Beetle Outbreaks Research Articles

Different triggers, different stories: Bark-beetle infestation patterns after storm and drought-induced outbreaks

In the recent decades, Norway spruce forests (Picea abiesKarst.) in Europe have been subject to large-scale tree mortality caused by the spruce bark beetle (Ips typographusL.). The outbreaks were induced by storm-felling events and periods of drought, which are becoming increasingly frequent due to climate change. Because storms and droughts spatially affect forests differently, the infestation patterns and configuration of the bark beetles might differ between storms and drought. In this study, we examined local and landscape factors associated with bark beetle-caused tree mortality after one storm (2005) and one drought-induced spruce bark beetle outbreak (2018), both occurring in southern Sweden. A total of 13,192 infested one-ha pixels after the storm and 6,425 one-ha pixels after the drought (in total 19,617) were compared regarding differences in infestation occurrence and size and associated forest structures and climate between the two different outbreaks, using a generalized linear model (GLM) approach. Based on our findings, we discovered that the allocation of infestation patch sizes (including four classes: 5–10, 11–25, 26–50 and >50 infested trees) for the two outbreaks were quite similar with a large proportion (>0.6) of small groups (≤10 trees).However, the outcomes from this study demonstrate that the drivers behind the spatial configuration of bark beetle infestations can differ considerably between outbreaks triggered by storms and droughts, and the main cause seems to be linked to the spatial distribution of susceptible trees. The most consistent differences for both occurrence and infestation size were that storm-induced infestations increased more with spruce volumes and area of protected forests (nature reserves) in the landscape; whereas for the drought-induced infestations, occurrence and size increased more with clear-cuts in the landscape and spruce heights across spatial scales. Soil moisture and mean drought index (SPEI; May-July) were important for both outbreaks, but generally more important for the infestation sizes after droughts than after storms and may involve a time-lagged effect.The reasoning behind the differences between storms and droughts may be that during storm-induced outbreaks, when the wind-felled trees are removed or not suitable anymore, bark beetles need to find specific susceptible standing trees, while after drought all trees are more or less stressed, which results in a selection of large trees in dry and warm landscapes as they have more resources and favorable reproduction conditions. Finally, we show that the previous infestation size influenced the later infestation size negatively within landscapes of 25 ha and this seems to be related to depletion of susceptible host trees.These results are important for the assessment of more specific outbreak predictions, which should be integrated in future risk mapping of bark beetle outbreaks.

Drought occurrence and forest management: Interrelated drivers of sanitary logging in central México temperate forests

Drought exacerbates forest pest outbreaks, causing widespread tree mortality. In México, to control bark beetle outbreaks, mandatory sanitary logging is carried out by the landowners who can use or sell the resulting wood. We analyzed the temporal trend of reported bark beetle outbreaks and their relationship to drought conditions; and assessed the intensity of the sanitary logging in relation to land tenure type and the pre-existing Forest Management Program (FMP) as a proxy to determine whether sanitary logging is being used only for its primary purpose: bark beetle population control. We used official records of bark beetle outbreaks (n = 11,151) on four major conifer species (Abies religiosa, Pinus hartwegii, P. pseudostrobus, and P. oocarpa) that triggered sanitary logging authorizations (n = 1,968) between 2009 and 2021 in two highly affected states: Michoacán and Estado de México. Results showed that reported bark beetle outbreaks increased from an average of 38 in 2009 to 1,051 in 2021, and were associated with droughts that occurred up to two years earlier. More severe droughts were recorded in Estado de México, but more sanitary logging authorizations were issued in Michoacán (829 and 1,139, respectively). In Michoacán, the area logged was smaller, but the sanitary logging was more intensive (2,623 ha, x̅ 139 m3/ha of wood extracted) compared to Estado de México (9,227 ha, x̅ 38 m3/ha of wood extracted). These sanitary logging interventions occurred equally on communal and private lands, but from 58% (Michoacán) to 69% (Estado de México) occurred on properties without a pre-existing FMP. While the results suggest that in some areas, particularly in Michoacán, sanitary logging may serve as a legal pass to harvest timber, further research is needed on ecological and social issues, such as: forest characteristics that may influence beetle reproduction and dispersal, local pest management practices, forest governance and conflicts, and the effectiveness of technical assistance and monitoring to control outbreaks, among others.

Physiological and biochemical indicators in Norway spruces freshly infested by Ips typographus: potential for early detection methods

IntoductionThe bark beetle Ips typographus currently represents the primary pest of Norway spruce (Picea abies) in Central Europe. Early detection and timely salvage cutting of bark beetle-infested trees are functional management strategies for controlling bark beetle outbreaks. However, alternative detection methods are currently being developed, and possible indicators of bark beetle infestation can be assessed through changes in the physiological, biochemical, and beetle-acceptance characteristics of trees.MethodThis study monitored infested and non-infested Norway spruce trees before and 3 weeks after Ips typographus natural attack. Permanently installed sensors recorded physiological features, such as sap flow, tree stem increment, bark surface temperature, and soil water potential, to monitor water availability. Defensive metabolism characteristics, beetle host acceptance, and attractiveness to trees were monitored discretely several times per season. The forest stand that was later attacked by bark beetles had lower water availability during the 2018–2020 seasons compared to the non-attacked stands.ResultsAfter the attack, sap flow and tree stem increment were significantly lower in infested trees than in intact ones, and bark surface temperature moderately increased, even when measured in the inner forest stand from the shadowed side. Infested trees respond to attacks with a surge in monoterpene emissions. In addition, freshly infested trees were more accepted by males in the no-choice bioassays, and a significantly higher number of beetles were caught in passive traps in the first week of infestation.ConclusionThe most promising characteristics for early detection methods of bark beetle-infested trees include tree bark temperature measured only in certain meteorological conditions, elevated monoterpene emissions, and significantly high catches in passive traps.

Mechanical properties of Norway spruce (Picea abies) infested by the bark beetle (Ips typographus)

ABSTRACT This study considered the mechanical properties of primary blue-stained sapwood of Norway spruce infested by the bark beetle Ips typographus. Nineteen stems from Stammerberg (ZH, Switzerland) were harvested in three different seasons and after different periods of bark beetle outbreaks. Density, Brinell hardness including elastic recovery after load, and key strength properties were tested on blue-stained wood and non-infected controls. Differences in mechanical properties were analysed according to the stand and storage time of the stems after the bark beetle attack. Spruce harvested at a late stage after the bark beetle attack (2–3 months) showed a clear reduction in impact bending strength, a significant loss in surface elastic recovery after indentation during Brinell hardness test, and a striking number of specimens with brittle fracture behaviour during Three-Point-Bending. Light microscopy and the results of the mechanical tests give clear evidence to suggest that wood-degrading fungi are already active in addition to the sap-staining fungi. Especially after larger outbreaks of bark beetles, it must be assumed that sapwood of Norway spruce does not only show the typical blue stain, but also incipient stages of wood degradation by wood-degrading fungi.

Genomic virulence features of Beauveria bassiana as a biocontrol agent for the mountain pine beetle population

BackgroundThe mountain pine beetle, Dendroctonus ponderosae, is an irruptive bark beetle that causes extensive mortality to many pine species within the forests of western North America. Driven by climate change and wildfire suppression, a recent mountain pine beetle (MPB) outbreak has spread across more than 18 million hectares, including areas to the east of the Rocky Mountains that comprise populations and species of pines not previously affected. Despite its impacts, there are few tactics available to control MPB populations. Beauveria bassiana is an entomopathogenic fungus used as a biological agent in agriculture and forestry and has potential as a management tactic for the mountain pine beetle population. This work investigates the phenotypic and genomic variation between B. bassiana strains to identify optimal strains against a specific insect.ResultsUsing comparative genome and transcriptome analyses of eight B. bassiana isolates, we have identified the genetic basis of virulence, which includes oosporein production. Genes unique to the more virulent strains included functions in biosynthesis of mycotoxins, membrane transporters, and transcription factors. Significant differential expression of genes related to virulence, transmembrane transport, and stress response was identified between the different strains, as well as up to nine-fold upregulation of genes involved in the biosynthesis of oosporein. Differential correlation analysis revealed transcription factors that may be involved in regulating oosporein production.ConclusionThis study provides a foundation for the selection and/or engineering of the most effective strain of B. bassiana for the biological control of mountain pine beetle and other insect pests populations.

The coconut rhinoceros beetle (Oryctes rhinoceros) outbreak is well established on Efate, Vanuatu

AbstractThe presence of coconut rhinoceros beetle (CRB: Oryctes rhinoceros) was reported on Efate, Vanuatu in 2019. The beetle population was determined to belong to the CRB‐S group (Clade II), similar to populations found in the Bismark Archipelago, Papua New Guinea. An introduction of the CRB‐G group (Clade IA) was discovered in 2021. Damage mapping, beetle trapping and genomic identification show that CRB‐S has now spread around about half of the island from the site of the original introduction. CRB‐G is mainly restricted to the seaport and surrounding areas of Port Vila with some outlying specimens. Through the efforts of Biosecurity Vanuatu, O. rhinoceros is still contained on the Efate group of islands and has not spread to the major copra‐producing islands in other parts of the country. Sanitation efforts appear to have reduced damage from coconut rhinoceros beetle within the zone of first arrival of the pest (Mangaliliu, Efate). Oryctes rhinoceros damage, however, is severe in areas with old plantations and limited sanitation activities (e.g. Mele/Devil's point, Efate). A concerted effort is necessary to contain the beetle on Efate and to reduce further spread and impact on the island.

Differential impacts of spruce beetle outbreaks on snowshoe hares and red squirrels in the southern Rocky Mountains

Spruce beetles (Dendroctonus rufipennis) have impacted millions of acres of Engelmann spruce (Picea engelmannii) – subalpine fir (Abies lasiocarpa) forest in North America over the past decade, resulting in the most extensive outbreak in recorded history. This dramatic alteration of forest composition and structure has precipitated numerous changes to forest ecology and ecosystem services. Among the least studied of these changes are impacts to wild mammals, including snowshoe hares (Lepus americanus) and red squirrels (Tamiasciurus hudsonicus). We sampled a chronosequence of spruce-fir stands along a gradient of ‘years elapsed since spruce beetle outbreak’ (YSO) in order to estimate impacts to abundance of these two species in the southern Rocky Mountains. Snowshoe hare abundance was not related to YSO, at least in the first decade post-outbreak. Instead, hare abundance during this period was positively related to horizontal cover, especially that due to stem density of small diameter subalpine fir. Notably, snowshoe hare abundance was negatively related to stem density of small diameter Engelmann spruce, suggesting that elements of horizontal cover may not be uniformly beneficial to hares. Hare abundance was also negatively related to ground cover, which could help explain the lack of relationship to YSO, assuming reduction in overstory canopy would lead to increases in ground cover. Red squirrel abundance was negatively related to YSO and outbreak severity (i.e., basal area of large diameter dead trees). This was likely due to diminished cone crops in impacted areas, which red squirrels cache and rely on heavily to sustain them through the winter. Basal area of remaining large live fir trees was not related to squirrel abundance, suggesting that regeneration of spruce and associated cone crops may be necessary for recovery of red squirrels, which may take several decades.

Assessing Combinations of Landsat, Sentinel-2 and Sentinel-1 Time series for Detecting Bark Beetle Infestations

ABSTRACT Bark beetle infestations are among the most substantial forest disturbance agents worldwide. Moreover, as a consequence of global climate change, they have increased in frequency and in the size and number of affected areas. Controlling bark beetle outbreaks requires consistent operational monitoring, as is possible using satellite data. However, while many satellite-based approaches have been developed, the full potential of dense, multi-sensor time series has yet to be fully explored. Here, for the first time, we used all available multispectral data from Landsat and Sentinel-2, Sentinel-1 SAR data, and combinations thereof to detect bark beetle infestations in the Bavarian Forest National Park. Based on a multi-year reference dataset of annual infested areas, we assessed the separability between healthy and infested forests for various vegetation indices calculated from the satellite data. We used two approaches to compute infestation probability time series from the different datasets: Bayesian conditional probabilities, based on the best-separating index from each satellite type, and random forest regression, based on all indices from each satellite type. Five different sensor configurations were tested for their detection capabilities: Landsat alone, Sentinel-1 alone, Sentinel-2 alone, Landsat and Sentinel-2 combined, and data from all satellite types combined. The best overall results in terms of spatial accuracy were achieved with Sentinel-2 (max. overall accuracy: 0.93). The detections of Sentinel-2 also were the closest to the onset of infestation estimated for each year. Sentinel-2 detected infested areas in larger contiguous patches with higher reliability compared to smaller patches. The results achieved with Landsat were somewhat inferior to those of Sentinel-2 (max. accuracy: 0.89). While yielding similar results, the combination of Landsat and Sentinel-2 did not provide any advantages over using Landsat or Sentinel-2 alone (max. accuracy: 0.87), while Sentinel-1 was unable to detect infested areas (max. accuracy: 0.62). The combined data of all three satellite types did not achieve satisfactory results either (max. accuracy: 0.67). Spatial accuracies were typically higher for Bayesian conditional probabilities than for random forest-derived probabilities, but the latter resulted in earlier detections. The approach presented herein provides a flexible disturbance detection pipeline well-suited for the monitoring of bark beetle outbreaks. Furthermore, it can also be applied to other disturbance types.

Sharing economy in the forestry sector: Opportunities and barriers

In the past years, the popularity of a sharing economy has increased, frequently in relation to accommodation and car sharing. There are many positive aspects to sharing, especially in cost reduction. The subject of our research was to find out whether a sharing economy and its principles have their place in the forestry sector, using the example of the Czech Republic. Three research questions were set to evaluate the potential of the sharing economy. To answer them, a mixed approach of quantitative and qualitative methods was used. These were represented by an online questionnaire survey and semi-structured interviews with representatives of the management of forestry companies. The data were further evaluated and analysed statistically. The main findings are that the representatives of companies from forestry are aware of the sharing economy from different economic fields. The principles of a sharing economy are perceived positively; however, there are barriers that currently prevent implementing such principles in forestry in the Czech Republic. This is primarily due to time and regional factors, salvage logging processing, ownership structures and the outsourcing of the main forestry operations. The great potential of a sharing economy lies in solving crises in forestry (bark beetle outbreak, lack of workforce and resources, etc.). The research results may bring changes in support for the forestry sector from European funds, especially in the planned area of cooperation between companies.

Exploring the potential to use in-between pixel variability for early detection of bark beetle attacked trees

Abstract. The European spruce bark beetle (Ips typographus L.) is a major disturbance agent in Norway spruce (Picea abies (L.) Karst) forests in Europe and it is estimated that a changing climate will result in more severe outbreaks in the future. To reduce the risk of large outbreaks it is important to have methods that enable early detection of bark beetle attacks to help forest managers to prevent population build-up, e.g by sanitary cutting. Several studies have been devoted to early detection of bark beetle attacks with Sentinel-2 data with a focus on spectral properties and vegetation indices for early detection with pixel-based methods. In this study we explore the potential to use changes in variability between pixels in windows of different sizes (3×3, 4×4 and 5×5 pixels). We compute the coefficient of variation for four vegetation indices (NDVI, NDWI, CCI and NDRS) in a time-series of Sentinel-2 data during a bark beetle outbreak in Sweden that was triggered by a drought in 2018. The results indicate that CCI is the most promising index for early detection and that the variability between pixels increase in windows with attacked trees from late July when the main swarming was the second week of May.

Spruce Bark Beetle Outbreak Prediction through Automatic Classification of Sentinel-2 Imagery

Extreme weather events are increasing in frequency and intensity, posing a threat to forest ecosystems and eliciting forest-pest outbreaks. In the southern Italian Alps, a dramatic windthrow called Vaia occurred in October 2018, shifting populations of the European spruce bark beetle (Ips typographus) from an endemic to an epidemic phase. Remote-sensing methods are often employed to detect areas affected by disturbances, such as forest-pest outbreaks, over large regions. In this study, a random forest model on the Sentinel-2 images acquired over the south-eastern Alps in 2021 and 2022 was used to detect the outbreak spots. The automatic classification model was tested and validated by exploiting ground data collected through a survey conducted in 2021 and 2022 in both healthy and infested spots, characterized by variable sizes and degrees of infestation. The model correctly identified the forest conditions (healthy or infested) with an overall accuracy of 72% for 2022 and 58% for 2021. These results highlight the possibility of locating I. typographus outbreaks, even in small spots (between 5 and 50 trees) or spots intermixed with healthy trees. The prompt detection of areas with a higher frequency of outbreaks could be a useful tool to integrate field surveys and select forest areas in which to concentrate management operations.

Tree canopy patterns in a temperate forest in Oaxaca, Mexico after bark beetle outbreaks: Implications for ecological resistance and resilience

Bark beetles (BB) are an insect that can become a forest pest and that principally affects temperate regions globally, massively killing tree hosts, mainly coniferous species. When tree hosts are the dominant species in the canopy, BB outbreaks may impose important changes in the structure, composition and diversity of forests, from the stand to the regional scale. In Mexico, during the current century, BB pests have become the main biotic threat in forests where Pinus species are dominant. Questions addressed in this study are: What is the structure, composition and diversity of the forest canopy after BB disturbances? and “What are the implications for ecological resistance and resilience of the forest”? The study site was located in the Sierra Norte region in Oaxaca, Mexico, where more a decade ago BB outbreaks affected the forest. In ninety plots of 500 m2 (4.5 ha) all live trees with ≥ 5 cm diameter at breast height (DBH) were located, their DBH was measured, and the species was identified. As well, all stumps resulting from previous sanitation logging and timber extraction (TE) were located and for each condition, basal diameter was estimated and the genera identified. For the post-disturbance canopy, density basal area, species composition and diversity (alpha and beta) were estimated. This data plus the analysis of the stumps was used to estimate the tree canopy structure before the BB outbreaks. Statistical comparisons among BB, TE disturbances and condition without disturbance were conducted. Based on interviews with local experts, a rank of risk to BB among Pinus species was established. A total of 4,053 live trees and 547 stumps were found. The live trees corresponded to 24 species from 8 genera. Pinus genus and, particularly P. patula predominated. Seventy-five percent of the individuals had < 20 cm DBH and a J-inverted diametric distribution resulted. Individuals with DBH ≥ 100 cm were found only in the condition without disturbance. Three of the nine Pinus species, were BB main-host species, and the least preferred host was P. ayacahuite. The three BB main-host species dominated the post-disturbance canopy (including regeneration). In contrast, P. ayacahuite and other non-host tree species dominated in the conditions without disturbance, and the three main-host species showed low density. As well, high species richness occurred in areas affected by BB. Since climate change will likely continue to exacerbate BB outbreaks, it is proposed that to increase ecological resistance and resilience forest management practices should focus on reducing density, promoting a mix of host and non-host species, increasing tree diversity and avoiding stands with monodominance of main-host species.

Ecological factors affecting the recent Picea abies decline in Slovenia: the importance of bedrock type and forest naturalness

Norway spruce (Picea abies [L.] Karst.) has been at the centre of controversy for many decades. Recent evidence of its profound disturbance-induced damage and consequent stock depletions across forest landscapes in Europe has reinforced doubts regarding the sustainability and prospects of this tree species in the future. Like many other European countries, Slovenia has experienced significant Norway spruce mortality and a decrease in growing stock primarily as the result of several disturbance agents (bark beetle outbreaks, an ice storm, windthrows). We investigated a countrywide spruce growing stock decline based on data between 2010 and 2018. Particular focus was placed on identifying the main ecological drivers of this decline, namely geological conditions, climatic parameters, soil attributes, topographic factors and forest stand characteristics. The effects of potential predictors on the relative change (%) in spruce volume (m3 ha-1) during the period 2010-2018 were analysed with Generalized Additive Models. Based on a national dataset including forest compartments (n = 6355) with a spruce growing stock decline > 10%, we found mixed support for ecology-based hypotheses. While spruce decline responded to bedrock type as predicted (i.e., greater relative decline in carbonate compared to silicate compartments), higher forest naturalness (preservation of tree species composition) was not associated with a lower decline. Spruce decline was amplified by higher potential evapotranspiration and soil clay content but showed a strong negative relationship with spruce proportion in the year 2010. General trends along the gradients of other selected predictors (stoniness/rockiness and heat load index) were less pronounced. The results suggest that most of these ecological predictors interact with geology and forest naturalness in affecting Norway spruce decline. Our analysis reveals that bedrock type can play an important role due to its mitigating effects. However, forest naturalness is of secondary significance as intensified large-scale forest disturbances likely override its buffering potential.

Tree aboveground biomass increment and mortality in temperate mountain forests: Tracing dynamic changes along 25-year monitoring period

Tree aboveground biomass and its increment are sensitive forest and landscape dynamics indicators. Although various methods can measure forest biomass dynamics, only in situ, direct monitoring can substantially reduce the uncertainty related to unknown or uncontrolled factors. Within the present study, we aimed to isolate drivers of tree biomass increment, mortality, and recruitment in the Gorce National Park (GNP), Outer Western Carpathians, Poland. We used a unique database consisting of information collected between 1992 and 2017 (in 5-year intervals) within 389 permanent monitoring plots regularly distributed in the GNP area. This allowed us to calculate tree biomass and its increments and model them using a set of explanatory variables: proportion of particular tree species, mean temperature of the coldest quarter (bio11), mean precipitation of the warmest quarter (bio18), elevation, topographic wetness index (TWI), stand basal area (BA), diameter heterogeneity expressed by the coefficient of variation (DBH CV), and conservation regime. We applied generalized linear mixed-effects models (GLMMs), assuming the beta distribution of response variables, i.e. biomass proportions of increment, recruitment, and mortality of three main tree species: Norway spruce Picea abies (L.) Karst, European beech Fagus sylvatica L., and silver fir Abies alba Mill.In a large part of the GNP area, tree biomass increased. In the central and southern parts of the park, tree mortality was higher than in other parts, especially between 2002 and 2007, due to bark beetle outbreaks and intense wind damage. Stand dynamics of all species depended mainly on species proportion in stand biomass. The increment of Abies alba increased with TWI but decreased with BA and DBH CV. Recruitment decreased with BA and slope but increased with TWI and DBH CV, while mortality decreased with DBH CV and TWI. For Fagus sylvatica, increment increased with TWI but decreased with BA, elevation, and bio18, while recruitment decreased with BA and increased with TWI. Mortality decreased with DBH CV, bio11, and TWI but increased with BA. Picea abies increment increased with elevation but decreased with BA, slope, and DBH CV and weakly increased with bio11. Recruitment increased with TWI but decreased with BA and DBH CV, while mortality decreased with DBH CV, BA, and bio18. Water retention at the plot level, approximated by TWI, decreased mortality and increased recruitment of the studied species. Therefore, it can serve as an indicator of suitable microsites for their persistence. In addition, the increment was lower in strictly protected forests, while mortality was higher. Our study provided quantitative evidence of how climate, geomorphology, and forest stand characteristics modify stand dynamics.

Model & Data Files for Analyses in "Differential impacts of spruce beetle outbreaks on snowshoe hares and red squirrels in the southern Rocky Mountains"

Model & Data Files for Analyses in "Differential impacts of spruce beetle outbreaks on snowshoe hares and red squirrels in the southern Rocky Mountains"

Bark beetle impacts on forest evapotranspiration and its partitioning

Insect outbreaks affect forest structure and function and represent a major category of forest disturbance globally. However, the resulting impacts on evapotranspiration (ET), and especially hydrological partitioning between the abiotic (evaporation) and biotic (transpiration) components of total ET, are not well constrained. As a result, we combined remote sensing, eddy covariance, and hydrological modeling approaches to determine the effects of bark beetle outbreak on ET and its partitioning at multiple scales throughout the Southern Rocky Mountain Ecoregion (SRME), USA. At the eddy covariance measurement scale, 85 % of the forest was affected by beetles, and water year ET as a fraction of precipitation (P) decreased by 30 % relative to a control site, with 31 % greater reductions in growing season transpiration relative to total ET. At the ecoregion scale, satellite remote sensing masked to areas of >80 % tree mortality showed corresponding ET/P reductions of 9–15 % that occurred 6–8 years post-disturbance, and indicated that the majority of the total reduction occurred during the growing season; the Variable Infiltration Capacity hydrological model showed an associated 9–18 % increase in the ecoregion runoff ratio. Long-term (16–18 year) ET and vegetation mortality datasets extend the length of previously published analyses and allowed for clear characterization of the forest recovery period. During that time, transpiration recovery outpaced total ET recovery, which was lagged in part due to persistently reduced winter sublimation, and there was associated evidence of increasing late summer vegetation moisture stress. Overall, comparison of three independent methods and two partitioning approaches demonstrated a net negative impact of bark beetles on ET, and a relatively greater negative impact on transpiration, following bark beetle outbreak in the SRME.

When tree diversity does not work: The interacting effects of tree diversity, altitude and amount of spruce on European spruce bark beetle outbreaks

The frequency of large-scale insect outbreaks is expected to increase with climate change, and the European spruce bark beetle (Ips typographus) is a major forest pest in Europe that has caused more extensive and widespread outbreaks in recent years. It is therefore important to evaluate forest management strategies for controlling bark beetle populations. While tree species diversity is often promoted as a way to improve forest resistance to specialist insects, the relationship between tree species diversity and bark beetle outbreaks is not well understood, partly due to the complex interactions with other factors. The aim of this study was to assess the impact of tree species diversity on European spruce bark beetle outbreaks, while controlling for host availability across different altitudes. We analysed data from a systematic 1 × 1 km grid in Slovenia. For each grid cell, we extracted data on the sanitary felling of Norway spruce that followed bark beetle outbreaks from a national data base covering the period 1996–2017. Additionally, we extracted data on resource availability (i.e., the percentage of Norway spruce in the grid cell), non-host tree species diversity and average altitude. We analysed the effect of altitude, forest cover, percentage of spruce and non-host tree species diversity during outbreak and non-outbreak periods on the probability of sanitary felling and on the amount of sanitary felling, separately. The probability of sanitary felling was higher at low altitudes and increased with an increasing proportion of spruce in the growing stock in the grid cell, but it was not affected by non-host tree species diversity. In contrast, the amount of sanitary felling decreased with increasing non-host tree species diversity, particularly at low altitudes. However, the effect of non-host tree species diversity on the amount of sanitary felling diminished as the percentage of Norway spruce increased. Thus, non-host tree species diversity per se is not the main driver of outbreak risk but strengthens biotic resistance with lower host availability at low altitudes where abiotic conditions are the least favourable to Norway spruce.

Deadwood: People, place, and neoliberal forest policy in British Columbia, Canada

This article describes the people and forests in traditional Secwépemc territory in the Interior Plateau of British Columbia (BC), Canada. It examines the intersection of neoliberal forest policy and profound ecological change in the south-central region of the province, with careful attention paid to the wider contexts of settler colonialism. The article critiques the implementation of forest policy reform (the Forest and Range Practices Act, enacted in 2004) as it coincided with one of the first major climate change events of landscape-altering magnitude in BC – the mountain pine beetle outbreak. Between 1998 and 2014, the outbreak resulted in tree mortality for roughly half of the mature lodgepole pine trees in the province. It left an expanse of ‘deadwood’ on the landscape, leading to heightened social and political conflict over how the beetle-affected areas were managed. Research methods include transcripts from a public hearing in Tk’emlúps/Kamloops, supplemented by eighteen semi-structured interviews. The article argues that the confluence of neoliberal forest policy and climate change has further entrenched the settler colonial and corporate capture of forests.

Fuels change quickly after California drought and bark beetle outbreaks with implications for potential fire behavior and emissions

BackgroundAn extreme drought from 2012–2016 and concurrent bark beetle outbreaks in California, USA resulted in widespread tree mortality. We followed changes in tree mortality, stand structure, and surface and canopy fuels over four years after the peak of mortality in Sierra mixed conifer and pinyon pine (Pinus monophylla) forests to examine patterns of mortality, needle retention after death, and snag fall across tree species. We then investigated how the tree mortality event affected surface and canopy fuel loading and potential impacts on fire hazard and emissions.ResultsDrought and beetle-related tree mortality shifted mortality patterns to be more evenly distributed across size classes and concentrated in pines. Substantial changes to surface fuel loading, stand density, canopy fuel loads, and potential wildfire emissions occurred within four years following peak levels of tree mortality, with the largest changes related to increases in coarse woody debris. Nearly complete needle fall occurred within four years of mortality for all species except red fir (Abies magnifica). Pine species and incense cedar (Calocedrus decurrens) snags fell more quickly than fir species. Potential fire behavior modelling suggested that crowning and torching hazard decreased as trees dropped dead needles and fell, but as canopy fuels were transferred to surface fuels, potential for smoldering combustion increased, causing greater emissions.ConclusionsOur study increases understanding of how extreme tree mortality events caused by concurrent disturbances alter canopy and surface fuel loading and have the potential to affect fire behavior and emissions in two compositionally different seasonally dry forest types. After a major tree mortality event, high canopy fuel flammability may only last a few years, but surface fuels can increase considerably over the same time period in these forest types. The accumulation of coarse woody surface fuels resulting from multi-year drought and concurrent bark beetle outbreaks combined with the increasing frequency of drought in the western U.S. have the potential to lead to heavy and dry fuel loads that under certain weather conditions may result in more extreme fire behavior and severe effects, particularly in forest types where decades of successful fire suppression has caused forest densification.

Carbon stored in live ponderosa pines in the Sierra Nevada will not return to pre-drought (2012) levels during the 21st century due to bark beetle outbreaks

Outbreaks of several bark beetle species can develop rapidly in response to drought and may result in large transfers of carbon (C) stored in live trees to C stored in dead trees (10s of Tg C yr-1 in the western U.S. alone), which over time will be released back to the atmosphere. The western pine beetle (WPB) outbreak incited by the 2012–2015 mega-drought in the Sierra Nevada, California, U.S., could portend more frequent and/or severe bark beetle outbreaks as the temperature warms and drought frequency and intensity increase in the future. However, changes in the frequency and/or severity (resultant levels of host tree mortality) of beetle outbreaks are difficult to predict as outbreaks are complex with non-linear and eruptive processes primarily driven by interactions among beetle populations, the demography of hosts and other tree species, and climate and weather. Using an insect phenology and tree defense model, we projected the future likelihood of WPB outbreaks in the Sierra Nevada with climate drivers from different Earth System Models. Our goal was to understand how host (ponderosa pine, PIPO) recovery and future warming and drought affect the frequency and severity of WPB outbreaks and their C consequences. Our projections suggested that by 2100 the C stored in live PIPO (mean: 1.98 kg C m-2, 95% CI: 1.74–2.21 kg C m-2) will not return to levels that occurred before the 2012–2015 drought (2012: ∼2.30 kg C m-2) due to future WPB outbreaks. However, differences in climate models indicate a wide range of possible WPB outbreak frequencies and severities. Our results suggest that total plot basal area is the most significant factor in the mortality rate of PIPO by WPB in any given year, followed by drought severity and temperature. High levels of host basal area, higher temperature, and extreme drought all contribute to the frequency and severity of future WPB outbreaks. While PIPO basal area may decline under increased drought and warming, limiting high-stand basal area (&amp;gt;60 m2 ha-1) may reduce the severity of future WPB outbreaks in the Sierra Nevada.