Insect interactions in Douglas-fir forests in the Northern Rockies
Disturbances, such as bark beetles, drought, and wildfire, are reshaping Douglas-fir forests throughout the Western United States. We are conducting a series of projects to help better understand the effects of these disturbances and to inform management actions that may be used to reduce damages to forests. Using multiple targeted case studies that were co-produced with forest managers, we are investigating how prior disturbances (drought, fire) and stand conditions influence the likelihood of insect outbreaks in Douglas-fir forests.
- Overview
- Research
- Bark Beetle Spillover
- Douglas-fir Hazard
- Climate Effects on Defoliator Outbreaks
- Defoliator and Bark Beetle Interactions
- People
- Resources
A patch of dead Douglas-fir caused by Douglas-fir beetle.
Douglas-fir (Pseudotsuga menziesii) is the most commercially valuable and abundant tree species in the Northern Rocky Mountains. Multiple disturbances stress and cause damage to Douglas-fir, including several insect species. The amount of tree mortality from insects and other disturbances has increased substantially in recent decades, contributing to critical losses of timber resources and high vulnerability to future losses.
Damages from a disturbance may also alter the dynamics of subsequent disturbances (i.e., disturbance interaction). Management actions can help reduce the chances of subsequent damage from disturbances. Understanding which disturbances interact to amplify damage and which management strategies may reduce timber loss is critical for conserving future timber resources. Through multiple case studies and tools, this collection of projects investigates the relationships between disturbances and silvicultural management strategies that help reduce tree mortality and improve forest health.
Keywords: Forest insects, Douglas-fir beetle, western spruce budworm, wildfire, disturbance interactions
A patch of dead Douglas-fir caused by Douglas-fir beetle.
We are conducting four projects that examine urgent forest health issues in Douglas-fir forests in the Northern Rocky Mountains. On-the-ground needs directly informed this work, and we developed all case studies in response to requests or discussions with entomologists and coordinators in the Forest Service’s Northern Region. The research context varies for each study, but the analytical approaches combine field data with geospatial analysis. Below are brief descriptions of each project. For further details, see the associated tab.
- Bark Beetle Spillover: There has been concern about the potential for populations of tree-killing Douglas-fir beetles (Dendroctonus pseudotsugae) to rapidly increase in fire-injured trees and then spread, or spill over, to unburned forests. This could cause otherwise healthy, green trees to die. After analyzing the pattern of tree mortality at multiple spatial and temporal scales, we found only limited evidence of Douglas-fir beetle spillover, particularly beyond 0.5 km of burned areas. Our research clarifies the likelihood of bark beetle spillover and helps managers understand what unburned areas are most likely to be affected by bark beetle spillover if it does occur.
- Douglas-fir Hazard: Forest managers can manipulate stand conditions by selectively thinning trees or using other silviculture practices to reduce tree mortality. However, managers often lack science-based tools to back their decision making. We are developing a hazard assessment model for multiple co-occurring damage agents of Douglas-fir to help managers prioritize desirable stand conditions for silvicultural treatment and support adaptive forest management.
- Climate and Defoliator Outbreaks: Specific climate conditions can initiate insect outbreaks across large geographic areas. Yet, the conditions that cause large outbreaks of insects, like western spruce budworm, which severely defoliates trees, are largely unknown. We are reconstructing outbreaks of western spruce budworm over multiple centuries to understand the climate conditions associated with the beginning and end of defoliator outbreaks. These findings may help managers anticipate defoliator outbreaks and plan management actions accordingly.
- Defoliator and Bark Beetle Interactions: When Douglas-fir beetles attack Douglas-fir trees, neighboring and seemingly identical trees may die from insect damage or be unaffected. To better understand the preferences of Douglas-fir beetles, we are examining how defoliation (removal of needles by insects) history and reduced tree vigor from drought may increase susceptibility to mortality from bark beetles using tree ring methods. Our results will assist forest managers in selecting individual trees with greater susceptibility to mortality for thinning or other management practices.
Bark Beetle Spillover
Bark beetles interact with fire in complex and sometimes poorly understood ways. Since 2000, increased fire activity in the Western United States has produced an abundance of fire-injured trees that are at risk of mortality due to bark beetle damage. Bark beetles may reproduce in these already stressed trees and can potentially move from fire-injured trees to healthy host trees outside the burned area. This is called “spillover.” Observed tree mortality from bark beetles surrounding burned areas has raised concerns among land managers about the likelihood of spillover and the clear need for further investigation.
Forest burned by the Rice Ridge Fire, 2017 (orange dashed line) and patches of Douglas-fir mortality outside the burn perimeter (yellow line) that may have resulted from Douglas-fir beetle dispersing from fire-injured trees. Douglas-fir beetles from fire-injured trees do not appear to initiate broad-scale outbreaks, but they may be associated with Douglas-fir mortality at short distances (0.25 km) from burned areas.
Douglas-fir beetle frequently colonizes fire-injured Douglas-fir trees. We investigated spatiotemporal patterns of Douglas-fir tree mortality from Douglas-fir beetle in unburned areas surrounding 61 wildfires that occurred from 2000 to 2017 in the Northern Rocky Mountains with a high likelihood of spillover. We developed a multiscale analytical framework to examine tree mortality that was potentially associated with spillover following fire.
A key marker for Douglas-fir beetle spillover from burned areas is high levels of tree mortality from bark beetles in unburned areas within the common flight distance of Douglas-fir beetles (about 0.5 km). We found similar levels of tree mortality within and beyond 0.5 km from burned areas over multiple years when examining all fires, suggesting minimal spillover. The similarity in tree mortality levels at multiple distances from burned areas suggests that broader scale processes, such as drought, mainly influence Douglas-fir beetle activity, rather than the proximity to burned trees. However, we could not rule out the influence of Douglas-fir beetle spillover into unburned areas that were within 0.5 km of burned areas. Beyond 0.5 km, Douglas-fir beetles were more likely to disperse from patches of beetle-caused mortality from the prior year.
Although we did not detect evidence for Douglas-fir beetle spillover across all fires, we did find some individual fires (8 to 15 percent) where Douglas-fir mortality from beetles was much higher surrounding the burned areas than farther away, suggesting the presence of Douglas-fir beetle spillover. In all fires, spillover was limited to an area of less than 1 km from burned areas. Importantly, tree mortality from spillover in our study fires, which we selected based on a high likelihood of spillover, was only 0.2 to 0.3 percent of total Douglas-fir mortality during the study period.
The extent of the study area in the Northern Rocky Mountains (thick black line), areas burned in 61 wildfires from 2000 to 2017 (orange), Douglas-fir (DF) mortality from Douglas-fir beetle (DFB) from 2000 to 2021 (purple), Random Forest-predicted DF forest susceptible to DFB (green), DF forest distribution (gray), locations of field sites (five plots per sampling area, triangles), and ecoregions (yellow lines). Inset: The study area in the Northern Rocky Mountains, spanning eastern Washington, northern Idaho, and western Montana (Canadian border to southern Montana, northeastern Washington to Big Belt mountains). Map by Robert Andrus.
For individual fires with evidence of spillover, we found that spillover was not associated with climate conditions that increase host tree stress. Rather, it was associated with higher levels of Douglas-fir mortality caused by Douglas-fir beetle in the prior year within the same unburned area, which complicates the link between tree mortality and Douglas-fir beetle spillover. Furthermore, field assessment of patches of tree mortality from Douglas-fir beetle in unburned areas surrounding burned areas suggested that other disturbances at the site, such as blowdown and root disease, were more likely causes for Douglas-fir beetle activity. To determine the risk of Douglas-fir beetles emigrating from burned areas, land managers may consider site-specific monitoring of postfire Douglas-fir beetle populations in susceptible, unburned Douglas-fir forests adjacent to fires. These findings inform postfire planning and clarify the ecological implications of disturbance interactions.
An open-source publication from this project is available here.
Douglas-fir Hazard
Forest Inventory and Analysis plots (n = 884) that we included in the analysis (black triangles) overlaid on the distribution of interior Douglas-fir (gray). West and east ecoregions are outlined in green and brown, respectively. In the map, we classify plots with less than or greater than 10-percent basal area mortality of Douglas-fir during the 10-year remeasurement period. Inset: The study area in the Northern Rocky Mountains, primarily in northern Idaho and western Montana. Map by Robert Andrus.
Land managers use susceptibility and risk rating systems to help plan for potential future damage and risks to forests. These rating systems are typically designed for individual damage agents. However, tree mortality often results from the interaction of multiple damage agents. In Northern Rocky Mountain Douglas-fir (Pseudotsuga menziesii var. glauca) forests, multiple damage agents are commonly associated with Douglas-fir mortality, including insects, disease, weather, and fire.
We investigated how recent mortality from insects and diseases shifted stand structure and composition in Douglas-fir forests. We specifically examined how susceptibility (e.g., stand structure and composition, topography, and spatial variability in climate) and risk (biotic agent pressure like insect and disease outbreaks) influenced mortality. We used Forest Inventory and Analysis program data from 884 plots—initially measured from 2003 to 2009 and remeasured 10 years later—in our multiscale analysis. Several spatial datasets supported the analysis.
Across a large, forested landscape, 60 percent of the plots had no new Douglas-fir mortality during the study period. Resource managers are concerned about stand health and timber production when mortality levels exceed 10 percent of basal area. We found that most plots (80 percent) experienced mortality that was less than 10 percent of basal area. We found severe tree mortality—more than 25 percent loss of Douglas-fir basal area—in only 6 percent of plots. Most dead trees (68 percent) were smaller diameter (12.7–29 cm diameter at breast height [DBH]), and mortality rates of smaller diameter trees were significantly higher than larger diameter trees (greater than 29 cm DBH). Following Douglas-fir mortality, average tree size increased in most plots, and only a small number of Douglas-fir-dominated plots with severe mortality shifted in dominance to another tree species.
Based on our models, basal area mortality (percentage of loss of initial basal area) was higher in stands with lower tree growth rates, larger average tree sizes, greater availability of Douglas-fir tree hosts for biotic agents, cooler and wetter topoclimatic locations, and higher Douglas-fir beetle populations within 1 km (as inferred from aerial surveys of tree mortality). The relative importance of each variable and the thresholds associated with higher mortality differed between stands west and east of the Continental Divide in the Northern Rocky Mountains.
The susceptibility and risk thresholds, along with the variables associated with higher Douglas-fir mortality that we identified in this project, will support adaptive forest management for multiple damage agents and help reduce tree mortality of this widespread and abundant conifer.
Climate Effects on Defoliator Outbreaks
(A) The study area in the Northern Rocky Mountains, primarily in northern Idaho and western Montana (bolded polygon) and the distribution of the dominant western spruce budworm (WSB) host tree species, including Douglas-fir or true fir/spruce. (B) Locations of the 23 sampled sites (triangles) where we reconstructed WSB outbreak histories from tree rings. The background layer shows the years with WSB activity (presence) as a percentage of the years flown (to account for variability in sampling) by the USDA Forest Service Aerial Insect and Disease Survey, 1962 to 2022. Not all locations were flown each year by the aerial survey, so the percentage of years was divided by the number of years that the aerial survey was flown at each location. (C) A time series of area of WSB activity (from B) divided by the areas flown east and west of the Continental Divide. Prior to 1999 (vertical dashed line), the extent of areas flown were not mapped and thus pseudoextents were developed, which likely resulted in higher estimates of WSB activity. WSB activity for three periods based on breaks in WSB activity by ecoregion: (D) 1962 to 1979, (E) 1980 to 1994, and (F) 2001 to 2020 (see legend in part B). Figures by Robert Andrus.
The western spruce budworm (Choristoneura occidentalis) is the most damaging defoliating insect in Western North American conifer forests. Budworms feed on new needles and buds of Douglas-fir and true firs, reducing timber yield and carbon uptake. Typically, budworm populations are small and cause relatively minor damage. However, when favorable climate conditions align with an abundance of suitable host trees (Douglas-fir and true firs), western spruce budworm populations may grow rapidly and cause severe damage and tree mortality. From 1962 to 2022, western spruce budworm affected more than 100,000 km2 of forestland in the Northern Rocky Mountains. About 30,000 km2 of forest land experienced 10 or more years of defoliation from 1962 to 2022. Understanding the climate conditions associated with the beginning and end of western spruce budworm outbreaks will help forest managers forecast outbreak periods and their impacts.
We are using tree ring methods to reconstruct budworm activity over multiple centuries at about 15 sites across a moisture gradient. Then, we will relate outbreak periods to climate variability in a large forest landscape that has experienced severe and chronic damage from western spruce budworm.
Defoliator and Bark Beetle Interactions
Field data collection sites (triangles) and areas with western spruce budworm defoliation (2011 to 2022) following Douglas-fir beetle attacks (2023 to 2024) (orange dots). We stratified field sites across a gradient of climatic water deficit (CWD). Higher CWD represents warmer and drier conditions, and lower numbers represent cooler and wetter conditions. CWD is clipped to the distribution of Douglas-fir, the study species. Inset: The study area, Northern Rocky Mountains spanning eastern Washington, northern Idaho, and western Montana (Canadian border to southern Montana, northeastern Washington to Big Belt mountains). Map by Robert Andrus.
Douglas-fir beetle is the most damaging biotic agent of Douglas-fir. Over large forest landscapes, defoliation by western spruce budworm and Douglas-fir tussock moth often precedes increases in Douglas-fir beetle activity. Defoliation reduces photosynthesis and carbon uptake, possibly increasing the susceptibility to bark beetle attack. However, research is needed to better understand the link between defoliation and greater susceptibility to bark beetle attack at the individual tree scale (rather than broad spatial extents). This will help determine whether defoliation duration or severity influences the likelihood of Douglas-fir beetle attack.
Additionally, Douglas-fir beetles are thought to prefer slower growing, low-vigor Douglas-fir trees, potentially with lower defense capacity, and stands with slower growth are more susceptible to mortality. The ways growth rates (i.e., vigor) influence the susceptibility to attack by bark beetles has been studied in multiple pine species but not in Douglas-fir.
Douglas-fir beetle activity has recently increased in the Northern Rocky Mountains, and managers are requesting indicators that help forecast tree mortality. We are sampling live and dead Douglas-fir trees that have been attacked by Douglas-fir beetle at multiple sites across a moisture gradient to understand how defoliation history and growth rates influence mortality from Douglas-fir beetle.
Investigator
- Robert Andrus, research fellow with the Oak Ridge Institute for Science and Education, hosted by the USDA Forest Service Western Wildland Environmental Threat Assessment Center
Collaborators
- Arjan Meddens, Washington State University
- Brytten Steed, formerly USDA Forest Service Northern Region, Forest Health Protection
- Henry Adams, Washington State University
- James Steed, USDA Forest Service Forest Inventory and Analysis
- Jeffrey Hicke, University of Idaho
- Joel Egan, formerly USDA Forest Service Northern Region, Forest Health Protection,
- John Goodburn, University of Montana
- Jordan Lestina, USDA Forest Service Northern Region, Forest Health Protection
- Patrick Bennet, formerly Rocky Mountain Research Station
Related Programs
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Center or Group
https://research.fs.usda.gov/pnw/centers/wwetac
Related Publications
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Andrus,
Robert A., Joel Egan, Nathan Ivy, Laura Lowrey, Cameron E. Naficy, Brytten Steed, and Arjan Meddens. 2025. “ Limited Tree Mortality in Unburned Areas Linked to Bark Beetle Spillover from Wildfires.” Ecological Applications 35(5): e70066. https://doi.org/10.1002/eap.70066