongly individualist responses of the basins. For our third hypotheses we found little support for increased sedimentary nitrogen, likely due to alterations to nutrient cycling from human activities. Therefore the host/non-host pollen ratio provides the most promising metric for detecting past outbreaks.

Reconstructing the biogeochemical consequences of disturbances

Paleo Reconstructions of Biogeochemical Environments (PROBE) Workshop;Manhattan, Kansas; 19–21 April 2012 Disturbances—discrete events that reduce plant biomass—commonly regulate material and energy flow in terrestrial ecosystems. Recent studies document an increase in the size and/or severity of disturbances such as native bark beetle outbreaks and large fires compared to the recent past. However, scientists cannot evaluate the potential consequences of these events for ecosystem dynamics without decadal to multimillennial records of disturbances and ecosystem response. The Paleo Reconstructions of Biogeochemical Environments (PROBE) workshop brought together ecosystem ecologists and paleoecologists for a 3‐day workshop at the Konza Prairie Biological Station in Manhattan, Kansas. The focus of the meeting was the reconstruction of the biogeochemical consequences of disturbances (e.g., beetle outbreaks, wildfires, windstorms, and droughts) on different timescales, the assessment of the state of current knowledge, and identification of challenges and opportunities for future research.

The Relative Abundance of Mountain Pine Beetle Fungal Associates Through the Beetle Life Cycle in Pine Trees

The mountain pine beetle (MPB) is a native bark beetle of western North America that attacks pine tree species, particularly lodgepole pine. It is closely associated with the ophiostomatoid ascomycetes Grosmannia clavigera, Leptographium longiclavatum, Ophiostoma montium, and Ceratocystiopsis sp.1, with which it is symbiotically associated. To develop a better understanding of interactions between beetles, fungi, and host trees, we used target-specific DNA primers with qPCR to assess the changes in fungal associate abundance over the stages of the MPB life cycle that occur in galleries under the bark of pine trees. Multivariate analysis of covariance identified statistically significant changes in the relative abundance of the fungi over the life cycle of the MPB. Univariate analysis of covariance identified a statistically significant increase in the abundance of Ceratocystiopsis sp.1 through the beetle life cycle, and pair-wise analysis showed that this increase occurs after the larval stage. In contrast, the abundance of O. montium and Leptographium species (G. clavigera, L. longiclavatum) did not change significantly through the MPB life cycle. From these results, the only fungus showing a significant increase in relative abundance has not been formally described and has been largely ignored by other MPB studies. Although our results were from only one site, in previous studies we have shown that the fungi described were all present in at least ten sites in British Columbia. We suggest that the role of Ceratocystiopsis sp.1 in the MPB system should be explored, particularly its potential as a source of nutrients for teneral adults.

Exploring associations between international trade and environmental factors with establishment patterns of exotic Scolytinae

Although invasion of exotic ambrosia beetles (fungus feeders) and bark beetles (phloem feeders) (Coleoptera: Curculionidae: Scolytinae) is considered a major threat to forest health worldwide, no studies have quantitatively investigated the anthropogenic and environmental factors shaping the biogeographical patterns of invasion by these insects across large spatial scales. The primary aim of this study was to assess the relative importance of international trade and several environmental variables of the recipient region on species richness of established exotic Scolytinae. As a reference, we also evaluated the relationships between the same environmental variables and species richness of native Scolytinae. Using an information-theoretic framework for model selection and hierarchical partitioning, we evaluated the relative importance of the potential drivers of species richness of native and exotic Scolytinae in 20 European countries and the 48 contiguous continental US states. Analyses were conducted separately for ambrosia and bark beetle species. Value of imports was a strong predictor of the number of exotic Scolytinae species in both regions. In addition, in the USA, warmer and wetter climate was positively linked to increased numbers of both native and exotic ambrosia beetles. Forest heterogeneity and climatic heterogeneity and secondarily forest area were key drivers in explaining patterns of species richness for native bark beetles but not for exotic species in both regions. Our findings suggest that if current infestation levels continue on imported plants and wood packaging material, increasing international trade will likely lead to more establishments of exotic Scolytinae with concomitant negative effects on forest health in both Europe and the USA. Compared to Europe the risk of invasion appears higher in the USA, especially for ambrosia beetles in the southeastern USA where the climate appears highly suitable for exotic establishment.

Evaluation of Insecticides for Control of Overwintering Hylurgopinus rufipes (Coleoptera: Curculionidae)

A bark disk bioassay was used to assess the effectiveness of different insecticides for control of overwintering adult native elm bark beetles, Hylurgopinus rufipes (Eichoff) (Coleoptera: Curculionidae). Insecticides were applied to the base of the trunks of living American elm trees, Ulmus americana L., and bark disk samples were removed at intervals with a hole saw. Adult H. rufipes were exposed in bioassay chambers for which the bark disks formed the floor of the chamber. In September 2005, chlorpyrifos, permethrin, and a water control were applied to trees in five replicated blocks, and bark disk samples were taken at intervals over the following 725 d. In September 2006, chlorpyrifos, permethrin, carbaryl, bifenthrin, and a water control were applied to trees in six replicated blocks, and disk samples were removed at intervals up to 1,163 d after application. In both experiments, beetle mortality was 100% on chlorpyrifos-treated disks removed up to 725 d after treatment and, in the 2006 trial, declined thereafter. The pattern of mortality in the bifenthrin treatment did not differ significantly from that in the chlorpyrifos treatment in the 2006 experiment. Average corrected mortality on disks treated with permethrin or carbaryl declined below 100% by 11 d after application. On disks removed in the May following treatment, 248 d after application, average corrected mortality was 42-44% on permethrin-treated disks and 26% on carbaryl-treated disks.

Efficacy of tree defense physiology varies with bark beetle population density: a basis for positive feedback in eruptive species

We evaluated the ability of constitutive and inducible defenses to protect trees and restrict herbivore reproduction across the endemic, incipient (i.e., transitory), and eruptive phases of a native bark beetle species. Host defenses were major constraints when mountain pine beetle (Dendroctonus ponderosae Hopkins) populations were low, but inconsequential after stand-level densities surpassed a critical threshold. We annually examined all lodgepole pines (Pinus contorta Douglas var. latifolia) in six 12–18 ha stands for 3–6 years for beetle attack and establishment as beetle densities progressed through various population phases. We also assayed a suite of tree physiological and chemical attributes and related them to subsequent attacks during that year. Rapidly inducible defenses appeared more important than constitutive defenses, and total monoterpenes were more important than particular constituents. Trees that exude more resin and accumulate higher monoterpene concentrations in response to simulated attack largely escaped natural attacks when populations were low. In stands where beetles had reached incipient densities, these defenses were ineffective. Larger diameter trees had more pronounced defenses than smaller diameter trees. As populations increased, beetles selected increasingly larger, more resource-rich trees, despite their better defenses. When populations were too low for cooperative attack, beetles exploited trees weakened by lower-stem insects. Behavioral plasticity allows beetles to persist at endemic levels until conditions shift, after which positive feedbacks predominate.

The Influence of Forest Management on Future Forest Structure Following a Moutain Pine Beetle Outbreak in Lodgepole Pine Stands in Northwestern Wyoming

Forest managers in the western U.S. are currently confronted with a bark beetle epidemic that is unprecedented in extent, severity, and duration. Several species of native bark beetle including the mountain pine beetle (MPB), spruce beetle, western balsam bark beetle, and Douglas-fir beetle are simultaneously affecting over 5 million ha of forest in the Intermountain West (Logan et al. 2003, Hicke et al. 2006). Estimates of the total area affected by the current bark beetle outbreak are updated annually, typically using aerial surveillance and manual mapping techniques. However, these estimates are often inaccurate and do not provide quantitative information on stand-level beetle activity and/or tree mortality. More importantly, these annual estimates of “spread” of the epidemic do not provide any information on the abundance of the surviving understory trees, which may be substantial, nor do they provide information on post-disturbance tree seedling establishment. This “advance forest regeneration” represents the future forests in these regions. Therefore, understanding the current structure of the forests is critical for making predictions about future structure and function.

Climate Change and Bark Beetles of the Western United States and Canada: Direct and Indirect Effects

Climatic changes are predicted to significantly affect the frequency and severity of disturbances that shape forest ecosystems. We provide a synthesis of climate change effects on native bark beetles, important mortality agents of conifers in western North America. Because of differences in temperature-dependent life-history strategies, including cold-induced mortality and developmental timing, responses to warming will differ among and within bark beetle species. The success of bark beetle populations will also be influenced indirectly by the effects of climate on community associates and host-tree vigor, although little information is available to quantify these relationships. We used available population models and climate forecasts to explore the responses of two eruptive bark beetle species. Based on projected warming, increases in thermal regimes conducive to population success are predicted for Dendroctonus rufipennis (Kirby) and Dendroctonus ponderosae Hopkins, although there is considerable spatial and temporal variability. These predictions from population models suggest a movement of temperature suitability to higher latitudes and elevations and identify regions with a high potential for bark beetle outbreaks and associated tree mortality in the coming century.

A Literal Use of “Forest Health” Safeguards against Misuse and Misapplication

A Literal Use of “Forest Health” Safeguards against Misuse and Misapplication