MPB Outbreaks Research Articles

Long-term efficacy of fuel reduction and restoration treatments in Northern Rockies dry forests.

Fuel and restoration treatments seeking to mitigate the likelihood of uncharacteristic high-severity wildfires in forests with historically frequent, low-severity fire regimes are increasingly common, but long-term treatment effects on fuels, aboveground carbon, plant community structure, ecosystem resilience, and other ecosystem attributes are understudied. We present 20-year responses to thinning and prescribed burning treatments commonly used in dry, low-elevation forests of the western United States from a long-term study site in the Northern Rockies that is part of the National Fire and Fire Surrogate Study. We provide a comprehensive synthesis of short-term (<4 years) and mid-term (<14 years) results from previous findings. We then place these results in the context of a mountain pine beetle (MPB; Dendroctonus ponderosae) outbreak that impacted the site 5-10 years post-treatment and describe 20-year responses to assess the longevity of restoration and fuel reduction treatments in light of the MPB outbreak. Thinning treatments had persistently lower forest density and higher tree growth, but effects were more pronounced when thinning was combined with prescribed fire. The thinning+prescribed fire treatment had the additional benefit of maintaining the highest proportion of ponderosa pine (Pinus ponderosa) for overstory and regeneration. No differences in understory native plant cover and richness or exotic species cover remained after 20 years, but exotic species richness, while low relative to native species, was still higher in the thinning+prescribed fire treatment than the control. Aboveground live carbon stocks in thinning treatments recovered to near control and prescribed fire treatment levels by 20 years. The prescribed fire treatment and control had higher fuel loads than thinning treatments due to interactions with the MPB outbreak. The MPB-induced changes to forest structure and fuels increased the fire hazard 20 years post-treatment in the control and prescribed fire treatment. Should a wildfire occur now, the thinning+prescribed fire treatment would likely have the lowest intensity fire and highest tree survival and stable carbon stocks. Our findings show broad support that thinning and prescribed fire increase ponderosa pine forest resilience to both wildfire and bark beetles for up to 20 years, but efficacy is waning and additional fuel treatments are needed to maintain resilience.

Modeled interactions of mountain pine beetle and wildland fire under future climate and management scenarios for three western US landscapes

BackgroundMountain pine beetle (MPB) is a native disturbance agent across most pine forests in the western US. Climate changes will directly and indirectly impact frequencies and severities of MPB outbreaks, which can then alter fuel characteristics and wildland fire dynamics via changes in stand structure and composition. To investigate the importance of MPB to past and future landscape dynamics, we used the mechanistic, spatially explicit ecosystem process model FireBGCv2 to quantify interactions among climate, MPB, wildfire, fire suppression, and fuel management under historical and projected future climates for three western US landscapes. We compared simulated FireBGCv2 output from three MPB modules (none, simple empirical, and complex mechanistic) using three focus variables and six exploratory variables to evaluate the importance of MPB to landscape dynamics.ResultsWe found that inclusion of MPB (empirical or mechanistic) in the simulations significantly changed past and future landscape dynamics and that the mechanistic MPB module had more cross-scale interactions that increased variability, and perhaps realism, of simulation results. We also evaluated impacts of fire and fuel management on MPB dynamics and found that fire suppression influenced fuel loadings more than MPB disturbance, but at a landscape scale, most fuel treatment programs did little to change fuel loadings, MPB dynamics, and burned area, except under high fire suppression.ConclusionsSynergistic interactions of climate, MPB, and wildfire catalyzed landscape-scale changes in vegetation distributions, fuels, and fire regimes in FireBGCv2 simulations. Models that simulate climate change on pine-dominated landscapes may be improved by including mechanistic MPB simulations to account for potentially important ecological interactions.

Does the legacy of historical thinning treatments foster resilience to bark beetle outbreaks in subalpine forests?

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

Alpine treeline ecotones are potential refugia for a montane pine species threatened by bark beetle outbreaks.

Warming-induced mountain pine beetle (Dendroctonus ponderosae; MPB) outbreaks have caused extensive mortality of whitebark pine (Pinus albicaulis; WBP) throughout the species' range. In the highest mountains where WBP occur, they cross alpine treeline ecotones (ATEs) where growth forms transition from trees to shrub-like krummholz, some of which survived recent MPB outbreaks. This observation motivated the hypothesis that ATEs are refugia for WBP because krummholz growth forms escape MPB attack and have the potential to produce viable seed. To test this hypothesis, we surveyed WBP mortality along transects from the ATE edge (locally highest krummholz WBP) downslope into the forest and, to distinguish if survival mechanisms are unique to ATEs, across other forest ecotones (OFEs) from the edge of WBP occurrence into the forest. We replicated this design at 10 randomly selected sites in the U.S. Northern Rocky Mountains. We also surveyed reproduction in a subset of ATE sites. Mortality was nearly absent in upper ATEs (mean±SE percent dead across all sites of 0.03%±0.03% 0-100m from the edge and 14.1%±1.7% 100-500m from the edge) but was above 20% along OFEs (21.4±5.2% 0-100m and 32.4±2.7% 100-500m from the edge). We observed lower reproduction in upper ATEs (16±9.9cones/ha and 12.9±5.3viable seeds/cone 0-100m from the edge) compared to forests below (317.1±64.4cones/ha and 32.5±2.5viable seeds/cone 100-500m from the edge). Uniquely high WBP survival supports the hypothesis that ATEs serve as refugia because krummholz growth forms escape MPB attack. However, low reproduction suggests ATE refugia function over longer time periods. Beyond our WBP system, we propose that plant populations in marginal environments are candidate refugia if distinct phenotypes result in reduced disturbance impacts.

Spatial and temporal fuels changes in whitebark pine (Pinus albicaulis) from mountain pine beetle (Dendroctonus ponderosae)

Mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) causes extensive tree mortality in whitebark pine (Pinus albicaulis Engelm) forests. Previous studies conducted in lodgepole pine (Pinus contorta Douglas), Douglas-fir (Pseudotsuga menziesii (mirb.) Franco), and Engelmann spruce (Picea engelmanni Parry ex. Engelm) have shown that litter, duff, and 1-h (<0.64 cm) and 10-h (0.64–2.54 cm) time lag fuels are altered significantly from MPB outbreaks, while coarse woody fuels are affected over a longer time frame. MPB activity in conifer stands also alters foliar fuel moisture content over the course of the bark beetle rotation. This study evaluated changes to fine surface fuels and foliar fuel moisture in and under whitebark pine trees infested by MPB at two sites in Montana and Wyoming, USA. Fuel loads and foliar moisture were measured for crown condition classes of green trees (healthy), red trees (within two years since initial MPB attack with at least 50% of needles remaining), and gray trees (more than two years since attack with approximately 15% to 45% needles remaining). Tree locations and condition class were mapped using 2011, 2013, and 2015 NAIP imagery, and spatial point patterns were identified. Duff depths were significantly shallower beneath green trees (10.7–11.9 mm) than red (16.7–17.7 mm) and gray (13.9–16.1 mm) trees. Foliar fuel moisture content was altered dramatically across crown condition classes. Red needled trees had the lowest fuel moisture content, which was less than 18%. Point pattern hot spot analysis revealed increased (hot-spots) fuel hazard in trees 2–3 years post MPB attack, and decreased fuel hazard (cold-spots) in trees more than 3 years after MPB attack because foliage fell from trees and left larger diameter aerial fuels which were less likely to ignite. MPB-attacked trees were often clustered, and thus fuels hazard was not uniform across the landscape following MPB attack.

Tree regeneration and soil responses to management alternatives in beetle-infested lodgepole pine forests

Recent mountain pine beetle (Dendroctonus ponderosae; MPB) outbreaks have caused one of the most widespread and dramatic changes in forest condition in North American forests in more than a century and highlighted challenges facing resource managers. To address uncertainty regarding the consequences of post-harvest woody residue management on soil productivity and tree regeneration following MPB outbreaks in lodgepole pine-dominated forests we compared three treatment prescriptions (bole-only harvest, whole-tree harvest, and whole-tree harvest with scarification) and uncut stands. The study was replicated at twelve sites across a range of operational project areas and stand conditions in northern Colorado. Salvage logging generated a new cohort of lodgepole pine at densities far above the threshold considered adequate to develop into well-stocked stands (1700–2300 t ha−1 in logged compared to 537 t ha−1 in uncut areas). Regeneration density was generally highest in whole-tree harvested areas. Growth of planted and naturally regenerating lodgepole pine recruits was best in the bole-only, residue-retention treatment, where soil moisture and inorganic nitrogen supply was also highest. However, we found no indication that whole-tree harvesting lowered soil moisture, soil nitrogen supply or pools relative to uncut stands. The density of trees regenerating beneath uncut stands indicates that post-outbreak forest structure should recover without management in these forests. The cohort of trees that regenerated following MPB-related overstory mortality, but prior to harvesting, comprise the fastest-growing component of the growing stock and 30% of its density. The broader watershed-scale outcomes of these treatments and their implications for wildfire behavior and other effects remain uncertain. However, the soil and tree patterns we report during the initial post-treatment period inform on-going decisions regarding harvest and residue retention and create a platform to guide future forest management research.

The Potential for Pollen Records to Detect Mountain Pine Beetle Disturbance in a Pine-Dominated Forest, Idaho, USA

Mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins) outbreaks are an important component in forest succession for Pinus (pine) forests in the western United States. The research presented here is the first attempt to use pollen data from lake sediment cores to identify MPB disturbance over the past 1600 years within a pine-dominated forest. With evidence of both current and presettlement MPB outbreaks in the surrounding forest, Fishstick Lake, a small subalpine lake in the Sawtooth Mountains of Idaho, is located in a Pinus contorta (lodgepole pine)–dominated forest. Previous work by Morris et al. (2010) and Morris and Brunelle (2012) used pollen data from lake sediment cores to identify Dendroctonus rufipennis (spruce beetle) outbreaks in a Picea engelmannii (Engelmann spruce) forest by identifying the tradeoff between host (P. engelmannii) and nonhost (Abies lasiocarpa [subalpine fir]) conifers following an outbreak. This research tested the hypothesis that pollen data can be used to identify disturbance events that affect Pinus within a Pinus-dominated forest. Because the abundance of non-pine taxa is low in pine-dominated systems, it was uncertain whether the response of non-pine, nonhost taxa would be detectable. This study used a normalized difference pollen index (NDPI) to test this hypothesis. The results indicate that proxy data (pollen) can be used to identify MPB disturbance in a pine-dominated forest.

Whitebark Pine in the National Parks of the Pacific States: An Assessment of Population Vulnerability

Whitebark pine (Pinus albicaulis) is a long-lived tree found in high-elevation forests of western North America that is declining due to the non-native white pine blister rust (Cronartium ribicola) and climate-driven outbreaks of mountain pine beetle (Dendroctonus ponderosae; MPB). The National Park Service established a monitoring program for whitebark pine in seven parks, including Sequoia & Kings Canyon, Yosemite, Lassen Volcanic, Crater Lake, Mount Rainier, Olympic, and North Cascades National Parks. Using these data, we summarized stand structure, presence of blister rust, and MPB prevalence to provide a baseline for future monitoring. Next, we used a stochastic, size-structured population model to speculate on future trends in the seven national park populations under conditions of increased MPB activity and ongoing blister rust infection observed in Crater Lake. We found that blister rust infected 29 to 54% of whitebark pine in all the parks except the two southernmost, Sequoia & Kings Canyon and Yosemite, where infections rates were 0.3% and 0.2%, respectively. The proportion of dead trees in Sequoia & Kings Canyon and Yosemite was low (0 to 1%), while they ranged from 10 to 43% in the other parks. Model projections suggested an average population decline of 25% in the parks over the next century using Crater Lake conditions, declines which are possible if blister rust continues to spread and climate change results in a significant increase in the frequency or severity of MPB outbreaks. Overall, our study describes conditions at seven western parks and illustrates potential rates of whitebark pine decline if pest outbreaks and/or blister rust infections worsen.

TRIA-Net: 10 years of collaborative research on turning risk into action for the mountain pine beetle epidemic

Forest insects are showing increasing intensity of outbreaks and expanded ranges, and this has become a major challenge for forest managers. An understanding of these systems often depends upon detailed examination of complex interactions involving multiple organisms. In 2013, a team of researchers formed TRIA-Net, an NSERC support Strategic Network, with the explicit goal of exploring such interactions in the mountain pine beetle (MPB; Dendroctonus ponderosae Hopkins, 1902) – pine (Pinus sp.) – blue stain fungi (Ophiostomatales) system. Contributions of this network include novel genetic and genomic resources and insights, as well as exploration of how landscape structure affects MPB movements. This review highlights the unique contributions of TRIA-Net to our understanding of the MPB outbreak system. We also highlight how the insights we generated can be used to inform management strategies — including assessing stand susceptibility, predicting spread, and developing better monitoring approaches — and how the approach taken by the TRIA project can be used as a model for tackling other challenging forest insect outbreaks.

Vegetation dynamics following compound disturbance in a dry pine forest: fuel treatment then bark beetle outbreak.

In the western United States, restoration of forests with historically frequent, low-severity fire regimes often includes fuel reduction that reestablish open, early-seral conditions while reducing fuel continuity and loading. Between 2001 and 2016, fuel reduction (e.g., thinning, prescribed burning, etc.) was implemented on over 26million hectares of federal lands alone in the United States, reflecting the urgency to mitigate risk from high-severity wildfire. However, between 2001 and 2012, nearly 20million hectares in the United States were impacted by mountain pine beetle (MPB; Dendroctonus ponderosae), compounding restoration effects in wildfire-hazard-treated stands. Knowledge of the effects of treatments followed by natural disturbance on long-term forest structure and communities is needed, especially considering that fuel treatments are increasingly being implemented and warming climate is predicted to exacerbate disturbance frequency and severity. We tested the interacting effects of treatments designed to reduce high-severity wildfire hazard in stands subsequently challenged by MPB outbreak on vegetation dynamics using a factorial experimental design (control, thin only, burn only, thin + burn) in a ponderosa pine (Pinus ponderosa)-dominated forest. Stands were treated by 2002, then impacted by MPB outbreak from 2005 to 2012. We assessed change in overstory and understory forest community structure, composition, and diversity over time. There were distinct thinning, burning, and year effects. Thinning immediately reduced overstory density; pine density then declined 4.5 times more in unthinned than thinned treatments due to MPB. Burning immediately reduced graminoid, shrub, and total understory cover by as much as 52%, resulting in greater species evenness than unburned treatments, but differences disappeared by 2016 due to growth and MPB outbreak. Similarly, multivariate analyses indicated forest communities were starkly different after treatment but became more similar over time, though key understory and overstory attributes still distinguish control and thin + burn. This study shows the value of long-term silvicultural experiments to evaluate treatment longevity and the compounded effects of treatment and natural disturbance. We demonstrate the homogenizing effects of treatment-induced growth coupled with MPB-caused tree mortality on management strategies that just treat the overstory (thinning) or understory (burning), showing that only combined treatments can provide the unique structural and compositional outcomes expected of restoration.

The effects of mountain pine beetle outbreaks on avian communities in lodgepole pine forests across the greater Rocky Mountain region

Disturbance is an integral component of forest ecosystems. Therefore, contemporary changes to the frequency and severity of disturbances in forests may have lasting ecological consequences. In recent decades, widespread mountain pine beetle (MPB; Dendroctonus ponderosae) outbreaks have contributed more to total tree mortality than other disturbances such as wildfire. Past studies have used a broad range of measurements to evaluate the local effects of beetle outbreaks on birds (e.g., dead trees per acre versus percent mortality) and this has led to some challenges and inconsistencies in generalizing results across larger spatial scales. We sought to address these inconsistencies by using a single data source estimating outbreak-related tree mortality across the western United States to better understand the effect of beetle outbreaks on avian communities in lodgepole pine forests. This study uses five years of avian monitoring data (2008–2012) from the Integrated Monitoring in Bird Conservation Regions program, collected across Colorado, Idaho, Montana, and Wyoming. We developed a hierarchical multispecies occupancy model to quantify changes in the forest bird community following beetle outbreaks while accounting for the imperfect detection of species. Mean species richness (N) did not differ between MPB outbreak sites (N = 19.35, 95%CRI = 17.00, 22.01) and sites with no outbreak (N = 20.23, 95%CRI = 16.83, 24.25). Furthermore, we found weak evidence for a community-level response to increasing outbreak severity and time since outbreak, due in part to the variability among species within each of the four nesting guilds analyzed in this study (canopy, cavity, shrub, and ground nesting birds). However, we did find evidence of turnover in community composition with just over 50% of species (28 of 55) having statistically different occurrence rates following MPB outbreaks. Our study underscores the notion that species richness alone does not fully express changes in the forest bird community. Our results also suggest many species, particularly cavity nesters, utilize beetle-killed forests and persist at higher occurrence rates for up to 10 years post-outbreak. This period includes the timeframe when beetle-killed trees are most readily salvageable and emphasizes the need for continued evaluation of snag-retention policies prior to the implementation of post-outbreak management activities.

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.

Ecological effects and effectiveness of silvicultural restoration treatments in whitebark pine forests

Silvicultural thinning treatments to restore whitebark pine (Pinus albicaulis) are widely used in subalpine forests throughout the western United States (US) and Canada. The objectives of these treatments are to (1) improve the condition of whitebark pine at all ages, (2) to improve seedling recruitment processes, and (3) mitigate the damage caused by mountain pine beetle (MPB; Dendroctonus ponderosae) and white pine blister rust (WPBR; caused by the fungus Cronartium ribicola). However, there is some disagreement about the ecological basis of restoration and a paucity of information on the effects these activities – few treatments have been monitored to assess their success. We investigated the ecological effects of silvicultural restoration treatments in whitebark pine forests and evaluated their success by retrospectively sampling five treatment sites in the western US 6–10 years after implementation. We found strong evidence of growth release at a site previously characterized by closed-canopy stands. Growth responses in more open, park-like stands, however, were variable: we found weak growth increases at one site, weak growth decreases at another and no response at two other sites. At the site with strong growth increases, trees with previous damage from WPBR infection had growth increases similar to uninfected trees. We found low rates of whitebark pine seedling recruitment overall, and no increase in whitebark pine recruitment associated with treatments at any site. However, at one site, treated stands had higher regeneration of non-target species than did untreated stands. Post-treatment mortality (mostly from the late 2000s MPB outbreak) was significantly lower in the treated stand at the closed-canopy site; at the other sites, there was no difference in mortality between treated and untreated stands. The treatments had little detectable effect on short-term growth-climate relationships, although our analyses revealed that whitebark pine growth at our sites was more temperature limited than water limited. While some management goals were achieved, many were not, and there were some unintended consequences. Our results call for a closer examination of the ecological basis of silvicultural restoration treatments in whitebark pine and an expanded use of adaptive management.

The Mountain Pine Beetle: A Study of Tourism Businesses’ Perceptions of the Risk of Ecological Disturbances

Recent increases in forest insect activities pose a potential risk to the ecosystem and often have social and economic consequences. In areas dependent on outdoor recreation and tourism, businesses such as parks, accommodations, restaurants, and attractions are central stakeholders in forest management, and it is important for policy makers to craft more effective policies that incorporate business needs. Management agencies understand the need to adapt the standard management strategies to focus on prevention, early detection, and prompt response. Yet, heterogeneity among stakeholders in terms of values, ideologies, capacity, and vested interests, make it difficult to get broad participation in management practices. The present study examines the attitudes and perceptions of tourism and outdoor recreation businesses regarding the mountain pine beetle (MPB, MPB Dendroctonus ponderosae) infestation in the Black Hills Forest, South Dakota, U.S. Despite significant efforts to stymie the infestation, the beetle has changed the forest ecosystem by killing pine trees. The dead trees have not only changed the aesthetic landscape but also have presented a safety hazard and financially impacted local businesses. The Health Belief Model served as a theoretical framework for this study. Data were generated from 417 businesses, such as accommodations, restaurants, convention and visitors bureaus (CVBs), and attractions, using a targeted online survey. While in general, owners and managers of tourism businesses in the Black Hills felt that the MBP had affected their business operations, the study found substantial variation in the range and degree of perceived impacts and potential risks. The use of the Health Belief Model facilitated the understanding of variations in support and participation in combating MPB activities among tourism businesses, with those having greater knowledge of MPB, higher perceived severity, higher self-efficacy, and higher perceived benefits being more likely to engage in the recommended ecological management activities. The level of perception was also influenced by the location of the business with relation to recent MPB outbreaks. The study highlights that in order to increase participation in combating the pine beetle outbreak, policy makers must develop programs that not only improve awareness but also increase stakeholder self-efficacy and the perceived benefits of participation. Management strategies should also be adaptable to a more local context in order to accommodate differences in risk perceptions related to an ecological disturbance. Subscribe to JPRA

Consequences of mountain pine beetle outbreak on forest ecosystem services in western Canada

After affecting millions of hectares of pine forests in western Canada, the mountain pine beetle (MPB; Dendroctonous ponderosae Hopkins) is spreading out of its native range and into Canada’s boreal forest. Impacts of outbreaks can be environmental, economic, and social, and an ecosystem services (ES) viewpoint provides a useful perspective for an integrated approach to assessing these impacts and may help to identify how possible management strategies could minimize these impacts. In this regards, a comprehensive overview of the ecosystem functions and socioeconomic factors that have been impacted by the current outbreaks in western Canada was carried out to facilitate a more general ES assessment. In addition to timber production, current MPB outbreaks have negative effects on provisioning services (water supply and food production) and aesthetic cultural services, while effects on regulating services (carbon and forest fire) are still in debate. Among the supporting services, nutrient cycling and aquatic habitat showed short- and long-term negative effects, while terrestrial habitat showed a mostly positive response. The overall impact on ES may be more severe if salvage logging is practiced as a post-MPB forest management strategy. The outcomes of this study may help to identify areas of greatest socioecological vulnerability to MPB and identify knowledge gaps and avenues for research to advance the ES framework for MPB outbreak management.

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.

Forest composition change after a mountain pine beetle outbreak, Rocky Mountain National Park, CO, USA

Recent severe and extensive mountain pine beetle (Dendroctonus ponderosae; MPB) outbreaks have created novel conditions in Southern Rocky Mountain lodgepole pine forests which historically had disturbance regimes dominated by extensive, stand-replacing fires. The goal of this study is to investigate patterns of and potential mechanisms in post-outbreak forest change in order to better understand the ecological legacy of the recent outbreak in the context of its implications for resilience to future disturbances and adaptation to climate change. To this end, we collected field data on forest structure and species composition in 2012 in lodgepole pine dominant forests in Rocky Mountain National Park. We then used a combination of modeling and statistical methods to identify possible mechanisms in post-outbreak forest conditions and evaluate the effect of the MPB outbreak on forest heterogeneity. We found that the outbreak initiated a shift in forest structure from single-cohort lodgepole pine stands to stands with greater diversity in age classes and species composition. This increase in landscape asynchrony may increase resiliency to future disturbances. However, this heterogeneity is a result of more spruce and fir on the landscape, species which are less adapted to projected future climate conditions. Our results indicate that disturbances do not necessarily increase the rate at which vegetation adapts to a changing climate, and that it is essential to consider disturbance type and available seed sources when predicting future forest conditions.

Fire severity and cumulative disturbance effects in the post-mountain pine beetle lodgepole pine forests of the Pole Creek Fire

Recent large scale mountain pine beetle (Dendroctonus ponderosae Hopkins, MPB) outbreaks have created concern regarding increased fuel loadings and exacerbated fire behavior and have prompted a desire to understand the effects of sequential disturbances on the landscape. However, previous research has focused on quantifying fuel loadings and using operational fire behavior models, rather than direct field measurements, to understand changes in fire severity following MPB. The 2012 Pole Creek Fire in central Oregon partially occurred in gray stage (8–15years post-MPB epidemic) lodgepole pine forests. We examined the combined effects of MPB and fire disturbances on stand structure, and investigated the influence of previous MPB severity and fire weather on subsequent fire severity and cumulative disturbance severity. We randomly selected and installed 52 plots over a gradient of MPB and fire severity combinations and measured stand structure and fire severity characteristics. Fire severity metrics representing both crown and surface fire decreased with increased MPB severity under extreme burning conditions, following expected trends for crown fire severity, but not surface fire severity. Cumulative basal area mortality increased with MPB severity under moderate burning conditions, while other cumulative disturbance severity metrics were unrelated or weakly related to MPB severity. High severity crown fire was common despite hypothesized low canopy fuel loadings during the gray stage, indicating the importance of understanding variable mortality density of MPB outbreaks. Although long-term studies are needed to understand ecosystem recovery trajectories over time, there was no indication that a loss of ecosystem resilience occurred as a result of two sequential disturbances in this landscape.

Long‐term history and synchrony of mountain pine beetle outbreaks in lodgepole pine forests

AbstractAimA subcontinental‐scale outbreak of mountain pine beetle (MPB; Dendroctonus ponderosae) has affected millions of hectares of forest in the western USA and Canada over the past 15 years. Little research has examined the long‐term and broad‐scale history of MPB outbreaks, which is necessary to provide a baseline for comparing current and future outbreak extents. We examine the long‐term history of MPB outbreaks in western Colorado, and analyse the synchrony of previous outbreaks across western North America.LocationWestern Colorado for tree‐ring analysis; western North America for comparative analysis.MethodsWe used dendroecological methods to reconstruct the history of MPB outbreaks since the 1700s in lodgepole pine (Pinus contorta) forests in Colorado. We then combined these new records with previously published data on MPB outbreaks to examine the historical synchrony of outbreaks across western North America.ResultsThe tree‐ring record indicated that multiple cross‐site MPB outbreaks occurred in Colorado lodgepole pine forests, initiating c. 1760s, 1780s, 1820s–1830s, 1860s, 1910s, 1960s and 1980s. Comparative analysis indicated that these outbreak dates coincide with documented and reconstructed MPB outbreaks in British Columbia, Colorado, Idaho, Montana, Wyoming and Utah.Main conclusionsSeveral subcontinental‐scale MPB outbreaks have occurred in lodgepole pine forests over the past three centuries. Although the current study does not address the intensity of past outbreaks, it does indicate that previous outbreaks appear to have been highly synchronous across western North America. The subcontinental, rather than purely local, nature of these outbreaks is important for putting recent and future outbreaks of MPB into a broader ecological context. This study presents the longest tree‐ring reconstruction of MPB outbreaks in the southern Rocky Mountains available to date, as well as the first subcontinental long‐term comparative analysis of MPB outbreaks in western North America.

Spatiotemporal dynamics of recent mountain pine beetle and western spruce budworm outbreaks across the Pacific Northwest Region, USA

Across the western US, the two most prevalent native forest insect pests are mountain pine beetle (MPB; Dendroctonus ponderosae; a bark beetle) and western spruce budworm (WSB; Choristoneura freemani; a defoliator). MPB outbreaks have received more forest management attention than WSB outbreaks, but studies to date have not compared their cumulative mortality impacts in an integrated, regional framework. The objectives of this study are to: (1) map tree mortality associated with MPB and WSB outbreaks by integrating forest health aerial detection surveys (ADS; 1970–2012), Landsat time series (1984–2012), and multi-date forest inventory data; (2) compare the timing, extent, and cumulative impacts of recent MPB and WSB outbreaks across forested ecoregions of the US Pacific Northwest Region (PNW; Oregon and Washington). Our Landsat-based insect atlas facilitates comparisons across space, time, and insect agents that have not been possible to date, complementing existing ADS maps in three important ways. The new maps (1) capture variation of insect impacts within ADS polygons at a finer spatial resolution (30m), substantially reducing estimated insect extent; (2) provide consistent estimates of change for multiple agents, particularly long-duration changes; (3) quantify change in terms of field-measured tree mortality (dead basal area). Despite high variation across the study region, spatiotemporal patterns are evident in both the aerial survey- and Landsat-based maps of insect activity. MPB outbreaks occurred in two phases – first during the 1970s and 1980s in eastern and central Oregon and then more synchronously during the 2000s throughout dry interior conifer forests of the PNW. Reflecting differences in habitat susceptibility and epidemiology, WSB outbreaks exhibited early activity in northern Washington and an apparent spread from the eastern to central PNW during the 1980s, returning to northern Washington during the 1990s and 2000s. At ecoregional and regional scales, WSB outbreaks have exceeded MPB outbreaks in extent as well as total tree mortality, suggesting that ongoing studies should account for both bark beetles and defoliators. Given projected increases of insect and fire activity in western forests, the accurate assessment and monitoring of these disturbances will be crucial for sustainable ecosystem management.