Mountain Pine Beetle Disturbance Research Articles

Cumulative disturbance converts regional forests into a substantial carbon source

British Columbia’s interior forests (∼400 000 km2) have experienced severe cumulative disturbance from harvesting, wildfires, and mountain pine beetle (MPB). Estimating their impacts on carbon dynamics is critical for effective forest management and climate-change mitigation strategies. This study quantifies the magnitude of historical cumulative forest disturbances and models the effect on regional carbon stocks and emissions using the Carbon Budget Model of the Canadian Forest Service from 1980 to 2018. The study region has been a sustained carbon source since 2003, with an estimated net biome production of −18.6 ± 4.6 gC m−2 yr−1 from 2003 to 2016, dropping to −90.4 ± 8.6 gC m−2 yr−1 in 2017 and 2018 due to large-scale wildfires. MPB affected areas emitted an estimated 268 ± 28 Mt C from 2000 to 2018. Harvesting transferred an estimated 153 ± 14 Mt C to forest products and these areas also emitted 343 ± 27 Mt C in 2000–2018. Areas disturbed by wildfire from 2000 to 2018 generated an estimated 100 ± 8 Mt C of emissions, 73% of which were from 2017 and 2018. Of the area burned between 2014 and 2018, 38% had been previously affected by MPB, highlighting landscape-level interactions of cumulative forest disturbance. Approximately half of decomposition carbon emissions from disturbances in 2000–2018 were calculated as incremental to the decomposition that would have otherwise occurred without MPB disturbance. The average net primary production was reduced by 10% to 335 ± 31 gC m−2 yr−1 from 2000 to 2018. We conclude that cumulative forest disturbance has driven the region’s forests to become a sustained carbon source over the past two decades. While MPB and harvesting were dominant and consistent drivers, recent severe wildfires have prolonged and strengthened the carbon source. Increased disturbances, driven in part by climate change, may limit the ability of regional forests to meet national carbon emission reduction targets.

Understory plant species and community changes following a mountain pine beetle outbreak in Rocky Mountain National Park, USA

AbstractAimsThe response of understory plants is an important, yet understudied element of forest dynamics associated with mountain pine beetle (MPB) disturbance. Here, we provide a detailed assessment of both the changes in understory species and communities over a 5‐year period following severe MPB‐induced lodgepole pine mortality. Specifically, we examine species turnover, rarity and mobility over time, and evaluate shifts in community composition, including the potential for post‐outbreak community homogenization or differentiation.LocationWestern Rocky Mountain National Park in the southern Rocky Mountains, Colorado.MethodsWe collected species occurrence data across lodgepole pine‐dominated forest understories using repeated sampling of the same study sites (during the peak of MPB activity and 5 years after).ResultsMore species appeared than disappeared 5 years after the peak of the outbreak. New species were comprised of both early‐ and late‐successional species, suggesting the presence of rather high microhabitat heterogeneity. Although most species remained relatively rare, the number of exceedingly rare species decreased, and the number of highly common species increased. Several species were able to take advantage of the new stand conditions and expand throughout the study area. Some species appeared to spread locally over short distances mainly via vegetative reproduction, others expanded primarily upwards in elevation, probably through long‐distance dispersal and some spread over both short and long distances. Non‐native species doubled in occurrence and tended to spread upslope, towards the interior of the park, yet remained a small component of the understory overall. Shifts in community composition were minimal, but a slight convergence of plant community groups suggests a trend towards community homogenization.ConclusionsOur results demonstrate that even relatively sparse and depauperate lodgepole pine forest understories may be altered in diverse ways following MPB‐induced overstory mortality, depending on the unique ability of species to respond to changes in site conditions and resource availability.

Understory vegetation response to mountain pine beetle disturbance in northern Colorado lodgepole pine forests

Understory plants are an important element of forests, having a considerable influence on ecosystem functioning and canopy-tree development following disturbance. Recent bark beetle outbreaks across western North American forests have caused extensive canopy mortality, creating new growing conditions that provide the opportunity for changes within the intact understory. Over a five-year period following peak mountain pine beetle (MPB) activity across lodgepole pine-dominated forests in Rocky Mountain National Park, Colorado, we measured the changes in plant diversity, cover, and dominance by lifeform and quantified tree regeneration rates. Average species richness and diversity increased, but overall plant cover did not change. Graminoids appeared to benefit the most, increasing in average cover, richness, and relative dominance. The rise in graminoid dominance was largely at the expense of shrubs, which showed little ability to benefit from overstory mortality within the first years following attack. Most plant responses were positively related to the total tree basal area lost since the peak of the outbreak, suggesting that increased resource availability following tree death may benefit understory plants. However, a negative relationship between several understory variables and tree sapling density provides evidence that understory plants compete with saplings for the newly available resources. Tree seedling density nearly doubled over the duration of the study, indicating a strong regeneration pulse. Among species, lodgepole pine displayed the greatest tree seedling establishment. This is one of the first studies to use repeated measurements to describe this often-overlooked component of forest change associated with MPB disturbance.

Mountain goat survival and mortality during a period of increased puma abundance in the Black Hills, South Dakota.

We investigated survival and cause-specific mortality for a mountain goat (Oreamnos americanus) population during a period when the puma (Puma concolor) population was growing in the Black Hills, South Dakota, 2006–2018. We obtained survival data from 47 adult goats (n = 33 females, n = 14 males). Annual survival varied from 0.538 (95% CI [0.285–0.773]) to 1.00 (95% CI [1.00–1.00]) and puma predation was the primary cause-specific mortality factor over a 12-year period. Cumulative hectares of mountain pine beetle (Dendroctonus ponderosae) disturbance was a covariate of importance (wi = 0.972; β = 0.580, 95% CI [0.302–0.859]) influencing survival. To our knowledge, this is the first account of puma being the primary mortality factor of mountain goats over a long-term study. The Black Hills system is unique because we could examine the expanded realized niche of puma in the absence of other large carnivores and their influence on mountain goats. We hypothesize that puma were being sustained at higher densities due to alternate prey sources (e.g., white-tailed deer; Odocoileous virginianus) and this small population of mountain goats was susceptible to predation by one or several specialized puma in the Black Hills. However, we also hypothesize a changing landscape with increased tree mortality due to insect infestation provided conditions for better predator detection by goats and increased survival. Alternatively, open canopy conditions may have increased understory forage production potentially increasing mountain goat survival but we did not evaluate this relationship. Survival and mortality rates of mountain goats should continue to be monitored as this small population may be highly susceptible to population declines due to slow growth rates.

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.

Documents as data: A content analysis and topic modeling approach for analyzing responses to ecological disturbances

We apply content analysis on government documents containing ecological information relevant to a significant ecological disturbance - mountain pine beetle (MPB) outbreaks in the United States. The intent is to demonstrate a semi-automated approach that applies topic modeling to investigate policy responses to ecological disturbances, using latent Dirichlet allocation (LDA) and hierarchical Dirichlet process (HDP), and term frequency–inverse document frequency (tf-idf) analysis. Results demonstrate how analysts and researchers are better able to understand what topics and focus areas government officials consider in relation to MPB disturbances. In the case study demonstrating the method’s utility, documents found from before 1960 and until recent years demonstrate focus on outbreak area, tree mortality, research and services, management, infestation, outbreak control, fire, insect control, outbreak factors, and tree population. Terms such as fire, mortality, treatment, and outbreak reflect more recent U.S. government focus on MPB, while disease and infestation have become less of a focus in recent years. There are also varying differences and interests between how different parts (i.e., federal agencies versus congress) of the U.S. government focus on MPB, where mostly interests and focus are not aligned or do not match temporally. As a term, temperature has become a greater recent government focus, but there is general avoidance of the term climate change. The methods applied demonstrate the utility of topic modeling and tf-idf for understanding discourse and content in policy related to ecological disturbances. The tool created in this effort is provided freely as a way for scientists and researchers to extend its utility in ecological policy research.

Improving ecosystem‐scale modeling of evapotranspiration using ecological mechanisms that account for compensatory responses following disturbance

AbstractMountain pine beetle outbreaks in western North America have led to extensive forest mortality, justifiably generating interest in improving our understanding of how this type of ecological disturbance affects hydrological cycles. While observational studies and simulations have been used to elucidate the effects of mountain beetle mortality on hydrological fluxes, an ecologically mechanistic model of forest evapotranspiration (ET) evaluated against field data has yet to be developed. In this work, we use the Terrestrial Regional Ecosystem Exchange Simulator (TREES) to incorporate the ecohydrological impacts of mountain pine beetle disturbance on ET for a lodgepole pine‐dominated forest equipped with an eddy covariance tower. An existing degree‐day model was incorporated that predicted the life cycle of mountain pine beetles, along with an empirically derived submodel that allowed sap flux to decline as a function of temperature‐dependent blue stain fungal growth. The eddy covariance footprint was divided into multiple cohorts for multiple growing seasons, including representations of recently attacked trees and the compensatory effects of regenerating understory, using two different spatial scaling methods. Our results showed that using a multiple cohort approach matched eddy covariance‐measured ecosystem‐scale ET fluxes well, and showed improved performance compared to model simulations assuming a binary framework of only areas of live and dead overstory. Cumulative growing season ecosystem‐scale ET fluxes were 8 – 29% greater using the multicohort approach during years in which beetle attacks occurred, highlighting the importance of including compensatory ecological mechanism in ET models.

Assessing spatiotemporal relationships between wildfire and mountain pine beetle disturbances across multiple time lags

AbstractNative bark beetles and wildfire are among the most prominent natural disturbances in the conifer forests of western North America. Using multiple approaches and scales, researchers have sought to understand how each disturbance may, or may not, influence the subsequent likelihood or severity of the other disturbance. Our objectives were to complement existing research on the linkage of mountain pine beetle and wildfire disturbances by combining large‐scale spatial data at variable temporal resolutions to characterize the discrepancy between observed and expected overlap areas of fire and beetle disturbances. Multiple time lags, differing by 1‐yr intervals, spanning the 30‐yr period 1984–2014 in the U.S. states of Washington and Oregon were examined for wildfire following bark beetle epidemics and for beetle epidemics following wildfire. We used existing spatial data sets of wildfires and insect pest disturbances to perform overlay analyses to characterize the two relationships. We also compared years of peak vs. non‐peak fires, as well as the influence of several individual, large disturbances. We found a clear association of increased bark beetle epidemic shortly after wildfire. The evidence of an association between increased fire extent and severity following bark beetle epidemics was less straightforward, although it suggested that the nature of the observed patterns varied with fire severity. In some cases, individual disturbance seasons or events were highly influential on the overall trends. Variability in the results highlights the challenges of quantifying landscape‐level relationships among disturbance types and supports the existing calls for further analyses with additional ecological variables.

Biotic disturbance facilitates range shift at the trailing but not the leading edge of lodgepole pine's altitudinal distribution

AbstractAimClimate change will alter the geographic distribution of many species, but in forest communities, long‐lived trees often persist after the climate is no longer suitable for recruitment. Disturbance events may accelerate distribution shifts in forested ecosystems by removing persistent trees and creating new recruitment opportunities for species better adapted to the current climate. Here, we investigate how biological disturbances such as insect outbreaks may interact with climate change to initiate tree species range shifts.LocationSouthern Rocky Mountains, Colorado, USA.MethodsWe compared tree and seedling data from long‐term monitoring transects established in 1992–1995 and re‐sampled in 2012 following a widespread mountain pine beetle (Dendroctonus ponderosae Hopkins) outbreak. Transects are located along elevational gradients at the altitudinal range margins of lodgepole pine (Pinus contorta).ResultsWe found that recruitment had declined through time at the trailing edge of lodgepole pine's distribution (lower elevation range margin), where there are now many fewer seedlings compared to the leading edge (upper elevation range margin). At the trailing edge, mountain pine beetle increased mortality of mature lodgepole pines relative to background levels. This mortality was associated with an increase in recruitment of Douglas‐fir (Pseudotsuga menziesii), a lower elevation tree species. At the leading edge of lodgepole pine's distribution, overstorey mortality was associated with only a modest increase in the density of lodgepole pine seedlings (mean increase of 362 seedlings·ha−1), and instead favoured more shade‐tolerant competitors (mean increase of 2182 seedlings·ha−1).ConclusionWhile there is some evidence that climate change is beginning to alter the altitudinal range of lodgepole pine, mountain pine beetle disturbance will likely facilitate this process only at the trailing (lower elevation) range margin. At the leading (upper elevation) range margin, range expansion may be limited by competition with trees and seedlings of species not susceptible to mountain pine beetle. A physical disturbance such as fire that can eliminate competing species may be required for the upward expansion of lodgepole pine's leading range margin. For tree species such as lodgepole pine that are affected by periodic insect outbreaks, ranges may shrink in the near term if this disturbance–climate interaction continues to drive range retractions that outpace range expansions.

Potential near‐future carbon uptake overcomes losses from a large insect outbreak in British Columbia, Canada

AbstractThe current capacity of northern high‐latitude forests to sequester carbon has been suggested to be undermined by the potential increase in fire and insect outbreaks. Here we investigate the response of the terrestrial ecosystems in the province of British Columbia (BC), Canada, to the recent large mountain pine beetle (MPB) outbreak that started in 1999 as well as changing climate and continually increasing atmospheric CO2 concentration up to 2050, in a combined framework, using a process‐based model. Model simulations suggest that the recent MPB outbreak results in BC's forests accumulating 328 Tg less carbon over the 1999–2020 period. Over this same period changing climate and increasing atmospheric CO2 concentration, however, yield enhanced carbon uptake equal to a cumulative sink of around 900–1060 Tg C, depending on the future climate change scenario, indicating that the reduced carbon uptake by land due to the MPB disturbance may already be surpassed by 2020.

Characterizing spectral–temporal patterns of defoliator and bark beetle disturbances using Landsat time series

Defoliators and bark beetles are natural disturbance agents in many forest ecosystems around the world. Mapping the spatial and temporal patterns of insect disturbance dynamics can help in understanding their impacts on forest ecosystem resilience and functioning, and in developing adaptive management strategies. In recent years, much progress has been made in landscape-level analyses of insect-induced disturbances using remotely sensed data. However, many studies have focused on single insect agents or aggregated different insect agents into a single group. In this study, we characterized the temporal-spectral patterns associated with bark beetle and defoliator disturbances using Landsat time series between 1990 and 2013, with the objective to test if the two insect disturbances can be separated with Landsat data. We analyzed a recent outbreak of mountain pine beetle (Dendroctonus ponderosae Hopkins) and western spruce budworm (Choristoneura freemani Razowski) in British Columbia, Canada. To characterize the disturbance and recovery trends associated with insect disturbances we used the LandTrendr segmentation algorithm. We fitted LandTrendr spectral trajectories to annual normalized burn ratio (NBR) and Tasseled Cap (TC) time series, from which we then extracted a set of disturbance metrics. With these disturbance metrics, two random forest models were trained to a) distinguish insect disturbances from harvest and fire disturbances; and to b) attribute the insect disturbances to the most likely agent, i.e. mountain pine beetle or western spruce budworm. Insect disturbances were successfully mapped with an overall accuracy of 76.8%, and agents were successfully attributed with overall accuracies ranging from 75.3% to 88.0%, depending on whether only pure host-stands or mixed stands with both insect hosts were considered. In the case of mixed host stands, nearly 45% of the western spruce budworm disturbances were falsely attributed to mountain pine beetle. Spectral metrics describing disturbance magnitude were more important for distinguishing the two insect agents than the disturbance duration. Spectral changes associated with western spruce budworm disturbances had generally lower magnitudes than mountain pine beetle disturbances. Moreover, disturbances by western spruce budworm were more strongly associated with changes in TC greenness, whereas disturbances by mountain pine beetle were more strongly associated with changes in TC brightness and wetness. The results reflect the ephemeral nature of defoliators versus the tree mortality impacts of bark beetles in our study area. This study demonstrates the potential of Landsat time series for mapping bark beetle and defoliator disturbances at the agent level and highlights the need for distinguishing between the two insect agents to adequately capture their impacts on ecosystem processes.

The impacts of mountain pine beetle disturbance on the energy balance of snow during the melt period

AbstractMountain snowpacks provide most of the annual discharge of western US rivers, but the future of water resources in the western USA is tenuous, as climatic changes have resulted in earlier spring melts that have exacerbated summer droughts. Compounding changes to the physical environment are biotic disturbances including that of the mountain pine beetle (MPB), which has decimated millions of acres of western North American forests. At the watershed scale, MPB disturbance increases the peak hydrograph, and at the stand scale, the ‘grey’ phase of MPB canopy disturbance decreases canopy snow interception, increases snow albedo, increases net shortwave radiation, and decreases net longwave radiation versus the ‘red’ phase. Fewer studies have been conducted on the red phase of MPB disturbance and in the mixed coniferous stands that may follow MPB‐damaged forests. We measured the energy balance of four snowpacks representing different stages of MPB damage, management, and recovery: a lodgepole pine stand, an MPB‐infested stand in the red phase, a mixed coniferous stand (representing one successional trajectory), and a clear‐cut (representing reactive management) in the Tenderfoot Creek Experimental Forest in Montana, USA. Net longwave radiation was lower in the MPB‐infested stand despite higher basal area and plant area index of the other forests, suggesting that the desiccated needles serve as a less effective thermal buffer against longwave radiative losses. Eddy covariance observations of sensible and latent heat flux indicate that they are of similar but opposite magnitude, on the order of 20 MJ m−2 during the melt period. Further analyses reveal that net turbulent energy fluxes were near zero because of the temperature and atmospheric vapour pressure encountered during the melt period. Future research should place snow science in the context of forest succession and management and address important uncertainties regarding the timing and magnitude of needlefall events. Copyright © 2015 John Wiley & Sons, Ltd.

Resilience of Ponderosa and Lodgepole Pine Forests to Mountain Pine Beetle Disturbance and Limited Regeneration

Resilience of Ponderosa and Lodgepole Pine Forests to Mountain Pine Beetle Disturbance and Limited Regeneration

Erratum: Corrigendum: Summertime climate response to mountain pine beetle disturbance in British Columbia

Corrigendum: Summertime climate response to mountain pine beetle disturbance in British Columbia

Impact of mountain pine beetle outbreaks on forest albedo and radiative forcing, as derived from Moderate Resolution Imaging Spectroradiometer, Rocky Mountains, USA

AbstractMountain pine beetle (Dendroctonus ponderosae) outbreaks in North America are widespread and have potentially large‐scale impacts on albedo and associated radiative forcing. Mountain pine beetle outbreaks in Colorado and southern Wyoming have resulted in persistent and significant increases in both winter albedo (change peaked 10 years post outbreak at 0.06 ± 0.01 and 0.05 ± 0.01, in lodgepole pine (Pinus contorta) and ponderosa pine (Pinus ponderosa) stands, respectively) and spring albedo (change peaked 10 years post outbreak at 0.06 ± 0.01 and 0.04 ± 0.01, in lodgepole pine and ponderosa pine stands, respectively). Instantaneous top‐of‐atmosphere radiative forcing peaked for both lodgepole pine and ponderosa pine stands in winter at 10 years post outbreak at −1.7 ± 0.2 W m−2 and −1.4 ± 0.2 W m−2, respectively. The persistent increase in albedo with time since mountain pine beetle disturbance combined with the continued progression of the attack across the landscape from 1994–2011 resulted in an exponential increase in winter and annual radiative cooling (MW) over time. In 2011 the rate of radiative forcing within the study area reached −982.7 ± 139.0 MW, −269.8 ± 38.2 MW, −31.1 ± 4.4 MW, and −147.8 ± 20.9 MW in winter, spring, summer, and fall, respectively. An increase in radiative cooling has the potential to decrease sensible and/or latent heat flux by reducing available energy. Such changes could affect current mountain pine beetle outbreaks which are influenced by climatic conditions.

Nitrogen Cycling Responses to Mountain Pine Beetle Disturbance in a High Elevation Whitebark Pine Ecosystem

Ecological disturbances can significantly affect biogeochemical cycles in terrestrial ecosystems, but the biogeochemical consequences of the extensive mountain pine beetle outbreak in high elevation whitebark pine (WbP) (Pinus albicaulis) ecosystems of western North America have not been previously investigated. Mountain pine beetle attack has driven widespread WbP mortality, which could drive shifts in both the pools and fluxes of nitrogen (N) within these ecosystems. Because N availability can limit forest regrowth, understanding how beetle-induced mortality affects N cycling in WbP stands may be critical to understanding the trajectory of ecosystem recovery. Thus, we measured above- and belowground N pools and fluxes for trees representing three different times since beetle attack, including unattacked trees. Litterfall N inputs were more than ten times higher under recently attacked trees compared to unattacked trees. Soil inorganic N concentrations also increased following beetle attack, potentially driven by a more than two-fold increase in ammonium (NH4 +) concentrations in the surface soil organic horizon. However, there were no significant differences in mineral soil inorganic N or soil microbial biomass N concentrations between attacked and unattacked trees, implying that short-term changes in N cycling in response to the initial stages of WbP attack were restricted to the organic horizon. Our results suggest that while mountain pine beetle attack drives a pulse of N from the canopy to the forest floor, changes in litterfall quality and quantity do not have profound effects on soil biogeochemical cycling, at least in the short-term. However, continuous observation of these important ecosystems will be crucial to determining the long-term biogeochemical effects of mountain pine beetle outbreaks.

Summertime climate response to mountain pine beetle disturbance in British Columbia

The present mountain pine beetle infestation in forests in British Columbia ranks among the largest ecological disturbances recorded in Canada so far. These recent outbreaks are thought to have been favoured by large-scale climatic shifts, and may foreshadow outbreaks of a similar magnitude in North American forests over the coming decades. The associated forest dieback could result in substantial shifts in evapotranspiration and albedo, thereby altering the local surface energy balance, and in turn regional temperature and climate. Here we quantify the impact of the Canadian pine beetle disturbance on the local summertime surface energy budget, using measurements of evapotranspiration, albedo and surface temperature, obtained primarily through remote sensing. We show that over the 170,000 km 2 of affected forest, the typical decrease in summertime evapotranspiration is 19%. Changes to the absorbed short-wave flux are negligible, in comparison. As a result, outgoing sensible and radiative heat fluxes increased by 8% and 1%, respectively, corresponding to a typical increase in surface temperature of 1 C. These changes are comparable to those observed for other types of disturbance, such as wildfire, and may have secondary consequences for climate, including modifications to circulation, cloud cover and precipitation.

Nitrogen cycling following mountain pine beetle disturbance in lodgepole pine forests of Greater Yellowstone

Widespread bark beetle outbreaks are currently affecting multiple conifer forest types throughout western North America, yet many ecosystem-level consequences of this disturbance are poorly understood. We quantified the effect of mountain pine beetle ( Dendroctonus ponderosae) outbreak on nitrogen (N) cycling through litter, soil, and vegetation in lodgepole pine ( Pinus contorta var. latifolia) forests of the Greater Yellowstone Ecosystem (WY, USA) across a 0–30 year chronosequence of time-since-beetle disturbance. Recent (1–4 years) bark beetle disturbance increased total litter depth and N concentration in needle litter relative to undisturbed stands, and soils in recently disturbed stands were cooler with greater rates of net N mineralization and nitrification than undisturbed sites. Thirty years after beetle outbreak, needle litter N concentration remained elevated; however total litter N concentration, total litter mass, and soil N pools and fluxes were not different from undisturbed stands. Canopy N pool size declined 58% in recent outbreaks, and remained 48% lower than undisturbed in 30-year old outbreaks. Foliar N concentrations in unattacked lodgepole pine trees and an understory sedge were positively correlated with net N mineralization in soils across the chronosequence. Bark beetle disturbance altered N cycling through the litter, soil, and vegetation of lodgepole pine forests, but changes in soil N cycling were less severe than those observed following stand replacing fire. Several lines of evidence suggest the potential for N leaching is low following bark beetle disturbance in lodgepole pine.

Observations and modeling of aboveground tree carbon stocks and fluxes following a bark beetle outbreak in the western United States

Bark beetle epidemics result in tree mortality across millions of hectares in North America. However, few studies have quantified impacts on carbon (C) cycling. In this study, we quantified the immediate response and subsequent trajectories of stand-level aboveground tree C stocks and fluxes using field measurements and modeling for a location in central Idaho, USA that experienced an outbreak of mountain pine beetle (Dendroctonus ponderosae Hopkins). We measured tree characteristics in lodgepole pine (Pinus contorta) plots spanning a range of structure and mortality conditions. We then initialized the forest vegetation simulator, an individual tree-based model, with these measurements and simulated the response of aboveground production of C fluxes as well as trajectories of C stocks and fluxes in the coming decades. Mountain pine beetles killed up to 52% of the trees within plots, with more larger trees killed. C stocks in lodgepole pine were reduced by 31-83% following the outbreak, and plot-level C fluxes decreased 28-73%. Modeled C stocks increased nearly continuously following the infestation, recovering to preoutbreak levels in 25 years or less. Simulated aboveground tree C fluxes increased following the immediate postoutbreak decrease, then subsequently declined. Substantial variability of C stocks and fluxes among plots resulted from the number and size of killed and surviving trees. Our study illustrates that bark beetle epidemics alter forest C cycling unlike stand-replacement wildfires or clear-cut harvests, due in part to incomplete mortality coupled with the preference by beetles for larger trees. The dependency of postoutbreak C stocks and fluxes on stand structure suggests that C budget models and studies in areas experiencing mountain pine beetle disturbances need to include size distribution of trees for the most accurate results.

ForestSimMPB: A swarming intelligence and agent-based modeling approach for mountain pine beetle outbreaks

The widespread outbreaks of Mountain Pine Beetle (MPB) are responsible for infestations of lodgepole pine forests since 1990 in Canada. In British Columbia, this forest insect disturbance has resulted in losses of more than 13 million hectares of pine trees. The complexity of the MPB emergence, aggregation and attack behaviour is captured by this study, using an intelligent agent-based model (ABM) of beetle outbreaks at a local scale of individual trees. Agent-based approach permits simulation of interactions that describe the ecological context in which insect populations spread. Intelligent reasoning is introduced by a swarm intelligence (SI) algorithm integrated with the ABM that depicts indirect communication, collective behaviour and self-organized aggregation of insects in a forest ecosystem. The objectives of this study are the following: 1) to develop ForestSimMPB model that integrates SI and ABM within a geographic information systems (GIS) framework; 2) to implement the proposed model on real datasets to simulate the MPB aggregation and mass attacks on lodgepole pine trees; and 3) to determine the spatial patterns and extents of these attacks. The ForestSimMPB is calibrated by fine tuning two model parameters, and implemented using data from three sites located in the Cariboo Regional District in the central interior of BC. The obtained results demonstrate the aggregation behaviour of MPB to collectively attack lodgepole pines, as well as portray the spatial clustering of dead trees resulting from infestation. Simulation outputs provide analysis and predictions of spatial patterns in the forest landscape structure as a result of a MPB outbreak. The developed model can be used to assist the improvement of methods for prevention and control of MPB disturbances.